Studies of Electron Transport Enzymes

Functional and structural insight into the flexibility of cytochrome P450 reductases from Sorghum bicolor and its implications for lignin composition

Plant NADPH-dependent cytochrome P450 reductase (CPR) is a multidomain enzyme that donates electrons for hydroxylation reactions catalyzed by class II cytochrome P450 monooxygenases involved in the synthesis of many primary and secondary metabolites. These P450 enzymes include trans-cinnamate-4-hydroxylase, p-coumarate-3′-hydroxylase, and ferulate-5-hydroxylase involved in monolignol biosynthesis. Because of its role in monolignol biosynthesis, alterations in CPR activity could change the composition and overall output of lignin. Therefore, to understand the structure and function of three CPR subunits from sorghum, recombinant subunits SbCPR2a, SbCPR2b, and SbCPR2c were subjected to X-ray crystallography and kinetic assays. Steady-state kinetic analyses demonstrated that all three CPR subunits supported the oxidation reactions catalyzed by SbC4H1 (CYP73A33) and SbC3′H (CYP98A1). Furthermore, comparing the SbCPR2b structure with the well-investigated CPRs from mammals enabled us to identify critical residues of functional importance and suggested that the plant flavin mononucleotide–binding domain might be more flexible than mammalian homologs. In addition, the elucidated structure of SbCPR2b included the first observation of NADP⁺ in a native CPR. Overall, we conclude that the connecting domain of SbCPR2, especially its hinge region, could serve as a target to alter biomass composition in bioenergy and forage sorghums through protein engineering.

Reduction potential and heme-pocket polarity in low potential cytochrome b5 of Giardia intestinalis

Although it lacks mitochondria and the ability to synthesize heme, the protozoan parasite Giardia intestinalis encodes several heme proteins. This includes four members of the cytochrome b5 family, three of which are of similar size to mammalian cytochromes b5 but with reduction potentials that are 140 to 180mV lower. While no structures have yet been determined for any of these proteins, homology modeling points to an increase in heme pocket polarity as a reason for their low potentials. To test this we measured the reduction potentials of four mutants of Giardia cytochrome b5 isotype-I (gCYTB5-I) in which polar residues at two candidate positions (C84, Y51) in the heme pocket were changed to nonpolar ones (C84A, C84F; Y51L, Y51F). All mutants were expressed with comparable levels of heme incorporation and had UV-visible spectra consistent with low spin bis-histidyl coordination. These mutations increased the reduction potential by 18 to 57mV and highlight the influence of C84, which is a residue unique to gCYTB5-I and whose mutation to alanine caused the largest increase. The influence of these two residues plus that of Y61 reported previously accounts for much of the reduction potential difference between gCYTB5-I and microsomal cytochrome b5. A complementary triple mutant of the latter with the hydrophilic residues found in gCYTB5-I bound heme less effectively but nonetheless had a reduction potential that was 135mV lower than wild type.

Heme proteins of Giardia intestinalis

Among the few organisms that cannot make the iron cofactor heme, some nonetheless possess heme proteins. This includes the protozoan parasite Giardia intestinalis, which encodes five known heme proteins: a flavohemoglobin and four members of the cytochrome b5 family. Giardia flavohemoglobin closely resembles those of the Enterobacteriaceae in structure and function, acting as a nitric oxide dioxygenase that is induced when trophozoites are exposed to reactive nitrogen species. The Giardia cytochromes b5 are soluble proteins having relatively low reduction potentials and lack several features that are expected to promote rapid electron transfer with redox partners. Only one potential electron donor, and no electron acceptors, have yet been identified in the Giardia genome, and the roles of these cytochromes are presently unknown. The answer may lie in the sequences that flank the heme-binding core of these proteins which could serve to localize them within the cell through reversible post-translational modifications and to promote specific protein-protein interactions.

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Purification and characterization of two forms of microsomal cytochrome b5 from sheep lung

Two forms of cytochrome b5 were purified from detergent solubilized sheep lung microsomes by three successive DEAE-cellulose, Sephadex G-100 and Sephadex G-200 column chromatographies. The specific contents of cytochromes b5-I and b5-II were determined to be 45.4 and 43.8 nmol b5/mg protein, which represented up to 567 and 547-fold purification compared with that of the lung microsomes. The most striking difference between b5-I and b5-II was observed in their elution pattern from the third DEAE-cellulose column. Cytochromes gave one protein band with a Mr of 16400 +/- 500 on SDS-PAGE. Both forms of reduced b5 showed a major peak at 423 nm while reduced b5-I had two minor peaks at 527 and 556 and reduced b5-II gave two well-defined peaks at 526 and 555 nm. The ability of the purified b5-I and b5-II fractions to transfer the electrons from NADH-cytochrome b5 reductase to cytochrome c was investigated. Apparent Km, 0.055 microM, of b5-II was found to be 38% lower than that of b5-I. In addition, cytochrome b5-I was found to be more sensitive to heat treatment than b5-II when cytochromes were subjected to 62 degrees C for varying periods of time and the coupling of b5 reduction to cytochrome c reduction was determined. These results may indicate that two forms may exist in lung endoplasmic reticulum.

