Retinoic acid can enhance the stimulation by thyroid hormone of heme oxygenase activity in the liver of thyroidectomized rats.

The effects of retinoic acid (RA) (50 micrograms/100 g body wt. per day) on hepatic heme oxygenase activity, delta-aminolevulinate synthase (ALAS) activity and on cytochrome P-450 content were determined in thyroidectomized rats treated with T3 (10 micrograms/100 g body wt. per day) or diluent. RA, when administered for 3 days, failed to influence significantly the activity of either heme oxygenase or ALAS, however, the retinoid depleted hepatic cytochrome P-450 content by 17% (P less than 0.01) and microsomal heme content by 47% (P less than 0.001). T3 administration enhanced heme oxygenase activity by 72% (P less than 0.001) and ALAS activity by 251% (P less than 0.001) above levels in diluent treated controls and depleted cytochrome P-450 levels by 55% (P less than 0.001) and heme levels by 75% (P less than 0.001). When RA and T3 were administered together, the retinoid markedly enhanced the T3 stimulation of heme oxygenase activity; 173% above controls (P less than 0.001), and 61% above T3 alone (P less than 0.001). However, RA failed to influence the effect of T3 on ALAS activity or cytochrome P-450 depletion. The results indicate that RA can influence the levels of hepatic cytochrome P-450 and can modulate the stimulation of heme oxygenase activity by thyroid hormone in vivo.     

Pronounced and differential effects of ionic strength and pH on testosterone oxidation by membrane-bound and purified forms of rat liver microsomal cytochrome P-450

The aim of this study was to determine the effects of ionic strength and pH on the different pathways of testosterone oxidation catalyzed by rat liver microsomes. The catalytic activity of cytochromes P-450a (IIA1), P-450b (IIB1), P-450h (IIC11) and P-450p (IIIA1) was measured in liver microsomes from mature male rats and phenobarbital-treated rats as testosterone 7 alpha-, 16 beta-, 2 alpha- and 6 beta-hydroxylase activity, respectively. An increase in the concentration of potassium phosphate (from 25 to 250 mM) caused a marked decrease in the catalytic activity of cytochromes P-450a (to 8%), P-450b (to 22%) and P-450h (to 23%), but caused a pronounced increase in the catalytic activity of cytochrome P-450p (up to 4.2-fold). These effects were attributed to changes in ionic strength, because similar but less pronounced effects were observed with Tris-HCl (which has approximately 1/3 the ionic strength of phosphate buffer at pH 7.4). Testosterone oxidation by microsomal cytochromes P-450a, P-450b, P-450h and P-450p was also differentially affected by pH (over the range 6.8-8.0). The pH optima ranged from 7.1 (for P-450a and P-450h) to 8.0 (for P-450p), with an intermediate value of 7.4 for cytochrome P-450b. Increasing the pH from 6.8 to 8.0 unexpectedly altered the relative amounts of the 3 major metabolites produced by cytochrome P-450h. The decline in testosterone oxidation by cytochromes P-450a, P-450b and P-450h that accompanied an increase in ionic strength or pH could be duplicated in reconstitution systems containing purified P-450a, P-450b or P-450h, equimolar amounts of NADPH-cytochrome P-450 reductase and optimal amounts of dilauroylphosphatidylcholine. This result indicated that the decline in testosterone oxidation by cytochromes P-450a, P-450b and P-450h was a direct effect of ionic strength and pH on these enzymes, rather than a secondary effect related to the increase in testosterone oxidation by cytochrome P-450p. Similar studies with purified cytochrome P-450p were complicated by the atypical conditions needed to reconstitute this enzyme. However, studies on the conversion of digitoxin to digitoxigenin bisdigitoxoside by liver microsomes, which is catalyzed specifically by cytochrome P-450p, provided indirect evidence that the increase in catalytic activity of cytochrome P-450p was also a direct effect of ionic strength and pH on this enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)     

Identification of the forms of cytochrome P-450 induced in rat liver by 2-acetylaminofluorene using immunoblotting and partial purification

The amounts of 5 different forms of cytochrome P-450 in liver microsomes from rats treated with 2-acetylaminofluorene were determined and compared with the corresponding patterns in microsomes from control, 3-methylcholanthrene- and phenobarbital-treated animals. 2-Acetylaminofluorene was found to increase the amount of cytochromes P-450b + e 10-fold and of cytochrome P-450d 3-fold, while there was a 54% increase in the level of cytochrome P-450 PB/PCN-E. Cytochrome P-450c was increased from a level too low to detect (less than 0.001 pmol/mg protein) to 0.019 pmol/mg protein. These findings were also confirmed by partial purification of cytochromes P-450b + e and c after 2-acetylaminofluorene treatment.     

