Separation, purification, and characterization of a novel form of hepatic cytochrome P-450 from rats treated with pregnenolone-16 alpha-carbonitrile

Differential regulation of liver P-450III cytochromes in choline-deficient rats: implications for the erythromycin breath test as a parameter of liver function

Progressive liver fibrosis in rats develops when they are fed a diet deficient in choline. This diet also results in a pronounced and selective decrease in the liver microsomal content of a phase I drug-metabolizing enzyme belonging to the cytochrome P-450III gene family. Because P-450III cytochromes characteristically catalyze the N-demethylation of erythromycin, we believed that the production of breath CO2 from erythromycin would be dramatically reduced in choline-deficient rats. However, when 12 choline-deficient rats were compared with 9 control rats, the reduction in CO2 production from erythromycin (mean decrease 71%) was essentially identical to that from aminopyrine (mean decrease 69%), a substrate believed to be metabolized normally by the hepatocyte in fibrotic liver disease. Furthermore, we found that the relative erythromycin and aminopyrine demethylase activities were comparable when measured in vitro in liver microsomes prepared from the choline-deficient rats. To determine the molecular basis for the erythromycin demethylase activity in the choline-deficient rats, the liver microsomes were subjected to immunoblot analysis using a variety of polyclonal and monoclonal antibodies capable of distinguishing individual P-450III-related proteins. Our studies confirm that a major erythromycin demethylase belonging to the P-450III family, termed P-450p, was greatly reduced in the choline-deficient rat liver. However, the specific concentration of a second P-450p-related protein was essentially normal and that of a third P-450p-related protein was actually increased in the choline-deficient rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic catecholestrogen synthases: differential effect of sex, inducers of cytochromes P-450 and of antibody to the glucocorticoid inducible cytochrome P-450 on NADPH-dependent estrogen-2-hydroxylase and on organic hydroperoxide-dependent estrogen-2/4-hydroxylase activity of rat hepatic microsomes

Formation of catecholestrogens (CE) by rat hepatic microsomes was re-examined because as recently shown; (1) CE formation can be catalyzed by an NADPH-dependent estrogen-4-hydroxylase (E-4-H(NADPH)) and by a peroxidatic, organic hydroperoxide-dependent estrogen-2/4-hydroxylase (E-2/4-H(OHP)), in addition to the established NADPH-dependent estrogen 2-hydroxylase (E-2-H(NADPH)); and (2) the indirect radiometric and the COMT-coupled radioenzymatic assays, used in many previous studies, may fail to provide an accurate measure, in particular, of 4-OH-CE. Using a direct product isolation assay, hepatic microsomes of both male and female rats were shown to express E-2/4-H(OHP) activity with properties similar to those of peroxidatic activity in other tissues. The activities of E-2/4-H(OHP) and E-2-H(NADPH) were affected differently by 5 out of 7 inducers of cytochromes P-450 administered in vivo. Phenobarbital and dexamethasone caused a 4- and 2-3-fold increase in E-2-H(NADPH) activity, respectively, but only a 38 and 20% increase in E-2/4-H(OHP) activity. Ketoconazol and beta-naphtoflavone caused a modest increase in E-2-H(NADPH) activity but a decrease in OHP-dependent activity. Clofibrate decreased peroxidatic activity by 50% and NADPH-dependent activity by approximately 20%. Both activities were increased by ethanol but decreased by isoniazide, an agent which induces the same form of cytochromes P-450 as ethanol. Polyclonal antibody against P-450p, a form of P-450 induced by glucocorticoids, inhibited E-2-H(NADPH) but not E-2/4-H(OHP) activity of untreated and of dexamethasone- and phenobarbital-treated rats. This study establishes that CE formation may occur in liver via the peroxidatic pathway and indicates that this pathway depends on forms of P-450 different from those mediating E-2-H(NADPH) activity. It also confirms and extends previous observations of the involvement of multiple, constitutive and induced forms of cytochrome P-450 in NADPH-dependent 2-hydroxylation in liver.

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.

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)

Developmental regulation of hepatic testosterone hydroxylases: simultaneous activation and repression of constitutively expressed cytochromes P450 in senescent rats

The aging process is generally associated with marked decreases in the activities of numerous enzymes as well as lower levels of sex hormones such as testosterone. We therefore examined testosterone metabolism in liver microsomes from individual 3- and 24-month-old male rats. Although the old rats exhibited lower 16 alpha-, 6 beta-, and 2 alpha-hydroxylase activities than the young rats, the old rats had a higher 7 alpha-hydroxylase activity. Immunoquantitation of P450a, a known 7 alpha-hydroxylase, showed that the level of this protein was elevated in the old rats, and was correlated with 7 alpha-hydroxylase activity. The mRNA for P450a was measured with a cDNA probe and its level was fivefold higher in the old rats, whereas levels of mRNA coding for a 6 beta-hydroxylase P450 were markedly decreased. The increased expression of cytochrome P450a demonstrates that the observed common decrease in cytochrome P450-catalyzed activities with senescence is not a universal phenomenon. Thus, constitutive expression of specific cytochrome P450 genes is repressed or activated in senescent rats.

