Separation and characterization of highly purified forms of liver microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls, phenobarbital, and 3-methylcholanthrene

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.

Isolation of a cytochrome P-450e gene variant and characterization of its 5' flanking sequences

A cytochrome P-450e gene variant has been isolated from the rat liver genomic library. It is a typical e gene clone but unique in having b-like single base substitutions at specific sites in the 5' flanking region. It also appears to have certain additional restriction sites in the introns. When compared with the cytochrome P-450b gene, the e gene has some of the repetitive motifs interrupted in the 5' flanking region. In addition, this region is characterized by the presence of alternating pyrimidine-purine stretch, steroid hormone regulatory elements, consensus eukaryotic enhancer sequence and sequences involved in general amino acid regulation.

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)

Retinol (vitamin A) inhibition of dimethylnitrosamine (DMN) and diethylnitrosamine (DEN) induced sister-chromatid exchanges in V79 cells and mutations in Salmonella/microsome assays

When the Chinese hamster cell line V79 and the tester strain of Salmonella typhimurium TA100 were treated with the precarcinogens dimethylnitrosamine (DMN) or diethylnitrosamine (DEN) in the presence of S9 mix, a dose-dependent increase of sister-chromatid exchanges (SCE) in V79 cells and His+ revertants in TA100 resulted. DMN was a far more efficient SCE inducer than DEN, while DEN was a more efficient inducer of His+ revertants than DMN. Retinol (Rol) effectively inhibited DMN and DEN induced SCE in V79 cells and His+ revertants in TA100. Concurrent treatment of V79 cells with Rol at various doses and one dose of DMN or DEN in the presence of S9 mix caused a significant reduction of SCE as compared to SCE induced by DMN or DEN without Rol. Rol inhibition of DMN-induced SCE was dose-dependent. Rol was less efficient in inhibiting DEN-induced SCE, and no consistent dose-dependent inhibition was observed. At all doses, Rol significantly inhibited DMN and DEN induced mutation frequencies in TA100. At the highest dose of Rol (40 micrograms/plate), the inhibition of DMN and DEN induced His+ revertants reached about 90% and 60%, respectively. The possibility that Rol exerts its antimutagenic activities by inhibiting certain forms of the cytochrome P-450 isoenzymes required for activation of precarcinogens such as DMN and DEN is discussed.

Evidence for distinct binding sites in the cumene hydroperoxide-dependent metabolism of benzo[a]pyrene catalyzed by cytochrome P-450

A few constitutive cytochrome P-450 isozymes in male rat liver microsomes catalyzed the metabolism of benzo[a]pyrene (BP) in cumene hydroperoxide (CHP)-dependent reactions, which produced predominantly 3-hydroxyBP and BP quinones. This process varied with the concentration of CHP. At 0.05 mM CHP, 3-hydroxyBP was the major metabolite. An increase in CHP concentration reduced 3-hydroxyBP formation but increased the level of BP quinones. This change in metabolic profile was reversed by preincubation with pyrene. Pyrene selectively inhibited quinone formation and enhanced 3-hydroxyBP formation. Naphthalene, phenanthrene and benz[a]anthracene nonspecifically inhibited total metabolism. BP binding to microsomal protein correlated with quinone formation, suggesting a common precursor reactive intermediate. BP metabolism by female rat liver microsomes also depended on CHP concentration but was much less effective than that in the male. With females, quinones were the major metabolites at all CHP concentrations, and their formation was again modulated by pyrene. These data indicate that two distinct binding sites are responsible for the formation of 3-hydroxyBP and BP quinones.

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.

A simple, non-chromatographic procedure to purify immunoglobulins from serum and ascites fluid

A simple, two-step procedure to purify the immunoglobulin G (IgG) fraction from mammalian sera and ascites fluid is described. In the first step, albumin and other non-IgG proteins are precipitated with caprylic acid (octanoic acid). In the second, the IgG fraction is precipitated with ammonium sulfate. Factors influencing the precipitation of serum proteins by caprylic acid are described, as are procedural modifications to purify the IgG fraction from sera with a high lipid content. The procedure can be used to purify the IgG fraction of serum from rabbit, sheep, goat, horse, rat and mouse, as well as monoclonal antibodies from mouse ascites fluid. Greater than 80% of the IgG in rabbit serum could be isolated by this procedure, with a purity equal to rabbit IgG purified by anion-exchange chromatography. In addition to its simplicity and low cost, the procedure described offers several advantages over other methods to purify IgG.