Biological activity of phenolic compounds. Hepatic cytochrome P-450, cytochrome b5, and NADPH cytochrome c reductase in chicks and rats fed phenolic monomers, polymers, and glycosides

Eight experiments were conducted to determine effects of a phenolic polymer (Kraft wood lignin, Indulin), phenolic glycosides (cane molasses and wood molasses), and phenolic monomers (vanillin, vanillic acid, ferulic acid, and p-coumaric acid) on liver cytochromes P-450, cytochrome b5, and NADPH cytochrome c reductase in chicks and rats. Chicks fed 6.0% lignin had a higher (P less than 0.01) cytochromes P-450 content than did chicks fed 0% fiber, 6.0% wood cellulose (Solka Floc), or 6.0% arenaceous flour. NADPH cytochrome c reductase activity was not affected by treatment. Chicks fed 12.0% wood molasses had a higher (P less than 0.06) cytochromes P-450 level than did chicks fed 0% fiber or 6.0% wood molasses. Cane molasses incorporated at both 6.0 and 12.0% of the diet induced (P less than 0.05) cytochromes P-450 content over those of control-fed birds. Chicks fed 6.0% lignin, with or without antibiotic (bacitracin:neomycin sulfate, 2:1), had a higher (P less than 0.01) cytochromes P-450 level than did chicks fed control diets, with or without antibiotic. Additionally, chicks fed 6.0% lignin had lower (P less than 0.01) intestinal diaminopimelic acid (DAP) levels than did chicks fed 0% fiber. Rats fed 0% fiber, 6.0% wood cellulose, 6.0% arenaceous flour, or 6.0% lignin exhibited no difference in cytochrome level or activity among treatments. Chicks fed 0.5% vanillin, 0.5% vanillic acid, 0.5% ferulic acid, or 0.5% p-coumaric acid had comparable cytochromes level and activity compared with chicks fed no phenolics. Chicks fed 0.5% p-coumaric acid had lower (P less than 0.05) rates of gain than did chicks fed control or other phenolic-containing diets. Rats fed these phenolics had similar cytochromes P-450 content among treatments.     

Time-course of effects of toluene on microsomal enzymes in rat liver, kidney and lung during and after inhalation exposure

Inhalation of toluene vapour of 2000 ppm increased the activities of aniline hydroxylase, aminopyrine N-demethylase, aryl hydrocarbon hydroxylase and NADPH-cytochrome c reductase and the concentrations of cytochromes P-450 and b5 in liver microsomes of adult male rats after an exposure period of 1 day or less. Repeated treatments, 8 h daily for 1-16 days, had only a slight further effect. In lung microsomes, the activities of monooxygenases and the concentration of cytochrome P-450 decreased after 6-24 h toluene exposure, but those of cytochrome b5 and NADPH-cytochrome c reductase did not change. In kidney microsomes the changes were mostly insignificant. After discontinuation of exposure the activities of enzymes and the concentrations of cytochromes returned to the control level in 1-4 days. The results obtained resemble the time-courses for the induction of monooxygenases by other inducers. The tissue differences suggest the unequal distribution of various cytochrome P-450 forms and their individual responsiveness to induction in liver, kidneys and lungs.     

Interaction of 9-hydroxyellipticine with two forms of partially purified rabbit lung cytochrome P-450. Inhibition of lung microsomal benzo[a]pyrene hydroxylase.

9-Hydroxyellipticine (9-OHE), a potent inhibitor of rat liver monooxygenase activities, binds to the various forms of partially purified lung cytochromes P-450 from untreated and 3-methylcholanthrene (3-MC)-treated rabbits. The spectral data (lambda max: 428 nm (ox.), 447 nm (red.), Ks: 10 microM and 5 muM for cytochrome I and cytochrome II from 3-MC-treated rabbits respectively) resemble those obtained with cytochrome P-450 purified from liver of Aroclor 1254-pretreated rats (lambda max: 428 nm (ox.), 445 nm (red.), Ks: 8 microM). 9-OHE has been shown to inhibit the benzo[a]pyrene hydroxylase activity of rat and rabbit lung microsomes. The inhibitory effect was higher towards the 3-MC-induced lung microsomes than with the control microsomes. However, the lung microsomes, as well as the liver microsomes of rabbits were less sensitive to inhibition by 9-OHE than the corresponding microsomes from rats. These results suggest that rabbit and rat cytochromes P-450 have subtle structural differences.     

