Competition for electron transfer between cytochromes P450scc and P45011 beta in rat adrenal mitochondria

Rat adrenal mitochondria contain approximately equal levels of P450scc and P45011 beta, each reduced by NADPH through adrenodoxin reductase (ADX-reductase) and adrenodoxin (ADX). Constitutive cholesterol side-chain cleavage (SCC) can be increased over 20-fold through a combination of hormonal activation and inhibition of cholesterol metabolism in vivo prior to isolation of the mitochondria. This stimulation, which results from accumulated reactive cholesterol, does not significantly affect either the dependence of activities on the concentration of isocitrate (IC) and succinate (SU) or the ratio of maximum activities [3:1] supported by these reductants. Thus, the rate of cholesterol SCC is determined independently by electron transfer and the amount of reactive cholesterol. Hydroxylation of deoxycorticosterone (11 beta and 18 positions) required much higher levels of each reductant, indicating less effective reductant transfer to P45011 beta. Reactions at P450scc and P45011 beta, mediated by IC, are enhanced by low concentrations of various dicarboxylates anions (fumarate, SU). The actions of SU dehydrogenase inhibitors and the activity of fumarate, a poor direct reductant, suggest that higher production of NADPH results from malate-enhanced uptake of isocitrate. Only synergistic combinations of reductants are sufficient to sustain maximum rates of 11-deoxycorticosterone (DOC) metabolism, whereas IC is fully effective for P450scc. Increased reaction at P450scc (cholesterol loading or addition of 20 alpha-hydroxycholesterol) decreased simultaneous DOC metabolism at P45011 beta in inverse proportion to the estimated intramitochondrial generation of NADPH (1 mM or 10 mM SU > 1 mM IC > 10 mM IC). These decreases were reversed by inhibition of P450scc. Crossover inhibition caused by maximum DOC metabolism was less pronounced. EGTA/albumin treatment, which enhanced activities at both P450scc and P45011 beta, presumably via increased NADPH, diminished this cross-competition. The differential dependence on reductants and the characteristics of crossover competition are consistent with a roughly three-fold more favorable partitioning of electron transfer to P450scc, possibly caused by preferential interaction of reduced adrenodoxin with P450scc.

Reversal of cytochrome P-4501A1 and P-450-EF expression in MCA-C3H/10T1/2 cell-derived tumors as compared to cultured cells

The 3-methylcholanthrene-transformed tumorigenic cell line, MCA-C3H/10T1/2 CL15 (MCA), expresses the novel benz(a)anthracene (BA)-inducible polycyclic aromatic hydrocarbon-metabolizing cytochrome P-450 (P-450-EF). The level of expression is comparable to that reported for the nontumorigenic C3H/10T1/2 CL8 (10T1/2) cells (Pottenger, L. H., Christou, M., and Jefcoate, C. R. Arch. Biochem. Biophys., 286: 488-497, 1991). Sarcomas (3-12 mm in diameter) generated in athymic "nude" mice by s.c. injection of MCA cells exhibited much lower 7,12-dimethylbenz(a)anthracene-metabolizing activities (5-15% of the levels in cultured cells), both constitutively and after in vivo treatment with BA. A sharp decrease in P-450-EF expression was observed both at the functional level (10- to 30-fold) (as determined by antibody inhibition studies) and at the apoprotein level (50- to > 100-fold) (as determined by Western immunoblots). However, in contrast to the BA-treated MCA cells in which P-450-EF comprises essentially the total spectrally detectable P-450 content (approximately 30 pmol/mg) with virtually undetectable cytochrome P-4501A1, tumors from these cells expressed substantial levels of P-4501A1 immunodetectable protein in response to BA treatment (approximately 0.5-3 pmol/mg). In these tumors, P-450-EF expression decreased to undetectable or barely detectable levels (< 0.2-0.5 pmol/mg). P-4501A1-expressing cells were localized in tumor sections immunocytochemically and were morphologically identical to other MCA cells, which formed the majority of the sarcoma. At the functional level, antibody inhibition studies and product ratios demonstrated that P-4501A1 accounted for only 40% of the total dimethylbenz(a)anthracene-metabolizing activity of BA-induced tumor microsomes, whereas the remaining activity was due to P-450-EF. This low catalytic activity for P-4501A1 (35-95 pmol/mg/h) indicated that the majority of BA-inducible P-4501A1 in the tumors (> 95%) was expressed as apoprotein. The contribution from P-450-EF was consistent with full expression of hemoprotein. Tumor size affected the total dimethylbenz(a)anthracene-metabolizing activities/mg microsomal protein (small tumors were 2- to 3-fold more active than large tumors) but had no effect on the ratio of activities dependent on, respectively, P-450-EF and P-4501A1 holoenzymes (1.5:1), thus suggesting controlled coexpression of these proteins. Reculturing of tumor-derived cells effected partial restoration of P-450-EF expression and eliminated the expression of P-4501A1, confirming a unique contribution from tumor environment to the regulation of these genes. Possible mechanisms for an environment-dependent concerted regulation of the P-4501A1 and P-450-EF genes are discussed.

Regulation of cholesterol movement to mitochondrial cytochrome P450scc in steroid hormone synthesis

Transfer of cholesterol to cytochrome P450scc is generally the rate-limiting step in steroid synthesis. Depending on the steroidogenic cell, cholesterol is supplied from low or high density lipoproteins (LDL or HDL) or de novo synthesis. ACTH and gonadotropins stimulate this cholesterol transfer prior to activation of gene transcription, both through increasing the availability of cytosolic free cholesterol and through enhanced cholesterol transfer between the outer and inner mitochondrial membranes. Cytosolic free cholesterol from LDL or HDL is primarily increased through enhanced cholesterol ester hydrolysis and suppressed esterification, but increased de novo synthesis can be significant. Elements of the cytoskeleton, probably in conjunction with sterol carrier protein(2) (SCP(2)), mediate cholesterol transfer to the mitochondrial outer membranes. Several factors contribute to the transfer of cholesterol between mitochondrial membranes; steroidogenesis activator peptide acts synergistically with GTP and is supplemented by SCP(2). 5-Hydroperoxyeicosatrienoic acid, endozepine (at peripheral benzodiazepine receptors), and rapid changes in outer membrane phospholipid content may also contribute stimulatory effects at this step. It is suggested that hormonal activation, through these factors, alters membrane structure around mitochondrial intermembrane contact sites, which also function to transfer ADP, phospholipids, and proteins to the inner mitochondria. Cholesterol transfer may occur following a labile fusion of inner and outer membranes, stimulated through involvement of cardiolipin and phosphatidylethanolamine in hexagonal phase membrane domains. Ligand binding to benzodiazepine receptors and the mitochondrial uptake of 37 kDa phosphoproteins that uniquely characterize steroidogenic mitochondria could possibly facilitate these changes. ACTH activation of rat adrenals increases the susceptibility of mitochondrial outer membranes to digitonin solubilization, suggesting increased cholesterol availability. Proteins associated with contact sites were not solubilized, indicating that this part of the outer membrane is resistant to this treatment. Two pools of reactive cholesterol within adrenal mitochondria have been distinguished by different isocitrate- and succinate-supported metabolism. These pools appear to be differentially affected in vitro by the above stimulatory factors.

Polycyclic aromatic hydrocarbon metabolism in rat adrenal, ovary, and testis microsomes is catalyzed by the same novel cytochrome P450 (P450RAP)

A novel ACTH-inducible P450, cytochrome P450RAP, is responsible for polycyclic aromatic hydrocarbon (PAH) metabolism in male rat adrenal microsomes. P450RAP is present at similar levels in male and female adrenal microsomes and is immunochemically distinct from P450IA1. Anti-P450RAP immunoblots a protein present in ovarian and testicular microsomes that is the same size as P450RAP and which coelutes with the P450 fraction during chromatography on an immobilized artificial membrane column made with phosphatidylcholine. Rat adrenal, ovarian, and testicular microsomes exhibit similar regioselectivities in the metabolism of two polycyclic aromatic hydrocarbons, dimethylbenz(a)anthracene (DMBA) and benzo(a)pyrene (BP). Unlike P450IA1, these microsomes form little or no 7-OH-DMBA and BP-4,5-diol but do catalyze the formation of a high proportion of the presumptive procarcinogen, DMBA-3,4-diol. The relative activities of PAH metabolism by untreated adrenal, testicular, and ovarian microsomes are approximately 60, 20, and 6 pmol/mg microsomal protein/min, respectively. PMSG induced PAH metabolism 2- to 5-fold in ovarian microsomes and also increased the P450RAP immunoblot. Hypophysectomy reduced PAH metabolism 3-fold in testicular microsomes while also decreasing the P450RAP immunoblot. This close correlation between PAH metabolism and expression of P450RAP indicates the involvement of the cytochrome in this activity. DMBA and BP metabolism by PMSG-treated rat ovarian microsomes and untreated testicular microsomes are each completely inhibited by anti-P450RAP but are not inhibited by anti-P450IA1. Essentially all of the PAH metabolism in rat adrenal, testis, and ovary is, therefore, catalyzed by P450RAP, which is hormonally elevated in each tissue by a variety of possible mechanisms, including induction and selective proliferation of cells that express this protein.

Selective suppression of the catalytic activity of cDNA-expressed cytochrome P4502B1 toward polycyclic hydrocarbons in the microsomal membrane: modification of this effect by specific amino acid substitutions

Human hepatoma HEPG2 cells were infected with recombinant vaccinia virus vectors containing cDNAs encoding both known and variant rat cytochromes P450 (CYP). CYP2B1 and CYP2B2 cytochromes were equally well expressed (110-140 pmol/mg of microsomal protein) and catalyzed metabolism of 7,12-dimethylbenz[a]anthracene (DMBA). Their regioselectivity for DMBA metabolism paralleled that of the respective purified rat liver enzymes and reproduced previously reported regioselective differences between CYP2B1 and CYP2B2 [Wilson et al. (1984) Carcinogenesis 5, 1475-1483]. CYP2A1 and CYP2A2 expressed in HEPG2 microsomes exhibited nearly equal DMBA-metabolizing activities that closely matched that of purified CYP2A1. Although purified rat liver CYP2B1 was 3 times more active than purified rat liver CYP2B2, the expressed recombinant microsomal CYP2B1 (rCYP2B1) was 20 times less active than rCYP2B2, where activity matched that of the purified cytochrome. Microsomal suppression of rCYP2B1 catalytic activity was also observed for benzo[a]pyrene. Specific amino acid substitutions at equivalent positions of the completely homologous NH2-terminal halves of rCYP2B1 and rCYP2B2 changed this suppression effect. Thus, a L58----F, I114----F double mutant exhibited 3 times the normal activity for rCYP2B1 while remaining inhibitory for rCYP2B2. The single substitutions produced very different effects. The L58----F substitution prevented expression of rCYP2B1, while the I114----F substitution was inhibitory for both rCYP2B1 and rCYP2B2 (40 and 70%). A single E282----V mutation produced a stimulation of rCYP2B1 activity comparable to that of the L58----F, I114----F double substitution.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of monoterpenoids on in vivo DMBA-DNA adduct formation and on phase I hepatic metabolizing enzymes