Reconstitution of testosterone oxidation by purified rat cytochrome P450p (IIIA1)

Cytochrome P450p (IIIA1) has been purified from rat liver microsomes by several investigators, but in all cases the purified protein, in contrast to other P450 enzymes, has not been catalytically active when reconstituted with NADPH-cytochrome P450 reductase and dilauroylphosphatidylcholine. We now report the successful reconstitution of testosterone oxidation by cytochrome P450p, which was purified from liver microsomes from troleandomycin-treated rats. The rate of testosterone oxidation was greatest when purified cytochrome P450p (50 pmol/ml) was reconstituted with a fivefold molar excess of NADPH-cytochrome P450 reductase, an equimolar amount of cytochrome b5, 200 micrograms/ml of a chloroform/methanol extract of microsomal lipid (which could not be substituted with dilauroylphosphatidylcholine), and the nonionic detergent, Emulgen 911 (50 micrograms/ml). Testosterone oxidation by cytochrome P450p was optimal at 200 mM potassium phosphate, pH 7.25. In addition to their final concentration, the order of addition of these components was found to influence the catalytic activity of cytochrome P450p. Under these experimental conditions, purified cytochrome P450p converted testosterone to four major and four minor metabolites at an overall rate of 18 nmol/nmol P450p/min (which is comparable to the rate of testosterone oxidation catalyzed by other purified forms of rat liver cytochrome P450). The four major metabolites were 6 beta-hydroxytestosterone (51%), 2 beta-hydroxytestosterone (18%), 15 beta-hydroxytestosterone (11%) and 6-dehydrotestosterone (10%). The four minor metabolites were 18-hydroxytestosterone (3%), 1 beta-hydroxytestosterone (3%), 16 beta-hydroxytestosterone (2%), and androstenedione (2%). With the exception of 16 beta-hydroxytestosterone and androstenedione, the conversion of testosterone to each of these metabolites was inhibited greater than 85% when liver microsomes from various sources were incubated with rabbit polyclonal antibody against cytochrome P450p. This antibody, which recognized two electrophoretically distinct proteins in liver microsomes from troleandomycin-treated rats, did not inhibit testosterone oxidation by cytochromes P450a, P450b, P450h, or P450m. The catalytic turnover of microsomal cytochrome P450p was estimated from the increase in testosterone oxidation and the apparent increase in cytochrome P450 concentration following treatment of liver microsomes from troleandomycin- or erythromycin-induced rats with potassium ferricyanide (which dissociates the cytochrome P450p-inducer complex). Based on this estimate, the catalytic turnover values for purified, reconstituted cytochrome P450p were 4.2 to 4.6 times greater than the rate catalyzed by microsomal cytochrome P450p.

High affinity phenacetin O-deethylase is catalysed specifically by cytochrome P450d (P450IA2) in the liver of the rat

Phenacetin is metabolized primarily by O-deethylation to paracetamol (POD activity), a reaction catalysed by cytochrome P450. The high affinity component of POD activity is inducible in rat liver by treatment of the animals with polycyclic aromatic hydrocarbons. Following treatment with hydrocarbons such as 3-methylcholanthrene (MC) and isosafrole (ISF) both cytochromes P450c (P450IA1) and P450d (P450IA2) are also induced in rat liver. Studies with the reconstituted enzymes have shown that both forms of P450 catalyse phenacetin O-deethylation at rates that exceeded that of the high affinity component of activity of hepatic microsomal preparations from 3-methylcholanthrene-treated rats (at 4 microM phenacetin: P450c, 440 +/- 40 pmol/nmol/min; P450d, 1030 +/- 10 pmol/nmol/min; microsomal fraction, 163 pmol/mg/min). Specific inhibitory antibodies (both monoclonal and monospecific polyclonal) were used to define the specificity of microsomal POD activity. These studies have shown that hepatic high affinity POD activity is exclusively catalysed by cytochrome P450d in both untreated rats and in rats pretreated with MC.

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.

Organic solvents as modifiers of aldrin epoxidase in reconstituted monooxygenase systems and in microsomes

To examine the response of individual cytochrome P-450 species catalysing the epoxidation of aldrin (Wolff T and Guengerich FP, Biochem Pharmacol 36: 2581-2588, 1987), monooxygenase systems reconstituted from these species were assayed in the presence of 5% (v/v) = 0.87 M ethanol. The activity of cytochromes P-450PB-B and P-450PB-D, two enzymes inducible by phenobarbital was increased seven-fold. The activity of two other P-450 enzymes purified from these animals was either inhibited by 50%, as observed for cytochrome P-450PB-C or remained unchanged, as noted with cytochrome P-450PCN-E. Two P-450 enzymes purified from untreated rats, cytochromes P-450UT-F and P-450UT-H, showed an inhibition by 50 and 20%, respectively, while the activity of cytochrome P-450UT-A was slightly increased by 50%. Indirect evidence that solvents enhance aldrin epoxidation by interacting with the hemoprotein was obtained by the finding that ethanol stimulated the activity of cytochrome P-450PB-B already, before addition of the lipid component, L-alpha-1,2-dilauroyl-sn-glycero-3-phosphocholine. The Km of cytochrome P-450PB-B for NADPH cytochrome P-450 reductase was not altered by ethanol indicating that the interaction between the two enzymes was not affected by the solvent. Other results indicate that the stimulatory solvent binds to a site, apart from the type I or type II binding site. The potency of various hydrophylic solvents to modify aldrin epoxidase activity was assayed in microsomes of rats pretreated with phenobarbital and of untreated male rats. Ethanol, n-propranol, n-butanol, acetone and tetrahydrofuran enhanced enzyme activity of phenobarbital pretreated rats to a maximal extent of two-fold and, at similar concentrations, inhibited the enzyme activity of untreated rats by 50%. The potency of these solvents correlated with their lipophilicity. Methanol and dimethylsulfoxide only slightly modified the activity of induced and noninduced animals. In the presence of 0.5 M n-propranol as the modifying agent, microsomal epoxidase activity of rats pretreated with pregnenolone-16 alpha-carbonitrile, dexamethasone, 3-methylcholanthrene and of control rats was inhibited by 60-80%, whereas the activity of animals pretreated with phenobarbital, DDT, or the polychlorinated biphenyl mixture, Clophen A 50, was stimulated between two- and three-fold. The results reveal that organic solvents frequently used to dissolve monooxygenase substrates may considerably modify the activity of cytochrome P-450 dependent reactions, in particular when purified enzymes are assayed.

