Cytochrome P-450 of adrenal mitochondria. Steroid binding sites on two distinguishable forms of rat adrenal mitochondrial cytochrome P-450sec

Role of the peripheral-type benzodiazepine receptor and the polypeptide diazepam binding inhibitor in steroidogenesis

Steroidogenesis begins with the metabolism of cholesterol to pregnenolone by the inner mitochondrial membrane cytochrome P450 side-chain cleavage (P450scc) enzyme. The rate of steroid formation, however, depends on the rate of (i) cholesterol transport from intracellular stores to the inner mitochondrial membrane and (ii) loading of P450scc with cholesterol. We demonstrated that a key element in the regulation of cholesterol transport is the mitochondrial peripheral-type benzodiazepine receptor (PBR) and that the presence of the polypeptide diazepam binding inhibitor (DBI) was vital for steroidogenesis. We also showed that DBI, as the endogenous PBR ligand, stimulates cholesterol transport. In addition, DBI directly promotes loading of cholesterol to P450scc. We review herein our studies on the structure, function, topography and hormonal regulation of PBR and DBI in steroidogenic cells. Based on these data we propose a model where the interaction of DBI with PBR, at the outer/inner membrane contact sites, is the signal transducer of hormone-stimulated and constitutive steroidogenesis at the mitochondrial level. Hormone-induced changes in PBR microenvironment/structure regulate the affinity of the receptor. PBR ligand binding to a higher affinity receptor results in increased cholesterol transport. In addition, hormone-induced release (processing?) of a 30,000 Mw DBI-immunoreactive protein from the inner mitochondrial membrane may result to the intramitochondrial production of DBI which directly stimulates loading of P450scc with cholesterol. Thus, in vivo, hormonal activation of these two mechanisms results in efficient cholesterol delivery and utilization and thus high levels of steroid synthesis.

Comparison of the multiple molecular forms of bovine adrenocortical P450scc with those of corpus luteum P450scc

1. Bovine adrenocortical P450scc was resolved into several fractions by chromatography on AH-Sepharose 4B followed by gel filtration on Toyopearl HW55S. All fractions contained P450scc of the same molecular size and the P450scc could be resolved into 3-4 major and more than 10 minor isoelectric point forms by isoelectric focusing on polyacrylamide gel in the presence of Emulgen 913. 2. Both the AH-Sepharose chromatography profile and the isoelectric focusing pattern of the adrenocortical P450scc were more complex than those of the corpus luteum P450scc. The corpus luteum P450scc was practically devoid of the neutral to acidic isoelectric point forms. 3. Three to four P450scc subfractions with different isoelectric focusing pattern were obtained from a purified preparation of adrenocortical P450scc by ion-exchange chromatography on DEAE-Toyopearl 650S or DEAE-Sephadex A25. These P450scc subfractions showed essentially the same spectral properties, catalytic activity, molecular weight and N-terminal amino acid sequence. 4. The most acidic (the latest eluting) subfraction was composed mostly of the neutral to acidic isoelectric point forms. The sedimentation characteristics of this subfraction was also studied. 5. The structural basis of the multiple molecular forms was discussed.

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.

Ascorbate as a source of reducing equivalents for the synthesis of aldosterone

The three steps in the synthesis of aldosterone (11 beta/18-hydroxylations and aldehyde synthetase) were examined in mitochondria from bovine glomerulosa and fasciculata to study the regulation of aldehyde synthetase. Ascorbate plus NADH shows synergism with malate in stimulating aldehyde synthetase without affecting 11 beta/-18-hydroxylations. The concentration of semidehydroascorbate reductase in mitochondria from glomerulosa is more than twice that from fasciculata. We propose that in glomerulosa, ascorbate provides a source of reducing equivalents that specifically support the last step in the synthesis of aldosterone.

Multiple molecular forms of cytochrome P-450SCC purified from bovine corpus luteum mitochondria

Cytochrome P-450 related to side-chain cleavage of cholesterol (P-450SCC) was isolated from bovine corpus luteum mitochondria in the form of its stable cholesterol complex. The isolation procedure included ammonium sulfate fractionation and chromatography on omega-aminohexyl-Sepharose (AH-Sepharose). Corpus luteum P-450SCC was resolved into one minor (AH-I) and two major (AH-II and AH-III) fractions by the chromatography. Results of re-chromatography suggested the possibility that AH-III Fraction was originally complexed with lipidic material. The two major fractions purified by the re-chromatography (AH-IIR and AH-IIIR Fractions) showed essentially a single band on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and their absorption spectra were indistinguishable from each other. Both fractions were further resolved into two major and some minor bands of P-450SCC by isoelectric focusing on polyacrylamide gel in the presence of a non-ionic detergent, as detected by protein staining, heme staining and immunoblot analysis with anti-bovine P-450SCC monoclonal antibody. Both AH-IIR and AH-IIIR Fractions were further resolved by high-performance liquid chromatography (HPLC) on SP-TSK gel column into two fractions, SP-I and SP-II. These fractions had the same N-terminal amino acid sequence, showed similar catalytic activity and resolved into one major and a few minor bands on isoelectric focusing on polyacrylamide gel. Much more heterogeneity was observed in purified P-450SCC preparations from bovine adrenal cortex mitochondria. These results indicated the presence of multiple molecular forms of corpus luteum P-450SCC as well as adrenal cortex P-450SCC. Computer simulation studies were carried out in order to analyze the mechanism of formation of multiple bands on isoelectric focusing. The multiple bands of corpus luteum P-450SCC could be explained by postulating the presence of two isozymes (or molecular forms) having a pair of sites each with or without a charged group.