A microsomal membrane component associated with iron reduction in NADPH-supported lipid peroxidation

This study was conducted to determine whether a factor responsible for reduced nicotinamide adenine dinucleotide phosphate (NADPH)-supported lipid peroxidation in rat liver microsomes is involved in iron reduction by cooperation with NADPH-cytochrome P450 reductase. Under anaerobic conditions, NADPH-dependent reduction of ferric pyrophosphate in microsomes was not dependent on cytochrome P450 levels and was not inhibited by carbon monoxide (CO). All of the iron complexes with chelators such as adenosine 5'-diphosphate, pyrophosphate, nitrilotriacetate, oxalate or citrate were reduced in microsomes, although in the reconstituted system containing purified NADPH-cytochrome P450 reductase little or no iron reduction was found. A cytochrome P450-free fraction from a cholate-solubilized preparation of microsomes after passage through a laurate sepharose column was required for reduction of iron pyrophosphate in the reconstituted system leading to lipid peroxidation. The iron reduction was not inhibited by CO and was destroyed by heat treatment or trypsin digestion of the fraction. All iron complexes were reduced in the presence of the fraction, using a reducing equivalent of NADPH via NADPH-cytochrome P450 reductase. The results indicate that a heat-labile component, which is probably a protein distinct from cytochrome P450, is associated with iron reduction responsible for lipid peroxidation in microsomes.

Cholate solubilization of liver microsomal membrane components which promote NADPH-supported lipid peroxidation

NADPH-supported lipid peroxidation monitored by malondialdehyde (MDA) production in the presence of ferric pyrophosphate in liver microsomes was inactivated by heat treatment or by trypsin and the activity was not restored by the addition of purified NADPH-cytochrome P450 reductase (FPT). The activity was differentially solubilized by sodium cholate from microsomes, and the fraction solubilized between 0.4 and 1.2% sodium cholate was applied to a Sephadex G-150 column and subfractionated into three pools, A, B, and C. MDA production was reconstituted by the addition of microsomal lipids and FPT to specific fractions from the column, in the presence of ferric pyrophosphate and NADPH. Pool B, after removal of endogenous FPT, was highly active in catalyzing MDA production and the disappearance of arachidonate and docosahexaenoate, and this activity was abolished by heat treatment and trypsin digestion, but not by carbon monoxide. The rate of NADPH-supported lipid peroxidation in the reconstituted system containing fractions pooled from Sephadex G-150 columns was not related to the content of cytochrome P450. p-Bromophenylacylbromide, a phospholipase A2 inhibitor, inhibited NADPH-supported lipid peroxidation in both liver microsomes and the reconstituted system, but did not block the peroxidation of microsomal lipid promoted by iron-ascorbate or ABAP systems. Another phospholipase A2 inhibitor, mepacrine, poorly inhibited both microsomal and pool-B'-promoted lipid peroxidation, but did block both iron-ascorbate-driven and ABAP-promoted lipid peroxidation. The phospholipase A2 inhibitor chlorpromazine, which can serve as a free radical quencher, blocked lipid peroxidation in all systems. The data presented are consistent with the existence of a heat-labile protein-containing factor in liver microsomes which promotes lipid peroxidation and is not FPT, cytochrome P450, or phospholipase A2.

Metabolism of nitroxide spin labels in subcellular fraction of rat liver. I. Reduction by microsomes

As part of an ongoing study of the role of subcellular fractions on the metabolism of nitroxides, we studied the metabolism of a set of seven nitroxides in microsomes obtained from rat liver. The nitroxides were chosen to provide information on the effects of the type of charge, lipophilicity and the ring on which the nitroxide group is located. Important variables that were studied included adding NADH, adding NADPH, induction of enzymes by intake of phenobarbital and the effects of oxygen. Reduction to nonparamagnetic derivatives and oxidation back to paramagnetic derivatives were measured by electron-spin resonance spectroscopy. In general, the relative rates of reduction of nitroxides were similar to those observed with intact cells, but the effects of the various variables that were studied often differed from those observed in intact cells. The rates of reduction were very slow in the absence of added NADH or NADPH. The relative effect of these two nucleotides changed when animals were fed phenobarbital, and paralleled the levels of NADPH cytochrome c reductase, cytochrome P-450, cytochrome b5 and NADH cytochrome c reductase; results with purified NADPH-cytochrome c reductase were consistent with these results. In microsomes from uninduced animals the rate of reduction was about 10-fold higher in the absence of oxygen. The products of reduction of nitroxides by microsomes were the corresponding hydroxylamines. We conclude that there are significant NADH- and NADPH-dependent paths for reduction of nitroxides by hepatic microsomes, probably involving cytochrome c reductases and not directly involving cytochrome P-450. From this, and from parallel studies now in progress in our laboratory, it seems likely that metabolism by microsomes is an important site of reduction of nitroxides. However, mitochondrial metabolism seems to play an even more important role in intact cells.