Characterization of a cDNA encoding a new member of the glucocorticoid-responsive cytochromes P450 in human liver

Adult human liver contains a form of cytochrome P450, termed HLp, that resembles the glucocorticoid-inducible cytochrome P450p in rat liver in its structure, function, and regulation and catalyzes the oxidation of such clinically important substrates as cyclosporin, nifedipine, erythromycin, and midazolam. Recent evidence, however, suggests that HLp may represent two or more closely related forms of cytochromes P450, one of which is termed P450nf. To search for additional members of the Class III human subfamily of HLp related genes, we screened a human liver cDNA library cloned in phage vector lambda gt11 with oligonucleotides and with a cDNA fragment related to HLp. We isolated a full-length cDNA (1709 nucleotides) encoding a new form of human cytochrome P450 termed HLp2. Analysis of HLp2 cDNA predicted a protein of 502 amino acids, weighing 57,294 Da 83% similar to HLp. HLp2 appears to represent a distinct gene as judged by partial sequence analysis of a cloned human gene and by hybridizations of Southern blots, under conditions of varying stringency, with a 3'-portion of HLp cDNA and with an oligonucleotide specific for HLp2. Northern blot analysis revealed that HLp/P450nf was present in all samples of liver mRNA from adult patients not treated with inducers of HLp, whereas HLp2 mRNA was undetectable in more than two-thirds. Human fetal liver RNA contained mRNA species 2.1 and 1.9 kb which hybridized with an HLp2 oligonucleotide. We conclude that HLp2 represents a third member of the Class III glucocorticoid-responsive gene family that is expressed in both fetal and adult human liver and may account for polymorphism in metabolism of clinically important drugs.

Human fetal and adult liver metabolism of ethylmorphine. Relation to immunodetected cytochrome P-450 PCN and interactions with important fetal corticosteroids

The N-demethylation of ethylmorphine was studied in liver microsomes from human fetuses and adult patients as well as from human fetal adrenals and kidneys. Unexpectedly the reaction was catalysed at the same rate in fetal (42.3-1277.4 pmol/mg/min in 11 individuals) and adult microsomes (414-1617.8 pmol/mg/min in two individuals), which also had similar values of the apparent Km (1.50, 1.72 mM respectively) and Vmax (1.33, 1.81 nmol/mg/min respectively) in studies of the enzyme kinetics. There was a close correlation (r = 0.96) between the semiquantitative immunoblotting assessment of cytochrome P-450 HL-p in fetal liver microsomes (with the use of a monoclonal antibody against pregnenolone-16-alpha-carbonitrile induced rat hepatic cytochrome P-450) and the catalytic activity. The fetal adrenal microsomal N-demethylation was only 11-30% of the hepatic activity when compared within three fetuses in which such a comparison was possible. No activity was measurable in the kidneys. Two drugs that are believed to be substrates of the cytochrome P-450 HLp were tested as inhibitors of the ethylmorphine N-demethylation in human fetal and adult liver microsomes and in rat liver microsomes. Midazolam was a potent inhibitor (100% at 0.4 mM) of the reaction in all specimens, whereas cyclosporin A inhibited the reaction clearly only in adult liver microsomes. Endogenous steroids of importance in the fetal circulation were also tested as inhibitors. Progesterone and dehydroepiandrosterone inhibited the reaction by 75-80% at a concentration of 0.4 mM, whereas pregnenolone and 17-alpha-hydroxyprogesterone were almost devoid of inhibitory potency. These results are of interest in the discussion about the physiological role of the human fetal cytochrome P-450 HLp which has an unprecedented relative abundance in the liver.

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.

Oxidative N-demethylation of N,N-dimethylaniline by purified isozymes of cytochrome P-450

The metabolism of N,N-dimethylaniline (DMA) by rabbit liver microsomes results in the formation of N-methylaniline (NMA) and formaldehyde. The N-oxide of DMA (DMA N-oxide) has been suggested as an intermediate in the cytochrome P-450-catalyzed demethylation reaction. The role of DMA N-oxide as an intermediate in demethylation has been investigated in a reconstituted system consisting of NADPH-cytochrome P-450 reductase, phospholipid, and several different purified isozymes of cytochrome P-450. The abilities of several cytochrome P-450 isozymes from rabbit liver (P-450 form 2 and P-450 form 4) and rat liver (P-450b and P-450c) to catalyze N-oxide formation and their abilities to catalyze demethylation of the N-oxide were determined and compared with their abilities to catalyze the demethylation of DMA. The metabolism of DMA by the purified isozymes of cytochrome P-450 in the reconstituted system did not result in the formation of measurable amounts of the N-oxide. The turnover numbers for the metabolism of DMA and DMA N-oxide to formaldehyde by the reconstituted system containing cytochrome P-450 form 2 were 25.6 and 3.4 nmol/min/nmol cytochrome P-450, respectively. The three other isozymes (P-450 form 4, P-450b, and P-450c) also exhibited significantly greater rates for the demethylation of DMA than for the N-oxide. If the N-oxide were an intermediate in the demethylation reaction, it should be metabolized at a rate greater than or at least equal to DMA. Therefore, these data, along with the inability to detect N-oxide formation during the cytochrome P-450-catalyzed demethylation of DMA, suggest that the N-oxide of DMA is not an intermediate in demethylation of DMA by these forms of cytochrome P-450 and that DMA N-oxidase activity is not associated with these isozymes.

