Regulation of three forms of cytochrome P-450 and epoxide hydrolase in rat liver microsomes. Effects of age, sex, and induction

Divergent effects of cycloheximide on the induction of class II and class III cytochrome P450 mRNAs in cultures of adult rat hepatocytes

We have previously reported that when hepatocytes isolated from adult male rats are cultured in serum-free medium on matrigel, a reconstituted basement membrane gel, it is possible to elicit a stimulation of gene expression for both Class II cytochrome P450b/e and Class III cytochrome P450p by phenobarbital treatment (E.G. Schuetz et al., 1990 J. Biol. Chem. 265, 1188-1192). In the present study, an investigation of the requirement of protein synthesis for the rise in mRNAs for these cytochromes, pretreatment of the cells with cycloheximide prior to adding phenobarbital or "phenobarbital-like" inducers to the culture medium inhibited induction of P450b/e mRNA (46-90%), whereas the accumulation of P450p mRNA was enhanced (2- to 19-fold). Heme depletion did not appear to explain these observations because the inhibitory effects of cycloheximide on the induction of P450b/e mRNA were not overcome by supplementation of the medium with exogenous heme or with delta-aminolevulinic acid. Because Class IIIA P450s are regulated by gender as well as by phenobarbital, we examined the basal expression of P450p mRNA in cultures of hepatocytes derived from male rats and found that cycloheximide treatment was without effect. However, in cultures of hepatocytes isolated from female rats, where P450p mRNA is barely detectable, cycloheximide treatment greatly enhanced expression of P450p mRNA. As was observed in the cultured cells, the treatment of living female rats with cycloheximide also increased the amounts of P450p mRNA to levels comparable to those found in livers of untreated male rats. Analysis of Northern blots hybridized with oligonucleotides specific for P450PCN1(IIIA1) and P450PCN2(IIIA2), respectively, revealed that untreated male rat liver and cultures of hepatocytes prepared from these animals expressed readily detectable amounts of P450PCN1(IIIA1) mRNA. Such analyses confirmed that cycloheximide treatment selectively increased P450PCN1(IIIA1) mRNA in female rat liver, whereas the amount of mRNA for P450PCN2(IIIA2), a closely related male-specific family member, was unaffected. We conclude that the pathways for the induction of P450b/e and P450p by phenobarbital, and the pathways for the gender-specific basal expression of P450PCN1(IIIA1) and P450PCN2(IIIA2) are not the same and can be distinguished by their differential response to inhibition of ongoing protein synthesis.

Drug metabolizing capacity in vitro and in vivo--II. Correlations between hepatic microsomal monooxygenase markers in phenobarbital-induced rats

Pretreatment with various doses of phenobarbital (PB) has been used to create a pool of rats with a wide range of hepatic microsomal monooxygenase activity to systematically examine relationships between and within in vivo and in vitro markers. The in vivo clearance of tolbutamide (TOL), theophylline (TH), antipyrine (AP) and its metabolites were determined in the same rats used for hepatic microsome preparation and assessment of P450 content and activities (via 7-ethoxycoumarin O-deethylase (ECOD), 7 ethoxyresorufin O-deethylase, 7-methoxycoumarin O-demethylase (MCOD) and aldrin epoxidase determinations). A graded dose-response relationship was found between PB treatment and most but not all parameters. The need for careful selection of in vivo and as well as in vitro markers is apparent from these studies. The most responsive parameters--TOL and AP clearances, MCOD and ECOD activities--were also those producing the strongest in vivo-in vitro correlations. Despite the diffuse nature of the PB induced response in P450 complement, good predictive relationships were apparent between ECOD and TOL clearance (r2 = 0.88).

Drug metabolizing capacity in vitro and in vivo--I. Correlations between hepatic microsomal monooxygenase markers in beta-naphthoflavone-induced rats

Relationships between and within in vivo and in vitro markers of drug oxidative metabolism have been investigated in rats displaying a wide range of hepatic microsomal monooxygenase activity due to prior treatment with various doses of the inducing agent beta-naphthoflavone (BNF). BNF induction produced large dose-related changes in the in vivo clearance (CL) of theophylline (TH), antipyrine (AP) and the individual AP metabolite formation clearances, 4-hydroxyantipyrine (4H) and norantipyrine, and the in vitro parameters, 7-ethoxycoumarin O-deethylase, 7-ethoxyresorufin and P450. No trends were observed with the formation clearance of 3-hydroxymethylantipyrine and 7-methoxycoumarin O-demethylase whilst a negative response was observed with aldrin epoxidase. The selectivity of the markers towards BNF induction was coincident with the degree of covariance observed between these parameters. Strong correlations were observed in particular between CL(TH) and CL(4H) and ECOD and EROD indicating the high predictive value of these parameters. These studies demonstrate that under the well controlled conditions which may be imposed in animal environments predictively useful relationships (r2 greater than 0.8) can be established between in vitro and in vivo markers of hepatic microsomal monooxygenase activity.

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)

The interference of aroclor 1254 with progesterone metabolism in guinea pig adrenal and testes microsomes

The effects of Aroclor 1254 on cytochrome P-450-mediated steroidogenic activities were investigated in adrenal and testis microsomes of male guinea pigs. A significant decrease was recorded in the tissue content of adrenal microsomal cytochrome P-450 as well as a significant reduction in the overall conversion of progesterone to steroid products. The effects of exposure to Aroclor 1254 on activities of cytochrome P450 21-hydroxylase and cytochrome P450 17 alpha-hydroxylase/C17,20-lyase were selective. Cytochrome P-450 21-Hydroxylase activity was inhibited, as reflected by a decrease in production of 11-deoxycortisol and 11-deoxycorticosterone, whereas the cytochrome P-450 17 alpha-hydroxylase/C17,20-lyase activities, represented by the production of 17 alpha-hydroxyprogesterone and androstenedione were elevated. The same and even more pronounced pattern of altered progesterone metabolism elicited by Aroclor 1254 was observed in vitro, when Aroclor 1254 was introduced into incubation mixtures prepared with adrenal microsomes from untreated animals. Under such experimental conditions, a decrease in the overall metabolism of progesterone was observed as well as a decrease in cytochrome P-450 21-hydroxylase activity, while there was significant elevation in the 17 alpha-hydroxylase/C17,20-lyase activities. The effect of Aroclor 1254 on the testes differed largely from its effect on the adrenal cortex. In testis microsomes, pretreatment with Aroclor 1254 resulted in no changes in the cytochrome P-450 content, contrary to the decrease observed in adrenal microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotransformation of lovastatin--III. Effect of cimetidine and famotidine on in vitro metabolism of lovastatin by rat and human liver microsomes

The effects of the H2-receptor antagonists, cimetidine and famotidine, on the microsomal metabolism of [14C]lovastatin were investigated. Liver microsomes were prepared from control, phenobarbital- and 3-methylcholanthrene-pretreated rats and humans (male and female). Concentration-dependent inhibition of the metabolism of lovastatin (0.1 mM) was observed with cimetidine (0.1 to 1.0 mM). In contrast, famotidine at a similar concentration was a very weak inhibitor. The formation of 6'beta-hydroxy-lovastatin, the major microsomal metabolite of lovastatin, was similarly inhibited. The results suggest that in vivo metabolic interaction with concomitantly administered lovastatin is less likely with famotidine than with cimetidine. Phenobarbital pretreatment produced 58% stimulation in overall metabolism, whereas 3-methylcholanthrene pretreatment had no effect relative to control rats (5.4 nmol/mg protein/min). Liver microsomes from phenobarbital-pretreated rats produced 67% more of the 6'beta-hydroxy-lovastatin but 63-66% less of the 3''-hydroxy and 6'-exomethylene metabolites. Liver microsomes from 3-methylcholanthrene-treated rats also produced less 3"-hydroxy-lovastatin (49%) but similar quantities of the other two metabolites. 6'beta-Hydroxy-lovastatin was a major metabolite with human liver microsomes. Interestingly with these microsomes, hydroxylation at the 3''-position of the molecule was a negligible pathway and hydrolysis to the hydroxy acid form was not observed. The formation of 6'-exomethylene-lovastatin was also catalyzed by human liver microsomes (0.5 to 0.8 nmol/mg protein/min).