Distributions and properties of cytochromes P-450 and cytochrome P-450 reductase from rat colon mucosal cells

Cytochrome P-450 reductase and cytochrome P-450 fractions have been separated and partially purified from colonic mucosal microsomes of rat pretreated with phenobarbital or beta-naphthoflavone. Colonic cytochrome P-450 reductase has a molecular weight of 76,000. The Km values of colonic cytochrome P-450 reductase for the artificial electron acceptors cytochrome c, ferricyanide, and dichlorophenolindophenol and the electron donor NADPH are 6, 50, 11 and 11 microM, respectively. Immunochemical techniques identified the presence of beta-naphthoflavone Forms 1, 4 and 5 after beta-naphthoflavone treatment but beta-naphthoflavone Forms 1 and 4 and phenobarbital Form 1 after phenobarbital treatment.

Further characterization of the polycyclic aromatic hydrocarbon binding properties of the 4S protein

A 4-S protein which specifically binds [3H]benzo(a)pyrene and other polycyclic aromatic hydrocarbons has been investigated in the rat using a hydroxylapatite assay and sucrose gradient analysis. Although there was significant interanimal variation, the specific polycyclic aromatic hydrocarbon binding activity appeared to be highest in 4-week-old male rats and declined with age. The specific [3H]benzo(a)pyrene binding activity was induced after pretreatment with either phenobarbital or isosafrole as evidenced by a 72 and 61% increase, respectively, over untreated controls. No apparent increase in specific binding activity was observed after pretreatment of animals with 3-methylcholanthrene. Pretreatment with either phenobarbital or isosafrole also resulted in the appearance of a small, nonspecific, benzo(a)pyrene binding peak at the 8- to 9-S region in the sucrose density gradients. This 8-S peak was not seen in untreated control animals and represented low affinity, high capacity binding sites. In contrast to the 8-S protein, the 4-S binding protein had low affinity for polychlorinated aromatic compounds such as tetrachlorodibenzodioxin and tetrachlorodibenzofuran. The addition of a 200-fold excess of tetrachlorodibenzofuran to incubations did not displace [3H]benzo(a)pyrene from the 4-S protein. The addition of sodium molybdate to isolation buffers, known to stabilize certain hormone receptors, did not alter the sedimentation coefficient or the specific binding activity of the 4-S protein. These experiments indicate that the 4-S protein does not appear to be a subunit of the 8-S protein. We conclude that in the rat the 4-S protein is distinct from the 8-S protein and the 4-S species may regulate the polycyclic aromatic hydrocarbon-induced expression of aryl hydrocarbon hydroxylase activity.

Effects of hypophysectomy and growth hormone treatment on sex-specific forms of cytochrome P-450 in relation to drug and steroid metabolisms in rat liver microsomes

Hypophysectomy decreased the content of a male specific cytochrome P-450, P-450-male, in male rats, while it expressed P-450-male and completely depressed a female specific cytochrome P-450, P-450-female, in female rats. Intermittent injections of human growth hormone (GH), which mimic the secretion in males, restored P-450-male in hypophysectomized (Hypox) male rats and partially restored P-450-female in Hypox female rats. Continuous infusion of GH, which mimics the female secretion pattern, into Hypox male rats caused a further decrease in P-450-male content, and it caused the expression of P-450-female. In Hypox female rats, the same treatment depressed P-450-male and expressed P-450-female to the level of an intact female. These results indicate that the diurnal changes in the pattern of serum growth hormone level regulate the expression of P-450-male and P-450-female. The activities of testosterone 2 alpha- and 16 alpha-hydroxylases were closely correlated to P-450-male content with the correlation coefficients (r) of 0.955 and 0.929, respectively. Benzo(a)pyrene hydroxylation was also correlated to P-450-male content (r = 0.850). Aminopyrine N-demethylation and propoxycoumarin O-depropylation were correlated to less extents (r = 0.692 and r = 0.720), while aniline hydroxylation and O-ethylresorufin O-deethylation were not correlated to P-450-male content. These results indicate that testosterone 2 alpha- and 16 alpha-hydroxylations are closely dependent, but the metabolism of a variety of drugs are dependent to different extents on P-450-male in rat liver microsomes.