Synergy of phenobarbital and 3-methylcholanthrene in "superinduction" of cytochrome P-450c mRNA but not enzyme activity

The combination of phenobarbital and 3-methylcholanthrene in the inductive process of the rat hepatic cytochrome P-450c gene was evaluated. Daily injections of phenobarbital (80 mg/kg, i.p.) had little or no effect on the amount of poly (A)+ RNA encoding cytochrome P-450c, whereas a single injection of 3-methylcholanthrene (25 mg/kg, i.p.) produced a significant accumulation at 15 hr in cytosolic mRNA coding for cytochrome P-450c. Four daily injections of phenobarbital followed by a single dose of 3-methylcholanthrene produced 5-24 times more poly (A)+ RNA coding for P-450c than 3-methylcholanthrene treatment alone. This superinduction of RNA transcripts was also observed for a species coding for cytochrome P-450d, which was increased 3-6 times over 3-methylcholanthrene treatment alone. However, the elevated concentration of transcripts for both the P-450d and P-450c RNA species did not result in an increase in the marker enzyme activity for cytochrome P-450c, 7-ethoxyresorufin O-deethylase. These data implicate a regulatory step in the induction of cytochrome P-450c enzyme activity which must occur at a level beyond transcription.

Effects of vitamin A and/or thyroidectomy on liver microsomal enzymes and their induction in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

Vitamin A and thyroid hormone status have been shown previously to alter the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats. In the present study, we have examined the effects of a vitamin A-excess and a vitamin A-deficient diet on thyroid hormone levels, on selected drug-metabolizing enzymes in liver microsomes, and on their inducibility by TCDD in male Sprague-Dawley rats. Except for a slight increase in serum T3 levels, none of these end points was affected by feeding rats the vitamin A-deficient diet. In contrast, excess dietary vitamin A caused a decrease in serum thyroxine (T4) and triiodothyronine (T3) levels, although the levels of T3 remained in the euthyroid range (60-80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b5 and the basal activity of benzo[a]pyrene hydroxylase and 7-ethoxyresorufin O-de-ethylase were unaffected by excess dietary vitamin A. This result is consistent with our previous observation that the basal activity of these enzymes is dependent more on T3 than on T4 levels. Vitamin A excess markedly suppressed the activity of liver microsomal UDP-glucuronosyl transferase toward 1-naphthol. However, no such enzyme suppression was observed in thyroidectomized rats. This suggests that the suppressive effect of vitamin A on UDP-glucuronosyl transferase activity may be dependent on T3. Neither vitamin A nor thyroid status had any major effect on the inducibility of UDP-glucuronosyl transferase and cytochrome P-450-dependent enzyme activities by TCDD. However, vitamin A and TCDD had a nearly additive effect on suppression of serum T4. It is concluded that liver microsomal enzyme induction is not associated with the modulatory effect of vitamin A and thyroid hormones on the toxicity of TCDD.

Incorporation of cytochrome b5 into rat liver microsomal membranes. Impairment of cytochrome P-450-dependent mixed function oxidase activity

Cytochrome b5 was purified to electrophoretic homogeneity from the liver microsomes of untreated rats and reincorporated into liver microsomes from phenobarbital-treated rats, resulting in an approximate three-fold enrichment of the cytochrome b5 specific content (1.5 nmol haemoprotein X mg-1 protein). Our results have shown that the N-demethylation of benzphetamine was progressively inhibited in cytochrome b5-fortified microsomal preparations. Using stopped flow, visible difference spectrophotometry, the NADPH-driven reduction kinetics of cytochrome P-450 were examined in the modified microsomes over the first few seconds of reaction. Increasing the amount of incorporated cytochrome b5 resulted in a progressive inhibition of the initial, fast phase reduction rate constant of microsomal cytochrome P-450, both in the absence and presence of the type I substrate benzphetamine. Although the initial rate of NADPH-driven cytochrome b5 reduction was the same for both native and cytochrome b5-fortified microsomes, the extent of cytochrome b5 reduction was greater in the fortified microsomes. If cytochrome b5 has a positive role to play in cytochrome P-450-dependent mixed function oxidase activity either as an effector or in electron transfer or both, the former haemoprotein must be already present in sufficient concentrations in the native microsomes.