Metabolism of halazepam by rat liver microsomes: stereoselective formation and N-dealkylation of 3-hydroxyhalazepam

Metabolism of halazepam [7-chloro-1,3-dihydro-5-phenyl-1-(2,2,2-trifluoroethyl)-2H-1,4-benzod iazepin- 2-one, HZ] was studied by incubation with liver microsomes prepared from untreated, phenobarbital (PB)-treated, and 3-methylcholanthrene (3MC)-treated male Sprague-Dawley rats. Metabolites of HZ were separated by normal-phase HPLC. Relative rates of HZ metabolism by liver microsomes prepared from untreated and treated rats were PB-treated much greater than untreated greater than 3MC-treated at low concentration of microsomal enzymes (0.25 mg protein per ml of incubation mixture) and PB-treated much greater than 3MC-treated approximately untreated at high concentration of microsomal enzymes (2 mg protein per ml of incubation mixture). The relative amounts of major metabolites were found to be 3-hydroxy-HZ (3-OH-HZ) greater than N-desalkylhalazepam (NDZ, also known as N-desmethyldiazepam and nordiazepam) much greater than oxazepam (OX) for all three rat liver microsomal preparations and the distribution of metabolites was independent of microsomal enzyme concentrations. Enantiomers of 3-OH-HZ were resolved by HPLC on a Chiralcel OC column (cellulose trisphenylcarbamate coated on silica gel, particle size 10 microns). 3-OH-HZ enantiomeres have racemization half-lives of approximately 150 min in pH 4, 7.5, and 10 aqueous solutions. 3-OH-HZ formed in the metabolism of HZ by liver microsomes prepared from untreated and treated rats were found to have 3R/3S enantiomer ratios of 37/63 (untreated), 55/45 (PB-treated), and 36/64 (3MC-treated), respectively. N-dealkylation of 3-OH-HZ by liver microsomes from PB-treated rats was substrate enantioselective; the 3R-enantiomer was N-dealkylated faster than 3S-enantiomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal modulation of adult rat hepatic microsomal benzo[a]pyrene hydroxylase activities by Aroclor 1254 or phenobarbital

The constitutive and Aroclor 1254-induced activities of hepatic microsomal benzo[a]pyrene hydroxylases in male and female rats were determined in animals from ages 11 to 120 days. In 11-day-old noninduced male rats, benzo[a]pyrenediones and 9-hydroxybenzo[a]pyrene were the major microsomal metabolites; in 21-day-old males benzo[a]pyrene-diones and benzo[a]pyrene-9,10-dihydrodiol were predominant. In 60- and 120-day-old animals 3-hydroxybenzo[a]pyrene was the major microsomal metabolite. A similar trend was observed for the development of benzo[a]pyrene hydroxylase activities in female rats. With the exception of 4,5-dihydrodiol formation, the highest induction of individual and total benzo[a]pyrene hydroxylase activities by Aroclor 1254 was observed in the 21-day-old immature male rats, in which there was a 330- and 4.5-fold increase in the formation of 3-hydroxybenzo[a]pyrene and quinone metabolites, respectively. The induction of benzo[a]pyrene total metabolite formation by Aroclor 1254 in female rats from 11 to 120 days of age was relatively constant (i.e., 13.3- to 10.1-fold induction); however, the relative induction of the individual benzo[a]pyrene hydroxylases was highly variable. In a second set of experiments, male and female rats were neonatally exposed to phenobarbital (600 mumol/kg) or Aroclor 1254 (100 mumol/kg), and the effects of these xenobiotics on neonatal imprinting of hepatic microsomal benzo[a]pyrene hydroxylase activities were determined in the 120-day-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of drug metabolism enzymes by dihalogenated biphenyls

The effects of pretreatment with symmetrically dihalogenated biphenyls (DXBs, X-F, Cl(C), Br(B) and I) on rat liver drug metabolism enzymes were investigated. 4,4'-DFB, -DCB, and -DBB as well as 2,2'-DFB appeared to be inducers of microsomal cytochrome P-450-linked monoxygenases (N-demethylases of aminopyrine and ethylmorphine). However, no structure-induction relationship was found. 4,4'-DXBs also induced a cytochrome P-448-linked mono-oxygenase (ethoxyresorufin O-deethylase), and their order of induction potential seemed to parallel the increase of the size of the halogen substituent. Therefore, 4,4'-DXB's may be categorized as mixed-type inducers, the cytochrome P-450 component being the more pronounced. Data on the cytochrome P-448 induction by dihalogenated biphenyls with only para substituents may be considered as a refinement of the previously described structure-activity relationship in this respect. All of the DXBs except 3,3'-DCB and 4,4'-DIB, enhanced, like phenobarbital, the activity of UDP-glucuronyltransferase toward 4-hydroxybiphenyl. Only 4,4'-DFB was able to induce the activity of glutathione S-transferase toward 1,2-epoxy-3-(p-nitrophenoxy)propane. Studies after 4,4'-DBB-treatment revealed, like phenobarbital, a preferential induction of ethylmorphine N-demethylase on rough endoplasmic reticulum-derived microsomes, whereas UDP-glucuronyltransferase activity toward 4-hydroxybiphenyl was induced to a larger extent on smooth endoplasmic reticulum microsomes, suggesting a dissimilar enzyme induction in microsomal subfractions.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Drug metabolism in Octodon degus: low inductive effect of phenobarbital

1. Differential effects of phenobarbital pre-treatment on liver microsomal drug metabolizing enzymes were registered in Octodon degus. 2. Glucuronidation reaction for morphine was decreased but that for p-nitrophenol was significantly increased. 3. Oxidative reactions such as naphthalene hydroxylation, morphine and aminopyrine N-demethylation were modestly increased. 4. In phenobarbital treated Octodon degus, testosterone metabolic pathways were decreased, not inducible or absent. 5. Spectral studies revealed two binding sites with different affinities for aniline in Octodon degus liver microsomes. 6. The poor phenobarbital induction on drug metabolism in Octodon degus may be a result of deficiency of androgen metabolic pathways associated to drug metabolizing enzymes.

Selective protein arylation by acetaminophen and 2,6-dimethylacetaminophen in cultured hepatocytes from phenobarbital-induced and uninduced mice. Relationship to cytotoxicity

To evaluate the mechanistic importance of covalent binding in acetaminophen (APAP)-induced hepatotoxicity, we compared the effects of 2,6-dimethylacetaminophen (2,6-DMA) to those of APAP in primary cultures of mouse hepatocytes. Immunochemical analysis of electrophoretically separated proteins has shown that the majority of covalent binding after a cytotoxic dose of APAP occurs on two major bands of 44 and 58 kD (Bartolone et al., Biochem Pharmacol 36: 1193-1196, 1987). At equimolar concentrations, 2,6-DMA bound proteins only 15% as extensively as did APAP and was not cytotoxic in hepatocytes from uninduced mice. However, when the hepatocytes were obtained from phenobarbital-induced mice, APAP administration resulted in increased protein arylation and a more rapid onset of cytotoxicity. Furthermore, in the cells from phenobarbital-induced mice, 2,6-DMA not only resulted in increased binding but also in overt cytotoxicity. Since our affinity-purified anti-APAP antibody did not cross-react with 2,6-DMA, a new antibody specific for 2,6-DMA was prepared and, after affinity purification, was used to detect 2,6-DMA protein adducts by Western blotting. Results indicated that, in hepatocytes from both phenobarbital-induced and non-induced mice, the binding of 2,6-DMA was also highly selective with the most prominent target being the 58 kD cytosolic protein. However, by contrast to APAP, only minimal binding to the 44 kD protein was detected after 2,6-DMA treatment. Although several additional protein adducts were increased in treated cells from phenobarbital-induced mice, the 58 kD protein was clearly the most prominently arylated target associated with both APAP and 2,6-DMA cytotoxicity. These data suggest that both the specificity of covalent binding as well as the extent of binding to the major targets may play an important role in the ensuing toxicity.