Lack of effect of phenobarbital on hepatocellular carcinogenesis initiated by N-nitrosodiethylamine or methylazoxymethanol acetate in male Syrian golden hamsters

Subchronic toxicity and long-term tumor-promoting effects of phenobarbital (PB) were investigated in male Syrian golden hamsters. In subchronic studies, PB was administered in drinking water to 5-week-old male hamsters for periods of 8 or 16 weeks at dosage levels of 250, 500, or 1000 ppm. No significant change in the ratio of liver weight to body weight was observed at 8 weeks; however, at 16 weeks there was a dose-dependent increase in the ratio of liver weight to body weight and a significant decrease in body weight gain among animals that received PB at 1000 ppm. The effect of PB on hepatic cytochrome P-450 and P-450-dependent aminopyrine N-demethylase activity was compared in male Syrian golden hamsters, F-344/NCr rats, and B6C3F1 mice. PB enhanced cytochrome P-450 activity in all three species; however, a significant increase (p less than 0.05) in aminopyrine N-demethylase activity was observed only in rats and mice. Potentially preneoplastic hepatocellular hyperplastic foci and hepatocellular neoplasms were studied in weanling male Syrian golden hamsters that received a single ip injection of either 100 mg N-nitrosodiethylamine (DEN)/kg body wt or 20 mg methylazoxymethanol acetate (MAM)/kg body wt at 5 weeks of age, followed by administration of 500 ppm PB in drinking water that began 2 weeks after the carcinogen injection and continued to 69 weeks of age. Groups of hamsters were killed at 25, 52, and 69 weeks of age; portions of liver and other organs with gross lesions were fixed in Formalin and examined histologically. MAM was a more potent hepatocarcinogen than DEN in male Syrian golden hamsters. PB failed to promote the development of either preneoplastic hepatocellular foci or hepatocellular neoplasms (adenomas or carcinomas) in either DEN- or MAM-initiated hamsters. Also, PB had no effect on the development of nonhepatic lesions occurring either spontaneously or induced by DEN or MAM in these animals.

Expression of cytochrome P-450d by Saccharomyces cerevisiae

Rat liver microsomal cytochrome P-450d was abundantly expressed in the yeast Saccharomyces cerevisiae by using a yeast-Escherichia coli shuttle vector consisting of rat liver P-450d cDNA and yeast acid phosphatase promoter. The expressed cytochrome P-450d was immunologically crossed with rat liver P-450d. The hydroxylase activity of estra-1,3,5(10)-triene-3, 17 beta-diol was 11 nmol/min per nmol P-450d, which is comparable to that reported previously for rat liver P-450d. The expressed P-450d content was nearlyt 1% of total yeast protein as estimated from immunoblotting, hydroxylase activity and optical absorpton of the reduced CO form.

Spectral and inhibitory interactions of (+/-)-3,4-methylenedioxyamphetamine (MDA) and (+/-)-3,4-methylenedioxymethamphetamine (MDMA) with rat hepatic microsomes

Incubation of racemic methylenedioxyamphetamine (MDA) or methylenedioxymethamphetamine (MDMA) with rat hepatic microsomes, in the presence of NADPH, generated a spectrally observed inhibitory complex with cytochrome P-450. The complex inhibited product formation from MDA and MDMA as well as other P-450 dependent reactions such as benzphetamine demethylation and CO binding. In the absence of NADPH, MDMA and MDA generated type I and type IIa difference spectra, respectively, suggesting differences in their binding to the enzyme active site. The N-demethylation of MDMA was partially inhibited by methimazole suggesting involvement of the hepatic flavin-containing monooxygenase.