Induction of cytochrome P450IA1 in mouse hepatoma cells by several chemicals. Phenobarbital and TCDD induce the same form of cytochrome P450

The mouse hepatoma cell line Hepa-1 was studied for aryl hydrocarbon hydroxylase (AHH) inducibility by sixteen compounds known to be inducers of cytochrome P450 of different "classes". Both 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and sodium phenobarbital induced AHH activity. A cytochrome P450IA1-specific (P1-450) mouse cDNA probe was used to quantitate mRNA induction. There was a good correlation between the amount of cytochrome P450IA1 mRNA induced and AHH activity. Immunoblots with monoclonal antibody 1-7-1, which recognizes rat liver P450IA1 and P450IA2 (P450c and P450d, respectively), showed that both phenobarbital and TCDD increase the amount of a P450 isozyme immunorelated to P450IA1 in this cell line. Hepa-1 mutants with no AHH inducibility (no functional P450IA1 structural gene; no Ah receptor; no nuclear translocation of the inducer-receptor complex; and presence of dominant repressor) did not respond to phenobarbital. The cytosolic receptor for TCDD (Ah receptor) was characterized to see if phenobarbital induced cytochrome P450IA1 mRNA and the hydroxylase enzyme through the same mechanism as TCDD. 20 mM Phenobarbital almost completely abolished the binding of 3H-TCDD to the cytosolic receptor. These data indicate that phenobarbital can be a weak ligand for the Ah receptor and thus induce cytochrome P450IA1 and AHH activity. The observation increases the list of different P450 forms inducible by phenobarbital.

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)

Differences between induction effects of 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene and phenobarbitone

The inductive effects of phenobarbitone (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) were compared in C57BL/6J mice. Induction parameters included six substrates: ethylmorphine (EM), benzphetamine (Bph), biphenyl, ethoxycoumarin (EtoC), pentoxyresorufin and dichloro-p-nitroanisole (DPNA). In order to validate this descriptive approach the comparison was extended to diazepam, rifampicin, warfarin, and pregnenolone-16 alpha-carbonitrile (PCN). All inducers were clearly distinguishable from each other. Warfarin was similar to PB, rifampicin was similar to PCN. TCPOBOP differed significantly from PB in relative liver weight, cytochrome P-450 content of liver microsomes, EM-, Bph- and DPNA-demethylations, biphenyl-hydroxylations, EtoC de-ethylation and absorption maximum of reduced CO-cytochrome P-450. TCPOBOP, as an inducer, was less "specific" than PB: total metabolic rates were excessively increased due to microsomal protein (1.5 times) and cytochrome P-450 (4 times) augmentation, whereas cytochrome P-450-related metabolic rates were less increased than those after PB. Thus TCPOBOP does not seem to be as similar to PB as was suggested in the first description of its inducing potency.

Developmental aspects of P450IIIA: prenatal activity and inducibility

We have used antiserum of defined specificity as well as a specific inducers and inhibitors of P450IIIA1(2) to determine the fetal occurrence and inducibility of this enzyme in rats. Apparently absent from uninduced fetal rat liver (or present in extremely low amounts) cytochrome P450IIIA1(2) becomes increasingly inducible as a function of gestational age. In adult rats, it is now apparent that there are at least two inducible members and one male-specific constitutive member of the IIIA subfamily. The ontogenesis of these enzymes from 2 weeks post partum to puberty has also been determined. The male-specific occurrence of P450IIIA2 subject to testosterone imprinting and maintenance has been proposed. Inconsistencies persist, however. Waxman et al. have proposed the perinatal occurrence in male and female rats with subsequent suppression in females, whereas others have not detected P450IIIA1(2) in uninduced perinatal rat liver. These differences remain unresolved and reflect the difficulties in defining the individual enzyme specificities for various substrates and of antiserum reactivity. Approaches recently applied to investigations of the IIB subfamily of cytochromes P-450 should contribute greatly to the elucidation of factors governing the ontogenesis of IIIA in rats and humans. Recently, cDNA probes capable of discriminating P450IIB1 and P450IIB2 (commonly referred to as P450s b and e, respectively) were utilized to discriminate the developmental regulation of these immune cross-reactive enzymes. cDNA probes specific for the constitutive and inducible P450IIIA enzymes should clarify the P450IIIA ontogeny in rats. However, in light of regulatory differences among the human and rat members of P450IIIA, it is apparent that the extrapolation of human biotransforming potential from results of animal models must be approached with great caution.

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)

Enhanced ethanol-induced changes in disposition and toxic response to dietary aflatoxin B1 due to Sacoglottis gabonensis bark extract, a Nigerian alcoholic beverage additive

The disposition of, and toxic response, to, dietary aflatoxin B1 (AFB1) was investigated in rats ingesting small doses of Sacoglottis gabonensis bark extract, ethanol, or both, in the drinking-water. Ingestion of ethanol alone or with the bark extract for 8 days resulted in a significant reduction in the level of AFB1 bound to serum albumin, but the level of unbound aflatoxin in the serum was significantly depressed only by concurrent ingestion of ethanol and the bark extract. In contrast, the bark extract alone or with ethanol significantly enhanced AFB1 binding to hepatic DNA. As with serum aflatoxin, concurrent ingestion of ethanol and the extract caused the most pronounced effect, suggesting synergism. All three treatments interfered with both the daily excretion pattern, and level, of aflatoxin in the urine. All three treatments enhanced AFB1-induction of liver gamma-glutamyltranspeptidase activity, suggesting potentiation of toxic response to AFB1. These data suggest that addition of the bark extract to alcoholic beverages may affect the biological response to dietary AFB1.