In vitro inhibition of hepatic drug oxidation by thioridazine. Kinetic analysis of the inhibition of cytochrome P-450 isoform-specific reactions

The phenothiazine tranquilizer thioridazine has been associated with drug interactions in man. This study investigated the capacity of the drug to inhibit hepatic drug oxidations mediated by cytochromes P-450 (P-450) in microsomes in vitro. Thioridazine was a potent linear mixed-type inhibitor of P-450b-dependent 7-pentoxyresorufin O-depentylase activity in phenobarbital-induced rat liver. The kinetic analysis revealed the enzyme-substrate dissociation constant (Ks) to be 1.6 microM whereas the dissociation constant of the enzyme-inhibitor complex (Ki) was 0.11 microM. In contrast, 7-ethoxyresorufin O-deethylase activity (mediated by P-450c) in beta-naphthoflavone-induced rat hepatic microsomes was inhibited to a lesser extent (Ki = 2.4 microM) in relation to the Ks value (0.5 microM). Spectral studies indicated that the efficiency of thioridazine binding in phenobarbital-induced microsomes was about 25-fold greater than in microsomes from beta-naphthoflavone-induced rat liver. This finding is consistent with the relative capacity of thioridazine to inhibit oxidase activities catalyzed by P-450b and P-450c. Mixed-function oxidase activities catalysed by other P-450s were also inhibited by thioridazine, although to a lesser extent than those catalysed by forms b and c. Thus, the 6 beta- and 16 beta-hydroxylations of androst-4-ene-3,17-dione in hepatic microsomes from untreated rats were inhibited to a similar extent (I50S = 52 and 43 microM, respectively). The 7 alpha- and 16 alpha-hydroxylase pathways were approximately only half as susceptible to inhibition by thioridazine. These findings demonstrate the capacity of thioridazine to inhibit a range of P-450-dependent drug oxidations, with those catalysed by forms b and c most susceptible. The present study strongly suggests that drug interactions elicited by thioridazine are most likely a consequence of inhibitory interactions with P-450 enzymes.

Effects of cytochrome P450-inducing agents on the monooxygenation of testosterone in long-term cultures of hepatocytes from male and female rats

Hepatocytes from male or female rats were cultured for up to 2 weeks in a modified Waymouth medium supplemented with 0.1 or 1.0 microM dexamethasone, 10 nM insulin, and 0.1 nM glucagon with or without addition of phenobarbital, methylcholanthrene, or isoniazid. The activities of testosterone hydroxylases were measured in the intact cell monolayer and in the corresponding microsomal fraction. Aniline hydroxylase was measured in cell homogenates. In the presence of 0.1 microM dexamethasone the testosterone hydroxylase activities varied differently in hepatocytes from male and female rats during the culture period. The activities of 6 beta- and 15 alpha-hydroxylases increased in female and were unchanged in male hepatocytes, while 16 alpha-hydroxylase activity increased in female and decreased in male, and 2 alpha- and 7 alpha-hydroxylase activities were unchanged in both male and female hepatocytes during the culture period. Increasing the dexamethasone concentration to 1.0 microM caused an increase in 6 beta- and 15 alpha-hydroxylase activities in cultures of hepatocytes from both sexes, whereas an increase of 2 alpha- and a decrease of 7 alpha- and 17-hydroxylase activities were found only in cultures of hepatocytes from female rats. Addition of phenobarbital caused an increase in the activity of 7 alpha-hydroxylase in both male and female hepatocytes, while the effect on the other hydroxylases differed with the sex. In hepatocytes from male rats phenobarbital addition decreased the activities of 2 alpha- and 16 alpha-hydroxylases, while these were increased or stable after addition of phenobarbital to hepatocytes from female rats. The activity of aniline hydroxylase was increased at Day 1 and declined afterward. The results demonstrate that the activities of different steroid hydroxylases are inducible and can be directly measured in monolayers of hepatocytes from rats.

Rat liver microsomal progesterone metabolism: evidence for differential troleandomycin and pregnenolone 16 alpha-carbonitrile inductive effects in the cytochrome P-450 III family

The effect of Troleandomycin (TAO) and pregnenolone 16 alpha-carbonitrile (PCN) on the hepatic microsomal progesterone metabolism in the rat is evaluated. Over thirteen hydroxylated progesterone derivatives are detected, including the novel 6 beta, 21-, 6 beta, 16 alpha-, 6 beta, 16 beta- and 2,21-dihydroxy derivatives, suggesting the induction of several cytochrome P-450 isozymes. PCN treatment results overall in an augmented production of progesterone metabolites whereas TAO treatment both induces and represses specific hydroxylase activities. Progesterone metabolism with purified isozymes isolated from liver microsomes from TAO and PCN treated rats differs significantly from that observed with intact microsomes, reflecting the complexity of the induction pattern of the cytochrome P-450 III family.

Suppression of male-specific cytochrome P450 2c and its mRNA by 3,4,5,3',4',5'-hexachlorobiphenyl in rat liver is not causally related to changes in serum testosterone

Rat cytochrome P450 2c (P450 gene IIC11) is a constitutive, male-specific hepatic enzyme which is suppressed greater than 90% by treatment with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) [H. N. Yeowell et al. (1987) Mol. Pharmacol. 32, 340-347]. HCB also decreases serum testosterone levels in adult male rats (greater than 98% loss). The present study assesses whether the suppression of P450 2c by HCB is a direct result of its effects on serum testosterone levels. Further, the site along the hypothalamic-pituitary-testicular axis at which HCB acts to depress testosterone secretion was examined. Administration of the synthetic androgen methyltrienolone to HCB-treated rats failed to prevent the suppression of P450 2c mRNA and its associated microsomal steroid 16 alpha-hydroxylase activity under conditions where it effectively reversed the large decrease in P450 2c mRNA and steroid 16 alpha-hydroxylase activity produced by castration. Hepatic steroid 6 beta-hydroxylase activity, which is catalyzed primarily by P450 2a (P450 gene IIIA2), was also suppressed by HCB and was not protected by methyltrienolone. Administration of either human chorionic gonadotropin, an analog of pituitary-derived luteinizing hormone, or the hypothalamic luteinizing hormone releasing hormone elevated serum testosterone levels to a much smaller extent in HCB-treated rats than in control rats. These results indicate that the effects of HCB on serum testosterone levels reflect its effects on testicular function rather than the pituitary or hypothalamus. However, the present study demonstrates that the consequential reduction in serum testosterone levels in HCB-treated rats is not causally related to the reduction in hepatic P450 2c levels. Thus, HCB must also act on some other regulatory mechanism involved in the expression of this protein.

Identification of cytochrome P450a (P450IIA1) as the principal testosterone 7 alpha-hydroxylase in rat liver microsomes and its regulation by thyroid hormones

In the preceding paper, evidence was presented that rat liver microsomes contain two structurally related isozymes of cytochrome P450, namely cytochromes P450a and P450m, that can both catalyze the 7 alpha-hydroxylation of testosterone. The aim of the present study was to determine the extent to which these two P450 isozymes are responsible for the 7 alpha-hydroxylation of testosterone catalyzed by rat liver microsomes. Four monoclonal antibodies against cytochrome P450a, designated A2, A4, A5, and A7, were prepared in BALB/c mice. Monoclonal antibodies A2 (an IgM), A4 (an IgG2b), and A5 (an IgG1) were determined to be distinct immunoglobulins, whereas A7 could not be distinguished from A5. All of the antibodies were highly specific for cytochrome P450a; none cross-reacted with cytochrome P450m or with 10 other P450 isozymes purified from rat liver microsomes. Competition experiments between unlabeled and horseradish peroxidase-conjugated antibodies revealed that each of the monoclonal antibodies recognized the same epitope on cytochrome P450a. None of the monoclonal antibodies bound to denatured cytochrome P450a, suggesting that they each bound to a spatial epitope. A monospecific, polyclonal antibody against cytochrome P450a was also prepared, as described in the preceding paper. The levels of cytochrome P450a in liver microsomes were determined by single radial immunodiffusion, Western immunoblot (with polyclonal antibody), and enzyme-linked immunosorbent assay with monoclonal antibody. The levels of cytochrome P450a declined with age in male but not female rats, and were inducible up to 10-fold by treatment of rats with various xenobiotics. The levels of cytochrome P450a (but not cytochrome P450m) were also elevated (approximately 3-fold) by thyroidectomy of mature male rats. Near normal levels of cytochrome P450a were restored by treatment of athyroid rats with triiodothyronine, whereas treatment with thyroxine was less effective in this regard. These changes in the levels of cytochrome P450a were highly correlated (r = 0.995) with changes in testosterone 7 alpha-hydroxylase activity. None of the monoclonal antibodies inhibited the catalytic activity of cytochrome P450a when reconstituted with NADPH-cytochrome P450 reductase and lipid. In contrast, the polyclonal antibody not only inhibited the catalytic activity of purified cytochrome P450a, but also completely inhibited (greater than 96%) the 7 alpha-hydroxylation of testosterone catalyzed by liver microsomes from immature and mature rats of both sexes and by liver microsomes from male rats treated with a variety of cytochrome P450 inducers.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification of two isozymes of rat liver microsomal cytochrome P450 with testosterone 7 alpha-hydroxylase activity