Decreased expression of cytochrome P-452 in the resistance phenotype characteristic of putative preneoplastic hepatocyte nodules during hepatocarcinogenesis

Hepatocyte nodules, a characteristic early step in the development of liver cancer in rats, has a distinctive resistance phenotype including a large decrease in total cytochromes P-450 and in two isozymes induced by phenobarbital and two by 3-methylcholanthrene. In this study, it has been observed that the nodules show a large decrease in an additional cytochrome P-450, cytochrome P-452, which is very active in the hydroxylation of lauric acid at C-11 and C-12. The decrease in activity of this microsomal cytochrome P-452 is of the same order of magnitude as the decreases in the other cytochrome P-450 components. These observations are consistent with the hypothesis that there is some more basic alteration in the synthesis or availability of heme and that the changes in the activities of the cytochromes P-450 are secondary.     

Carbon tetrachloride and 2-isopropyl-4-pentenamide-induced inactivation of cytochrome P-450 leads to heme-derived protein adducts

When CCl4 was incubated with rat liver microsomes from phenobarbital-treated rats in an aerobic or anaerobic atmosphere, over 69% of the heme moiety of cytochrome P-450 was destroyed. At least 45% of the degraded heme under both reaction conditions was accounted for as heme-derived products irreversibly bound to microsomal proteins. Furthermore, 33% of the irreversibly bound products were bound specifically to a 54-kDa form of cytochrome P-450. A structurally different compound, 2-isopropyl-4-pentenamide, also destroyed the heme moiety of cytochrome P-450 and produced heme-derived adducts of microsomal proteins that accounted for 28% of the destroyed heme. These results represent a novel mechanism for the destruction of cytochromes P-450 by xenobiotics.     

Depressed guinea pig testicular microsomal cytochrome P-450 content by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Human exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can result in hirsutism and chloracne, symptoms that suggest an alteration in endocrine regulation. Consequently, the effect of TCDD on guinea pig testicular cytochrome P-450 has been investigated. Twelve hours after a single, oral dose of TCDD (1 μg/kg), testicular microsomal cytochrome P-450 content was depressed by approximately 36%. Microsomal cytochrome P-450 content reached a maximal depression at approximately 1 day (52% of control) and remained at this level for 9 days. No appreciable alterations of testicular microsomal heme levels or activity of testicular δ-aminolevulinic acid (ALA) synthetase were observed.     

Effect of cis-platinum on kidney cytochrome P-450 and heme metabolism: evidence for the regulatory role of the pituitary hormones

A novel action of the gonadotropic hormones of the adenohypophysis on the regulation of kidney heme metabolism and cytochrome P-450 concentrations is described. The treatment of rats with cis-platinum for 7 days caused a greater than twofold increase in the microsomal cytochrome P-450 and heme concentrations in the kidney. The sodium dodecyl sulfate-gel electrophoresis of the microsomal preparation revealed increased levels of both apocytochrome P-450 and heme in the molecular weight region corresponding to cytochrome P-450. In hypophysectomized rats, similar increases in heme and the cytochrome contents in the kidney were observed. Conversely, the treatment of rats with human chorionic gonadotropin (hCG) fully reversed the effect of cis-platinum on heme and cytochrome P-450 concentrations. The cellular basis of increases in concentrations of heme and the hemoprotein was explored by measuring the incorporation of [14C]glycine-labeled hemoglobin heme into the kidney microsomal heme fractions. In comparison with the control rats, the specific 14C activity of heme in microsomal fraction was not increased. Moreover, the effect of cis-platinum on kidney cytochrome P-450 appeared to be unrelated to alterations in the activities of the rate-limiting enzymes of heme biosynthesis and degradation pathways, delta-aminolevulinate synthetase, and heme oxygenase, respectively. On the other hand, ferrochelatase activity and the concentration of total porphyrins in the kidney were profoundly altered by cis-platinum treatment; a twofold increase in ferrochelatase activity and a marked reduction (40%) in the total porphyrin concentration were observed. Also, the activities of uroporphyrinogen-I synthetase and delta-aminolevulinate dehydratase were decreased in cis-platinum-treated animals. The latter effects reflect a direct inhibitory action of cis-platinum. It appears that the cis-platinum-mediated increase in the microsomal heme concentrations involves an accelerated rate of heme production as a consequence of increased ferrochelatase activity. This, in turn, could increase the production of cytochrome P-450. It is suggested that the anterior pituitary hormones control the concentration of the cytochrome P-450 in the kidney, and this process may be interrupted by cis-platinum.     