We have previously demonstrated the anticarcinogenic effects of monocyclic monoterpenes such as limonene when given during the initiation phase of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary cancer in Wistar-Furth (WF) rats. Here we investigated the possible mechanisms for this chemoprevention activity including limonene's effects on DMBA-DNA adduct formation and hepatic metabolism of DMBA. Twenty-four hours after carcinogen administration, there were approximately 50% of the total DMBA-DNA adducts found in control animals formed in the liver, spleen, kidney and lung of limonene-fed animals. While circulating levels of DMBA and/or its metabolites were not different in control and limonene-fed rats, there was a 2.3-fold increase in DMBA and/or DMBA-derived metabolites in the urine of the limonene-fed animals. Studies of the effects of limonene and sobrerol, a hydroxylated monocyclic monoterpenoid with increased chemoprevention activity, on phase I metabolizing enzymes revealed that these terpenoids modulated cytochrome P450 (CYP) and epoxide hydratase (EH) activity. The 5% limonene diet increased total CYP to the same extent as phenobarbital (PB) treatment when compared to control, while 1% sobrerol (isoeffective in chemoprevention to 5% limonene) did not. However, both 5% limonene and 1% sobrerol diets greatly increased the levels of microsomal EH protein and associated hydrating activities towards benzo[a]pyrene 4,5-oxide when compared to control and PB treatment. These changes also modified the rate and regioselectivity of in vitro microsomal DMBA metabolism when compared to PB treatment or control. Identification of the specific isoforms of CYP induced by these terpenoids was performed using antibodies to CYP isozymes in Western blot analysis and inhibition studies of microsomal DMBA metabolism. Five per cent limonene was more effective than 1% sobrerol at increasing the levels of members of the CYP2B and 2C families but was equally effective at increasing EH. Furthermore, both terpenoid diets caused increased formation of the proximate carcinogen, DMBA 3,4-dihydrodiol. While these terpene-induced changes in hepatic CYP and EH do not explain the anticarcinogenic mechanism of these chemopreventive agents, or the ability of limonene systemically to reduce DMBA-DNA binding, they do reveal novel and selective induction mechanisms of hepatic enzymes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits steroidogenesis in the rat testis by inhibiting the mobilization of cholesterol to cytochrome P450scc

Testosterone synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is decreased because pregnenolone production by the testis is inhibited. This inhibition can only be caused by a reduction in the activity of the mitochondrial enzyme which converts cholesterol into pregnenolone (cytochrome P450scc), and/or by an impairment in the multistep process by which luteinizing hormone (LH) stimulates the mobilization of cholesterol to this enzyme. Seven days after rats were treated with 100 micrograms TCDD/kg, testicular cytochrome P450scc activity (assayed with 20 alpha-hydroxycholesterol as substrate) was decreased to 45% of control. If this decrease were responsible for the inhibition of testicular steroidogenesis in vivo, substrate pools for cytochrome P450scc in the testis would be increased. Yet TCDD decreased the amount of cholesterol that was readily available to cytochrome P450scc in isolated testis mitochondria (the reactive cholesterol pool), even when steroidogenesis was maximally stimulated in vivo with the LH analogue human chorionic gonadotropin (hCG). These decreases in substrate pools were not due to a reduction in mitochondrial capacity for reactive cholesterol. We conclude that the 55% decrease in cytochrome P450scc activity is not severe enough to inhibit testicular steroidogenesis in vivo. Instead, TCDD must act by inhibiting the LH-stimulated mobilization of cholesterol to cytochrome P450scc. This conclusion is supported by two observations. First, when pregnenolone formation was blocked by treating rats with the cytochrome P450scc inhibitor aminoglutethimide, TCDD greatly reduced the rate at which hCG caused reactive cholesterol to accumulate in testis mitochondria in vivo. Second, TCDD inhibited both testosterone synthesis and the mobilization of cholesterol to cytochrome P450scc within 1 day. The steroidogenic inhibition does not appear to be due to an LH receptor defect, because TCDD inhibited dibutyryl cAMP- and hCG-stimulated steroid secretion by isolated perfused testes to comparable extents. We conclude that TCDD inhibits testicular steroidogenesis predominantly if not exclusively by inhibiting the mobilization of cholesterol to cytochrome P450scc, and that this inhibition occurs subsequent to cAMP formation.

Cytochrome P-450scc induces vesicle aggregation through a secondary interaction at the adrenodoxin binding sites (in competition with protein exchange

Addition of bovine adrenal cytochrome P-450scc to small unilamellar dioleoylphosphatidylcholine vesicles (DOPC-SUV) produces a complex sequence of interactions, indicating exceptional cytochrome mobility. First, cholesterol transfer from cytochrome to vesicles indicated rapid dissociation of P-450scc oligomers and integration of monomers into the membrane (delta A 390-420 nm; t1/2 = 2 s). After 10-15 s, P-450scc-induced aggregation of the vesicles starts, as indicated by increased turbidity (delta A 448 or 520 nm; complete in 6-8 min). Fluorescence quenching experiments indicate that this aggregation does not lead to measurable vesicle fusion during this period. Aggregation is prevented by mild heat denaturation of P-450scc, by addition of anti-P-450scc IgG, and also by 1:1 complex formation with the electron donor adrenodoxin (ADX). P-450scc, therefore, links two vesicles through two separate domains involved in, respectively, membrane integration (lipophilic) and ADX binding (charged). Although completely bound by DOPC-SUV, as evidenced by Sephadex elution, P-450scc has access within 1 min to cholesterol in secondary SUV. This is indicated by spectral changes (cholesterol complex formation) and by metabolism of secondary vesicle cholesterol. Since cholesterol equilibrates slowly between vesicles (t1/2 = 1-2 h), these changes arise from P-450scc transfer. This transfer was maximally slowed after a 5-min preincubation with primary vesicles, reflecting more extensive integration into the membrane than is necessary for the rapid initial cholesterol transfer to P-450scc. P-450scc transfer probably results from simultaneous interaction of P-450scc with two vesicles that may also initiate aggregation. Weaker integration into primary dimyristoylphosphatidylcholine vesicles facilitates exchange but prevents aggregation. Integration and aggregation are both enhanced by incorporation of 10% phosphatidylinositol into SUV, while exchange is slowed. This mobility of P-450scc is most probably a consequence of the absence of amino-terminal anchoring. P-450scc-induced association of inner mitochondrial membrane segments may contribute to the exceptionally vesiculated structure of adrenal and ovarian mitochondria that parallels increased P-450scc content.

Purification and immunological characterization of a novel cytochrome P450 from C3H/10T1/2 cells

The major polycyclic aromatic hydrocarbon-metabolizing cytochrome P450 in the mouse embryo fibroblast-derived C3H/10T1/2 CL8 cell line (P450-EF) has been partially purified from benz[a]anthracene (BA)-induced 10T1/2 cells (40 pmol P450/mg). The purification of P450-EF was carried out by sequential chromatography of solubilized microsomes over hydrophobic aminohexyl-Sepharose 4B, anion exchange DE-52 cellulose, and cation exchange carboxymethyl trisacryl columns. The final preparation (1700 pmol/mg) appeared as a single major 55-kDa band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Reconstitution of detergent-free partially purified P450-EF yielded a relatively high turnover for 7,12-dimethylbenz[a]anthracene (DMBA) metabolism (5.4 nmol/nmol/min). Polyclonal antibodies to purified P450-EF (anti-P450-EF), raised in, respectively, rabbit and chicken, detected a single 55-kDa band in 10T1/2 cell microsomes that was highly inducible by BA (approximately 20-fold) and TCDD (approximately 5-fold). Rabbit anti-P450-EF was much more effective than the corresponding chicken antibody at binding denatured P450-EF protein on Western blots. Conversely, only the chicken antibody was effective at inhibiting DMBA metabolism catalyzed by microsomal P450-EF. This antibody did not inhibit P450IA1-mediated DMBA metabolism. Rabbit anti-P450-EF recognized very weakly (less than 1% of homologous protein response) pure P450IA1, IIB1, IIC7, IIE1, and IIIA1 proteins on Western blots but exhibited substantial cross-reactivity (approximately 10%) with pure P450IIA1 and very strong cross-reactivity (approximately 75%) with a hormonally regulated rat adrenal P450. Polyclonal antibodies to several major P450 subfamilies either did not recognize P450-EF (anti-P450IA, IIB, and IIC) or recognized it very weakly (anti-P450IIA1). P450-EF is probably distantly related to the P450IIA subfamily and may belong to a new P450 subfamily.

Succinate stimulation of isocitrate supported deoxycorticosterone metabolism in rat adrenal mitochondria by a synergistic mechanism

The relative effectiveness of succinate and isocitrate in supplying NADPH for cholesterol and deoxycorticosterone (DOC) metabolism by rat adrenal mitochondria has been investigated. As previously seen for cholesterol metabolism, isocitrate supported DOC metabolism at a higher rate than succinate. Maximal rates of DOC metabolism, however, required 10 times more precursor (10mM) than cholesterol metabolism. Addition of DOC to mitochondria inhibited cholesterol metabolism, indicating competition for NADPH between these pathways. Coaddition of these reducing precursors resulted in a substantially greater than additive rate of DOC hydroxylation, but not cholesterol metabolism. The synergistic effect was seen with both 11 beta- and 18 hydroxylation. For both precursors, the synergism was maximal upon addition of only 1 mM of the second precursor. The synergistic effect was far more resistant to added KCN and malonate than succinate supported DOC metabolism, and neither inhibitor affected isocitrate supported DOC metabolism. These results suggest that while cytochromes P450scc and P450(11 beta) use a common supply of NADPH generated by each precursor, there is a pool of NADPH that is only effectively synthesised upon coaddition of precursors and only utilised by cytochrome P450(11 beta). This second NADPH pool may be produced in response to potentiated isocitrate dehydrogenase activity or activation of a different NADPH generating system.

Heterogeneous pools of cholesterol side-chain cleavage activity in adrenal mitochondria from adrenocorticotropic hormone-treated rats: reconstitution of the isocitrate response with succinate and low concentrations of isocitrate

Cholesterol side-chain cleavage in isolated adrenal mitochondria requires unique energy requirements that may determine not only electron transport to P450 but also cholesterol availability. In mitochondria from ACTH-treated rats, two approximately equal pools of reactive cholesterol are indicated by the partial effectiveness of succinate (SU; Type A), and the metabolism of residual cholesterol by 1 mM isocitrate (IC; Type B). Type A metabolism is associated with relatively few initial cholesterol-P450scc complexes and is rapidly and selectively lost when mitochondria are preincubated without an energy source. We now show that cholesterol metabolism supported by IC resolves into equal high and low affinity components (EC50 = 10 and 250 microM) exhibiting, respectively, Type A and Type B characteristics. SU and 50 microM IC, in combination, provided nearly the same activity characteristics as 1 mM IC, including resistance to preincubation and increased turnover of cholesterol-P450scc complexes. Much higher (three to six times) and more sustained pregnenolone formation was seen, with all reductants, following either enhancement of the reactive cholesterol pool or addition of 20-alpha-hydroxycholesterol, indicating that adrenocorticotropic hormone-mitochondria are limited by substrate availability. ATP generation was most effectively supported by SU, and IC was maximally active at 50 microM, emphasizing differences between respiratory and steroidogenic energy requirements. ATP production and the maintenance of uniform suppression after in vivo cycloheximide treatment indicate the integrity of the mitochondrial interaction with all reductants. Inhibitors of SU oxidation (KCN, malonate) strongly inhibited SU-supported cholesterol metabolism but had little effect on SU synergism with IC. Fumarate (but not alpha-ketoglutarate or oxaloacetate) was equally effective as a synergist, but was totally ineffective as a reductant. SU or fumarate, therefore, act by a nonreductive pathway to boost NADPH production from low concentrations of IC. This decrease in apparent Km for IC may be mediated by stimulation of mitochondrial uptake of the reductant through the specific transporters.