Cytochrome P450a was purified to electrophoretic homogeneity from liver microsomes from immature male Long-Evans rats treated with Aroclor 1254. Rabbit polyclonal antibody raised against cytochrome P450a cross-reacted with cytochromes P450b, P450e, and P450f (which are structurally related to cytochrome P450a). The cross-reacting antibodies were removed by passing anti-P450a over an N-octylamino-Sepharose column containing these heterologous antigens. The immunoabsorbed antibody recognized only a single protein (i.e., cytochrome P450a) in liver microsomes from immature male rats treated with Aroclor 1254 (i.e., the microsomes from which cytochrome P450a was purified). However, the immunoabsorbed antibody recognized three proteins in liver microsomes from mature male rats, as determined by Western immunoblot. As expected, one of these proteins (Mr 48,000) corresponded to cytochrome P450a. The other two proteins did not correspond to cytochromes P450b, P450e, or P450f (as might be expected if the antibody were incompletely immunoabsorbed), nor did they correspond to cytochromes P450c, P450d, P450g, P450h, P450i, P450j, P450k, or P450p. One of these proteins was designated cytochrome P450m (Mr approximately 49,000), the other cytochrome P450n (Mr approximately 50,000). Like cytochrome P450a, cytochrome P450n was present in liver microsomes from both male and female rats. However, whereas cytochrome P450a was detectable in liver microsomes from 1-week-old rats, cytochrome P450n was barely detectable until the rats were at least 3 weeks old. Furthermore, in contrast to cytochrome P450a, the levels of cytochrome P450n did not decline appreciably with age in postpubertal male rats. Cytochrome P450m was detectable only in liver microsomes from postpubertal (greater than 4 week-old) male rats. Cytochromes P450m and P450n were isolated from liver microsomes from mature male rats and purified to remove cytochrome P450a. When reconstituted with NADPH-cytochrome P450 reductase and lipid, cytochrome P450n exhibited little testosterone hydroxylase activity, whereas cytochrome P450m catalyzed the 15 alpha-, 18-, 6 beta-, and 7 alpha-hydroxylations of testosterone at 10.8, 4.6, 2.0, and 1.9 nmol/nmol P450/min, respectively. The ability of cytochrome P450m to catalyze the 7 alpha-hydroxylation of testosterone was not due to contamination with cytochrome P450a, which catalyzed this reaction at approximately 25 nmol/nmol P450a/min. Cytochrome P450m also converted testosterone to several minor metabolites, including androstenedione and 15 beta-, 14 alpha-, and 16 alpha-hydroxytestosterone.(ABSTRACT TRUNCATED AT 400 WORDS)

Redox cycling of resorufin catalyzed by rat liver microsomal NADPH-cytochrome P450 reductase

The O-dealkylation of 7-alkoxyresorufins to the highly fluorescent compound, resorufin (7-hydroxyphenoxazone), provides a rapid, sensitive, and convenient assay of certain forms of liver microsomal cytochrome P450. The results of this study indicate that NADPH-cytochrome P450 reductase catalyzes the reduction of resorufin (and the 7-alkoxyresorufins) to a colorless, nonfluorescent compound(s). The reduction of resorufin by NADPH-cytochrome P450 reductase was supported by NADPH but not NADH, and was not inhibited by dicumarol, which established that the reaction was not catalyzed by contaminating DT-diaphorase (NAD[P]H-quinone oxidoreductase). In addition to the rate of reduction, the extent of reduction of resorufin was dependent on the concentration of NADPH-cytochrome P450 reductase. The maintenance of steady-state levels of reduced resorufin required the continuous oxidation of NADPH, during which molecular O2 was consumed. When NADPH was completely consumed, the spectroscopic and fluorescent properties of resorufin were fully restored. These results indicate that the reduction of resorufin by NADPH-cytochrome P450 reductase initiates a redox cycling reaction. Stoichiometric measurements revealed of 1:1:1 relationship between the amount of NADPH and O2 consumed and the amount of H2O2 formed (measured fluorometrically). The amount of O2 consumed during the redox cycling of resorufin decreased approximately 50% in the presence of catalase, whereas the rate of O2 consumption decreased in the presence of superoxide dismutase. These results suggest that, during the reoxidation of reduced resorufin, O2 is converted to H2O2 via superoxide anion. Experiments with acetylated cytochrome c further implicated superoxide anion as an intermediate in the reduction of O2 to H2O2. However, the ability of reduced resorufin to reduce acetylated cytochrome c directly (i.e., without first reducing O2 to superoxide anion) precluded quantitative measurements of superoxide anion formation. Superoxide dismutase, but not catalase, increased the steady-state level of reduced resorufin and considerably delayed its reoxidation. This indicates that superoxide anion is not only capable of reoxidizing reduced resorufin, but is considerably more effective than molecular O2 in this regard. Overall, these results suggest that NADPH-cytochrome P450 reductase catalyzes the one-electron reduction of resorufin (probably to the corresponding semiquinoneimine radical) which can either undergo a second, one-electron reduction (presumably to the corresponding dihydroquinoneimine) or a one-electron oxidation by reducing molecular O2 to superoxide anion.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of 7-alkoxyresorufins by NADPH-cytochrome P450 reductase and its differential effects on their O-dealkylation by rat liver microsomal cytochrome P450

Antibody-inhibition experiments established that the induction of cytochrome P450c is largely responsible for the marked increase in liver microsomal 7-ethoxyresorufin O-dealkylation in rats treated with 3-methylcholanthrene, whereas the induction of cytochrome P450b and/or P450e is largely responsible for the marked increase in 7-pentoxy- and 7-benzyloxyresorufin O-dealkylation in rats treated with phenobarbital. When reconstituted with NADPH-cytochrome P450 reductase and lipid, purified cytochrome P450c catalyzed the O-dealkylation of 7-ethoxyresorufin at a rate of approximately 30 nmol/nmol P450/min, which far exceeded the rate catalyzed by either purified cytochromes P450b and P450e or microsomal cytochrome P450c. In contrast, purified cytochrome P450b and P450e were poor catalysts of the O-dealkylation of 7-pentoxy- and 7-benzyloxyresorufin. However, purified cytochrome P450b is an excellent catalyst of several other reactions, such as the N-demethylation of benzphetamine, the hydroxylation of testosterone, and the O-dealkylation of 7-ethoxycoumarin. The low rate of 7-pentoxyresorufin O-dealkylation catalyzed by purified cytochrome P450b did not reflect a requirement for cytochrome b5, and could not be ascribed to an artifact of the method used to measure the formation of resourufin. The catalytic activity of purified cytochrome P450b toward 7-pentoxyresorufin was consistently low over a range of substrate and lipid concentrations, and was not stimulated by sodium deoxycholate (which stimulates the N-demethylation of benzphatamine by purified cytochrome P450b). Evidence is presented which indicates that cytochrome P450c catalyzes the O-dealkylation of both the oxidized and reduced forms of 7-ethoxyresorufin, with perhaps a slight preference for the reduced form. In contrast, cytochrome P450b preferentially catalyzes the O-dealkylation of the oxidized form of 7-pentoxyresorufin. Conditions that favored formation of the reduced form of 7-ethoxyresorufin tended to stimulate its O-dealkylation by purified cytochrome P450c, whereas conditions that favored formation of the reduced form of 7-pentoxyresorufin decreased its rate of O-dealkylation by purified cytochrome P450b. Such conditions included a molar excess of NADPH-cytochrome P450 reductase over cytochrome P450, the presence of superoxide dismutase, and the presence of DT-diaphorase (liver cytosol).(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and characterization of human fetal liver cytochrome P450HLp2: a third member of the P450III gene family