Induction of a high spin form of microsomal cytochrome P-448 in rat liver by 4-aminoazobenzene derivatives

Male Sprague-Dawley rats were treated with 4-aminoazobenzene derivatives or other drug metabolizing enzyme inducers such as phenobarbital, 3-methylcholanthrene and isosafrole. The expression of hepatic microsomal cytochrome P-450 of the rats, principally that of a high spin form of cytochrome P-448 (cytochrome P-448H), was assessed by a bacterial mutation test and by immunological methods. The results of the mutation test with use of Salmonella typhimurium TA9 and 3 aromatic amine substrates showed that 2-methoxyl, 3-methoxyl and 2',3-dimethoxyl derivatives of AAB and methyl derivatives of AAB such as o-aminoazotoluene, N-methyl-4-aminoazobenzene and N,N-dimethyl-4-aminoazobenzene have a large capacity for the selective induction of cytochrome P-448H. Activity of the cytochrome increased by 6 hr after an azo dye treatment, reached a maximum after 24 hr, and then declined. In contrast, 4'-methoxy-AAB has a small, and AAB has no, capacity for the cytochrome induction. The aminoazo dye-induced enzymes differ in their substrate specificities from those induced with 3-methylcholanthrene or phenobarbital, and the induced enzyme was identified to be cytochrome P-448H, as determined by an enzyme-linked immunosorbent assay and immunoblotting with use of anti-cytochrome P-448 monoclonal antibodies. These observations indicate that several methoxyl and methyl derivatives of 4-aminoazobenzene are potent and selective inducers of cytochrome P-448H in the rat.

Induction of mixed-function oxidase activity in mouse lymphoid tissues by polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbon (PAH) exposure can cause mixed-function oxidase (MFO) enzyme induction in certain tissues of various organisms. Measurement of such induction might serve as a useful bioindicator of human exposure to PAHs, provided readily obtainable human tissues can be utilized for such measurements. We have investigated the MFO activity in various lymphoid tissues of the C3H mouse as a model system and have studied the effect of systemic PAH treatment on such enzyme activity. An MFO enzyme assay was used to measure the activity of 7-ethoxyresorufin deethylase, an enzyme activity that may be specific for the cytochrome P-448 subset of MFO enzymes (those enzymes that are induced in cells or tissues following PAH administration). Intraperitoneal injection of mice with 180 mg/kg (4.6 mg) benzo[a]pyrene (BaP) or 160 mg/kg (4.0 mg) 3-methylcholanthrene (MC) produced a significant induction in MFO activity in mouse spleen S9 fractions 48 h after the injection. Induction ratios (induced activity/control activity) between 4 and 5 were seen with BaP; MC produced induction ratios of 2.5-3.0. Enzyme activity was not induced in the spleen within 16 h following BaP or MC administration. Other experiments indicated that MFO activity could be induced in thymus cells 48 h after either BaP or MC treatment. Treatment with BaP or MC did produce significant enzyme induction in the liver and lung tissues from the animals both 16 and 48 h after chemical treatment.

Monoclonal antibodies to liver microsomal cytochrome P-450E of the marine fish Stenotomus chrysops (scup): cross reactivity with 3-methylcholanthrene induced rat cytochrome P-450

Hybridomas were prepared from myeloma cells and spleen cells of BALB/c female mice immunized with hepatic cytochrome P-450E purified from the marine fish, Stenotomus chrysops (scup). Nine independent hybrid clones produced MAbs, either IgG1, IgG2b, or IgM, that bound to purified cytochrome P-450E in radioimmunoassay. Antibodies from one clone MAb (1-12-3), also strongly recognized rat cytochrome P-450MC-B (P-450BNF-B; P-450c). The nine antibodies inhibited reconstituted aryl hydrocarbon hydroxylase (AHH) and ethoxycoumarin O-deethylase of scup cytochrome P-450E to varying degrees, and inhibited AHH activity of beta-naphthoflavone-induced scup liver microsomes in a pattern similar to that in reconstitutions, indicating that cytochrome P-450E is identical to the AHH catalyst induced in this fish by beta-naphthoflavone. MAb 1-12-3 also inhibited the reconstituted AHH activity of the major BNF-induced rat isozyme. Conversely, MAb 1-7-1 to rat cytochrome P-450MC-B had little effect on AHH activity of scup cytochrome P-450E, and did not recognize cytochrome P-450E in radioimmunoassay nor in an immunoblot. Scup cytochrome P-450E and rat cytochrome P-450MC-B thus have at least one common epitope recognized by MAb 1-12-3, but the epitope recognized by Mab 1-7-1 is absent or recognized with low affinity in cytochrome P-450E. The various assays indicate that the nine MAbs against cytochrome P-450E are directed to different epitopes of the molecule. These MAbs should be useful in determining phylogenetic relationships of the BNF- or MC-inducible isozymes and their regulation by other environmental factors.