Heterogeneous pools of cholesterol side-chain cleavage activity in adrenal mitochondria from ACTH-treated rats: differential responses to different reducing precursors

Side-chain cleavage (SCC) of endogenous cholesterol in adrenal mitochondria isolated from ACTH-treated rats indicates that the size of the reactive cholesterol pool depends on the reducing precursor. At optimal concentrations of reductant, this pool was typically at least 2 times greater for isocitrate than for succinate. Succinate-supported reactions were rapidly completed, were highly sensitive to a 2-min preincubation, and failed to deplete spectrally detected P-450SCC-cholesterol complexes. Cholesterol SCC with 1 mM isocitrate exhibited 2-3 times more fast-phase metabolism, a pronounced slow phase, insensitivity to preincubation, and 60% depletion of spectrally detected cholesterol-P-450SCC complexes. Addition of bovine serum albumin (BSA) and EDTA, either during homogenization or directly to the incubation, prevented preincubation losses in response to succinate and removed most of the difference between succinate and isocitrate activities. This effect of BSA/EDTA was reversed within 5 min by octanoate by a mechanism that was enhanced by Ca2+. These distinct reductant characteristics suggest that only a subpopulation of mitochondria or of pools of activity within individual mitochondria can support cholesterol SCC with succinate while isocitrate is necessary for the remainder. The rapid responses of succinate-supported metabolism to preincubation or to octanoate suggest depletion of a critical factor for cholesterol metabolism. Metabolism of added 20 alpha-hydroxycholesterol or deoxycorticosterone established that NADPH remained fully available after succinate-supported cholesterol metabolism had stopped or after preincubation. Cessation of pregnenolone formation, therefore, results from a failure to supply cholesterol, not inadequate NADPH. The preincubation effect suggests loss of an energy-dependent component that enhances this supply of cholesterol. One possibility tested was that GTP, an activator of intermembrane cholesterol transfer (Xu et al. (1989) J. Biol. Chem. 264, 17674-17680), was being lost. Added GTP slightly activated succinate-supported pregnenolone production but did not prevent preincubation-induced losses. alpha-Ketoglutarate, which can generate matrix GTP, is an effective reductant that, in combination with succinate, prevents preincubation-induced losses.

Differences in the modulation of P450IA1 and epoxide hydratase expression by benz[a]anthracene and 2,3,7,8-tetrachlorodibenzo-p-dioxin in mouse embryo versus mouse hepatoma-derived cell lines

Polycyclic aromatic hydrocarbon (PAH)-induced C3H/10-T1/2/CL8 mouse embryo fibroblasts (10T1/2) and mouse hepatoma-derived Hepa 1c1c7 cells (Hepa-1), exhibit comparable total cytochrome P450 levels and total PAH-metabolizing activities but very different distributions of PAH metabolites. Based on anti-P450IA1-IgG inhibition data, P450IA1 contributes essentially all PAH metabolism in Hepa-1 microsomes but is not involved in PAH metabolism by 10T1/2 cells. In addition, the microsomal epoxide hydratase (EHm) in Hepa-1 cells is far less effective in dihydrodiol (diol) formation compared to that in 10T1/2 microsomes [Pottenger, L.H. and Jefcoate, C.R. Carcinogenesis, 11, 321-327 (1990)]. In the present study, the levels of expression of P450IA1 and EHm proteins and the corresponding mRNAs, both prior to and following exposure to benz[a]anthracene (BA) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), have been correlated with microsomal PAH metabolism by each cell type. In 10T1/2 cells, P450IA1 protein (56 kd) and mRNA (2.6 kb) were detectable at extremely low levels in only two of five cell preparations and then only after maximum induction by TCDD and BA. Thus although 10T1/2 cells contain functional Ah receptors, their capacity to induce P450IA1 is highly suppressed, representing at most 2% of the total P450. TCDD (10 nM) was 4-fold more effective than BA (10 microM) in inducing P450IA1 mRNA, while the levels of immunodetectable protein were comparable. An even greater discrepancy between P450IA1 mRNA and protein levels was seen in BA-induced Hepa-1 cells, where a 4-fold increase in mRNA was paralleled by a 20-fold increase in protein. This difference is probably due to the greater effect of BA depletion on mRNA compared to protein levels. In 10T1/2 cells, BA and TCDD were equally effective at increasing expression of an unidentified 1.9 kb mRNA sequence that blotted very weakly with the P450IA1 cDNA probe. The expression of this mRNA was independent from that of P450IA1. A similar band was visible in Hepa-1 cells less than 1% of the P450IA1 mRNA. EHm mRNA was almost 3-fold higher in 10T1/2 compared to Hepa-1 cells and was unaffected by cell treatments. In Hepa-1 cells, BA and TCDD elevated EHm protein and hydrating activity to levels comparable to those expressed in 10T1/2 cells. It is, therefore, suggested that the relative ineffectiveness of Hepa-1, compared to 10T1/2 EHm, to hydrate low levels of PAH-epoxides is due to differences between the two proteins or their disposition in the microsomal membrane.

Selective expression of cytochrome P450 isozymes by 4-n-alkyl-methylenedioxybenzenes in rat lung cells

Rat lung preparations were analyzed for cytochrome P450 expression as a function of pretreatment with 1 mmol/kg/day of various substituted n-alkyl-methylenedioxybenzenes (MDBs). Lung P450s were quantitated by Western blotting and visualized by immunocytochemical techniques using polyclonal antibodies directed against P450IA1/IA2 and P450IIB1/IIB2. Results demonstrated that n-hexyl and n-octyl-MDB (but not n-butyl-MDB) increased P450IA1 in lung microsomes (50 pmol/mg microsomal protein) compared to untreated controls and n-butyl-MDB-treated animals (6 pmol/mg). However, treatment with all of these MDBs concomitantly decreased P450IIB1 in rat lung microsomes, from 40 pmol/mg in untreated control lungs to below the detection limit of this method (less than 2 pmol/mg). In vitro metabolism assays with rat lung microsomes, utilizing 7,12-dimethylbenz[a]anthracene as substrate, confirmed the increase in P450IA1 by selected MDBs and the concomitant decrease of catalytically active P450IIB1. Immunocytochemistry of rat lungs with these same IgGs indicated selective cellular responses. n-Hexyl-MDB (but not n-butyl-MDB) increased P450IA1-like immunoreactivity, but the increase was specifically localized to Clara cells. Immunocytochemical results further showed that P450IIB1-like immunoreactivity disappeared entirely from alveolar type II cells, yet appeared unchanged in Clara cells relative to untreated controls. Increases in P450IA1 due to treatment with n-hexyl-MDB were associated with increases in P450IA1 mRNA as indicated by Northern blot experiments. In contrast, the decreased levels of P450IIB1, consequent to treatment with n-alkyl-MDBs, were not associated with altered levels of pulmonary P450IIB1 mRNA as determined by Northern blotting and solution hybridization assays. These results imply that n-alkyl-MDBs regulate pulmonary cytochromes P450 by both transcriptional and post-transcriptional mechanisms.

Selective induction of cytochrome P450 isozymes in rat liver by 4-n-alkyl-methylenedioxybenzenes

To examine the structural requirements of cytochrome P450 induction by 4-n-alkyl-substituted methylenedioxybenzenes (MDBs), rats were treated in vivo with a series of MDBs that differed in alkyl carbon side-chain length (0, 1, 2, 3, 4, 5, 6, or 8). Expression patterns of specific P450 isozymes were evaluated with Western and Northern blotting, enzymatic assays, and solution hybridization assays. As determined by carbon monoxide difference spectroscopy, maximal hepatic induction of total P450 content occurred when rats were treated with MDB derivatives with alkyl chain lengths of five or six carbons. However, maximum induction of the specific P450s--P450IA1, P450IIB1, and P450IIB2--occurred with n-hexyl-MDB. In contrast to effects observed with phenobarbital, treatment with MDBs resulted in higher levels of P450IIB2 than of P450IIB1 in rat hepatic microsomes. Western blot quantitation of MDB-induced hepatic P450IIB1 and P450IIB2 apoenzymes did not correlate to measured levels of the corresponding P450 mRNAs. In fact, P450IIB1 and P450IIB2 apoenzyme levels were consistently lower than expected based on Northern blot and solution hybridization measures of the respective mRNAs. These data suggest that the n-alkyl-MDBs effect increases in levels of hepatic P450 in a complex manner, producing accumulation of P450 mRNAs concomitant with alterations in processes regulating steady-state levels of P450 apoenzyme.

Characterization of a novel cytochrome P450 from the transformable cell line, C3H/10T1/2

The cytochrome P450 in the transformable C3H/10T1/2 (10T1/2) cell line has been characterized and compared to the major polycyclic aromatic hydrocarbon (PAH)-inducible hepatic form, cytochrome P450IA1 (P450IA1). The mouse hepatoma cell line, Hepa-1, was used as an in vitro model for P450IA1 expression and regulation by PAH. Microsomes from uninduced and benz[a]anthracene (BA)-induced 10T1/2 cells provided PAH mono-oxygenated product profiles that were totally different from metabolite profiles produced by microsomes from uninduced and BA-induced Hepa-1 cells even though total activities were similar. The proximate carcinogen, 7,12-dimethylbenz[a]anthracene-3,4-diol (DMBA-3,4-diol) was a major product for the 10T1/2 microsomes, while Hepa-1 formed less than 2% of this metabolite. Hepa-1 converted benzo[a]pyrene (BP) to BP-4,5-diol and DMBA to 7-hydroxymethyl-12-methyl-BA, while 10T1/2 did not produce either product. Polyclonal antibody to rat hepatic P450IA1 did not inhibit metabolism of either PAH substrate by 10T1/2 microsomes, but totally inhibited such metabolism by Hepa-1 microsomes. Western immunoblot analysis of BA-induced 10T1/2 microsomes showed that less than 1% of total P450 was P450IA1. The PAH-metabolizing activity of 10T1/2 microsomes was highly inducible (14-fold) by pre-treatment of non-confluent intact cells with BA, but was only half as inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin. In contrast, the P450IA1 activity of Hepa-1 cells was highly inducible by both compounds. The distinct metabolite profiles, antibody inhibition data and lack of immunoreactivity all indicate that PAH metabolism in 10T1/2 cells is catalyzed by a form of P450 distinct from P450IA1. The anomalous induction patterns suggest that this novel isozyme is predominantly regulated by a mechanism other than the Ah receptor.