We purified from human fetal livers a form of cytochrome P450, termed HLp2, related to adult human liver cytochrome P450HLp by monitoring the purification procedures by immunoblots developed with a monoclonal antibody to HLp. The preparation of HLp obtained was judged to be homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found to have an apparent monomeric molecular weight slightly greater than that of HLp, 51,500 versus 51,000. Immunoblot and Ouchterlony double-diffusion analyses indicated that HLp and HLp2 are immunochemically related. However, ferrous-CO versus ferrous difference spectra yielded different Soret maxima for HLp and HLp2 (448.5 and 449.5 nm, respectively). In addition, structural comparisons between HLp and HLp2 indicated that they were distinct isozymes. Specifically, the 28 amino acid amino-terminal sequence determined for HLp2 was found to be 79% homologous to that of HLp. In addition, peptide mapping of HLp and HLp2 by limited proteolysis with three proteases yielded similar but different patterns indicating that HLp and HLp2 are structurally distinct. These results demonstrate that HLp2 and HLp are highly related but distinct proteins and that HLp2 is a member of the steroid-inducible P450III gene family.

Polychlorinated biphenyls (PCBs): mutagenicity and carcinogenicity

The potential mutagenicity and carcinogenicity of commercial PCBs has been investigated in both in vivo and in vitro systems and several conclusions can be drawn from these studies. (1) PCBs can covalently adduct DNA both in vivo and in vitro (using a source of metabolic activation); the more highly chlorinated biphenyls are poorly metabolized and these compounds tend to exhibit very low binding to DNA. Based on the structure-activity relationships for PCBs (Safe, 1984) it is unlikely that the more toxic compounds such as 3,3',4,4',5-penta- and 3,3',4,4',5,5'-hexachlorobiphenyl, would form covalent adducts with DNA. (2) PCB mixtures and individual compounds exhibit minimal mutagenic activity in most assay systems. (3) The more highly chlorinated PCB mixtures (i.e. greater than 50% Cl by weight) are hepatocarcinogens in rodents whereas data from a limited number of studies suggest that the lower chlorinated mixtures are not carcinogenic. (4) In some model systems, the higher chlorinated PCB mixtures act as promoters of preneoplastic lesions and hepatocellular carcinomas in rodents treated with a variety of initiators. (5) Aroclor 1254 acts as a promoter of skin papilloma formation in HRS/J hairless mice and structure-activity and genetic studies suggest that the Ah receptor is necessary but not sufficient for the activity of halogenated aryl hydrocarbons as promoters in hairless mice. (6) Individual PCB congeners and higher chlorinated commercial mixtures also exhibit anti-carcinogenic activity in the CD-1 mouse skin cancer model. (7) Results from occupational studies suggest that individuals exposed to PCBs may have an excess of cancer at some sites, however, the most comprehensive study (Brown, 1987) suggests that there are no significant increases in the overall cancer rate in workers exposed to PCBs. Follow-up and continuing epidemiological studies on the PCB-exposed workers are required to further clarify the potential carcinogenic effects of PCBs on humans. In several strains of rats and mice, there is a high incidence of hepatic preneoplastic lesions and carcinomas and these lesions can be induced by diverse promoting agents (Schulte-Hermann et al., 1983; Weinstein, 1984). Since PCBs are not mutagenic and do not readily form covalent adducts with cellular DNA, it is likely that the higher chlorinated biphenyls are not genotoxic and act as promoters of carcinogenesis in rodents. A comparable mechanism has been suggested for 2,3,7,8-TCDD (Shu et al., 1987; Weinstein, 1984). For PCBs, the role of the Ah receptor in mediating their activity as promoters has not been delineated.(ABSTRACT TRUNCATED AT 400 WORDS)

The phenobarbital-type induction of rat liver microsomal monooxygenases by perfluorodecalin

Induction of perfluorodecalin (PFD) of the liver microsomal system of metabolism of xenobiotics has been studied and compared with the inductions by phenobarbital (PB) and 3-methylcholanthrene (MC). It has been shown that PFD increases the content of cytochrome P-450, NADPH-cytochrome c reductase activity. Like PB, PFD induces the activities of benzphetamine-N-demethylase, aldrine-epoxidase, 16 beta-androstendion-hydroxylase. Using specific antibodies against cytochromes P-450b and P-450c (which are the main isoenzymes of cytochrome P-450 in the PB- and MC-microsomes respectively), an immunological identity of the cytochrome P-450 isoforms during PFD and PB induction has been found. According to the rocket immunoelectrophoresis the content of cytochrome P-450 in PFD-microsomes, which is immunologically indistinguishable from P-450b, was approximately 70% of the total cytochrome P-450. Two forms of cytochrome P-450 were isolated from the liver microsomes of PFD-induced rats and purified to homogeneity. A comparison of these forms with cytochromes P-450b and P-450e obtained from the PB-induced rat liver microsomes revealed their similarity in a number of properties, e.g., chromotographic behavior on DEAE-Sephacel column, molecular weight determined by sodium dodecyl sulphate (SDS) electrophoresis in polyacrylamide gel, immunoreactivity, peptide mapping, catalytic activity. The data presented demonstrate that PFD induced in rat liver microsomes the cytochrome P-450 forms whose immunological properties and substrate specificity correspond to those of the PB-type cytochrome P-450. These findings suggest that PFD and PB, which differ in their chemical structure, induce in the rat liver microsomes identical forms of cytochrome P-450.

Induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by a series of 4'-substituted-2,3,4,5-tetrachlorobiphenyls

We have shown previously that 2,3,4,5-tetrachlorobiphenyl is ineffective as an inducer of rat liver microsomal cytochrome P-450. Addition of a single para-chloro substituent in the otherwise unsubstituted phenyl ring, to give 2,3,4,4',5-pentachlorobiphenyl, produces a potent cytochrome P-450 inducer with both phenobarbital- and 3-methylcholanthrene-type characteristics. In the present study, 2,3,4,5-tetrachlorobiphenyl was substituted in the para(4') position with 12 other functional groups. The 4'-X-C12H5Cl4 derivatives were tested as inducers of cytochromes P-450a--P-450e and epoxide hydrolase, by immunochemical analysis of liver microsomes prepared 4 days after a single treatment (500 mumol/kg) of 1-month-old male Long Evans rats. When the para' substituent was a halogen (F, Cl, Br or I), the derivative induced both cytochromes P-450b and P-450e, and cytochromes P-450c and P-450d, which are the major phenobarbital- and 3-methylcholanthrene-inducible isozymes, respectively. A similar type of induction was observed with a second group of derivatives substituted with CN, NO2 or CF3. However, a derivative containing CH3CO--(which is also a meta-directing, ring-activating substituent) failed to induce cytochromes P-450a-P-450e at the dosage and time tested. Members of a third group of derivatives, which contained an ortho/para-directing, ring-activating substituent) were either ineffective inducers (OH, CH3, CH3O--), or were inducers of cytochromes P-450c and P-450d (isopropyl or t-butyl). Hence, 4'-substitution with a bulky lipophilic substituent conferred 3-methylcholanthrene- but not phenobarbital-type characteristics on 2,3,4,5-tetrachlorobiphenyl. Some of the derivatives tested, namely those substituted with Cl, Br, I and CF3, were remarkably effective inducers of cytochrome P-450a, causing a 10-11-fold induction of this isozyme. Data on the induction of cytochrome P-450c were analyzed by multiparameter linear regression in an attempt to correlate the biological activity of the 4'-X-C12H5Cl4 derivatives with the physiochemical properties of the various substituents. From these results, and those reported recently, we propose that binding of the 4'-X-C12H5Cl4 derivatives to the rat cytosolic Ah receptor is favored by increasing the electronegativity, lipophilicity and hydrogen bonding characteristics of the 4' substituent, whereas enzyme induction (both in vivo and in cultured rat hepatoma cells) is also governed by a fourth characteristic, the STERIMOL factor, which gives a measure of the width of the substituent.(ABSTRACT TRUNCATED AT 400 WORDS)