Purification and characterization of two constitutive cytochromes P-450 (F-1 and F-2) from adult female rats: identification of P-450F-1 as the phenobarbital-inducible cytochrome P-450 in male rat liver

Two hepatic microsomal cytochromes P-450, P-450F-1 and P-450F-2 were purified to electrophoretic homogeneity from untreated adult female rats by high-performance liquid chromatography (HPLC) with anion-exchange, cation-exchange, and hydroxyapatite columns. Cytochromes P-450F-1 and P-450F-2 were not adsorbed with the anion-exchange column, but were retained on a cation-exchange column and were separated poorly. These forms separated on hydroxyapatite HPLC. The molecular weights of cytochromes P-450F-1 and P-450F-2 were 50,000 and 49,000, respectively. The absolute spectrum of the oxidized forms indicated that they had the low-spin state of heme, and the CO-reduced spectral maxima of cytochromes P-450F-1 and P-450F-2 were at 450 and 448 nm, respectively. Both forms catalyzed the N-demethylation of benzphetamine and had low catalytic activity for 7-ethoxycoumarin. Cytochrome P-450F-1 had low 2 alpha-hydroxylation activity toward testosterone. Cytochrome P-450F-2 had low 15 alpha-hydroxylation activity. On the basis of these results and those of NH2-terminal sequence analysis, cytochrome P-450F-2 seemed to be the typical female-specific cytochrome P-450. The NH2-terminal sequence of cytochrome P-450F-1 was identical to that of cytochrome P-450PB-2 purified from hepatic microsomes of male rats treated with phenobarbital. Cytochromes P-450F-1 and P-450PB-2 had identical chromatographic properties, minimum molecular weight, spectral properties, and peptide maps. Furthermore, the antibody to phenobarbital-inducible cytochrome P-450PB-2 gave a single immunoprecipitin band with cytochrome P-450F-1 by Ouchterlony double-diffusion analysis.     

The in vivo turnover of rat liver microsomal epoxide hydrolase and both the apoprotein and heme moieties of specific cytochrome P-450 isozymes

The in vivo turnover rates of liver microsomal epoxide hydrolase and both the heme and apoprotein moieties of cytochromes P-450a, P-450b + P-450e, and P-450c have been determined by following the decay in specific radioactivity from 2 to 96 h after simultaneous injections of NaH14CO3 and 3H-labeled delta-aminolevulinic acid to Aroclor 1254-treated rats. Total liver microsomal protein was characterized by an apparent biphasic exponential decay in specific radioactivity, with half-lives of 5-9 and 82 h for the fast- and slow-phase components, respectively. Most (approximately 90%) of the rapidly turning over microsomal protein fraction was immunologically distinct from membrane-associated serum protein, and thus appeared to represent integral membrane proteins. The existence of two distinct populations of cytochrome P-450a was suggested by the apparent biphasic turnover of both the heme and apoprotein moieties of the holoenzyme. The half-lives of the apoprotein were estimated to be 12 and 52 h for the fast- and slow-phase components, respectively, and 7 and 34 h for the heme moiety. The turnover of cytochromes P-450b + P-450e was identical to that of cytochrome P-450c, with half-lives of 37 and 28 h for the apoprotein and heme moieties, respectively. In all cases, the shorter half-lives of the heme component compared to the protein component were statistically significant. In contrast to the cytochrome P-450 isozymes, epoxide hydrolase (t1/2 = 132 h) turned over slower than the "average" microsomal protein (t1/2 = 82 h). The differential rates of degradation of these major integral membrane proteins during both the rapid and slow phases of total microsomal protein turnover argue against the concepts of unit membrane degradation and unidirectional membrane flow of liver endoplasmic reticulum.     