Mechanism of interaction between cytochromes P-450 RLM5 and b5: evidence for an electrostatic mechanism involving cytochrome b5 heme propionate groups

The role of cytochrome b5 heme propionate groups in the functional interactions between cytochromes P-450 RLM5 and b5 has been investigated by comparing the capacity of RLM5 to interact with both native b5 and a b5 derivative in which the native heme was replaced with ferric protoporphyrin IX dimethyl ester (DME-b5). Both forms of b5 interacted with RLM5 causing an increase in the RLM5 spin state from 28 to 68% high-spin RLM5 at saturation, as judged using uv-visible spectrophotometry. However, DME-b5 exhibited a 7-fold weaker affinity for RLM5. The apparent dissociation constant (Kd) for the interaction between RLM5 and b5 was also shown to be a strong function of ionic strength, in a manner consistent with the involvement of electrostatic attraction in complex formation. Reconstitution of b5 into an RLM5-dependent monooxygenase system stimulated the p-nitroanisole demethylase rate about 25-fold and 7-ethoxycoumarin deethylase about 6-fold. DME-b5, however, produced only 30% of the stimulation of RLM5-dependent turnover of p-nitroanisole observed at equivalent concentrations of native b5 without a change in Km. With 7-ethoxycoumarin, turnover was 50% diminished. The diminished capacity of DME-b5 to stimulate RLM5-dependent substrate turnover was shown not to be due to impairment of electron flow between NADPH-cytochrome P-450 reductase and DME-b5, since the Km of reductase for DME-b5 is 2.5-fold lower, and the Vmax is actually increased, but rather to an impairment of some aspect of functional interaction between the DME-b5 and RLM5. The data show that complex formation between cytochrome P-450 and b5 involves electrostatic attraction mediated in part by cytochrome b5 heme propionate groups.

Reduced serum thyroid hormone levels in hexachlorobenzene-induced porphyria

The effect of feeding 0.1% hexachlorobenzene (HCB) for 55 days on mortality, body weight, urinary porphyrin excretion, serum thyroid hormones and induction of liver microsomal enzymes was studied in female Sprague-Dawley rats. This dosage regimen, followed by 42 days of a regular diet, resulted in 33% mortality with a mean time to death of 67 +/- 4 days. Body weight of survivors was not affected by dietary HCB, whereas non-survivors underwent a rapid weight loss (wasting) prior to death. At the end of the dosing period (day 55), rats fed the HCB diet exhibited an increase in the excretion of urinary porphyrins (4-fold) and a significant decrease in the levels of serum thyroxine (T4) and triiodothyronine (T3). When rats were returned to a regular diet the excretion of urinary porphyrins continued to rise (approx. 100 times higher than controls) and serum thyroid hormones remained suppressed. At the end of the experiment (day 97), the concentration of liver microsomal cytochrome P-450 and cytochrome bs and the activity of ethoxyresorufin-O-deethylase, pentoxyresorufin-O-dealkylase, aminopyrine-N-demethylase and UDP-glucuronosyl transferase were significantly induced, whereas the activity of NADPH-cytochrome c reductase and benzo[a]pyrene hydroxylase was not. Results demonstrate that HCB-induced lethality and porphyria occur by different mechanisms, reduced T4 and T3 serum levels accompany induction of porphyria by HCB, and induction of aryl hydrocarbon hydroxylase (with benzo[a]pyrene as substrate) is not a sensitive indicator of HCB exposure.

Effect of thyroid hormones on liver microsomal enzyme induction in rats exposed to 2,3,7,8,-tetrachlorodibenzo-p-dioxin

The effect of thyroidectomy and thyroid hormone replacement therapy on liver microsomal enzyme induction was studied in 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats (100 micrograms/kg). Treatment of non-thyroidectomized rats with TCDD had no effect on the concentration of liver microsomal cytochrome b5. In contrast, cytochrome b5 content was increased by TCDD treatment of thyroidectomized rats, regardless of replacement therapy with either T3 or T4. TCDD treatment increased the concentration of cytochrome P-450 (2-3-fold) and the activities of benzo[a]pyrene hydroxylase (4-7-fold), ethoxyresorufin O-de-ethylase (50-70-fold) and UDP-glucuronosyltransferase (5-7-fold) in non-thyroidectomized and thyroidectomized as well as thyroidectomized thyroid hormone treated rats; indicating the induction of these liver microsomal enzyme activities is independent of thyroid status. Because thyroid status alters the toxicity of TCDD but does not alter the ability of TCDD to induce microsomal enzymes, it appears that TCDD toxicity may not be directly related to microsomal enzyme induction.