Further characterization of RLM2 and comparison with a related form of cytochrome P-450, RLM2b

We have extended the characterization of RLM2, a constitutive form of rat liver cytochrome P-450, using immunochemical means to quantitate its presence in microsomes, to follow its development in maturing male and female rats, and to determine its response to prototypical P-450 inducers. In addition, RLM2 is compared to RLM2b, a form of P-450 with similar migration on SDS-PAGE and NH2-terminal amino acid sequence. RLM2b is expressed in both sexes at a level of 0.08 nmol/mg microsomal protein at 2 weeks of age. In female rats, this level is unchanged with maturation. However, in the male, the level declined with maturation to reach 0.02 nmol/mg protein by 12 weeks of age. RLM2 is a male-specific form of cytochrome P-450. Originally absent in the 2-week-old rat, it reached a level of 0.03 nmol/mg protein in the adult male, its appearance and increase coinciding with the onset of puberty. Both phenobarbital and 3-methylcholanthrene induced microsomal levels of RLM2b in the adult male and female rat. RLM2, however, was suppressed in the male rat, 58 and 42%, respectively, by the same treatments. RLM2b and RLM2 each catalyze a unique spectrum of hydroxytestosterone metabolites. RLM2b is highly site specific. In contrast, RLM2 produces several isomeric products in the same region of the testosterone molecule. Substitution of the acetyl group of progesterone for the 17-hydroxy group of testosterone did not alter the site specificity of RLM2b, but did alter it for RLM2, indicating, further, a difference in the active site conformation of the two enzymes. Although RLM2b and RLM2 responded differently to inducers and to a changing physiology during maturation, and were functionally quite distinct, the proteins showed a high degree of immunologic relatedness which is suggestive of significant structural similarities. Structural differences do exist, however, as alpha-chymotryptic digestion formed a number of peptide fragments that differed between the two proteins.

Inhibition of steroid 5 alpha-reductase and its effects on testosterone hydroxylation by rat liver microsomal cytochrome P-450

It has been shown previously that liver microsomal steroid 5 alpha-reductase activity increases with age in female but not male rats, which coincides with a female-specific, age-dependent decline in the cytochrome P-450-dependent oxidation of testosterone to 1 beta-, 2 alpha-, 2 beta-, 6 alpha-, 6 beta-, 7 alpha-, 15 beta-, 16 alpha-, 16 beta-, and 18-hydroxytestosterone and androstenedione. To determine whether the increase in steroid 5 alpha-reductase activity is responsible for the decrease in testosterone oxidation, we have examined the effects of the steroid 5 alpha-reductase inhibitor, 4-MA (17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one), on the pathways of testosterone oxidation catalyzed by rat liver microsomes. We have also determined which hydroxytestosterone metabolites are substrates for steroid 5 alpha-reductase. At concentrations of 0.1 to 10 microM, 4-MA completely inhibited steroid 5 alpha-reductase activity without inhibiting the pathways of testosterone oxidation catalyzed by liver microsomes from rats of different age and sex, and from rats induced with phenobarbital or pregnenolone-16 alpha-carbonitrile. 4-MA (10 microM) had little or no effect on the oxidation of testosterone catalyzed by liver microsomes from mature male rats (which have low steroid 5 alpha-reductase activity). In contrast, the hydroxylated testosterone metabolites formed by liver microsomes from mature female rats (which have high steroid 5 alpha-reductase activity) accumulated to a much greater extent in the presence of 4-MA. Evidence is presented that 4-MA increases the accumulation of hydroxytestosterones by two mechanisms. First, 4-MA inhibited the 5 alpha-reduction of those metabolites (such as 6 beta-hydroxytestosterone) that were found to be excellent substrates for steroid 5 alpha-reductase. In the absence of 4-MA, these metabolites eventually disappeared from incubations containing liver microsomes from mature female rats. Second, 4-MA inhibited the formation of 5 alpha-dihydrotestosterone, which otherwise competed with testosterone for oxidation by cytochrome P-450. This second mechanism explains why 4-MA increased the accumulation of metabolites (such as 7 alpha-hydroxytestosterone) that were found to be poor substrates for steroid 5 alpha-reductase. Despite its marked effect on the accumulation of hydroxylated testosterone metabolites, 4-MA had no effect on their initial rate of formation by liver microsomes from either male or female rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Nutrition-related alterations in liver microsomal testosterone hydroxylases

The oxidation products of testosterone formed by liver microsomes from normal-fed and protein-energy malnourished male rats have been analysed by HPLC. Microsomes from normal-fed rats oxidized testosterone at a rate of 4.52 nmol/min/mg protein. The major products formed were: 6 beta-, 7 alpha- and 16-alpha-hydroxytestosterone; these three metabolites represented 65% of the total testosterone metabolism. Microsomes from protein-energy malnourished rats oxidized testosterone at a reduced rate of 2.03 nmol/min/mg protein. The major product formed was 7 alpha-hydroxytestosterone, which accounted for 43% of total testosterone oxidation. Microsomes from protein-energy malnourished rats showed a CO-reduced cytochrome P-450 spectra with a maxima at 452 nm, and a 38% decrease in the total content of cytochrome P-450. Some testosterone hydroxylases were drastically affected by protein-energy malnutrition but others, such as 7 alpha-hydroxylase, remained unchanged. The present results suggest that nutritional status can modify the relative amounts of individual cytochrome P-450 isozymes, thus explaining the observed changes in several testosterone hydroxylases. Protein-energy malnutrition seems to be an excellent tool with which to obtain a microsomal fraction containing predominantly P-450 isozymes, which are probably involved in key mono-oxygenations of physiological substrates.

Complexation of cytochrome P-450 isozymes in hepatic microsomes from SKF 525-A-induced rats

Potassium ferricyanide-elicited reactivation of steroid hydroxylase activities, in hepatic microsomes from SKF 525-A-induced male rats, was used as an indicator of complex formation between individual cytochrome P-450 isozymes and the SKF 525-A metabolite. Induction of male rats with SKF 525-A (50 mg/kg for three days) led to apparent increases in androst-4-ene-3,17-dione 16 beta- and 6 beta-hydroxylation to 6.7- and 3-fold of control activities. Steroid 7 alpha-hydroxylase activity was decreased to 0.8-fold of control and 16 alpha-hydroxylation was unchanged. Ferricyanide-elicited dissociation of the SKF 525-A metabolite-P-450 complex revealed an even greater induction of 16 beta- and 6 beta-hydroxylase activities (to 1.8- and 1.6-fold of activities in the absence of ferricyanide). Androst-4-ene-3,17-dione 16 alpha-hydroxylase activity increased 2-fold after ferricyanide but 7 alpha-hydroxylase activity was unaltered. An antibody directed against the male-specific cytochrome P-450 UT-A decreased androst-4-ene-3,17-dione 16 alpha-hydroxylase activity to 13% of control in hepatic microsomes from untreated rats. In contrast, 16 alpha-hydroxylase activity in microsomes from SKF 525-A-induced rats, before and after dissociation with ferricyanide, was reduced by anti UT-A IgG to 32 and 19% of the respective uninhibited controls. Considered together, these observations strongly suggest that the phenobarbital-inducible cytochrome P-450 isozymes PB-B and PCN-E are present in an inactive complexed state in microsomes from SKF 525-A-induced rat liver. Further, the increased susceptibility of androst-4-ene-3,17-dione 16 alpha-hydroxylase activity to inhibition by an antibody to cytochrome P-450 UT-A, following ferricyanide treatment of microsomes, suggests that this male sexually differentiated enzyme is also complexed after in vivo SKF 525-A dosage. In contrast, the constitutive isozyme cytochrome P-450 UT-F, which is active in steroid 7 alpha-hydroxylation, does not appear to be complexed to any extent in microsomes from SKF 525-A-induced rats.