Growth hormone depresses ethylmorphine demethylase activity: correlation with decreased levels of fast-turnover cytochrome P-450 in hypophysectomized female rats

The hepatic monooxygenase system was studied in hypophysectomized female rats infused for 5 days with ovine growth hormone (GH). At 7.5 micrograms.h-1 GH decreased the total cytochrome P-450 by 16%; at 10 micrograms.h-1 it reduced both cytochrome P-450 (31%) and the activity of ethylmorphine demethylase (31%). GH did not alter the activities of NADPH cytochrome c reductase or aniline hydroxylase. The lower GH dose decreased the amount of fast- and slow-turnover P-450 by 11 and 38%, respectively, while the higher dose decreased both by 49%. The loss of demethylase activity therefore correlates with the loss of fast-turnover P-450. This component is relatively more abundant in the female (fast: slow turnover of 4.3) than the male (fast:slow turnover of 2.5). GH did not affect the half-lives of the P-450 components, suggesting that it decreases their synthesis. The P-450 concentration in microsomes from GH-treated animals did not increase after incubation with hemin, suggesting that in vivo the hormone does not lower P-450 synthesis via depression of heme. Puromycin mimicked the effect of GH and when given with the hormone their effects on the P-450 levels were multiplicative (p less than 0.05), suggesting different modes of action and that GH does not decrease P-450 by acting at translation.     

Studies on the rate-determining factor in testosterone hydroxylation by rat liver microsomal cytochrome P-450: evidence against cytochrome P-450 isozyme:isozyme interactions

The aim of the present study was to examine a recent proposal that inhibitory isozyme:isozyme interactions explain why membrane-bound isozymes of rat liver microsomal cytochrome P-450 exert only a fraction of the catalytic activity they express when purified and reconstituted with saturating amounts of NADPH-cytochrome P-450 reductase and optimal amounts of dilauroylphosphatidylcholine. The different pathways of testosterone hydroxylation catalyzed by cytochromes P-450a (7 alpha-hydroxylation), P-450b (16 beta-hydroxylation), and P-450c (6 beta-hydroxylation) enabled possible inhibitory interactions between these isozymes to be investigated simultaneously with a single substrate. No loss of catalytic activity was observed when purified cytochromes P-450a, P-450b, or P-450c were reconstituted in binary or ternary mixtures under a variety of incubation conditions. When purified cytochromes P-450a, P-450b, and P-450c were reconstituted under conditions that mimicked a microsomal system (with respect to the absolute concentration of both the individual cytochrome P-450 isozyme and NADPH-cytochrome P-450 reductase), their catalytic activity was actually less (69-81%) than that of the microsomal isozymes. These results established that cytochromes P-450a, P-450b, and P-450c were not inhibited by each other, nor by any of the other isozymes in the liver microsomal preparation. Incorporation of purified NADPH-cytochrome P-450 reductase into liver microsomes from Aroclor 1254-induced rats stimulated the catalytic activity of cytochromes P-450a, P-450b, and P-450c. Similarly, purified cytochromes P-450a, P-450b, and P-450c expressed increased catalytic activity in a reconstituted system only when the ratio of NADPH-cytochrome P-450 reductase to cytochrome P-450 exceeded that normally found in liver microsomes. These results indicate that the inhibitory cytochrome P-450 isozyme:isozyme interactions described for warfarin hydroxylation were not observed when testosterone was the substrate. In addition to establishing that inhibitory interactions between different cytochrome P-450 isozymes is not a general phenomenon, the results of the present study support a simple mass action model for the interaction between membrane-bound or purified cytochrome P-450 and NADPH-cytochrome P-450 reductase during the hydroxylation of testosterone.     