The role of sterol carrier protein 2 in stimulation of steroidogenesis in rat adrenal mitochondria by adrenal cytosol

Cholesterol side-chain cleavage (CSCC) in isolated rat adrenal mitochondria is enhanced by prior corticotropin (ACTH) stimulation in vivo (8-fold). Part of this stimulation is retained in vitro by addition of cytosol from ACTH-stimulated adrenals to mitochondria from unstimulated rats (2.5- to 6-fold). In vivo cycloheximide (CX) treatment fully inhibits the in vivo response and resolves the in vitro cytosolic stimulation into components: (i) ACTH-sensitive, CX-sensitive; (ii) ACTH-sensitive, CX-insensitive; and (iii) ACTH-insensitive, CX-insensitive. These components contribute approximately equally to stimulation by ACTH cytosol. Components (i) and (iii) most probably correspond to previously identified cytosolic constituents steroidogenesis activator peptide and sterol carrier protein 2 (SCP2). SCP2, as assayed by radioimmunoassay or ability to stimulate 7-dehydrocholesterol reductase, was not elevated in adrenal cytosol or other subcellular fractions by ACTH treatment. Complete removal of SCP2 from cytosol by treatment with anti-SCP2 IgG decreased cytosolic stimulatory activity by an increment that was independent of ACTH or CX treatment. Addition of an amount of SCP2, equivalent to that present in cytosol, restored activity to SCP2-depleted cytosol but had no effect alone or when added with intact cytosol, suggesting the presence of a factor in cytosol that potentiates SCP2 action. Pure hepatic SCP2 stimulated CX mitochondrial CSCC 1.5- to 2-fold (EC50 0.7 microM) but was five times less potent than SCP2 in adrenal cytosol. Two pools of reactive cholesterol were distinguished in these preparations characterized, respectively, by succinate-supported activity and by additional isocitrate-supported activity. ACTH cytosol and SCP2 each stimulated cholesterol availability to a fraction of mitochondrial P450scc that was reduced by succinate but failed to stimulate availability to additional P450scc reduced only by isocitrate.

Cholesterol metabolism by purified cytochrome P-450scc is highly stimulated by octyl glucoside and stearic acid exclusively in large unilamellar phospholipid vesicles

Cholesterol side-chain cleavage (CSCC) catalyzed by purified bovine adrenal mitochondrial cytochrome P-450scc is highly dependent on the vesicles that supply cholesterol. Six-fold higher rates are achieved with large unilamellar dioleoylphosphatidylcholine vesicles (diameter 150 nm) prepared by octyl glucoside (OG) dialysis (DOPC-LUV) than with small sonicated vesicles (diameter 30 nm) (DOPC-SUV) (Vmax = 25 and 4 min-1, respectively. Extensive dialysis that may remove OG decreased Vmax rates for DOPC-LUV almost to rates seen with DOPC-SUV. These dialyzed DOPC-LUV were, however, very sensitive to addition of OG (EC50 = 2.5 microM, 4.3-fold stimulation) while DOPC-SUV were only weakly affected (EC50 = 100 microM, 1.6-fold stimulation). This enhancement of CSCC in LUV by OG only occurred when the cholesterol:DOPC exceeded 0.1 and was associated with a 15-fold increase in the Km for cholesterol. Structural changes in both SUV and LUV at high cholesterol:DOPC ratios (0.1-1) were indicated by decreases in internal volume that were insensitive to OG and did not affect the external diameters. Stearic acid produced a similar stimulation of CSCC in LUV (EC50 = 50 microM) and had no effect on SUV. The Vmax for CSCC, produced by OG activation of DOPC-LUV, is comparable to the highest attained for cytochrome P-450scc (Tween 20/cholesterol). In LUV, a minor proportion of OG (1-5% of cholesterol) is thus sufficient to generate a domain of reactive cholesterol that maintains a near-optimum turnover. This increased CSCC was paralleled by increased binding of cholesterol to P-450scc, suggesting that this cholesterol is more readily donated by the membrane to the cytochrome.

Epoxide hydratase: sex specific expression and rate-limiting role in DMBA metabolism

The rate of generation of the proximate carcinogen 7,12-dimethylbenz[a]anthracene 3,4-dihydrodiol (DMBA 3,4-diol) from DMBA, was 3-fold lower in rat liver microsomes (RLM) from female, compared to male Sprague-Dawley rats. However, the sum of products that potentially derive from the common intermediate DMBA 3,4-oxide namely, DMBA 3,4-diol, 3- and 4-hydroxy DMBA, was comparable between the two sexes (18 versus 20 pmol/mg/min). Addition of purified microsomal epoxide hydratase (EHm) (150 nM) to female RLM increased the rate of DMBA 3,4-diol formation to a level comparable to that obtained in male RLM. This activity was not increased when equivalent amounts of EHm were added to male RLM. Female RLM contained 2-fold lower levels of EHm protein compared to male RLM, based on Western blot analyses, and exhibited correspondingly lower hydrating activities towards both benzo[a]pyrene 4,5-oxide (BP 4,5-oxide) and DMBA 5,6-oxide. The rate of DMBA 3,4-diol formation was also limited by the availability of EHm in lung and adrenal microsomes from male rats. Addition of exogenous EHm to both tissues stimulated DMBA 3,4-diol formation by 2 and 6-fold respectively. EHm activity was still more deficient in the formation of the K-region, DMBA 5,6-diol, and this limitation was seen in both female and male-RLM as well as in microsomes from extrahepatic tissues. These limiting effects were enhanced by 3-methylcholanthrene treatment, through increases in DMBA monooxygenase but not EHm activities, but were diminished by phenobarbital treatment where EHm was also induced. Manipulation of EHm activity (through addition of pure EHm or its inhibitor trichloropropylene oxide) demonstrated a direct relationship between EHm activity and total DMBA metabolism. A deficiency in EHm apparently caused a selective loss of products resulting from initial DMBA 5,6- and 8,9-monooxygenation. Substantial reduction of DMBA 5,6-oxide to DMBA was measured in RLM under anaerobic conditions. This reduction was inhibitable by ambient oxygen levels. These data indicate that insufficient cellular EHm levels result in the reversion of microsomally generated DMBA 5,6-oxide (and 8,9-oxide) back to DMBA and also lead to reduced activation of DMBA via the carcinogenic bay region diol epoxide.

Selective potent restriction of P450b- but not P450e-dependent 7,12-dimethylbenz[a]anthracene metabolism by the microsomal environment

The prototypic members of the rat liver cytochrome P450IIB subfamily, P450b and P450e, differ by only 13 amino acids and yet purified P450b is considerably more active than P450e for all known substrates. A unique regioselectivity difference between cytochromes P450b and P450e for the metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) and a genetic deficiency in P450e expression in the Marshall (M520/N) rat strain have been exploited to determine the microsomal contributions of the respective forms toward the metabolism of DMBA. The total contribution to metabolism by each isozyme has been assessed based on the sensitivity to rabbit anti-P450b/e IgG and comparison with microsomal P450b and P450e content as measured by Western blots. Liver microsomes from untreated M520/N rats do not express detectable levels of P450e but express P450b at a level that is 2-fold higher than that of P450e in liver microsomes from untreated F344 rats (50 pmol/mg). However, only 4% of the constitutive DMBA metabolizing activity of liver microsomes from the M520/N rat strain could be inhibited by anti-P450b/e IgG. A 30-fold induction of hepatic P450b by phenobarbital (PB) was also completely ineffective in increasing P450b-dependent DMBA metabolism. PB treatment had no appreciable effect on either the levels of expression of P450b protein or P450b-dependent DMBA metabolism, in M520/N lung and adrenal microsomes. In contrast, PB treatment of F344 rats considerably increased P450b/e-dependent metabolism by liver, lung, and adrenal microsomes. The regioselectivity of the anti-P450b/e-sensitive metabolism (predominantly 12-methyl hydroxylation), however, indicated a much greater contribution from P450e than P450b in every tissue examined despite a several fold higher expression of P450b than of P450e. P450b was expressed constitutively in lung microsomes from both strains but again failed to exhibit appreciable DMBA metabolizing activity. Based on these activities and microsomal P450b contents, P450b consistently exhibited turnover numbers (0.02-0.15 nmol/nmol P450b/min) that were at least 10-fold lower than those of pure P450b. In contrast, the calculated turnover numbers for microsomal P450e were consistently comparable to those of pure P450e (approximately 1 nmol/nmol P450e/min).

The expression and metabolic activity of cytochrome P-450 isozymes in control and phenobarbital-induced primary cultures of rat hepatocytes

The expression and activity of the phenobarbital (PB)-inducible P-450 isozymes, P-450b and P-450e, and the major 3-methylcholanthrene (MC)-inducible form, P-450c, were studied in primary cultures of adult rat hepatocytes in T1, Leibovitz L-15 (L-15), and a modification of Waymouth 752/1 (Way) media. P-450 isozymes in initially isolated hepatocytes and control and PB-treated cultures were quantitated by Western blot analysis, and activity was determined with 7,12-dimethylbenz[a]anthracene (DMBA) as substrate. Data from the Western blot analysis correlated well with the metabolic activity toward DMBA. P-450b was consistently induced by PB in hepatocytes in T1 and to a lesser extent in Way. P-450e protein was constitutive in initially isolated cells, expressed in control cultures at a reduced level, and increased or maintained by PB in all three media. DMBA metabolite formation associated with P-450b and P-450e activity was induced by PB in hepatocytes in T1 and Way and was inhibited by antibodies to P-450b. P-450c was only infrequently expressed in freshly prepared hepatocytes, but was detected in all control and PB-treated cultures although at a much higher level in T1. Thus, the amounts of P-450 isozymes, their inducibility by PB, and their activity toward DMBA were found to be dependent on the medium. We have demonstrated enzyme induction and increased activity of the major PB-inducible isozymes in hepatocytes in T1; these are also associated with a change in the control of P-450c expression leading to enhanced constitutive expression and inducibility by phenobarbital.

Synergistic stimulation of pregnenolone synthesis in rat adrenal mitochondria by n-hexane and cardiolipin

n-Hexane and cardiolipin each stimulate pregnenolone production by isolated rat adrenal mitochondria. Following corticotropin (ACTH) stimulation, mitochondrial cholesterol metabolism exhibits a fast phase lasting 2 min, followed by a 10-fold slower metabolism. ACTH suppression by dexamethazone or cycloheximide (CX) treatment removes this fast phase. n-Hexane, at concentrations approaching 80% of the aqueous solubility limit (approximately 0.08 mM), selectively stimulates the slow phase of metabolism, while cardiolipin (100 microM) stimulates only the fast phase. Other alkanes and ethers are effective. The effect of n-hexane is dependent on mitochondrial integrity, as evidenced by decreased effects in hypoosmotically shocked mitochondria (outer membrane disrupted) and ineffectiveness in sonicated mitochondria (both membranes disrupted). n-Hexane apparently enhances the transfer of outer membrane cholesterol to inner membrane P-450scc. Stimulation by cardiolipin is retained by disrupted mitochondria and may involve enhanced availability of P-450scc to inner membrane cholesterol. When added together, these agents produce more than additive effects on cholesterol metabolism. Preincubation with n-hexane did not increase reactive cholesterol, suggesting that enhanced cholesterol transport occurs only in concert with metabolism of inner membrane cholesterol. Uptake of alkanes into mitochondrial membranes may effect structural changes that facilitate outer to inner membrane cholesterol transfer, but major changes are excluded by the effectiveness of isocitrate as a reductant for P-450scc. In combination, n-hexane and cardiolipin reproduce the effect of the ACTH-sensitive sterol regulatory peptide on mitochondria [R. C. Pedersen and A. C. Brownie (1983) Proc. Natl. Acad. Sci. USA 80, 1882-1886], suggesting that peptide action on adrenal mitochondria may resolve into two analogous components.