Induction of rat hepatic microsomal cytochrome P-450 by 2,3',4,4',5,5'-hexachlorobiphenyl

The following evidence suggests that 2,3',4,4',5,5'-hexachlorobiphenyl resembles isosafrole as an inducer of hepatic microsomal cytochrome P-450d in the immature male Wistar rat. First, the major hepatic microsomal polypeptide (Mr = 52,000), intensified after treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with cytochrome P-450d (i.e. the major isosafrole-inducible polypeptide) but had an electrophoretic mobility intermediate between cytochrome P-450b (Mr approximately equal to 51,500) and cytochrome P-450c (Mr = 56,000) (i.e. the major phenobarbital- and 3-methylcholanthrene-inducible polypeptides respectively). Second, when pairs of various xenobiotics were coadministered to rats at doses effecting maximal induction of hepatic microsomal cytochrome P-450, the inductive effects of 2,3',4,4',5,5'-hexachlorobiphenyl were additive with those of phenobarbital, 3-methylcholanthrene and pregnenolone-16 alpha-carbonitrile but not with those of isosafrole. The inductive effects of phenobarbital, 3-methylcholanthrene, pregnenolone-16 alpha-carbonitrile and isosafrole were all expressed additively with each other. Third, in contrast to phenobarbital and pregnenolone-16 alpha-carbonitrile treatment, treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene failed to increase markedly the proportion of total cytochrome P-450 capable of forming a 446 nm-absorbing complex with metyrapone. Fourth, the in vitro metabolism of isosafrole, catalyzed by hepatic microsomes from rats treated with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene, produced complexes between ferrous cytochrome P-450 and a methylenedioxyphenyl metabolite, the spectra of which were between 400 and 500 nm and were similar to each other but which were readily distinguishable from the spectra of the product adducts formed during the metabolism of isosafrole by hepatic microsomes from rats treated with corn oil (control), phenobarbital, or pregnenolone-16 alpha-carbonitrile.

Two homologous cytochromes b5 in a single cell

The amino acid sequence of the heme-binding domains of rat liver cytochromes b5 from outer mitochondrial membranes and from microsomes has been determined by a combination of automatic and manual degradation of fragments generated by trypsin digestion and by cleavage at tryptophan. Tryptic peptides were separated by high-pressure liquid chromatography. The sequence of microsomal cytochrome b5 is identical with the one published by Ozols and Heinemann after completion of this study [Biochim. Biophys. Acta (1982) 704, 163-173]. The sequence of outer membrane cytochrome b5 differs from the microsomal one at 38 positions out of 91. There are 40 positions invariant between this sequence and the eight microsomal sequences published thus far. The non-conservative substitutions are located at the surface of the known three-dimensional structure of calf microsomal cytochrome b5 except for the substitution of histidine-15 by arginine. This paper brings the final proof that two iso-cytochromes b5 exist in the same cell. Their high degree of similarity as well as their differential cellular localization raise some questions which are briefly discussed.

Relationship between malondialdehyde production and arachidonate consumption during NADPH-supported microsomal lipid peroxidation

Fatty acid concentrations and malondialdehyde formation were determined before and during NADPH-supported lipid peroxidation in liver microsomes from rat, mouse, guinea pig and rabbit. In agreement with earlier reports, malondialdehyde production was greatest for rat, followed by mouse, and much less for guinea pig and rabbit. The microsomal content of total unsaturated fatty acids (18:1, 18:2, 18:3, 20:3, 20:4, 20:5, 22:5, 22:6) was approximately the same for rat and mouse and was lower in guinea pig and rabbit. Lipid peroxidation caused a time-dependent decrease in the polyunsaturated fatty acids, particularly 20:4 and 22:6, for all species. These decreases were most pronounced for rat and mouse. Alterations in the dietary regime for rat produced marked changes in microsomal fatty acid content as reported by others, and also caused changes in the rates of malondialdehyde production and polyunsaturated fatty acid consumption during lipid peroxidation. A comparison between the rates of malondialdehyde production and the rates of individual unsaturated fatty acid consumption was performed for each animal species and for rats fed different diets. A linear relationship was found between malondialdehyde production and 20:4 disappearance in individual microsomal preparations and in different species. A similar relationship was seen for the initial microsomal concentration of 20:4 and the initial rate of malondialdehyde formation. Other unsaturated fatty acids did not exhibit linear relationships. Various microsomal mixed-function oxidase variables were measured for the different species. No direct relationship between these values and malondialdehyde production was found.