Human liver microsomal steroid metabolism: identification of the major microsomal steroid hormone 6 beta-hydroxylase cytochrome P-450 enzyme

Cytochrome P-450-dependent steroid hormone metabolism was studied in isolated human liver microsomal fractions. 6 beta hydroxylation was shown to be the major route of NADPH-dependent oxidative metabolism (greater than or equal to 75% of total hydroxylated metabolites) with each of three steroid substrates, testosterone, androstenedione, and progesterone. With testosterone, 2 beta and 15 beta hydroxylation also occurred, proceeding at approximately 10% and 3-4% the rate of microsomal 6 beta hydroxylation, respectively, in each of the liver samples examined. Rates for the three steroid 6 beta-hydroxylase activities were highly correlated with each other (r = 0.95-0.97 for 25 individual microsomal preparations), suggesting that a single human liver P-450 enzyme is the principal microsomal 6 beta-hydroxylase catalyst with all three steroid substrates. Steroid 6 beta-hydroxylase rates correlated well with the specific content of human P-450NF (r = 0.69-0.83) and with its associated nifedipine oxidase activity (r = 0.80), but not with the rates for debrisoquine 4-hydroxylase, phenacetin O-deethylase, or S-mephenytoin 4-hydroxylase activities or the specific contents of their respective associated P-450 forms in these same liver microsomes (r less than 0.2). These correlative observations were supported by the selective inhibition of human liver microsomal 6 beta hydroxylation by antibody raised to either human P-450NF or a rat homolog, P-450 PB-2a. Anti-P-450NF also inhibited human microsomal testosterone 2 beta and 15 beta hydroxylation in parallel to the 6 beta-hydroxylation reaction. This antibody also inhibited rat P-450 2a-dependent steroid hormone 6 beta hydroxylation in uninduced adult male rat liver microsomes but not the steroid 2 alpha, 16 alpha, or 7 alpha hydroxylation reactions catalyzed by other rat P-450 forms. Finally, steroid 6 beta hydroxylation catalyzed by either human or rat liver microsomes was selectively inhibited by NADPH-dependent complexation of the macrolide antibiotic triacetyloleandomycin, a reaction that is characteristic of members of the P-450NF gene subfamily (P-450 IIIA subfamily). These observations establish that P-450NF or a closely related enzyme is the major catalyst of steroid hormone 6 beta hydroxylation in human liver microsomes, and furthermore suggest that steroid 6 beta hydroxylation may provide a useful, noninvasive monitor for the monooxygenase activity of this hepatic P-450 form.

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.

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.

Effect of 17 alpha-ethynylestradiol on the induction of cytochrome P-450 by 3-methylcholanthrene in cultured chick embryo hepatocytes

This study investigated the effects of estrogens on the induction of cytochrome P-450 by polycyclic aromatic hydrocarbons in primary cultures of chick embryo hepatocytes. Exposure to polycyclic aromatic hydrocarbons, such as 3-methylcholanthrene led to 2- to 3-fold increases of cytochrome P-450. The amount of cytochrome P-450 induced by 3-methylcholanthrene was increased 40-50% when the synthetic estrogen, 17 alpha-ethynylestradiol, was also present. The rate of decay of cytochrome P-450 in the presence of cycloheximide as measured spectrophotometrically was similar in cells previously treated with either 3-methylcholanthrene or 3-methylcholanthrene plus 17 alpha-ethynylestradiol, suggesting that 17 alpha-ethynylestradiol did not affect the stability of the 3-methylcholanthrene-induced cytochrome P-450. In contrast, 17 alpha-ethynylestradiol did not potentiate the induction of cytochrome P-450 by phenobarbital-like inducers, such as 2-propyl-2-isopropylacetamide, as indicated by a lack of increase in both the content of cytochrome P-450 and benzphetamine demethylase activity. The naturally occurring estrogens, 17 beta-estradiol and estrone, and the synthetic estrogen, diethylstilbestrol, did not affect cytochrome P-450 induction by 3-methylcholanthrene, suggesting that the effect of 17 alpha-ethynylestradiol was not mediated via the estrogen receptor. We investigated whether the amount of cytochrome P-450 increased in the presence of 17 alpha-ethynylestradiol was the same or different from that induced by 3-methylcholanthrene. Treatment with 17 alpha-ethynylestradiol alone resulted in a small increase in ethoxyresorufin deethylase activity. The enzymatic activities of 7-ethoxyresorufin and aryl hydrocarbon hydroxylase, when expressed per cytochrome P-450 content, were identical in microsomes from cells treated with either 3-methylcholanthrene or the combination of 3-methylcholanthrene and 17 alpha-ethynylestradiol. The data suggest that the additional cytochrome P-450 induced by the combination of 17 alpha-ethynylestradiol and 3-methylcholanthrene was the same isozyme as that induced by 3-methylcholanthrene alone.

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.

Effects of pregnenolone-16 alpha-carbonitrile on drug metabolizing enzymes in hypophysectomized female rats

Treatment of intact and hypophysectomized female rats with pregnenolone-16 alpha-carbonitrile (PCN) resulted in a significant increase in hepatic aryl hydrocarbon hydroxylase (AHH) activity. However, the total cytochrome P-450 concentration, as measured by CO difference spectra, was increased to a greater extent in hypophysectomized rats than in intact rats. Total cytochrome P-450 was found to be 0.82 +/- 0.16 vs 2.43 +/- 0.31 nmoles/mg protein for control and PCN-treated hypophysectomized rats, respectively, and 0.68 +/- 0.23 vs 1.28 +/- 0.05 nmoles/mg protein for control and PCN-treated intact rats respectively. The concentration of metyrapone complex in microsomes from intact control and PCN-treated rats was found to be 0.4 +/- 0.11 vs 1.88 +/- 0.23 M respectively. Treatment of hypophysectomized rats with PCN resulted in an approximate 10-fold increase in the concentration of the metyrapone complex (0.42 +/- 0.15 M for control and 4.46 +/- 0.44 M for PCN-treated). Microsomal NADPH and NADPH cytochrome c reductase activities were also altered by PCN-treatment. Aminopyrine demethylase activity was stimulated approximately three-fold by PCN treatment in both intact and hypophysectomized rats. Benzphetamine demethylase activity was not significantly affected by PCN treatment. The results of these studies suggest that the absence of the pituitary gland can markedly influence PCN induction of cytochrome P-450 in the liver in female rats. PCN also differentially affects microsomal mixed-function oxidase activities associated with drug and xenobiotic metabolism.