Hepatic mitochondrial cytochrome P-450 system. Identification and characterization of a precursor form of mitochondrial cytochrome P-450 induced by 3-methylcholanthrene

Hepatic mitoplasts from 3-methylcholanthrene-treated rats contain cytochrome P-450 which can metabolize polycyclic aromatic hydrocarbons like benzo(a)pyrene. Mitochondrial cytochrome P-450 was partially purified and reconstituted in vitro using adrenodoxin and the adrenodoxin reductase electron transfer system and [3H]benzo(a)pyrene as the substrate. A polyclonal antibody to purified microsomal P-450c (a major 3-methylcholanthrene-inducible form) inhibited the activity of mitochondrial enzyme in a concentration-dependent manner and also reacted with a 54-kDa protein on the immunoblots. A monoclonal antibody having exclusive specificity for P-450c, on the other hand, did not inhibit the aryl hydrocarbon hydroxylase activity of the mitochondrial enzyme and showed no detectable cross-reaction with the 54-kDa mitochondrial protein. Similarly, two-dimensional analysis and immunodetection using the polyclonal antibody showed distinct molecular properties of the mitochondrial enzyme different from the similarly induced microsomal P-450c with respect to the isoelectric pH. In vitro translation of free polysomes from 3-methylcholanthrene-induced liver, transport of precursor proteins by isolated mitochondria in vitro, and immunoprecipitation with the polyclonal antibody showed the presence of a 57-kDa putative precursor which is transported and processed into mature 54-kDa species. These results present evidence for the true intramitochondrial location of the P-450c-antibody reactive isoform detected in 3-methylcholanthrene-induced rat liver mitochondria.

Expression and function of three cytochrome P-450 isozymes in rat extrahepatic tissues

Western blots using a polyclonal and a monoclonal antibody raised against rat liver cytochrome P-450b indicate tissue-specific expression of low levels of cytochrome P-450's b and e. P-450b and P-450e were expressed very selectively in, respectively, lung and adrenal microsomes of untreated rats but neither isozyme was detected in the corresponding kidney or small intestine microsomes. The regioselectivity of microsomal metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) as well as the sensitivity to inhibition by anti P-450b/e IgG established that low levels of "b-like" P-450's are functional in lung and adrenal microsomes from uninduced rats, but not in microsomes from the kidney or small intestine. Functional P-450c was also detected at low levels in liver, lung, kidney, and adrenals of untreated rats. Among the extrahepatic tissues examined, DMBA metabolism was the highest in rat adrenal microsomes. However, only 30% of this activity was due to P-450's b, e, or c. Phenobarbital (PB) treatment of rats increased microsomal DMBA metabolism in all extrahepatic tissues examined. The selectivity of this increase for 12-methyl hydroxylation of DMBA and the near complete inhibition by anti-P-450b/e are consistent with induction of P-450e even though P-450b was preferentially induced in each of the extrahepatic tissues examined. The levels of expression of P-450b were increased by PB in all sets of adrenal, lung, and intestinal microsomes and in three out of six sets of kidney microsomes. The levels of P-450e were also increased by PB in all sets of adrenal microsomes. Following PB treatment, P-450e became immunoquantifiable (greater than 2 pmol/mg protein) in three of six sets of lung and kidney microsomes but remained below detection in all sets of intestinal microsomes. Based on the activity of purified P-450e, undetectable levels (less than 1 pmol/mg protein) could account for increased DMBA metabolism in this tissue. The high constitutive level of P-450b in the lung (approximately 40 pmol/mg), was remarkably inactive in DMBA metabolism and was only slightly increased by PB treatment (50%). In contrast, PB treatment caused a 2.5- to 10-fold increase in 12-methyl hydroxylation of DMBA that was highly sensitive to anti-P-450b/e. A protein comigrating with P-450e was well above detection (6-7 pmol/mg) in two of six preparations of lung microsomes that showed highest induction of this activity.(ABSTRACT TRUNCATED AT 400 WORDS)

ACTH control of cholesterol side-chain cleavage at adrenal mitochondrial cytochrome P-450scc. Regulation of intramitochondrial cholesterol transfer

Rat adrenal mitochondria exhibit a linear 2-fold accumulation of cholesterol for 20 min following either in vivo ether stress or ACTH administration, providing cholesterol metabolism is inhibited by aminoglutethimide (AMG). Additional cycloheximide (CX) pretreatment only slightly decreases this increase, but the location of accumulation shifts from the inner membrane to the outer membrane, implying a decreased cholesterol transfer from outer to inner membrane. Although the capacity of outer mitochondrial membranes was saturated after a 10-min treatment with CX, a 20-min treatment resulted in further retention of cholesterol in intact mitochondria that was not recovered in the isolated membranes. An additional pool of loosely bound cholesterol is proposed for CX mitochondria. These studies provide evidence that the CX-sensitive step of adrenal steroidogenesis attributed to loss of a labile ACTH regulatory protein (Pedersen, R.C. and Brownie, A.C. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1882-1886) involves cholesterol transfer from the outer to the inner mitochondrial membrane. ACTH also enhances the PI and PE content of the outer membranes by a CX-sensitive mechanism that may contribute to intramitochondrial cholesterol transport. CX treatment does not affect cholesterol uptake by the inner membrane from phospholipid vesicles. The initial rate of endogenous metabolism in isolated inner membranes is insensitive to pretreatment (2 nmol/nmol P-450/min). The duration of this linear rate was increased 4-fold by AMG treatment while this increase was prevented by CX treatment. The kinetics indicate differences in inner membrane reactive cholesterol levels. Inner membranes also contained a fraction of unreactive cholesterol that is insensitive to pretreatment. Cholesterol-P-450scc complex formation for all pretreatments fits a single hyperbolic function of the reactive cholesterol content of the inner mitochondrial membrane (Kd = 0.025 mol cholesterol/mol phospholipid), and is activated over 5-fold upon mitochondrial disruption. All changes in inner membranes caused by CX can, therefore, be attributed solely to the restricted cholesterol access in vivo.

Differential expression and function of three closely related phenobarbital-inducible cytochrome P-450 isozymes in untreated rat liver

The levels of expression of cytochromes P-450b and P-450e (both inducible by phenobarbital (PB) and differing by only 14 of 491 amino acids) in liver microsomes from untreated male rats were separately quantitated by Western blotting with a polyclonal antibody raised against P-450b that is equally effective against P-450e (anti P-450b/e). A protein with mobility identical to P-450e was detected in all microsomal samples. Microsomes from uninduced livers of individual male rats from five different strains exhibited only minor interstrain and interindividual variability in the expression of P-450e (17 +/- 5 pmol P-450e/mg microsomal protein) with the exception of the Brown Norway strain (8.5 +/- 0.5 pmol P-450e/mg). Expression of P-450b varied widely from undetectable levels (less than 2 pmol/mg) in most Sprague-Dawley rats to about 50% of P-450e levels in Fischer and Brown Norway strains. Anti P-450b/e inhibited total metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) by uninduced microsomes, to an extent dependent on rat strain (15-30%), predominantly through inhibition of formation of 12-hydroxymethyl-7-methyl BA (12HOMMBA) (65-85%), the major metabolite of purified P-450e. A specific activity for P-450e-dependent DMBA metabolism was calculated from four sets of microsomes where the P-450b content was either undetectable or very low (0.7-1.0 nmol/nmol P-450e/min-1). Comparable calculated activities were, however, obtained from other untreated rat liver microsomes where P-450b levels were significant. Polymorphism in P-450b was detected but did not affect total P-450b expression or the sensitivity of DMBA metabolism to anti P-450b/e. A fourth band of greater mobility than P-450b (apparent Mr less than 50,000), was also recognized by anti P-450b/e. The intensity of this band did not vary among individual rats or among the different strains and therefore did not correlate with the sensitivity of microsomal DMBA metabolism to anti P-450b/e. A monoclonal antibody (MAb) against P-450b (2-66-3) recognized P-450's b, b2, and e on Western blots but did not react with this higher mobility band. MAb 2-66-3 and two other MAbs produced against P-450b inhibited 12-methylhydroxylation of DMBA by untreated rat liver microsomes to the same extent as anti P-450b/e. Following PB induction, P-450b was induced to about double the level of P-450e in most rat strains examined.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunological identification and effects of 3-methylcholanthrene and phenobarbital on rat pulmonary cytochrome P-450

Rabbit antibodies to the phenobarbital (PB) inducible rat liver microsomal cytochrome P-450s b and e and to 3-methylcholanthrene (MC) inducible P-450c were used to examine the expression of these isozymes in rat lungs. Western blots of total lung microsomes demonstrated that about 40 pmol P-450b/mg protein (and no detectable P-450e) were present in lungs from control or MC treated rats and that pretreatment with PB caused a small but significant (P less than 0.05) increase in the expression of P-450b. Microsomes from control and PB treated lung contained minimal levels of P-450c, and MC induced this isozyme to 185 pmol/mg. Immunocytochemistry was used to demonstrate immunoreactivity to these isozymes in specific cell types. Neither P-450b nor P-450c was detectable in endothelial cells from control or PB treated lungs, but MC increased immunoreactivity to P-450c in pulmonary endothelial cells. Type II alveolar cells showed distinct immunoreactivity to P-450b and weak immunoreactivity to P-450c in control or PB treated rats. Individual Clara cells stained for either P-450c or P-450b in control, MC treated, and PB treated rats, and colocalization was observed in some cells. An increase in type II alveolar cell and Clara cell immunoreactivity to P-450c was observed after MC induction. Mast cells, identified by metachromatic Giemsa staining, appeared to react nonspecifically with both antisera. In conclusion, P-450c is highly inducible by MC in rat lung (detected in microsomes by Western blot), specifically in endothelial cells, Clara cells, and alveolar type II cells (as visualized by immunocytochemistry); and P-450b is present in rat lung microsomes, and immunoreactivity to this isozyme is localized in alveolar type II and Clara cells.

Product inhibition of benzo[a]pyrene metabolism in uninduced rat liver microsomes: effect of diol epoxide formation

Conversion of benzo[a]pyrene (BP) to BP 7,8-dihydrodiol 9,10-oxides (DE) (measured as 7,10/8,9-tetrols) by untreated (UT) rat liver microsomes is over 10 times slower than following 3-methylcholanthrene (MC) induction. Time courses have been subjected to a kinetic analysis analogous to that previously reported for metabolism by MC-induced microsomes (J. Biol. Chem., 259 (1984) 13770-13776). Competition between BP and 7,8-dihydrodiol for P-450 is the major determinant of the rate of DE formation. Glucuronidation of quinones and phenols only increases the isolated BP metabolites including DE by 40%. This indicates far less inhibition by these products than for metabolism in MC-microsomes (4-6-fold). Thus stimulation may result from a decreased quinone-mediated oxidation of metabolites. In the presence of DNA, UT-microsomes metabolize BP to approximately equal amounts of 9-phenol-4,5-oxide (9-PO) and DE/DNA adducts. Addition of uridine diphosphoglucuronic acid (UDPGA) fails to enhance modification of DNA by DE, but formation of the 9-PO adduct is reduced as a result of lower free 9-phenol levels. The kinetic characteristics of BP metabolism by UT-microsomes are highly sensitive to the presence of very small but variable amounts (2-25 pmol/mg) of the very active cytochrome P-450c, which is the predominant form in MC-microsomes. The major effect of elevated levels of P-450c is an 8-fold increase in DE formation at low concentrations of BP due to a lowering of Km (7.9-2.6 microM) and an increase in the regioselectivity for DE formation from 7,8-dihydrodiol (5-15% of total BP metabolites). The formation of DE was directly correlated with the content of P-450c (r = 0.94). The presence of increased levels of P-450c in UT-microsomes is probably due to previous exposure of the animals to environmental inducers and is minimized by controlled housing and feeding.