Cytochrome P-450 isozymes from the marine teleost Stenotomus chrysops: their roles in steroid hydroxylation and the influence of cytochrome b5

Two new cytochrome P-450 forms were purified from liver microsomes of the marine fish Stenotomus chrysops (scup). Cytochrome P-450A (Mr = 52.5K) had a CO-ligated, reduced difference spectrum lambda max at 447.5 nm, and reconstituted modest benzo[a]pyrene hydroxylase activity (0.16 nmol/min/nmol P-450) and ethoxycoumarin O-deethylase activity (0.42 nmol/min/nmol P-450). Cytochrome P-450A reconstituted under optimal conditions catalyzed hydroxylation of testosterone almost exclusively at the 6 beta position (0.8 nmol/min/nmol P-450) and also catalyzed 2-hydroxylation of estradiol. Cytochrome P-450A is active toward steroid substrates and we propose that it is a major contributor to microsomal testosterone 6 beta-hydroxylase activity. Cytochrome P-450A had a requirement for conspecific (scup) NADPH-cytochrome P-450 reductase and all reconstituted activities examined were stimulated by the addition of purified scup cytochrome b5. Cytochrome P-450B (Mr = 45.9K) had a CO-ligated, reduced difference spectrum lambda max at 449.5 nm and displayed low rates of reconstituted catalytic activities. However, cytochrome P-450B oxidized testosterone at several different sites including the 15 alpha position (0.07 nmol/min/nmol P-450). Both cytochromes P-450A and P-450B were distinct from the major benzo[a]pyrene hydroxylating form, cytochrome P-450E, by the criteria of spectroscopic properties, substrate profiles, minimum molecular weights on NaDodSO4-polyacrylamide gels, peptide mapping and lack of cross-reaction with antibody raised against cytochrome P-450E. Cytochrome P-450E shares epitopes with rat cytochrome P-450c indicating it is the equivalent enzyme, but possible homology between scup cytochromes P-450A or P-450B and known P-450 isozymes in other vertebrate groups is uncertain, although functional analogs exist.

Preparation and characterization of monoclonal antibodies to pregnenolone 16-alpha-carbonitrile inducible rat liver cytochrome P-450

Hybridomas were prepared from mouse myeloma cells and spleen cells derived from BALB/c female mice immunized with purified rat hepatic pregnenolone 16-alpha-carbonitrile (PCN) induced cytochrome P-450 2a/PCN-E. The monoclonal antibodies (MAbs) thus obtained were screened for binding to the purified P-450 2a/PCN-E by radioimmunoassay. Eleven independent hybrid clones produced MAbs, each of which was of a single mouse immunoglobulin subclass of the IgG1, IgG2a or IgG2b type. Each of the MAbs produced by the eleven individual hybrid clones bound strongly to P-450 2a/PCN-E as assessed by radioimmunoassay and immunoprecipitation of P-450 2a/PCN-E in Ouchterlony double-immunodiffusion plates. Of the eleven MAbs, three also bound strongly to the phenobarbital-inducible rat liver cytochrome P-450 PB-4. Thus, two classes of MAbs were obtained, one class specific for P-450 2a/PCN-E and a second class that bound to both PCN- and phenobarbital-inducible P-450 forms. The reactivities of one MAb from each class toward eight highly purified rat hepatic cytochromes P-450 were examined using solid phase enzyme-linked immunosorbent analyses. The MAb designated C2 was found to be specific for P-450 2a/PCN-E and did not cross-react with seven other P-450 forms. This MAb was shown to be an effective probe for monitoring, by Western blotting, the induction of microsomal P-450 2a/PCN-E by PCN and phenobarbital. The MAb designated C1 reacted both with P-450 2a/PCN-E and with the two major phenobarbital-inducible P-450 forms, PB-4 and PB-5. None of the MAbs was inhibitory towards P-450 2a/PCN-E-dependent aryl hydrocarbon hydroxylase, benzphetamine N-demethylase, ethoxycoumarin O-deethylase or ethymorphine N-demethylase activity, indicating that the epitopes recognized by these MAbs are not directly associated with catalytic activity. The strong reactivities of three of the MAbs with both P-450 2a/PCN-E and P-450s PB-4 and PB-5 indicate that these two structurally quite different cytochrome P-450 families share at least one common epitope. These new MAbs are additions to our library of MAbs to different cytochromes P-450 and should help further our understanding of the relationship of cytochrome P-450 phenotype and multiplicity to inter-individual differences in drug and carcinogen metabolism and sensitivity.