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)

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.

Differences between the regulation of cholesterol side-chain cleavage in Leydig cells from mice and rats

A Leydig cell culture system has been used to study the in vitro modulation by luteinizing hormone (LH) of steroidogenesis in Leydig cells isolated from mice and immature rats. Mouse Leydig cells precultured for 24 h in the presence of increasing concentrations of LH (1 ng-1 microgram/ml) showed a dose-dependent decrease of the maximal LH-stimulated testosterone production. After pretreatment with 1 microgram LH/ml, maximal LH-stimulated testosterone production. After production in the presence of excess 20 alpha-hydroxycholesterol (a cholesterol side-chain cleavage substrate) were reduced to approx. 50% of control values. The possible site of action of LH is probably prior to pregnenolone, because testosterone production in the presence of excess pregnenolone was not affected by the LH pretreatment. Immature rat Leydig cells showed no decrease of maximal steroid production after 24 h culture in the presence of 1 microgram LH/ml. These results indicate that the regulation of the cholesterol side-chain cleavage activity during long-term LH action is different in mouse and rat Leydig cells. The properties of the cholesterol side-chain cleavage enzyme in mouse and rat Leydig cells were further investigated with different hydroxylated cholesterol derivatives as substrates. Steroid production by mouse Leydig cells in the presence of (22R)-22 hydroxycholesterol was similar as in the presence of LH. In contrast, steroidogenesis in rat Leydig cells in the presence of (22R)-22 hydroxycholesterol was at least 10-fold higher than in the presence of LH. It is concluded that the cholesterol side-chain cleaving enzyme in the mouse Leydig cell operates at its maximal capacity during short-term LH stimulation and can be inhibited after long-term LH action, whereas in the rat Leydig cell only a fraction of the potential activity is used during short-term LH stimulation, which is not affected during long-term LH action.

Temperature effects on optical absorption and circular dichroism of cytochrome P-450scc from bovine adrenocortical mitochondria

Temperature-dependent spin changes of the heme iron atom on cytochrome P-450scc were studied by optical absorption and circular dichroism measurements. The optical absorption and circular dichroism spectra of cholesterol-free cytochrome P-450scc did not change between 10 and 26 degrees C. In contrast, the absorbance at 390 nm and the ellipticity at 330 nm of cholesterol-bound cytochrome P-450scc decreased upon temperature elevation, and the absorbance at 424 nm correspondingly increased. These spectral changes were reversible in respect of temperature. The far-ultraviolet circular dichroism spectra of both cholesterol-bound and -free cytochrome P-450scc were not affected by temperature. In addition, bound cholesterol molecule is not released from the cytochrome molecule by increasing temperature. From these results, we propose that temperature modulates specific interactions between the heme protein and bound cholesterol rather than the gross secondary structural changes of the protein.

The domain structure of the cholesterol side-chain cleavage cytochrome P-450 from bovine adrenocortical mitochondria

A homogeneous cytochrome P-450scc preparation with a specific enzyme content of 18 nmol/1 mg protein has been obtained using affinity chromatography on adrenodoxin-Sepharose under optimal conditions of the protein adsorption onto and desorption from the affinity sorbent. The data on the N-terminal amino acid sequence of the enzyme, along with the results of electrophoretic and spectrophotometric analyses favoured the multistage cholesterol transformation to pregnenolone to be catalyzed by single species of cytochrome P-450scc consisting of one polypeptide chain. Limited proteolysis of cytochrome P-450scc with trypsin resulted, at the initial stages, in the formation (in an equimolar ratio) of two large polypeptide fragments, I and II, with Mr 27000 and 22000, respectively. Prolonged action of trypsin led to the digestion of fragment II and the formation of a stoichiometric amount of fragment III, Mr of about 14000. Cytochrome P-450scc converted by trypsin into equimolar mixtures of fragments I and II or I and III retained the major spectral and functional properties of the native protein. The aspartyl-prolyl linkages, sulphhydryl groups, and surface tyrosine residues are distributed nonuniformly among fragments I and II. These data, as well as a different resistance of the fragments to the action of trypsin, suggest that cytochrome P-450scc consists of two independently folded domains linked with a short loop of the polypeptide chain, the domains being rigidly associated under neutral conditions.