Rat liver cytochrome P-450 isozymes as catalysts of aldrin epoxidation in reconstituted monooxygenase systems and microsomes

To explore which rat liver cytochrome P-450 species are involved in aldrin epoxidation, we have studied the catalytic activities of a series of cytochrome P-450 isozymes purified from untreated and inducer-treated Sprague-Dawley rats. Of ten cytochrome P-450 forms analyzed, seven isozymes, listed in order of decreasing activity, catalyzed aldrin epoxidation: P-450UT-A, P-450PB-C, P-450UT-H, P-450PB-B, P-450PCN-E, P-450UT-F, and P-450PB-D. P-450UT-I, P-450BNF-B, and P-450ISF-G were not very active at all. A novel aldrin metabolite, endo-dieldrin, was formed by cytochrome P-450UT-F in a 6-fold excess over dieldrin, which is the exo-isomer. The activity of aldrin epoxidase furthermore was assayed in liver microsomes from Sprague-Dawley rats of diverse physiological status and after pretreatment with various inducers resulting in a peculiar pattern of cytochrome P-450 isozymes. Untreated animals, at an age of 3 weeks, showed similar enzyme activities in both genders. During maturation, the activity of males increased by 3-fold, while the activity in females did not significantly change during this period. Pretreatment with pregnenolone-16-alpha-carbonitrile or dexamethasone strongly increased the activity in females. Pretreatment with dexamethasone did not increase the activity of males. A 50% depression of epoxidase activity was noted for males pretreated with 5,6-benzoflavone. Phenobarbital pretreatment increased the activity of females by 12-fold and of males by 2-fold. Males responded to pretreatment with polychlorinated biphenyls in a strain dependent fashion: enzyme activity was increased 2-fold in Sprague-Dawley rats but was not altered in Wistar rats. "Theoretical" values of microsomal epoxidase activity were calculated for weanling and adult Sprague-Dawley rats from turnover numbers and published data on the relative abundance of aldrin epoxidizing P-450 isozymes (Waxmann et al., Biochemistry 24, 4409, 1985). These values agreed with the activities determined. A similar statement can be made for male rats of both strains pretreated with inducers, when the ratio of enzyme activity of pretreated to control animals was used as a basis of comparison. The activity ratio of females pretreated with pregnenolone-16-alpha-carbonitrile, dexamethasone and phenobarbital, however, was much higher than the ratio calculated. Our results reveal that aldrin epoxidation is a reaction indicative of male specific and of phenobarbital-inducible cytochrome P-450 isozymes in rat liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in the constitutive forms of hepatic cytochrome P-450 in male and female adult beagle dogs

Cytochrome P-450 isoenzymes were prepared from the solubilized liver microsomes of untreated adult male and female dogs, then separated into groups by high-performance liquid chromatography (HPLC). Partial purification was also completed through DEAE-52 cellulose and phosphocellulose ion-exchange chromatography. For comparison, solubilized hepatic cytochromes P-450 were obtained from rats dosed with phenobarbital (PB), beta-naphthoflavone (BNF) or pregnenolone-16 alpha-carbonitrile. Minimal molecular masses of cytochrome P-450 subpopulations were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. HPLC and ion-exchange chromatography results suggested the presence of two or three major and several minor cytochrome P-450 subpopulations. Three distinct groups were predominant in the female and two major and two or three minor subpopulations were found in the male. One of two isoenzymes prominent in BNF-dosed rats was present in the male but was missing in the female dog; another minor canine cytochrome similar to one found in PB-dosed rats was missing from the male. These data indicate qualitative and quantitative sex-dependent differences in the constitutive cytochrome P-450 populations of the dog and suggest that HPLC analysis may be useful for the interpretation of toxicological studies where microsomal enzyme induction is suspected.

Regulation of testosterone hydroxylation by rat liver microsomal cytochrome P-450

The pathways of testosterone oxidation catalyzed by purified and membrane-bound forms of rat liver microsomal cytochrome P-450 were examined with an HPLC system capable of resolving 14 potential hydroxylated metabolites of testosterone and androstenedione. Seven pathways of testosterone oxidation, namely the 2 alpha-, 2 beta-, 6 beta-, 15 beta-, 16 alpha-, and 18-hydroxylation of testosterone and 17-oxidation to androstenedione, were sexually differentiated in mature rats (male/female = 7-200 fold) but not in immature rats. Developmental changes in two cytochrome P-450 isozymes largely accounted for this sexual differentiation. The selective expression of cytochrome P-450h in mature male rats largely accounted for the male-specific, postpubertal increase in the rate of testosterone 2 alpha-, 16 alpha, and 17-oxidation, whereas the selective repression of cytochrome P-450p in female rats accounted for the female-specific, postpubertal decline in testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity. A variety of cytochrome P-450p inducers, when administered to mature female rats, markedly increased (up to 130-fold) the rate of testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylation. These four pathways of testosterone hydroxylation were catalyzed by partially purified cytochrome P-450p, and were selectively stimulated when liver microsomes from troleandomycin- or erythromycin estolate-induced rats were treated with potassium ferricyanide, which dissociates the complex between cytochrome P-450p and these macrolide antibiotics. Just as the testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity reflected the levels of cytochrome P-450p in rat liver microsomes, so testosterone 7 alpha-hydroxylase activity reflected the levels of cytochrome P-450a; 16 beta-hydroxylase activity the levels of cytochrome P-450b; and 2 alpha-hydroxylase activity the levels of cytochrome P-450h. It is concluded that the regio- and stereoselective hydroxylation of testosterone provides a functional basis to study simultaneously the regulation of several distinct isozymes of rat liver microsomal cytochrome P-450.