3-(Trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine photolabels a substrate-binding site of rat hepatic cytochrome P-450 form PB-4

Hepatic microsomes isolated from untreated male rats or from rats pretreated with phenobarbital (PB) or 3-methylcholanthrene (3-MC) were labeled with the hydrophobic, photoactivated reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID). [125I]TID incorporation into 3-MC- and PB-induced liver microsomal protein was enhanced 5- and 8-fold, respectively, relative to the incorporation of [125I]TID into uninduced liver microsomes. The major hepatic microsomal cytochrome P-450 forms inducible by PB and 3-MC, respectively designated P-450s PB-4 and BNF-B, were shown to be the principal polypeptides labeled by [125I]TID in the correspondingly induced microsomes. Trypsin cleavage of [125I]TID-labeled microsomal P-450 PB-4 yielded several radiolabeled fragments, with a single labeled peptide of Mr approximately 4000 resistant to extensive proteolytic digestion. The following experiments suggested that TID binds to the substrate-binding site of P-450 PB-4. [125I]TID incorporation into microsomal P-450 PB-4 was inhibited in a dose-dependent manner by the P-450 PB-4 substrate benzphetamine. In the absence of photoactivation, TID inhibited competitively about 80% of the cytochrome P-450-dependent 7-ethoxycoumarin O-deethylation catalyzed by PB-induced microsomes with a Ki of 10 microM; TID was a markedly less effective inhibitor of the corresponding activity catalyzed by microsomes isolated from uninduced or beta-naphthoflavone-induced livers.(ABSTRACT TRUNCATED AT 250 WORDS)

Age- and sex-related expression of cytochromes p450f and P450g in rat liver

We have previously shown that rat hepatic cytochromes P450f, P450g, P450h, and P450i possess a high degree of immunochemical and, presumably, structural relatedness. Polyclonal antibodies directed against cytochromes P450f and P450g were made monospecific by immunoabsorption against the cross-reactive proteins. The specificity of the immunoabsorbed antibodies was established by using Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays (ELISA), and immunoblots. Since factors regulating the expression of cytochromes P450f and P450g are unknown, a competitive ELISA employing the monospecific antibodies was developed to quantitate each of these isozymes in hepatic microsomes from control and treated rats. The results obtained showed that expression of cytochrome P450f is developmentally regulated in both male and female rat liver. Cytochrome P450f levels rise from less than 1% in young animals to approximately 7 and 14% of total cytochrome P450 in adult male and female rats, respectively. Cytochrome P450g is sex-specific since it is expressed only in male rat liver where it also is developmentally regulated. Levels of cytochrome P450g rise from less than 1% in 3-week-old male rats to an average value of 17% of total cytochrome P450 in 6-week-old adult animals. However, there appear to be at least two subpopulations of adult male Long Evans rats, one of which expresses low levels (less than 1%) of cytochrome P450g and the other high levels (greater than or equal to 10%). This expression appears to be independent of serum testosterone levels. Treatment of immature and adult male rats with 20 xenobiotics that are known inducers of certain cytochrome P450 isozymes revealed that cytochromes P450f and P450g are relatively refractory to induction, although Kepone appears to be a weak inducer of cytochrome P450f.

Drug metabolizing activity in rats with chronic liver injury induced by carbon tetrachloride: relationship with the content of hydroxyproline in the liver

The drug metabolizing activity of rat liver during long-term administration of carbon tetrachloride (CCl4), and its relationship with the content of hydroxyproline (Hyp) in the liver were examined. The contents of cytochrome P-450 (P-450) and cytochrome b5 (b5) and the metabolization of aniline, aminopyrine, 7-ethoxycoumarin (7-EC) and benzo(a)pyrene (B(a)P) in the microsomal fraction were examined five days after the final administration of CCl4. The contents of P-450 and b5 and the activity to metabolize the four substrates were gradually reduced as the Hyp content in the liver increased. However, aminopyrine N-demethylation and B(a)P hydroxylation, particularly the latter, was more reduced than aniline hydroxylation and 7-EC O-deethylation in the early stage of hepatic fibrosis. Such differences may be due mainly to the different P-450 subtypes affected by CCl4.