Qualitative and quantitative differences in the induction and inhibition of hepatic benzo[a]pyrene metabolism in the rat and hamster

The present study compared the induction and inhibition of the metabolism of the prototype polycyclic aromatic hydrocarbon, benzo[a]pyrene (BaP), in rat and hamster liver microsomes. The production of total polar metabolites was quantitated by separating 3H-metabolites from [3H]-BaP using reverse-phase thin-layer chromatography. The rate of hepatic microsomal BaP metabolism was similar in the rat and hamster (0.81 vs 0.72 nmol/min/nmol cytochrome P-450 respectively). In the rat, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 5 micrograms/kg, i.p.) and 3-methylcholanthrene (3-MC; 50 mg/kg, i.p., X 3 days) pretreatments doubled the rate of BaP metabolism, whereas phenobarbital pretreatment (PB; 80 mg/kg, i.p., X 3 days) had no effect. In contrast, hamster hepatic microsomal BaP metabolism was elevated 2.3-fold by PB pretreatment, whereas TCDD and 3-MC pretreatments had no effect. Isosafrole pretreatment (ISO; 150 mg/kg, i.p., X 3 days) elevated the rate by almost 2-fold in each species. Another cytochrome P-448-mediated activity, 7-ethoxyresorufin O-deethylase (EROD), was induced by the same compounds that induced BaP metabolism in the rat. In hamster liver microsomes, in contrast to BaP metabolism, EROD was induced by TCDD and 3-MC but not PB or ISO pretreatments. The results suggest differences in the substrate specificity of the cytochromes P-448-450 induced by TCDD, 3-MC and PB in these species. This was supported by the different selectivity of the in vitro inhibitors, metyrapone and 7,8-benzoflavone, towards BaP metabolism and EROD in hepatic microsomes from TCDD- or PB-pretreated rats and hamsters. Reverse-phase HPLC analysis indicated that, while 3-hydroxy-BaP was the major metabolite formed by the untreated rat, untreated hamster liver microsomes formed predominantly BaP-4,5-diol. Microsomes from TCDD-treated rats generated elevated levels of all BaP-diols, diones and 3-hydroxy-BaP, with the major metabolites being BaP-9,10- and BaP-7,8-diols. In contrast, the metabolite profile from TCDD-pretreated hamsters was unchanged from the control. PB-treated hamster microsomes produced elevated levels of BaP-diones and 3-hydroxy-BaP. However, the major hepatic metabolite formed by PB-pretreated hamsters was BaP-4,5-diol, while BaP-9,10- and BaP-7,8-diols were not detected. The results of the study indicate differences in the induced cytochrome P-450s and the generation of toxic BaP metabolites in the liver of the rat and hamster.

Ethanol-, fasting-, and acetone-inducible cytochromes P-450 in rat liver: regulation and characteristics of enzymes belonging to the IIB and IIE gene subfamilies

Two major forms of hepatic microsomal cytochrome P-450 were purified from starved and acetone-treated rats. On the basis of amino acid sequence analysis, they were identified as P-450j and P-450b. Ethanol or acetone treatment of rats caused a 9-fold increase in the amount of P-450j in liver microsomes accompanied by similar increases in the rate of NADPH-dependent metabolism of carbon tetrachloride, acetone, and benzene. Immunological experiments indicated that P-450j constitutes the major catalyst of the microsomal metabolism of the latter agents and contributes by about 50% to microsomal P-450-dependent ethanol oxidation under the conditions used. The P-450j-dependent catalytic activities had a high rate of turnover. In contrast, this was not the case for the immunodetectable P-450j, indicating the occurrence of inactive forms of this protein in microsomes. Starvation or ethanol or acetone treatment caused 10-30-fold increases in the amount of both mRNA and apoprotein of P-450b,e compared to control. Run-on experiments and the concomitant increases of the P-450b,e gene products at the mRNA and protein levels indicated the appearance of mainly a transcriptional activation by acetone, ethanol, or starvation. Fasting exerted, in addition, a pronounced synergistic effect on acetone-dependent induction of P-450b,e mRNA (3-fold), apo-P-450b,e (4.3-fold), P-450j mRNA (2-fold), and apo-P-450j (2-fold). No increase of mRNA coding for P-450j, compared to control, was seen after acetone or ethanol treatment alone. The results indicate that effects of ethanol, acetone, and/or starvation on drug and xenobiotic metabolism are caused by the induction of P-450 forms belonging to at least two gene subfamilies.

First evidence of cytochrome P-450 induction in the mouse brain by phenytoin

Long-term administration of phenytoin (60 mg/kg) to male C57Bl/6J mice causes an average drug serum level of 16-19 micrograms/ml. Cytological changes consisting of focal axonal swellings in the deep cerebellar nuclei are characterized by an increasing accumulation of vesiculotubular membranes in the dense, but ribosome-deficient axoplasm. Microsomal fractions of brain and liver were prepared at intervals until the 80th day of treatment to determine their cytochrome P-450 activities and isoenzyme characteristics. The cerebellar tissue was shown to be distinguished by a 15-fold enhancement of cytochrome P-450 PB3a activity, that means twice the extent of that found in liver and cerebrum.

Induction of microsomal epoxide hydrolase by nitrosamines in rat liver. Effect on messenger ribonucleic acids

Nitrosomethylethylamine and nitrosomethylpropylamine were found to be more potent inducers of rat liver microsomal epoxide hydrolase (styrene oxide hydrolase) than nitrosodiethylamine or nitrosodimethylamine. The time course of induction following a single administration of nitrosodimethylethylamine, nitrosomethylpropylamine or nitrosodiethylamine each showed a delay of 24 hr during which enzyme activity was unaltered. After that time activity increased and reached a maximum at between 72 and 120 hr. Increased enzyme activity following NDEA was paralleled by changes in the content of epoxide hydrolase in microsomes as measured by Western blots. Nitrosamines caused an increase of mRNA for epoxide hydrolase which was detected by probing Northern blots with a [32]-P labelled epoxide hydrolase cDNA and by in vitro translation of polyadenylated mRNA. Both methods showed a maximal increase at 72 hr after nitrosodiethylamine treatment but a significant increase was also observed at 24 hr although at this time no increase in enzyme activity was apparent.

Purification and characterization of liver cytochrome P-446 isolated from protein energy malnourished rats

A liver cytochrome P-450 isozyme has been purified to homogeneity from protein-energy malnourished rats induced with beta-naphthoflavone (beta-NF). The purification steps included chromatography on DEAE-Sephadex-A-25, DEAE-cellulose (DE-53), hydroxylapatite (HA) and carboxymethyl-sephadex (CM) columns. The reduced carbon monoxide difference and absolute spectra showed a Soret peak at 446.5 nm. The wavelength maxima for the oxidized and reduced spectra were at 416 and 408 nm, respectively. Cytochrome P-446 appears to have a predominantly low spin ferric iron, migrates as a single band of molecular weight 56,000 in sodium dodecyl sulfate polyacrylamide gels and has a specific content of 14 nmol/mg of protein. P-446 oxidized various substrates at different rates in a reconstituted system with NADPH-cytochrome P-450 reductase and dilauroyl-phosphatidylcholine. In this system turnover rates for benzo[alpha]pyrene, testosterone and benzphetamine oxidation were: 81.10; 1.85 and 1.42 nmoles product/min/nmol P-446 respectively. While NH2 terminal amino acid sequence analysis of 18 of the first 20 residues suggests that the cytochrome P-446 isolated from malnourished rats is identical with form c, the catalytic activities suggest that this isozyme may be a more effective or efficient catalyst for some substrates.