Amino-steroids as inhibitors and probes of the active site of cytochrome P-450scc. Effects on the enzyme from different sources

A series of analogues of cholesterol, each having a primary amine attached to a shortened side chain, were tested for their effects on cytochrome P-450scc from several different sources. Reconstituted enzyme systems using disrupted mitochondria from bovine adrenal and placenta, adult human adrenal and placenta, neonatal human adrenal, and rat adrenal and testis were used to assay for inhibitory effects on the side chain cleavage of cholesterol to pregnenolone. Two of the derivatives tested, 22-amino-23,24-bisnor-5-cholen-3 beta-ol and 23-amino-24-nor-5-cholen-3 beta-ol, were found to be potent inhibitors of this reaction; the derivatives in which the amine was attached closer to or further from the steroid ring, (20 R and S)-20-amino-5-pregnen-3 beta-ol and 24-amino-5-cholen-3 beta-ol, were much weaker inhibitors. In addition, spectral studies with rat adrenal mitochondria and a soluble preparation of human placental cytochrome P-450scc showed that binding of the 22-amine derivative to the enzyme produces difference spectra characteristic of nitrogen bonding to the heme; this indicates that the heme is positioned close to C-22 in the steroid-enzyme complex. These findings on the relative effectiveness of the amino-steroid inhibitors and the type of complex formed are similar to results obtained with purified bovine adrenocortical cytochrome P-450scc. This establishes that the proximity of the substrate binding site and the heme-iron catalytic site is a feature common to the enzyme from several sources and is therefore likely to be a necessary property of the active site structure.

Polypeptide activators of cholesterol side-chain cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."

Magnetic and natural circular dichroism spectra of cytochgromes P-450(11) beta and P-450scc purified from bovine adrenal cortex

Magnetic (MCD) and natural circular dichroism (CD) spectra various complexes of cytochrome P-450(11) beta (P-450(11) beta) and cytochrome P-450scc (P-450scc) from bovine adrenal cortex were measured from 250 nm to 700 nm. MCD and CD spectral contours of cytochromes P-450(11) beta and P-450scc in the Soret and visible regions were, as a whole, analogous to those of cytochromes P-450 from rabbit liver microsomes and also from Pseudomonas putida in their high-spin ferric, high-spin ferrous and ferrous-CO complexes. MCD spectrum of the low-spin ferric P-450scc free from the substrate, cholesterol, was very similar to that caused by addition of 20 alpha-hydroxycholesterol, a reaction intermediate. However, it was distinct from those of the low-spin ferric P-450(11) beta and P-450scc complexes caused by addition of external nitrogenous ligands. The electronic states of the heme in the low-spin ferric P-450 free from substrates seemed to be subtly different from those of low-spin complexes coordinated with external nitrogenous ligands. Soret CD spectra of ferric low-spin complexes were not so different from each other. Upon reduction of high-spin ferric P-450(11) beta or P-450scc, the Soret CD magnitudes increased significantly in contrast with those of other P-450s, the Soret CD magnitudes of which decrease upon reduction. This may reflect an increased proximity of the neighbouring aromatic groups upon reduction of high-spin P-450(11) beta or P-450scc. High substrate specificity of adrenal P-450s compared with liver P-450s can be explained in view of the above findings. THe CD spectra in the near ultraviolet region (250-350 nm) were found to be quite sensitive to the spin change for ferric P-450scc, while the MCD spectra in this region did not reflect substantially the spin state of the enzyme. MCD parameters of cytochrome P-450s were compared to those of other hemoproteins in diagrams describing selected MCD spectral values of hemoproteins so far available and were discussed in connection with the structures of the heme environment of P-450.

Effects of cholesterol, hydroxycholesterols and calcium on pregnenolone production rates in mitochondrial fractions from rat testes

The in vitro regulation of the mitochondrial conversion of cholesterol to pregnenolone in rat testis tissue has been further investigated. Pregnenolone production rates by isolated mitochondrial fractions could be stimulated by the addition of cholesterol. The stimulation was always highest in mitochondria isolated from lutropin-treated testes relative to control and cycloheximide treated testes. Addition 20- or 25-hydroxycholesterol resulted in a greater stimulation of pregnenolone production rates and these rates were unaffected by prior treatment with cycloheximide. When both cholesterol and 20- or 25-hydroxycholesterol were present in the incubation medium, pregnenolone production rates were mainly influenced by the hydroxycholesterol, even in the presence of a ten-fold excess of cholesterol. Ca2+ in vitro stimulated pregnenolone production rates from endogenous cholesterol as well as from added cholesterol. However, pregnenolone production rates in the presence of hydroxycholesterol were not influence by the addition of Ca2+ in vitro.