Regioselectivity in rat microsomal metabolism of benzo[a]pyrene: evidence for involvement of two distinct binding sites

The metabolic profile of benzo[a]pyrene (BP) in cumene hydroperoxide-(CHP)-dependent reaction by male rat liver microsomes was dependent on CHP concentration. At 0.05 mM CHP, 3-hydroxy-BP was the major metabolite. Increase in CHP reduced 3-hydroxy-BP formation but increased BP quinone formation simultaneously. This change in metabolic profile was reversed by preincubation with pyrene. Pyrene (PY) selectively inhibited quinone formation but enhanced 3-hydroxy-BP formation. Naphthalene (NP) had no effect on BP quinone formation but inhibited BP 3-hydroxylation. Phenanthrene (PA) and benz[a]anthracene (BA) inhibited effectively 3-hydroxy-BP formation but only slightly quinone formation. BP binding to microsomal protein correlated to quinone formation and not BP 3-hydroxylation. BP metabolism by female rat liver microsomes also depended on CHP concentration but was much less efficient than the male. Quinones were consistently predominant metabolites and their formation was also inhibited by pyrene. Our data provide evidence that regioselectivity in BP metabolism involves at least two distinct binding sites. One site recognizes the benzo region of BP in BP 3-hydroxylation and the other recognizes the pyrene region in quinone formation. The different ratios of 3-hydroxy-BP to quinone formation by male and female rat liver microsomes suggest that the two binding sites are probably located at separate cytochrome P-450 isozymes.

Molecular dimensions of the substrate binding site of cytochrome P-448

The molecular geometries of specific substrates, inhibitors and inducers of cytochrome P-448 activity were determined using computer-graphic techniques for use in defining the molecular dimensions of the substrate binding site of this enzyme. Specific substrates of cytochrome P-448 are essentially planar molecules characterised by a small depth and a large area/depth ratio. In contrast, compounds that do not serve as substrates of cytochrome P-448 are bulky, non-planar molecules characterised by small area/depth ratios and greater flexibility in molecular conformation. Specific inhibitors of cytochrome P-448 whose effect is mediated through interaction with the haem still meet the dimensional criteria for substrates indicating that they must also interact with the substrate binding-site, which is probably located in proximity to the haem. Inducers of cytochrome P-448 activity exhibit similar molecular geometries to the substrates from which it may be inferred that the cytosolic receptor associated with the induction of cytochrome P-448 activity is structurally related to the active site of the cytochrome.

Mutagenic activation of aflatoxin B1 by several forms of purified cytochrome P-450

Metabolic activation by several forms of purified cytochrome P-450 of aflatoxin B1 to a product(s) mutagenic to Salmonella typhimurium TA100 was examined. Of the 5 forms of cytochrome P-450 purified from liver microsomes of untreated and PCB-treated male rats, a constitutive form purified from untreated male rats, P-450-male, and a high-spin form of cytochrome P-450, P-448-H, from PCB-treated rats were highly active.

In vitro activation of isophosphamide and trophosphamide to metabolites mutagenic for bacteria

The ability of S9 liver fractions from uninduced rats to activate isophosphamide (IP) and trophosphamide (TP) to metabolites mutagenic for bacteria was compared to that of S9 fractions prepared from rats pretreated in vivo with three inducers of hepatic monooxygenase. Pretreatment of rats with phenobarbital (PB) and Aroclor 1254 increased IP and TP mutagenic activation by S9 fractions as compared to control and 3-methylcholanthrene (3-MC)-induced rat liver S9. Furthermore, the effect of mixed-function oxidase inhibitors, such as alpha-naphthoflavone, metyrapone and SKF 525-A on S9-mediated mutagenic activation of IP and TP was investigated. The data obtained suggest the involvement of a PB-inducible form of cytochrome P-450 in the activation of IP and TP to mutagenic species.