Altered regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

A single treatment of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (50 micrograms/kg) produced two distinct effects on adrenal steroidogenesis in rats 13 days post-treatment. In unstressed rats, the very low corticosterone levels early in the light phase (AM) increased 4-fold relative to ad libitum-fed control (ALC) rats, but the peak level of corticosterone that is seen late in the light phase (PM) decreased up to 40% relative to ALC rats. The AM stimulation was also observed in rats pair-fed to compensate for the diminished feed intake of TCDD-treated animals, indicating that the change results from nutritional deprivation. The PM suppression, however, was not observed in pair-fed rats. In rats given a lower dose of TCDD (15 micrograms/kg), there was no AM stimulation, whereas the suppression of the PM diurnal peak of corticosterone was retained. Plasma adrenocorticotropin (ACTH) levels and adrenal size were not changed by these treatments, indicating that TCDD affects adrenal responsiveness. TCDD did not, however, have a significant effect on corticosterone secretion in rats receiving high doses of ACTH. In control animals, the availability of cholesterol to cytochrome P-450scc limits the rate of steroidogenesis. While the specific content of the cytochrome was unaffected by TCDD, cholesterol turnover by this enzyme appeared to be affected following TCDD treatment, as evidenced by small increases in the mitochondrial levels of free cholesterol, reactive cholesterol, and in the proportion of P-450scc complexed with cholesterol relative to both ad libitum- and pair-fed controls. This accumulation of mitochondrial cholesterol following TCDD treatment is consistent with an inhibition of cholesterol metabolism at cytochrome P-450scc in vivo that is removed upon isolation of the mitochondria. These TCDD-induced increases were enhanced substantially in ACTH-stimulated rats, probably because ACTH enhances cholesterol influx into the mitochondria. Normally, substrate availability is rate limiting in cholesterol side-chain cleavage, and the AM stimulation of steroidogenesis by TCDD may result from such increased cholesterol transfer. The inhibition of cholesterol side-chain cleavage resulting from TCDD treatment may, however, only become rate limiting for corticosterone synthesis when cholesterol transfer is more substantially activated, as for peak PM secretion.

Induction of mammary cytochromes P-450: an essential first step in the metabolism of 7,12-dimethylbenz[a]anthracene by rat mammary epithelial cells

Rat mammary epithelial cells (RMEC) in culture have been shown to activate polycyclic aromatic hydrocarbon (PAH) carcinogens. This study investigates the role of mammary cytochrome P-450 monooxygenases in these metabolic processes. Monooxygenation of 7,12-dimethylbenz[a]anthracene (DMBA) by RMEC in culture exhibited a 6-h lag period before reaching a constant rate. The mechanism for this time-dependent expression of DMBA monooxygenase activity was investigated in lysed cells, where both conjugation and in situ induction of P-450 are prevented. Although metabolism of DMBA by untreated RMEC lysates was undetectable (less than 1 pmol/mg cell protein/h), prior exposure of cultured cells to benz[a]anthracene (BA) induced DMBA metabolism, (approximately 100 pmol/mg cell protein/h). BA pretreatment also eliminated the lag period for metabolism of DMBA by cultured RMEC but did not prevent additional induction of DMBA monoxygenase activity by the substrate. The distribution of monooxygenated DMBA metabolites formed by BA-induced cell lysates was clearly different from that obtained with purified P-450c, the predominant PAH-inducible isozyme in rat liver. For example, the carcinogen precursor DMBA 3,4-dihydrodiol, which is not formed by P-450c, was a clearly detectable product in RMEC. The low epoxide hydratase activity of BA-induced lysate (approximately 400-fold lower compared to that in the liver) limited formation of all DMBA dihydrodiols. The formation of DMBA 3,4-dihydrodiol increased by 5-fold following addition of exogenous purified epoxide hydratase. The DMBA monooxygenase activity of BA-induced RMEC lysates was completely inhibited by alpha-naphthoflavone but was only partially inhibited (50%) by a polyclonal antibody raised against cytochrome P-450c. Anti P-450c completely inhibited formation of some of the metabolites, partially inhibited formation of others and notably stimulated formation of DMBA 3,4-dihydrodiol by 60%. A polyclonal antibody that recognized both rat hepatic P-450a and a group of P-450 isozymes related to P-450h, and which totally inhibited DMBA 3,4-dihydrodiol formation by rat liver microsomes, did not inhibit formation of any DMBA metabolite in RMEC, including DMBA 3,4-dihydrodiol. Western blot analyses of RMEC homogenates demonstrated that BA pretreatment induces P-450c, but not P-450a or any of the P-450h-related isozymes. We conclude that metabolism of DMBA by RMEC depends on induction of P-450c and at least one additional form of cytochrome P-450 which is immunochemically distinct from rat hepatic P-450a and P-450h related isozymes, but is sensitive to alpha-naphthoflavone.

ACTH regulation of cholesterol movement in isolated adrenal cells

Confluent bovine adrenal cell primary cultures respond to stimulation by adrenocorticotropin (ACTH) to produce steroids (initially predominantly cortisol and corticosterone) at about one-tenth of the output of similarly stimulated rat adrenal cells. The early events of steroidogenesis, following ACTH stimulation, have been investigated in primary cultures of bovine adrenal cortical cells. Steroidogenesis was elevated 4-6-fold within 5 min of exposure to 10(-7) M ACTH and increased linearly for 12 h and declined thereafter. Cholesterol side-chain cleavage (SCC) activity was increased 2.5-fold in mitochondria isolated from cells exposed for 2 h to ACTH and 0.5 mM aminoglutethimide (AMG), even though cytochrome P-450scc only increases after 12 h. Mitochondrial-free cholesterol levels increased during the same time period (16.5-25 micrograms/mg of protein), but then both cholesterol levels and SCC activity declined in parallel. More prolonged exposure to ACTH prior to addition of AMG caused the elevation in mitochondrial cholesterol to more than double, possibly due to enhanced binding capacity. Early ACTH-induced effects on cellular steroidogenesis result from these changes in mitochondrial-free cholesterol. The maximum rate of cholesterol transport to mitochondria in AMG-blocked cells was consistent with the maximum rate of cellular steroidogenesis. Cycloheximide (0.2 mM) rapidly blocked (less than 10 min) cellular steroidogenesis, cholesterol SCC activity, and access of cholesterol to cytochrome P-450scc without affecting mitochondrial-free cholesterol. Exposure of confluent cultures to the potent environmental toxicant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (10(-8) M), for 24 h prior to ACTH addition decreased the rates of ACTH- and cAMP-stimulated steroidogenesis but did not affect the basal rate. In both cases, the effectiveness of TCDD increased with time of exposure to the stimulant. Although cholesterol accumulated in the presence of ACTH and AMG (13-28 micrograms/mg), pretreatment of cells with TCDD caused a decrease in mitochondrial cholesterol (13-8 micrograms/mg). The effect of TCDD was produced relatively rapidly (t1/2 approximately 4 h). Since even in the absence of TCDD, the mitochondria of ACTH-stimulated cells also eventually lose cholesterol (after 2 h) TCDD pretreatment may increase the presence of a protein(s) that cause this mitochondrial-cholesterol depletion following stimulation by ACTH or cAMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypercholesterolemia and the regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Plasma and adrenal cholesterol disposition have been examined to gain further insight into the mechanisms by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment decreases the diurnal peak in plasma corticosterone concentrations. TCDD induces an increase in plasma cholesterol concentration that is nearly complete on Day 2, at least 2 days before the most pronounced increase in adrenal cholesterol concentration (Days 4-6). This adrenal increase involves both free cholesterol and cholesterol esters, in contrast to the response to dietary hypercholesterolemia where only cholesterol esters increase. Although adrenocorticotropin (ACTH) does not increase adrenal mitochondrial cholesterol in normal rats (cholesterol turnover is faster than cholesterol uptake), this response changes between Days 6 and 9 after TCDD treatment such that ACTH then stimulates accumulation of mitochondrial cholesterol. This additional cholesterol is fully available to cytochrome P-450SCC, as judged both by active cholesterol metabolism in isolated mitochondria and by increased cholesterol-P-450SCC complex formation. The accompanying in vivo suppression of the peak plasma corticosterone concentration suggests a TCDD-induced inhibition of cholesterol side-chain cleavage (SCC). Consistent with this hypothesis, similar effects on adrenal mitochondrial cholesterol were produced by in vivo administration of the cholesterol side-chain cleavage inhibitor, aminoglutethimide, to ACTH-stimulated rats. Although the putative TCDD-induced inhibitory factor is apparently readily lost from mitochondria during preparation, inhibition may be retained in isolated cells. TCDD, therefore, affects adrenal cholesterol regulation by at least two mechanisms. Adrenal cholesterol content increases in part as a consequence of elevated plasma cholesterol, and cholesterol side-chain cleavage becomes partially inhibited in vivo.

Selective interactions of cytochromes P-450 with the hydroxymethyl derivatives of 7,12-dimethylbenz[a]anthracene

Competition between a hydroxylated metabolite and the parent polycyclic aromatic hydrocarbon (PAH) for metabolism at cytochromes P-450 may result in the generation of hydroxylated dihydrodiol epoxides. The effectiveness of the competition between 7-hydroxymethyl-12-methylbenz[a]anthracene (7HOMMBA) or 12-hydroxymethyl-7-methylbenz[a]anthracene (12HOMMBA) and 7,12-dimethylbenz[a]anthracene (DMBA) is highly dependent on the form(s) of cytochrome P-450 in the microsomes. The inhibitory effects of exogenously added 7HOMMBA or 12HOMMBA on DMBA metabolism were 30- to 50-fold greater in 3-methylcholanthrene (MC)-induced rat liver microsomes (Ki = 0.4 microM) compared to either uninduced or phenobarbital (PB)-induced liver microsomes (Ki = 14 and 11 microM, respectively). Similarly, product inhibition of total DMBA metabolism by metabolites generated in situ was significant only in MC-induced liver microsomes (Ki' = 2.5 microM). Metabolism of 7HOMMBA in these microsomes was strongly restricted by an unusual substrate inhibition derived from the inhibitory binding of a second molecule of 7HOMMBA. This same phenomenon was observed with reconstituted cytochrome P-450c but not with PB-induced or uninduced microsomes. Complex formation by binding of DMBA, 7HOMMBA, and 12HOMMBA to purified P-450c reconstituted in phospholipid micelles was determined by optical spectroscopy and fluorescence quenching. Binding affinities of both the 7HOMMBA and 12HOMMBA (Kd = 95 and 110 nM, respectively), were 2.5-fold higher compared to that of DMBA (Kd = 265 nM). These results provide a first demonstration that hydroxylation of a PAH can lead to preferential metabolism through an increased affinity for cytochrome P-450.