Purification of rat liver microsomal cytochrome P-450b without the use of nonionic detergent

Sodium cholate, Emulgen 911, and (3-[(-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate) (CHAPS) were selected to examine the effects of ionic, nonionic, and zwitterionic detergents on testosterone hydroxylation catalyzed by four purified isozymes of rat liver microsomal cytochrome P-450, namely P-450a, P-450b, P-450c, and P-450h, in reconstituted systems containing optimal amounts of dilauroylphosphatidylcholine and saturating amounts of NADPH-cytochrome P-450 reductase (reductase). The major phenobarbital-inducible form of rat liver microsomal cytochrome P-450, designated P-450b, was extremely sensitive to the inhibitory effects of Emulgen 911, which is used in several procedures to purify this and other forms of cytochrome P-450. In contrast, sodium cholate and CHAPS had little effect on the catalytic activity of cytochrome P-450b, even at ten times the concentration of Emulgen 911 effecting 50% inhibition (IC-50). By substituting the zwitterionic detergent CHAPS for Emulgen 911, we purified cytochrome P-450b without the use of nonionic detergent. The protein is designated cytochrome P-450b* to distinguish it from cytochrome P-450b purified with the use of Emulgen 911. NADPH-cytochrome P-450 reductase was also purified both with and without the use of nonionic detergent. The absolute spectra of cytochrome P-450b and P-450b* were indistinguishable, as were the carbon monoxide (CO)- and metyrapone-difference spectra of the dithionite-reduced hemoproteins. When reconstituted with NADPH-cytochrome P-450 reductase and dilauroylphosphatidylcholine, cytochromes P-450b and P-450b* catalyzed the N-demethylation of benzphetamine and aminopyrine, the 4-hydroxylation of aniline, the O-dealkylation of 7-ethoxycoumarin, the 3-hydroxylation of hexobarbital, and the 6-hydroxylation of zoxazolamine. Both hemoproteins catalyzed the 16 alpha- and 16 beta-hydroxylation of testosterone, as well as the 17-oxidation of testosterone to androstenedione. Both hemoproteins were poor catalysts of erythromycin demethylation and benzo[a]pyrene 3-/9-hydroxylation. The rate of biotransformation catalyzed by cytochrome P-450b* was up to 50% greater than the rate catalyzed by cytochrome P-450b when reconstituted with either reductase or reductase*. The activity of cytochrome P-450b and P-450b* increased up to 50% when reconstituted with reductase* instead of reductase. In addition to establishing the feasibility of purifying an isozyme of rat liver microsomal cytochrome P-450 without the use of nonionic detergent, these results indicate that the catalytic activity of cytochrome P-450 is not unduly compromised by residual contamination with the nonionic detergent Emulgen 911.

Studies on the pregnenolone-16 alpha-carbonitrile-inducible form of rat liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase

Treatment of rats with pregnenolone-16 alpha-carbonitrile (PCN) markedly induces rat liver microsomal cytochrome P-450p and UDP-GT-dt1, a glucuronosyltransferase active towards the digitoxin metabolite, digitoxigenin monodigitoxoside. The present study characterizes the regulation of these two enzymes in rats treated with different xenobiotics. Like PCN, treatment of rats with dexamethasone, spironolactone, troleandomycin or erythromycin estolate markedly induced both UDP-GT-dt1 and cytochrome P-450p (measured as erythromycin demethylase and testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activities). However, compared to PCN and dexamethasone, both troleandomycin and erythromycin estolate preferentially induced cytochrome P-450p, whereas spironolactone preferentially induced UDP-GT-dt1. Treatment of rats with the polychlorinated biphenyl mixture, Aroclor 1254, increased both cytochrome P-450p and UDP-GT-dt1 activity to about 40% of that in liver microsomes from rats induced with PCN or dexamethasone. Treatment of rats with phenobarbital or chlordane caused a relatively small increase in cytochrome P-450p and UDP-GT-dt1 activity. Neither enzyme was induced by treatment of rats with 3-methylcholanthrene, rifampin or digitoxin. The induction of cytochrome P-450p and UDP-GT-dt1 by PCN followed similar dose-response curves. Although cytochrome P-450p and UDP-GT-dt1 are differentially affected by the age and the sex of rats, the enzymes responded similarly, but not identically, to xenobiotic treatment. This suggests that cytochrome P-450p and UDP-GT-dt1 are co-inducible but not coordinately regulated.

Isozyme specificity of testosterone 7 alpha-hydroxylation in rat hepatic microsomes: is cytochrome P-450a the sole catalyst?

In the present study we show that monospecific antibody against cytochrome P-450a completely inhibits testosterone 7 alpha-hydroxylation in hepatic microsomes of untreated male or female rats or rats of either sex treated with dexamethasone. These data are in contrast with those of K. Nagata et al. (1987, J. Biol. Chem. 262, 2787-2793) who recently reported that an antibody prepared against cytochrome P-450a completely inhibited testosterone 7 alpha-hydroxylase activity in microsomes from untreated or 3-methylcholanthrene-treated rats but only inhibited 50% of the activity in microsomes from dexamethasone-treated rats. They proposed that dexamethasone treatment of rats induced another testosterone 7 alpha-hydroxylase in rat liver. The discrepancy in the two sets of data was due, at least in part, to the use of a chromatography system by Nagata et al. that is incapable of resolving a number of testosterone metabolites. Dexamethasone treatment of rats leads to a marked increase in the production of several testosterone metabolites, including 15 beta-hydroxytestosterone which is cochromatographic with 7 alpha-hydroxytestosterone in their chromatography system. Our results indicate that cytochrome P-450a accounts for all of the testosterone 7 alpha-hydroxylase activity in microsomes from dexamethasone-treated rats, and that testosterone 7 alpha-hydroxylation continues to be a useful marker for monitoring cytochrome P-450a in rat hepatic microsomes.

The effect of dexamethasone, insulin and triiodothyronine on microsomal NADPH-cytochrome-c (P-450) reductase in primary cultures of isolated hepatocytes

NADPH-cytochrome-c (P-450) reductase, a flavoprotein, is a constituent of the hepatic microsomal polysubstrate monooxygenase and catalyzes the transfer of electrons from NADPH to cytochrome P-450. The hormonal regulation of NADPH-cytochrome-c reductase activity and protein has been examined in insolated hepatocytes cultured as monolayers for 48 h in Waymouth's MB752/1 medium fortified with insulin, dexamethasone and triiodothyronine. No similarity between the response of NADPH-cytochrome-c reductase and of tyrosine aminotransferase and malate dehydrogenase activity to dexamethasone and triiodothyronine treatment could be detected. In the absence of hormones about 65% of the original NADPH-cytochrome-c reductase activity and protein estimated by the immunochemical staining technique was retained. Culture of hepatocytes in insulin (10.0 mU/ml) or dexamethasone (100 nM) alone but not triiodothyronine improved the retention of reductase activity and protein. Only when hepatocytes were cultured in insulin, triiodothyronine and dexamethasone could NADPH-cytochrome-c reductase activity and protein be maintained at the original level. Dexamethasone alone was found to enhance consistently retention of reductase protein, but not reductase activity, to approximately the same level as in freshly isolated hepatocytes. The results suggest that microsomal NADPH-cytochrome-c reductase activity and protein can be maintained in isolated hepatocytes at the original level by culturing the cells in dexamethasone, insulin and triiodothyronine.

Polymorphisms of four hepatic cytochromes P-450 in twenty-eight inbred strains of rat

Two-dimensional gel electrophoresis of hepatic microsomes from phenobarbital-treated animals was used to analyze electrophoretic/regulatory polymorphisms for cytochromes P-450b, P-450e, P-450g, and P-450h in 28 inbred strains of rat. Previous studies with outbred rats revealed the existence of four electrophoretic variants for P-450b, two for P-450e, and three for P-450h as well as two regulatory alleles for P-450g. With the exception of one allozymic form of P-450h, all of these alleles as well as a novel (null) allele for P-450e were found to be homozygous in at least two of the inbred strains tested. Eight phenotypes for combinations of these four cytochromes P-450 were observed. Inbred strains were identified that can be used in studies on the structure/function of unique cytochrome P-450-allozymes and in genetic crosses to map the four distinct cytochrome P-450 genes.

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.

Stereoselective formation of benzo(c)phenanthrene (+)-(3S,4R) and (+)-(5S,6R)-oxides by cytochrome P450c in a highly purified and reconstituted system

The principal oxidative metabolites formed from benzo(c)phenanthrene (B(c)Ph) by the cytochromes P450 in liver microsomes from control and treated rats are the 3,4- and 5,6-arene oxides. A procedure is described which allows determination of the enantiomer composition and absolute configuration of these arene oxides based on HPLC separation of isomeric thiolate adducts formed with N-acetyl-L-cysteine in base. Incubation of [3H]-B(c)Ph with highly purified cytochrome P450c in a reconstituted monooxygenase system followed by trapping of the metabolically formed arene oxides as above indicated that the 3,4-oxide was predominantly the (+)-(3S,4R)-enantiomer (90%) and that the 5,6-oxide consisted mainly of the (+)-(5S,6R)-enantiomer (76%). The results are discussed in terms of their implications about the catalytic binding site of cytochrome P450c.