Potent induction of rat liver microsomal, drug-metabolizing enzymes by 2,3,3',4,4',5-hexabromobiphenyl, a component of fireMaster

The multistep synthesis and purification of 2,3,3',4,4',5-hexabromobiphenyl (HBBp) is described. Capillary gas chromatography revealed that HBBp comprises 0.05% of the industrial polybrominated biphenyl (PBB) mixture, fireMaster BP-6 (lot 7062). When administered to immature male Wistar rats, HBBp caused a dose-dependent increase in (a) the activity of benzo[a]pyrene (B[a]P) hydroxylase (AHH) and 4-chlorobiphenyl (4-CBP) hydroxylase and (b) the concentration of cytochrome P-450. Sodium dodecyl sulfate (SDS)-gel electrophoresis indicated that these increases in cytochrome P-450 and cytochrome P-450-dependent monooxygenase activities were accompanied by a dose-dependent intensification of a protein of relative molecular weight (Mr) 55 000 which comigrated with the major 3-methylcholanthrene(MC)-inducible form of cytochrome P-450 (i.e., cytochrome P-448). Like MC, but in contrast to phenobarbitone (PB), HBBp competitively displaced 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin ([3H]-TCDD) from the cytosolic protein thought to be the receptor for cytochrome P-448 induction. The results indicate that HBBp is a potent inducer of cytochrome P-448 and as such is the third MC-type inducer identified in fireMaster BP-6.

Polychlorinated biphenyls as inducers of hepatic microsomal enzymes: effects of di-ortho substitution

All of the 13 possible polychlorinated biphenyl (PCB) isomers and congeners substituted at both para positions, at least two meta positions (but not necessarily on the same ring) and at two ortho positions have been synthesized and tested as rat hepatic microsomal enzyme inducers. The effects of these compounds were evaluated by measuring microsomal benzo-[a]pyrene (B[a]P) hydroxylase, 4-chlorobiphenyl (4-CBP) hydroxylase, 4-dimethylaminoantipyrine (DMAP) N-demethylase and NADPH-cytochrome c reductase activities, the cytochrome b5 content and the relative peak intensities and spectral shifts of the carbon monoxide(CO)- and ethylisocyanide(EIC)-difference spectra of ferrocytochrome P-450. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC) and PB plus MC (coadministered). At dose levels of 150 mumol . kg-1, all of the PCB congeners, except 2,3',4,4',5',6-hexachlorobiphenyl, significantly enhanced hepatic microsomal cytochrome P-450 content, B[a] P hydroxylase and/or DMAP N-demethylase activities compared to the control (corn oil-treated) animals. Only 5 of these compounds, namely 2,3,4,4',5,6-hexa-, 2,2',3,3',4,4'-hexa-, 2,2',3',4,4',5-hexa-, 2,3,3',4,4',6-hexa-and 2,2',3,3',4,4',5-heptachlorobiphenyl, enhanced microsomal B[a]P hydroxylase, 4-CBP hydroxylase, NADPH-cytochrome c reductase and DMAP N-demethylase activities in a manner consistent with a mixed pattern of induction. The results suggest that PCB isomers and congeners substituted at both para positions, at least two meta positions, at two ortho positions and containing a 2,3-4-trichloro substitution pattern on one ring are mixed-type inducers; in addition the effects of 2,3,4,4',5,6-hexachlorobiphenyl were also consistent with a mixed pattern of induction.

Aryl hydrocarbon hydroxylase (AHH) induction by polybrominated biphenyls (PBBs): enchancement by photolysis

Irradiation of the commercial polybrominated biphenyl (PBB( mixture, fireMaster BP-6, in cyclohexane solution at 300 nm for 930 min resulted in a marked diminution of the major components of the mixture. Administration of the photolyzed PBB mixture of fireMaster BP-6 to immature male Wistar rats caused both dose-related decreases in thymus weight and increase in hepatic microsomal benzo[a]pyrene hydroxylase (AHH), 4-dimethylaminoantipyrine N-demethylase and NADPH-cytochrome c reductase activities and cytochrome P-450 content. The dose effecting half-maximal AHH induction for the photolyzed PBBs (9 mg . KG-1) was approximately 6 times lower than that of fireMaster BP-6 (50 mg. kg-1). Furthermore, the concentration of photolyzed PBBs (2 micrometers) required to displace 50% of the specifically-bound [3H] TCDD from its high-affinity cytosolic Ah receptor was approximately 150 times lower than that required for fireMaster BP-6 (300 micrometers), as measured by sucrose density gradient centrifugation analysis. The results suggest that the photolysis of the commercial PBB mixture yields products which possess increased biologic activity.