Inhibition of ACTH action on cultured bovine adrenal cortical cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin through a redistribution of cholesterol

The conversion of cholesterol to cortisol by cultured bovine adrenal cortical cells is stimulated 6-fold by adrenocorticotropin and is limited by the movement of cholesterol to the mitochondria (DiBartolomeis, M.J., and Jefcoate, C.R. (1984) J. Biol. Chem. 259, 10159-10167). Exposure of confluent cultures to the potent environmental toxicant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (10(-8)M), for 24 h prior to adrenocorticotropin (ACTH) addition decreased the rate of ACTH-stimulated steroidogenesis but did not affect the basal rate. TCDD was more effective against stimulation at 10(-11) M ACTH (4-fold) than at 10(-7) M ACTH (10%), consistent with an increase in EC50 for ACTH. Stimulation of bovine adrenal cortical cells by cAMP was similarly decreased by TCDD. In both cases the effectiveness of TCDD increased with time of exposure to the stimulant. The transfer of cholesterol to mitochondria in intact cells was quantitated by means of the 2-h accumulation of mitochondrial cholesterol in the presence of aminoglutethimide, an inhibitor of cholesterol side chain cleavage. Although cholesterol accumulated in the presence of ACTH (13 to 28 micrograms/mg), pretreatment of cells with TCDD caused a decrease in mitochondrial cholesterol (13 to 8 micrograms/mg). The effect of TCDD was produced relatively rapidly (t1/2 approximately 4 h). In absence of TCDD, the mitochondria of ACTH-stimulated cells also eventually lose cholesterol (after 2 h). It is concluded that TCDD pretreatment may increase the presence of a protein(s) that cause mitochondrial cholesterol depletion when the cells are stimulated by ACTH or cAMP. TCDD-enhanced cholesterol efflux from mitochondria diminishes cholesterol side chain cleavage when mitochondrial cholesterol is sufficiently depleted (after 2-4 h).

Regulation of rat and bovine adrenal metabolism of polycyclic aromatic hydrocarbons by adrenocorticotropin and 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on polycyclic aromatic hydrocarbon (PAH) metabolism and steroidogenesis in primary cultures of bovine adrenal cortical (BAC) and rat adrenal cortical (RAC) cells have been examined. Remarkably TCDD is an ineffective inducer (15-50%) of PAH metabolism in confluent BAC cells and completely antagonizes a 5-fold induction by benz[alpha]anthracene (BA). In the same concentration range (EC50 5 X 10(-11) M) TCDD suppresses steroidogenesis through an effect on cholesterol metabolism. Adrenocorticotropin (ACTH) and cAMP also suppress PAH metabolism at concentrations which stimulate steroidogenesis (10(-7) M). In RAC cells ACTH potently induces PAH metabolism (7-fold) at a comparable concentration to the stimulation of steroidogenesis. Parallel stimulation of PAH metabolism and steroidogenesis by cAMP suggest that ACTH induction of PAH metabolism is mediated by cAMP. TCDD induces PAH metabolism (2.8-fold, EC50 8 X 10(-11) M) at similar concentrations to the inhibitory effect in BAC cells and this action is additive with ACTH induction. In male rats in vivo TCDD induces adrenal microsomal PAH metabolism (72%) and is more effective in this respect than 3-methylcholanthrene (3MC). Rabbit antibodies against rat liver cytochrome P-450c (the major TCDD-inducible liver form) inhibited the TCDD-induced adrenal metabolism of 7,12-dimethylbenz[alpha]anthracene (DMBA), which also exhibited regioselectivity typical of metabolism by P-450c. Constitutive adrenal microsomal metabolism, which exhibited regioselectivity of DMBA metabolism comparable to the ACTH-sensitive cellular metabolism, was not affected by anti-P-450c. It is concluded that ACTH and TCDD induce distinct forms of cytochrome P-450 in RAC cells and that the latter represents a typical Ah-receptor mediated response. The anomalous effect on PAH metabolism in BAC cells that parallels inhibition of steroidogenesis may derive from repression of a distinct adrenal form of P-450 by the TCDD-Ah-receptor complex.

Control of steroid synthesis in adrenal fasciculata cells

The control of steroid synthesis in adrenal fasciculata cells is considered in terms of two types of control by ACTH: control of cholesterol availability to inner mitochondrial cytochrome P-450scc. This process controls total steroid synthesis and is rapidly activated by ACTH. The several steps in cholesterol transfer are examined. partitioning of metabolism by means of competition between enzymes for limiting amounts of steroid intermediates. Changes in such competition determine the ratio of steroid products from the adrenal cells. Such changes typically are a slower response to ACTH. A critical aspect of such competition is the modulation of multiple activity P-450 cytochromes: P-450(17 alpha) (17 alpha-hydroxylation and 17,20 lyase) and P-450(11 beta) (11 beta- and 18-oxidases). Factors such as substrate binding, electron transfer steps and lipid environment are considered in addition to new enzyme synthesis. The ACTH stimulation of steroid synthesis in bovine adrenal cell primary cultures is examined as a model for both types of regulation.

Selectivity in the binding of hydroxylated benzo[a]pyrene derivatives to purified cytochrome P-450c

The interaction of rat liver microsomal cytochrome P-450c with potential benzo[a]pyrene (BP) metabolites has been compared with the binding of BP by optical and fluorescence spectroscopy. Fluorescence quenching of the phenolic derivatives of BP derives from 1:1 complex formation with P-450c, is a function of the position of the hydroxyl substituent, and correlates with the concomitant increase in high-spin cytochrome observed in parallel optical titrations. The proportion of high-spin cytochrome seen when P-450c was reconstituted in dilauroylphosphatidylcholine vesicles (60 micrograms/mL) ranged from about 7% for the 3- and 7-phenols to 75% for 11- and 12-phenols. BP and all 12 methyl-BP derivatives have comparable high affinities for P-450c (50-70% high spin). Kd determinations with purified P-450c indicated very strong binding of BP phenols that induce high-spin complexes (4-, 5-, 9-, 10-, 11-, and 12-phenols; Kd = 3-25 nM). Inhibition of n-octylamine binding by the 3- and 7-phenols indicated weak interactions (Kd = 80-90 nM), even though low-spin complexes were formed. Inhibition of BP metabolism catalyzed by P-450c with BP phenols correlated with their respective dissociation constants. These results suggest that phenolic substitution at certain positions on BP (1, 2, 3, 7, or 8) interferes with binding to the active site while substitutions at the other positions either enhance or have no effect on binding. BP dihydrodiols [including the (+)- and (-)-BP 7,8-dihydrodiols] were relatively ineffective in forming high-spin complexes (approximately 20%), and fluorescence quenching of dihydrodiols by P-450c also saturated at low levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of benzo[a]pyrene by purified cytochrome P-450c

Benzo[a]pyrene (BP) fluorescence-emission intensities in phospholipid micelles are quantitatively described over a broad range of lipid and BP concentrations by excitation that is linearly dependent upon BP concentration and an offsetting excimer quenching that is dependent upon the square of the BP concentration. The fluorescence of BP is quenched by the presence of cytochrome P-450c in proportion to the concentration of the cytochrome in the micelles and in accord with stoichiometric complex formation. Parallel optical titrations indicate a change in spin state of P-450c to a predominantly high-spin state that correlates directly with the percentage fluorescence quenching of complexed BP. Neither change occurs with five other purified forms of rat liver P-450 that have low activity in BP metabolism. N-Octylamine, a ligand that binds to the heme of P-450, competitively inhibits both the spin-state changes and the fluorescence quenching in equal proportion. The Kd for the interaction of BP with P-450c is exceptionally low (10 nM) relative to the Km for monooxygenation (ca. 1 microM). Decreasing the concentration of either dilauroylphosphatidylcholine or dioleoylphosphatidylcholine concomitantly increases the high-spin state (from 30% to 80%) of fully complexed P-450c and the fluorescence quenching (50-100%) of the complexed BP (half-maximal at 80 micrograms of lipid/mL). It is concluded that spin state and fluorescence quenching both reflect the same changes in the interaction of the BP with the P-450 heme.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenal microsomal hydroxylating system: purification and substrate binding properties of cytochrome P-450C-21

The substrate-cytochrome P-450C-21 binding reaction has been investigated in detail by using the purified cytochrome. The apparent substrate dissociation constant (KDapp) depended on the enzyme concentration, indicating that the binding reaction does not follow simple two-component mass action equilibrium. However, the binding data fit reasonably well to a model in which the P-450C-21 exists in a monomer-dimer equilibrium and the substrate does not bind to the dimer. The intrinsic dissociation constant (K1) and the dissociation constant for the dimerization reaction (K2) were calculated from the titration data by a pattern search procedure. K1 and K2 were found to be essentially independent of the enzyme concentration, indicating the appropriateness of the assumed model. In the present study, all factors that increased the dissociation of the dimer, as indicated by an increase in K2, decreased KDapp so that it approached the intrinsic constant K1. These results suggest that there is mutual interaction of the substrate binding and self-association reactions of cytochrome P-450C-21 in the purified preparation.

Benzo(a)pyrene activation to 7,8-dihydrodiol 9,10-oxide by rat liver microsomes. Control by selective product inhibition

Metabolism of benzo(a)pyrene (BP) and 7,8-dihydrodiol by 3-methylcholanthrene (MC)-induced rat liver microsomes are both subject to severe inhibition by primary metabolites of BP, which was analyzed by determining individual inhibition constants for all primary BP metabolites for both BP and 7,8-dihydrodiol metabolism. Monooxygenation of 7,8-dihydrodiol was, surprisingly, 5 to 10 times more sensitive than monooxygenation of BP to inhibition by all primary metabolites, even though both reactions require the same enzyme, cytochrome P-450c. Two representative products, 1,6-quinone and 9-phenol, were both strong, competitive inhibitors of BP metabolism with Ki values of 0.12 and 0.74 microM, respectively. The total effect of product inhibition on the overall reactions was determined by fitting progress curves of BP, 7,8-dihydrodiol, and anti-7,8-dihydrodiol 9,10-oxide (determined as 7,10/8,9-tetrol) over a range of BP concentrations to integrated steady-state equations using experimental Vmax and Km values. The effective product inhibition factors for BP and 7,8-dihydrodiol metabolism, determined from progress curve fits, were only 2-fold higher than the corresponding calculated theoretical values. The effective product inhibition factors, obtained from progress curve analysis, confirmed that 7,8-dihydrodiol metabolism was substantially more sensitive to inhibition by primary BP metabolites than BP metabolism itself. This difference probably reflects the much higher affinity of cytochrome P-450c for BP (Kd = 6 nM), as compared to 7,8-dihydrodiol (Kd = 175 nM) that was established spectrophotometrically both for the purified cytochrome and for MC microsomes. The Km for BP metabolism is 50 to 100 times higher than the Kd, while the Km is similar to the Kd for 7,8-dihydrodiol metabolism. The discrepancy for BP between Km and Kd suggests that standard Michaelis-Menten kinetics may be perturbed by either slow substrate or product dissociation.

Binding and metabolism of benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene by seven purified forms of cytochrome P-450

Seven different forms of cytochrome P-450 have been purified from rat liver microsomes. The major 3-methylcholanthrene (MC) inducible cytochrome P-450 (form c) exhibits the greatest activity toward both benzo[a]pyrene (BP) (58 min-1) and 7,12-dimethylbenz[a]anthracene (DMBA) (29 min-1) and forms substantially high spin, high affinity complexes (Kd = 10 nM) with both hydrocarbons. Cytochrome P-450d, a minor MC-inducible form, has far lower activity for metabolism of both polycyclic aromatic hydrocarbons (PAH), yet also forms high affinity complexes (Kd approximately 100 nM) with both PAH, retaining the full high spin state of the free cytochrome. Although two phenobarbital (PB)-induced forms (P-450's b and e) differ by only 13 amino acids, they exhibit significant differences in metabolism of PAH and in complex formation. Whereas P-450b is only active in metabolism of DMBA (9.8 min-1 versus 1.9 min-1 for BP), P-450e has low activity for both substrates (3.3 and 1.2 min-1). Nevertheless, P-450e forms a high affinity complex (Kd approximately 100 nM) with both PAH that enhances the proportion of the high spin state (from 30% to 70%). Failure to displace n-octylamine (NOA) suggests binding that is removed from the heme. P-450b remains low spin in the presence of PAH and NOA is again not displaced. In addition, the two forms can be distinguished by their regioselectivities for both PAH. P-450's a, h, and pregnenolone-16 alpha-carbonitrile (PCN) exhibit little activity toward BP or DMBA, but P-450 PCN does form a low spin complex with BP (not DMBA). Regioselectivity in metabolism of DMBA by PB-induced microsomes does not agree with that of the major constituent forms. Only the minor, less active purified forms (e and a) mediate substantial 12-hydroxylation and 3,4-epoxidation of DMBA. Thus, additional factors in microsomal reactions must contribute to these differences.