Hydroxylations in biosynthesis of bile acids. Isolation of a cytochrome P-450 from rabbit liver mitochondria catalyzing 26-hydroxylation of C27-steroids

A cytochrome P-450 catalyzing 26-hydroxylation of C27-steroids was purified from liver mitochondria of untreated rabbits. The enzyme fraction contained 10 nmol of cytochrome P-450/mg of protein and showed only one protein band with a minimum Mr = 53,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified mitochondrial cytochrome P-450 showed apparent molecular weight similar to microsomal cytochromes P-450LM4 but differed in spectral and catalytic properties from these microsomal isozymes. The purified cytochrome P-450 catalyzed 26-hydroxylation of cholesterol, 5-cholestene-3 beta,7 alpha-diol, 7 alpha-hydroxy-4-cholesten-3-one, 5 beta-cholestane-3 alpha,7 alpha-diol, and 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol up to 1000 times more efficiently than the mitochondria. The cytochrome P-450 required both ferredoxin and ferredoxin reductase for catalytic activity. Microsomal NADPH-cytochrome P-450 reductase could not replace ferredoxin and ferredoxin reductase. The cytochrome P-450 was inactive in 7 alpha-, 12 alpha- and 25-hydroxylations of C27-steroids. The results suggest that mitochondrial 26-hydroxylation of various C27-steroids is catalyzed by the same species of cytochrome P-450.     

Characterization of a major form of rat hepatic microsomal cytochrome P-450 induced by isoniazid

Cytochrome P-450j has been purified to electrophoretic homogeneity from isoniazid-treated adult male rats; and this enzyme appears to be a major protein induced in hepatic microsomes after administration of isoniazid, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hemoprotein has a minimum molecular weight of approximately 51,500, and the ferrous-carbonyl complex of cytochrome P-450j has a Soret maximum at 451-452 nm. The oxidized heme iron appears to be predominately in the high spin state as deduced from the Soret maximum at 395 nm. Ethylisocyanide binds to ferrous cytochrome P-450j to yield spectral maxima at approximately 458 and 430 nm with a resultant 458/430 ratio of 0.7 at pH 7.4. Cytochrome P-450j has no measurable catalytic activity for the metabolism of benzo[a]pyrene (3- and 9-hydroxylation), hexobarbital, testosterone, and 5 alpha-androstane-3 alpha,17 beta-diol-3,17-disulfate. Low, but detectable, catalytic activity is obtained for the metabolism of 7-ethoxycoumarin, benzphetamine, p-nitroanisole, zoxazolamine, and 2-hydroxylation of 17 beta-estradiol. In contrast, cytochrome P-450j effectively catalyzes p-hydroxylation of aniline with a turnover of 12.7 nmol/min/nmol cytochrome P-450j. Hydroxyl radical scavengers, Fe-EDTA, superoxide dismutase, and catalase have no effect on aniline p-hydroxylation catalyzed by cytochrome P-450j. Cytochrome P-450j is distinct from nine other rat hepatic microsomal cytochromes P-450 (P-450a-P-450i) previously purified in this laboratory, as well as different isozymes described by other investigators, based on several parameters including minimum molecular weight, spectral properties, and catalytic activity. In Ouchterlony double diffusion plates, antibodies against cytochromes P-450a-P-450f show no cross-reaction with cytochrome P-450j. Structural differences among cytochromes P-450a-P-450j are apparent from the NH2-terminal sequence of cytochrome P-450j, as well as the electrophoretic profiles of proteolytic digests of the hemoproteins.     

Regioselective progesterone hydroxylation catalyzed by eleven rat hepatic cytochrome P-450 isozymes

Quantitative high-pressure liquid chromatographic assays were developed that separate progesterone and 17 authentic monohydroxylated derivatives. The assays were utilized to investigate the hydroxylation of progesterone by 11 purified rat hepatic cytochrome P-450 isozymes and 8 different rat hepatic microsomal preparations. In a reconstituted system, progesterone was most efficiently metabolized by cytochrome P-450h followed by P-450g and P-450b. Seven different monohydroxylated progesterone metabolites were identified. 16 alpha-Hydroxyprogesterone, formed by 8 of the 11 isozymes, was the only detectable metabolite formed by cytochromes P-450b and P-450e. 2 alpha-Hydroxyprogesterone was formed almost exclusively by cytochrome P-450h, and 6 alpha-hydroxyprogesterone and 7 alpha-hydroxyprogesterone were only formed by P-450a. 6 beta-hydroxylation of progesterone was catalyzed by four isozymes with cytochrome P-450g being the most efficient, and 15 alpha-hydroxyprogesterone was formed as a minor metabolite by cytochromes P-450g, P-450h, and P-450i. None of the isozymes catalyzed 17 alpha-hydroxylation of progesterone, and only cytochrome P-450k had detectable 21-hydroxylase activity. 16 alpha-Hydroxylation catalyzed by cytochrome P-450b was inhibited in the presence of dilauroylphosphatidylcholine (1.6-80 microM), while this phospholipid either stimulated (up to 3-fold) or had no effect on the metabolism of progesterone by the other purified isozymes. Results of microsomal metabolism in conjunction with antibody inhibition experiments indicated that cytochromes P-450a and P-450h were the sole 7 alpha- and 2 alpha-hydroxylases, respectively, and that P-450k or an immunochemically related isozyme contributed greater than 80% of the 21-hydroxylase activity observed in microsomes from phenobarbital-induced rats.     