Octachloronaphthalene induction of hepatic microsomal aryl hydrocarbon hydroxylase activity in the immature male rat

Administration of octachloronaphthalene to immature male Wistar rats resulted in a dose-dependent increase in several enzymic, electrophoretic and spectral parameters associated with induction of the hepatic microsomal enzymes. Compared to corn-oil (control)treated animals octachloronaphthalene (150 mumol . kg-1) induced hepatic cytochrome P-450 (1.5-fold), benzo [alpha] pyrene hydroxylase (18-fold) and 4-chlorobiphenyl hydroxylase (18-fold) enzyme activities. In addition to increases in the relative peak intensities of the reduced microsomal cytochrome P-450 : CO and ethylisocyanide (EIC) difference spectra the peak maxima were observed at 448.5 and 452.2/428.0 nm, respectively. The effects of administering octachloronaphthalene to the rat were similar to those observed after pretreatment with 3-methylcholanthrene (MC) and electrophoresis of the induced microsomal proteins showed that both compounds enhance heme-staining peptides with comparable electrophoretic mobilities. Moreover coadministration of MC (3 x 100 mumol . kg-1) and octachloronaphthalene (2 x 150 mumol . kg-1) indicated that their inductive effects were not additive. It was concluded that octachloronaphthalene was an MC-type inducer of hepatic microsomal enzymes.

Polychlorinated biphenyls as inducers of hepatic microsomal enzymes: structure-activity rules

A number of highly purified polychlorinated biphenyl (PCB) isomers and congeners were synthesized and administered to male Wistar rats at dosage levels of 30 and 150 mumol . kg-1. The effects of this in vivo treatment on the drug-metabolizing enzymes were determined by measuring the microsomal benzo[a]pyrene (B[a]P) hydroxylase, dimethylaminoantipyrine (DMAP) N-demethylase and NADPH-cytochrome c reductase enzyme activities, the cytochrome b5 content and the relative peak intensities and spectral shifts of the reduced microsomal cytochrome P-450 CO and ethylisocyanide (EIC) binding difference spectra. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC) and PB plus MC (coadministered) to the test animals. The synthetic PCB congeners used in this study included 3,4,4',5-tetrachlorobiphenyl (TCBP-1), 2,3',4,4'-tetrachlorobiphenyl (TCBP-2), 2,3',4,4',5'-pentachlorobiphenyl (PCBP-1), 2,3,4,-4',5-pentachlorobiphenyl (PCBP-2), 2,3,3',4,4',5-hexachlorobiphenyl (HCBP-1) 2,3',4',5,6-hexachlorobiphenyl (HCBP-2), 2,3,3',5,5',6-hexachlorobiphenyl (HCBP-3), 2,2',3,5,5',6-hexachlorobiphenyl (HCBP-4) and 2,3,3',4,5,5'-hexachlorobiphenyl (HCBP-5) and were used to reappraise the structure-activity rules for PCBs as hepatic microsomal enzyme inducers. The results suggested that (a) PCBs which induce MC or mixed-type activity must be substituted at both para positions, at least two meta positions but not necessarily on the same phenyl ring and can also contain one ortho chloro substituent; (b) due to the considerable structural diversity of the PB-type inducers the rules for induction of this activity by PCB congeners are not readily defined.

Polychlorinated biphenyl isomers and congeners as inducers of both 3-methylcholanthrene- and phenobarbitone-type microsomal enzyme activity

Highly purified synthetic polychlorinated biphenyls substituted in the meta and para positions of both phenyl rings and at one ortho position were administered to male Wistar rats and the effects of these compounds on the microsomal drug-metabolising enzymes were evaluated. The in vivo effects of these compounds were determined by measuring the microsomal benzo[a]pyrene hydroxylase, dimethylaminoantipyrine N-demethylase and NADPH-cytochrome c reductase enzyme activities, the cytochrome b5 content and the relative peak intensities and spectral shifts of the reduced microsomal cytochrome P-450 : CO and ethylisocyanide binding difference spectra. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC), 2,2',4,4'-tetrachlorobiphenyl (TCBP-II) (a PB-type inducer), 3,3',4,4'-tetrachlorobiphenyl (TCBP-I) (an MC-type inducer), PB plus MC (coadministered) and TCBP-II + TCBP-I (coadministered) to the test animals. At dosage levels of 30 and 150 mumol . kg-1, pretreatment with 2,3,3',4,4'-pentachlorobiphenyl (PCBP-II), 2,3',4,4',5-pentachlorobiphenyl (PCBP-I), 2,3,3',4,4',5-hexachlorobiphenyl (HCBP-II) and 2,3,3',4,4',5-hexachlorobiphenyl (HCBP-III) gave hepatic microsomes with enzymic and spectral properties consistent with a mixed pattern of induction. These polychlorinated biphenyl (PCB) isomers and congeners have been identified as either major or minor components of the commercial PCB mixtures and must contribute to their activity as MC-type inducers. The only PCB isomer in this series which was not a mixed type inducer was 2,3',4,4',5,5'-hexachlorobiphenyl (HCBP-I) which appeared to be a PB-type inducer. This contrasted to the mixed-type activity observed for 2,3',4,4',5,5'-hexabromobiphenyl which was isolated from a commercial polybrominated biphenyl (PBB) mixture.