Hepatic mitochondrial cytochrome P-450 system. Distinctive features of cytochrome P-450 involved in the activation of aflatoxin B1 and benzo(a)pyrene

Rat liver mitoplasts containing less than 1% microsomal contamination contain cytochrome P-450 at 25% of the microsomal level and retain the capacity for monooxygenase activation of structurally different carcinogens such as aflatoxin B1 (AFB1), benzo(a)pyrene (BaP), and dimethylnitrosamine. Both phenobarbital (PB) and 3-methylcholanthrene (3-MC) induce the level of mitochondrial cytochrome P-450 by 2.0- to 2.5-fold above the level of control mitoplasts. The enzyme activities for AFB1 (3-fold) and BaP (16-fold) metabolism were selectively induced by PB and 3-MC, respectively. Furthermore, the metabolism of AFB1 and BaP by intact mitochondria was supported by Krebs cycle substrates but not by NADPH. Both PB and 3-MC administration cause a shift in the CO difference spectrum of mitoplasts (control, 448 nm; PB, 451 nm; and 3-MC, 446 nm) suggesting that they induce two different forms of mitochondrial cytochromes P-450. Mitoplasts solubilized with cholate and fractionated with polyethylene glycol exhibit only marginal monooxygenase activities. The activity, however, was restored to preparations from both PB-induced and 3-MC-induced mitochondrial enzymes (AFB1 activation, ethylmorphine, and benzphetamine deamination and BaP metabolism) by addition of purified rat liver cytochrome P-450 reductase, and beef adrenodoxin and adrenodoxin reductase. The latter proteins failed to reconstitute activity to purified microsomal cytochromes P-450b and P-450c that were fully active with P-450 reductase. Monospecific rabbit antibodies against cytochrome P-450b and P-450c inhibited both P-450 reductase and adrenodoxin-supported activities to similar extents. Anti-P-450b and anti-P-450c provided Ouchterlony precipitin bands against PB- and 3-MC induced mitoplasts, respectively. We conclude that liver mitoplasts contain cytochrome P-450 that is closely similar to the corresponding microsomal cytochrome P-450 but can be distinguished by a capacity to interact with adrenodoxin. These inducible cytochromes P-450 are of mitochondrial origin since their levels in purified mitoplasts are over 10 times greater than can arise from the highest possible microsomal contamination.

The role of secondary metabolism in the metabolic activation of 7,12-dimethylbenz[a]anthracene by rat liver microsomes

The effect of induction by either phenobarbital (PB) or 3-methylcholanthrene (MC) on the kinetics of both primary and secondary metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) have been studied. PB and MC induction stimulate the initial rate of total DMBA metabolism 4- and 8-fold, respectively. Individual metabolites exhibited distinct time courses dependent on the inducer. With both induced and uninduced microsomes, formation of DMBA 5,6-dihydrodiol exhibited a 2-5 min time lag before reaching a maximum rate while, in contrast, formation of 7-hydroxymethyl-12-methylbenz[a]anthracene (7HOMMBA) declined substantially from linearity at a very early point in the reaction. This was most apparent after 2 min and total levels decreased after 5 min. Regioselectivity for both DMBA and 7HOMMBA were measured based on the initial rates of mono-oxygenation derived from these kinetics. In each set of microsomes, prior 7-hydroxylation redirected metabolism towards DMBA 3,4-dihydrodiol formation and away from 5,6-dihydrodiol. Large differences were noted for the effectiveness and regioselectivity of secondary metabolism via 7HOMMBA. MC-induced liver microsomes exhibited a stronger preference for secondary metabolism of 7HOMMBA than PB-induced and control microsomes. In all cases, 7HOMMBA dihydrodiols, formed from DMBA, appeared in the same ratio as when formed directly from 7HOMMBA, suggesting formation from 7HOMMBA rather than from 7-hydroxylation of primary dihydrodiols. The extent of secondary metabolism of primary products was calculated as the difference between the calculated formation (determined from the appropriate regioselectivity and DMBA disappearance) and the quantitated level at a particular reaction time. However, for MC-induced microsomes, quantitation of 7HOMMBA mono-oxygenated metabolites accounted for less than 50% of the calculated 7HOMMBA secondary metabolism. This discrepancy results from selective further oxidation of 7HOMMBA phenols. For control and PB-induced liver microsomes, quantitation of identifiable 7HOMMBA mono-oxygenated metabolites agreed well with calculated secondary 7HOMMBA metabolism. Based on the results presented here, factors that favor hydroxylation of PAH prior to bay-region diol-epoxide formation are discussed.

Characterization of the acute stimulation of steroidogenesis in primary bovine adrenal cortical cell cultures

The early events of steroidogenesis, following adrenocorticotropin (ACTH) stimulation, were investigated in primary cultures of bovine adrenal cortical cells. Steroidogenesis was elevated 4-fold within 5 min of exposure to 10(-7) M ACTH and increased linearly for 12 h and declined thereafter. Cholesterol side-chain cleavage (SCC) activity was increased 2.5-fold in mitochondria isolated from cells exposed for 2 h to ACTH and 0.5 mM aminoglutethimide, even though cytochrome P-450scc only increases after 12 h. Mitochondrial free cholesterol levels increased during the same time period (16.5 to 25 micrograms/mg of protein), but then both cholesterol levels and SCC activity declined in parallel. It is concluded that early ACTH-induced effects on cellular steroidogenesis result from these changes in mitochondrial free cholesterol. The maximum rate of cholesterol transport to mitochondria in aminoglutethimide-blocked cells (8.6 micrograms/mg of protein/h) was consistent with both the maximum rate of mitochondrial cholesterol SCC and cellular steroidogenesis (6.0 micrograms of pregnenolone/mg/h and 5.5 micrograms of steroid/mg of mitochondria/h, respectively). Cycloheximide (0.2 mM) rapidly blocked (less than 10 min) cellular steroidogenesis, cholesterol SCC activity, and access of cholesterol to cytochrome P-450scc without affecting mitochondrial free cholesterol. The distribution of steroid products fell into three distinct phases during a 24-h period following ACTH stimulation: an initial increase in SCC activity (0-4 h), elevation of androstenedione in place of corticosterone (4-12 h), and then in place of cortisol (12-24 h). The changes from 4 to 24 h result from a progressive stimulation by ACTH of 17 alpha-hydroxylase activity (but not 21-hydroxylase or C17:20 lyase activities) that is maintained even when stimulation of total steroidogenesis has stopped.

The interrelationship of polycyclic hydrocarbon metabolism and steroidogenesis in primary cultures of bovine adrenal cortical cells

The interrelationship between adrenal steroidogenesis and polycyclic aromatic hydrocarbon metabolism has been examined in cultured bovine adrenal cortical (BAC) cells. Adrenocorticotropin (ACTH) selectively induced steroidogenic cytochrome P-450-dependent enzyme activities from BAC cell cultures. In the presence of 10(-7) M ACTH, steroid production requiring 17 alpha-hydroxylation (cortisol + androgens) was increased 5-fold over the formation of 17- deoxysteroids (corticosterone). The effect of 10 microns benz[a]anthracene on steroidogenesis was characterized by suppression of both steroid 17 alpha-hydroxylation (90%) and total steroidogenesis (50%), with a concomitant rise in 17- deoxysteroid formation. The order of stimulation of steroidogenic enzyme activities by ACTH (17 alpha-hydroxylase greater than side chain cleavage greater than 21-hydroxylase) paralleled the order of suppression by benz[a]anthracene. BAC cell cultures incubated with Su-10603, a specific 17 alpha-hydroxylase inhibitor, exhibited similar changes in the pattern of steroidogenesis, as did benz[a]anthracene-treated cells, suggesting that benz[a] anthracene also inhibits steroidogenesis as an inhibitor of 17 alpha-hydroxylase. In addition, benz[a]anthracene induced benzo[a]pyrene metabolism 4- to 6-fold over control levels in these cells. The profile of benzo[a]pyrene metabolites revealed predominantly water-soluble products (nonhydrolyzable greater than sulfates greater than glucuronides), 9,10- monooxygenation products, and 3-phenol. ACTH (10(-7) M) and 0.5 mM cyclic AMP each decreased benzo[a]pyrene metabolism by more than 50%. Both benz[a]anthracene-induced and uninduced benzo[a]- pyrene metabolism were equally reduced in response to ACTH and cyclic AMP. In the presence of 0.2 mM aminoglutethimide, which completely inhibited steroidogenesis, ACTH decreased benz[a]anthracene induction of benzo[a]pyrene metabolism to the same extent as ACTH treatment alone. It is concluded that the suppression of benzo[a]pyrene metabolism by ACTH is mediated by cyclic AMP and does not involve steroids generated in response to ACTH. These studies demonstrate that cytochrome P-450 isozymes involved in steroidogenesis and polycyclic aromatic hydrocarbon metabolism are regulated, in opposing directions, by ACTH.

Polyphosphoinositide activation of cholesterol side chain cleavage with purified cytochrome P-450scc

Highly purified beef adrenal cytochrome P-450 specific for cholesterol side chain cleavage (P-450-scc) has been reconstituted with sonicated vesicles containing cholesterol and either dimyristoyl phosphatidylcholine (DMPC) or dioleoyl phosphatidylcholine (DOPC). When cholesterol was present in DMPC vesicles at 1:15 molar ratio, cardiolipin and L-alpha-phosphatidylinositol 4-monophosphate (DPI) increased side chain cleavage by at least 5-fold (0.7 min-1-3.5 min-1). In DOPC vesicles, a smaller increase was observed (2.8 min-1-5.0 min-1). Activator phospholipids increased the rate of transference of cholesterol both to and from the cytochrome when, respectively, cholesterol-free P-450scc and cholesterol-P-450scc complex are combined with either DMPC or DOPC vesicles. Transfer of cholesterol to and from cytochrome P-450 occurred with similar first order rate constants and was also independent of the concentrations of cholesterol vesicles and P-450. It is suggested that transfer in both directions is limited by the rate of insertion of P-450scc into the membrane. Phospholipid stimulatory effects for both cholesterol transfer and for activation of side chain cleavage occurred with the same ranking, even though cholesterol transfer, following reconstitution, was 5-10 times slower than the turnover of side chain cleavage. DPI increased Vmax for side chain cleavage in both DMPC and DOPC vesicles to the same rate (12 min-1) without effect on the Km for cholesterol, while cardiolipin both produced a similar increase in Vmax and decreased Km (cholesterol). This activation by DPI is attributed to more favorable incorporation of P-450scc in these membranes and is consistent with previously reported effects of acidic phospholipids on other mitochondrial proteins.