Inhibition of testicular cytochrome P-450-dependent steroid biosynthesis by cis-platinum. Reversal by human chorionic gonadotropin

The treatment of rats with cis-platinum for 7 days caused a profound, and seemingly selective, decrease (70-80%) in the microsomal cytochrome P-450 levels in the testis. This decrease was accompanied by marked reductions (70-80%) in steroid 17 alpha-hydroxylase activity and in plasma testosterone concentration. The treatment of rats with human chorionic gonadotropin partially restored the cytochrome P-450 concentration and 17 alpha-hydroxylase activity and permitted the plasma testosterone level to approach control values. The effect of cis-platinum on the testicular cytochrome P-450 appeared unrelated to deficiencies in heme metabolic processes, in so far that neither was the activity of delta-aminolevulinate synthetase decreased, nor was that of heme oxygenase increased. These enzymes are rate-limiting in heme biosynthesis and degradation pathways, respectively. Also, the activities of uroporphyrinogen I synthetase, delta-aminolevulinate dehydratase, and ferrochelatase and the concentration of total porphyrins in the testis remained unchanged. The sodium dodecyl sulfate-gel electrophoresis of the microsomal preparation did not reveal a diminished level of apocytochrome; however, in this preparation, heme could not be detected in molecular weight regions corresponding to cytochrome P-450. The microsomal cytochrome b5 and the mitochondrial heme concentrations were not decreased in cis-platinum-treated rats. It is suggested that the mechanism of depletive action of cis-platinum on microsomal cytochrome P-450 involves an impairment of the effective assembly of heme and apoprotein moieties. It is further suggested that the anterior pituitary hormones control the factor(s) involved in this assembly, a process which is interrupted by cis-platinum.     

Rat liver cytochrome P-450b, P-420b, and P-420c are degraded to biliverdin by heme oxygenase

In this report we provide data, for the first time, demonstrating the conversion of the heme moiety of certain cytochrome P-450 and P-420 preparations, to biliverdin, catalyzed by heme oxygenase. We have used purified preparations of cytochromes P-450c, P-450b, P-450/P-420c, or P-450/P-420b as substrates in a heme oxygenase assay system reconstituted with heme oxygenase isoforms, HO-2 or HO-1, NADPH-cytochrome c (P-450) reductase, biliverdin reductase, NADPH, and Emulgen 911. With cytochrome P-450b or P-450/P-420b preparations, a near quantitative conversion of degraded heme to bile pigments was observed. In the case of cytochrome P-450/P-420c approximately 70% of the degraded heme was accounted for as bilirubin but only cytochrome P-420c was appreciably degraded. The role of heme oxygenase in this reaction was supported by the following observations: (i) bilirubin formation was not observed when heme oxygenase was omitted from the assay system; (ii) the rate of degradation of the heme moiety was at least threefold greater with heme oxygenase and NADPH-cytochrome c (P-450) reductase than that observed with reductase alone; and (iii) the presence of Zn- or Sn-protoporphyrins (2 microM), known competitive inhibitors of heme oxygenase, resulted in 70-90% inhibition of bilirubin formation.     

Light-dependent cytochrome P-450 changes in mung beans (Phaseolus aureus).

Maximum concentrations of microsomal cytochrome P-450 are present in 3-4 day-old mung beans (Phaseolus aureus). On illumination of dark-grown seedlings, cytochrome P-450 and later cytochrome P-450 undergo a rapid decrease in concentration in vivo, with an apparent half-time of about 6 h. Conversely light-grown seedlings, transferred to darkness, show a slow accumulation of cytochrome P-450, doubling time of about 30 h, with a later accumulation of cytochrome P-420. Microsomal cytochromes b559, b560.5 and b562.5 do not significantly alter on light-dark transitions. Possible functions for dark-induced cytochrome P-450 are discussed.