The detection of enzyme induction by rat liver microsomes prepared by isoelectric precipitation

A comparison was made of various indices of the hepatic drug-metabolizing apparatus associated with the postmitochondrial superntant, microsomes harvested by differential centrifugation, and microsomes harvested by isoelectric precipitation from control, phenobarbitone-pretreated and 3-methylcholanthrene-pretreated rats. The metabolic capabilities and distinctions between control and induced rats for each of the three liver preparations compared favourably as determined by the concentration of cytochrome b5 and P-450 and by the activity of NADPH-cytochrome c reductase, 4-dimethylaminoantipyrine N-demethylase and aryl hydrocarbon (benzo[a]pyrene) hydroxylase. The results suggest that the relatively simple isoelectric precipitation technique is a useful alternative to the conventional method of differential centrifugation for the preparation of hepatic microsomes.

Haem accessibility in cytochrome P-450 from rabbit liver. A proton magnetic relaxation study by stereochemical probes

Cytochrome P-450 was solubilized from phenobarbital induced rabbit liver and purified by affinity chromatography. The longitudinal proton magnetic relaxation rates of this ferric, low-spin sample (as confirmed by ESR) in 20% glycerol aqueous solution are very large compared with low-spin methaemoglobin and myoglobin derivatives. Similarly high rates were measured in a deuterated solution using the aliphatic protons of glycerol as stereochemical markers, which strongly suggests that the haem iron in cytochrome P-450 is much more accessible to the solvent than in harmoglobin or myoglobin. Type I substate (Spasman) produced small but significant increases in NMR rates both in the H2O and in the 2H2O solution, while binding of aniline (Type II substrate) doubled the rates.

NADH-dependent aryl hydrocarbon hydroxylase in rat liver mitochondrial outer membrane

NADH-dependent 3,4-benzpyrene hydroxylase activity was detected in the purified mitochondrial outer membrane fraction from the livers of rats treated with 3-methylcholanthrene. The specific activity in the outer membrane fraction is nearly equal to that of microsomes, a level too high to be accounted for only by the microsomal contamination. On the other hand, the NADPH-dependent 3,4-benzpyrene hydroxylase activity in the outer membrane fraction is about 50% of that of microsomes. The ratio of the specific activity of NADPH- to NADH-dependent 3,4-benzpyrene hydroxylase in microsomal fraction was about 3.5, while that of the outer membrane fraction was about 1.5. Moreover, it was found that NADH-dependent 3,4-benzpyrene hydroxylase activity in mitochondrial outer membrane from control rat liver was cyanide-insensitive, while that in microsomes was cyanide-sensitive. These results suggest the presence in the mitochondrial outer membrane fraction of aryl hydrocarbon hydroxylase activity which uses as electron donor NADH nearly to the same extent as NADPH. The hydroxylase system is composed of cyanide-insensitive cytochrome P-450 and is inducible markedly by 3-methylcholanthrene treatment. The probable electron transfer pathways in the mitochondrial outer membrane cytochrome P-450 oxidase system are discussed.

Binding of homogeneous cytochrome b5 to rat liver microsomes. Effect on N-demethylation reactions

Incubation of rat homogeneous detergent-solubilized cytochrome b5 with rat liver microsomes resulted in specific binding of the hemoprotein which was rapidly reduced by NADH. The NADH cytochrome c reductase activity in these preparations increased in proportion to the amount of cytochrome bound. However, the extra-bound detergent-solubilized cytochrome b5 did inhibit NADPH-dependent N-demethylations, the NADH synergism and NADPH cytochrome P-450 reductase activity. Manganese protoporphyrin-apocytochrome complex when bound to microsomes in amounts equivalent to detergent-solubilised cytochrome b5 showed no effect on N-demethylation activity. Furthermore, the binding of cytochrome b5 preparations reconstituted from heme and apocytochrome b5 had no effect on either the NADPH-dependent N-demethylation of aminopyrene or ethylmorphine or the NADH synergism observed with rat liver microsomes. In addition, homogeneous cytochrome b5 eluted from three additional Sephadex G-100 columns showed no inhibitory effects when bound to liver microsomes. Spectral analyses of the acid-acetone extract of the hemoprotein showed an absorption peak at 278 nm suggesting that the homogeneous b5 contains contaminating amounts of tightly bound detergent which is responsible for the observed inhibition of mixed function oxidase activity and which is removed during extraction of the heme from the apocytochrome and during further gel filtration applications.