Immunohistochemical localization of cytochrome P-450 in rat brain

The presence of cytochrome P-450 in rat brain was studied by immunohistochemistry, using antibodies to cytochrome P-450 purified from livers of phenobarbital- or 3-methylcholanthrene-treated rats. Immunoreactive nerves were observed only in brain sections incubated with immunoglobulin-G to 3-methylcholanthrene-induced cytochrome P-450. This immunoreactivity was abolished by preabsorption of the antibody with highly purified rat liver cytochrome P-450c, the major cytochrome P-450 isozyme induced by 3-methylcholanthrene, but was not affected by other cytochrome P-450 isozymes induced by phenobarbital, isosafrole or pregnenolone-16 alpha-carbonitrile. The most abundant concentration of nerve fibers with cytochrome P-450 immunoreactivity was observed in the globus pallidus. Immunoreactive fibers were also observed in the caudate putamen, amygdala, septum, ventromedial nucleus of the hypothalamus, medial forebrain bundle, ansa lenticularis, and ventromedial portion of the internal capsule and crus cerebri. Cell bodies with cytochrome P-450 immunoreactivity were observed in the caudate putamen and in the perifornical area of the hypothalamus. The cytochrome P-450 immunoreactive fibers in the globus pallidus and caudate putamen do not appear to emanate from cell bodies in the substantia nigra, since there was no reduction in the density of these fibers after unilateral stereotaxic electrolytic destruction of the substantia nigra (zona compacta and reticulata). Our data suggest that these striatal nerve processes are derived from cell bodies within the caudate putamen itself. The present results indicate that rat brain contains a form of cytochrome P-450 with antigenic relatedness to the hepatic 3-methylcholanthrene-inducible cytochrome P-450c.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity and cross-reactivity of monoclonal and polyclonal antibodies against cytochrome P-450E of the marine fish scup

Monoclonal antibody (MAb) 1-12-3 generated against liver cytochrome P-450E (P-450E), an aryl hydrocarbon hydroxylase of the marine fish Stenotomus chrysops (scup), reacted only with P-450E when tested in immunoblot analysis with five P-450 fractions from scup liver. This and six other MAbs against P-450E recognized purified P-450E, as well as a single band in beta-naphthoflavone (BNF)-induced scup microsomes that comigrated with authentic P-450E. Like MAb 1-12-3, polyclonal anti-P-450E reacted with P-450E but not with other scup P-450 fractions and reacted strongly with a band coincident to P-450E in BNF-treated scup microsomes. However, the polyclonal antibody (PAb) also faintly recognized additional microsomal proteins. MAb 1-12-3 recognized P-450E induced by 3,3',4,4',5,5'-hexachlorobiphenyl and by polychlorinated biphenyl mixtures in scup, and a single band induced by BNF or 3-methylcholanthrene (MC) in microsomes of other teleosts, including two trout species, killifish and winter flounder. The content of the P-450E counterpart in these fish and also in untreated scup coincided with induced ethoxyresorufin O-deethylase (EROD) activity. Induced EROD activity in scup and trout was strongly inhibited by MAb 1-12-3, further demonstrating the relationship between P-450E and induced P-450E in trout. MAb 1-12-3, two other MAbs, and anti-P-450E PAb recognized a band comigrating with P-450c in BNF-induced rat microsomes. MAb 1-12-3 also recognized purified rat P-450c. MAb 1-12-3 and anti-P-450E PAb recognized a second band of lower molecular weight than P-450c in BNF rat microsomes which may correspond to P-450d, the MC- and isosafrole-inducible rat isozyme. The results firmly establish the identity of scup P-450E, the relationship of BNF-induced P-450 in other teleosts with P-450E, and the immunochemical relationship of P-450E with rat P-450c. Furthermore, results with untreated fish suggest that effects of environmental chemicals may be detected by immunoblotting with monoclonal anti-P-450E.

Expression in Saccharomyces cerevisiae of chimeric cytochrome P450 cDNAs constructed from cDNAs for rat cytochrome P450c and P450d

Three chimeric cytochrome P450 cDNAs were constructed by replacing the central region, carboxy-terminal region, or both central and carboxy-terminal regions of cytochrome P450c cDNA with the corresponding regions of cytochrome P450d cDNA. These were inserted between the alcohol dehydrogenase I promoter and terminator of yeast expression vector pAAH5 to form expression plasmids pACDC2, pACCD1, and pACDD2. On introduction of each of these plasmids into Saccharomyces cerevisiae AH22 cells, chimeric cytochrome P450 proteins were expressed in AH22/pACDC2, AH22/pACCD1, and AH22/pACDD2 cells at the level of at least 10(5), 4 X 10(5) molecules per cell, respectively. The reduced CO-difference spectra showed that AH22/pACCD1 and AH22/pACDD2 cells contained 4 X 10(5) and 10(5) molecules per cell of the corresponding chimeric cytochrome P450 hemoproteins, designated as cytochrome P450ccd and cytochrome P450cdd, respectively. Cytochrome P450ccd exhibited higher monooxygenase activities toward 7-ethoxycoumarin, acetanilide, and benzo[alpha]pyrene than cytochrome P450c, although the substrate specificity of cytochrome P450ccd seemed to be the same as that of cytochrome P450c. Cytochrome P450cdd exhibited lower activities toward 7-ethoxycoumarin and benzo[alpha]pyrene, and a higher activity toward acetanilide as compared with those of cytochrome P450c and cytochrome P450ccd. Therefore, the substrate specificity of cytochrome P450cdd seemed to be the same as that of cytochrome P450d. These results suggest that the central one-third region of cytochrome P450c and cytochrome P450d is responsible for substrate-binding, and that the carboxy-terminal third of both cytochromes P450 plays an important role in electron transport.

Effect of dexamethasone on cytochrome P-450 mediated metabolism of 2-acetylaminofluorene in cultured rat hepatocytes

The metabolism of 2-acetylaminofluorene (AAF) to its six oxidative metabolites has been used to investigate the effect of dexamethasone on cytochrome P-450 activity in cultured rat hepatocytes. In control hepatocytes the metabolism of AAF to its 1-, 5-, 7-, 9- and N-hydroxylated metabolites rapidly declined in culture over the first 24 hr while 3-hydroxylation remained relatively constant. These activities either remained unchanged or increased slightly during the next 48 hr in culture. The addition of dexamethasone (100 nM) to the culture medium had little effect in arresting the initial decline but by 72 hr the 7-, 5- and 3-hydroxylations increased to values 2.5, 16 and 21 times the respective 24-hr values. The inductive effect of dexamethasone on the 3- and 5-hydroxylations of AAF was maximal at 100 nM whereas the 7-hydroxylation increased linearly as a function of the dexamethasone concentration up to 1 microM. Cortisol and corticosterone and the non-glucocorticoids fluoxymesterone and methyltestosterone induced a pattern of AAF metabolism resembling that in dexamethasone-treated cultures, suggesting that a range of steroids not restricted to glucocorticoids may induce multiple cytochrome P-450 isozymes via related mechanisms. Pregnenolone 16 alpha-carbonitrile induced only the 7-hydroxylation of AAF probably reflecting induction of cytochrome P-450p. While dexamethasone was a strong inducer of the 3- and 5-hydroxylations of AAF in hepatocyte culture, assay of these activities in freshly isolated cells after in vivo treatment with dexamethasone showed a strong induction of 7-hydroxylation but only small effects on 3- and 5-hydroxylations. Indeed the profile of AAF metabolism induced in culture by dexamethasone resembles more closely the profile induced by 3-methylcholanthrene in vivo. These data suggest that factors yet to be identified strongly influence the steroid-induced pattern of cytochrome P-450 gene expression.

Relationship between paracetamol binding to and its oxidation by two cytochromes P-450 isozymes--a proton nuclear magnetic resonance and spectrophotometric study

From the hepatic cytochrome P-450 isozymes b and c isolated from rats treated with phenobarbital and 3-methylcholanthrene respectively, only cytochrome P-450c was found to be active in the oxidation of paracetamol, in the presence of glutathione ultimately leading to the formation of the 3-glutathionyl conjugate. Paracetamol interacted with both cytochrome P-450b and c, as shown by difference spectrophotometry. Cytochrome P-450b was found to have a higher affinity for paracetamol than cytochrome P-450c and demonstrated a type I spectral change, whereas in the case of cytochrome P-450c a reverse type I spectral change was observed. Proton n.m.r. longitudinal relaxation rate measurements revealed that in the case of cytochrome P-450c, paracetamol was orientated with its phenolic hydroxyl group in closest proximity to the central haem iron ion. In the case of cytochrome P-450b, the acetylamino group of paracetamol most closely approached the haem iron ion.

Metabolism of n-pentane by ethanol-inducible cytochrome P-450 in liver microsomes and reconstituted membranes

The cytochrome P-450-dependent metabolism of n-pentane was studied by head-space gas chromatographic analysis of incubations with liver microsomes and reconstituted systems from rat and rabbit liver. The alkane was metabolized in liver microsomes from imidazole-treated rabbits at an apparent Vmax of 3 nmol mg-1 min-1 and a Km of 35 microM and in liver microsomes from acetone-treated rats at an apparent Vmax of 10 nmol mg-1 min-1 and a Km of 9 microM. The rate of microsomal n-pentane metabolism was enhanced fivefold by acetone treatment of rats, compared to the rate observed in liver microsomes from control rats. The reaction was inhibited in microsomes by compounds that are known to interact with the acetone and ethanol-inducible form of liver microsomal cytochrome P-450, like acetone, imidazole, ethanol and benzene. Effective inhibition was also accomplished when microsomal incubations were performed in the presence of IgG against this form of cytochrome P-450 from either rat or rabbit liver. In reconstituted membrane vesicles containing NADPH--cytochrome P-450 reductase, ethanol-inducible P-450 from rat liver (P-450j) was a fivefold more efficient catalyst of pentane metabolism than was the corresponding P-450 form from rabbit liver (P-450 LMeb). The rabbit enzyme metabolized the hydrocarbon with an apparent Vmax of 4 nmol nmol-1 min-1 and a Km of 8 microM, By contrast, phenobarbital-inducible P-450 LM2 or 3-methylcholanthrene-inducible P-450 LM4 from rabbit liver were quite ineffective catalysts of n-pentane metabolism. It is concluded that n-pentane constitutes a good substrate for cytochrome P-450 and, in particular, for the ethanol-inducible form of this hemoprotein. It is suggested that n-pentane is an important substrate for P-450 under in vivo conditions and that pentane measurements in expired air as an indicator of lipid peroxidation must be interpreted with caution.

Effect of pregnenolone carbonitrile, promethazine and antipyrine pretreatment on antipyrine metabolite formation in rats

The effect of three inducers of cytochrome P-450-mediated drug oxidations (Pregnenolone carbonitrile, promethazine and antipyrine) on antipyrine metabolite kinetics has been investigated using the urinary metabolite pattern and 14CO2 exhalation rate (CER)-time profile following [N-methyl-14C]antipyrine administration. The CER-time profiles showed the characteristic changes associated with induction, namely, increased maximum CER and decreased half-life, previously observed in phenobarbitone and beta-naphthaflavone-induced rats. Calculation of formation rate constants based on urinary recovery of 3-hydroxymethyl-, 4-hydroxy- and nor-antipyrine indicated no clear selectivity of induction by any pretreatment. However, the percentage increase of the latter two metabolites was two- to four-fold greater than for the former metabolite. The use of the metabolite ratio (3-hydroxymethylantipyrine/norantipyrine) is proposed to assess the qualitative nature of induction of antipyrine metabolism.