Changes in cytochrome P450 molecular species in rat liver in chloroform intoxication

The effect of CHCl3 on the composition of hepatic microsomal cytochrome P450 species was compared with that of CCl4 in rats pretreated with phenobarbital (PB) or 3-methylcholanthrene (3MC). The administration of CHCl3 hardly affected cytochrome P450 content in non-treated rat liver, but caused a similar degree of depletion in the content as observed after CCl4 administration in PB-pretreated rats. In the pretreatment with 3MC, the administration of CHCl3 brought about a marked decrease in the content to 24% of control after 12 hr, while CCl4 reduced the content only to one-half of control. It was demonstrated by SDS-polyacrylamide gel electrophoresis and Whatman DE-52 anion-exchange chromatography that 3MC-induced P450 species decreased with CHCl3, while it was affected little by CCl4 treatment. The activity of benzo[a]pyrene hydroxylase was altered together with the change in the content of cytochrome P450 species. The administration of CHCl3 to PB-pretreated rats caused the depletion in PB-induced P450. These findings indicate that cytochrome P450 species induced with 3MC as well as PB are highly susceptible to CHCl3 intoxication, whereas the administration of CCl4 depletes the PB-induced species without affecting the 3MC-induced species.

Characterization of two cysteine residues in cytochrome P-450scc: chemical identification of the heme-binding cysteine residue

It was found that there were only two cysteine residues in highly purified cytochrome P-450scc molecule from bovine adrenocortical mitochondria by titration with 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB) in denatured conditions. Only one cysteine residue at position 303 of cytochrome P-450scc could be specifically modified with DTNB in the native state. The resulting cytochrome P-450scc-5-thio-2-nitrobenzoic acid complex (cytochrome P-450scc-TNB) showed no distinct differences in absorption spectra, cholesterol binding, or electron transferring from adrenodoxin, compared to those of untreated cytochrome P-450scc. These observations indicated that the 303rd cysteine residue does not play a role in heme binding, cholesterol (substrate) binding or adrenodoxin binding. The other cysteine residue at 461 could be modified with DTNB only in a denatured condition. These assignments of cysteine residues were made by the subsequent S-cyanylation with KCN followed by incubation in 6 M guanidine hydrochloride at alkaline pH, which causes enhanced cleavage of peptide bonds adjacent to the cyanylated cysteine residues. Analyses of fragmented polypeptides by SDS-polyacrylamide gel electrophoresis confirmed that there were only two cysteine residues in the molecule and indicated that the cleavage rate of the peptide bond between 460 and 461 becomes high only when both cysteine residues (303 and 461) are cyanylated. These results clearly established that the 461st cysteine residue in cytochrome P-450scc plays a role as the heme fifth ligand on the basis of the general agreement that a thiolated cysteine residue coordinates to the heme iron.

Regulation of cytochrome P-450c by glucocorticoids and polycyclic aromatic hydrocarbons in cultured fetal rat hepatocytes

The actions of polycyclic aromatic hydrocarbons and glucocorticoids to regulate the synthesis of cytochrome P-450c (the major isozyme induced by polycyclic aromatic hydrocarbons) were investigated in fetal rat hepatocytes maintained in primary monolayer culture. Treatment of hepatocytes in culture with 1,2-benzanthracene resulted in a 50-fold increase in 7-ethoxycoumarin O-deethylase activity. The level of P-450c increased in the cells in a time-dependent fashion as determined by immunoelectrophoretic analysis. The inductive effect of BA was potentiated approximately 1.6- to 2.3-fold when 1 microM dexamethasone was included in the culture medium. However, dexamethasone alone had little or no effect on the induction of P-450c. The rate of synthesis of P-450c was examined by immunoisolation of the specific isozyme from total cellular proteins radiolabeled with [35S]methionine and from the protein products formed during in vitro translation of the isolated mRNA. In addition, the amount of mRNA specific for cytochrome P-450c was determined by Northern blot analysis of RNA extracted from cultured cells. The changes in the rates of synthesis and mRNA levels were found to parallel the changes in enzyme activity. The concentration of dexamethasone required to cause a half-maximal increase in P-450c content in the presence of 1,2-benzanthracene was between 10(-8) and 10(-7) M. It is concluded that glucocorticoids act synergistically with polycyclic aromatic hydrocarbons to increase the levels of P-450c expressed in the fetal rat liver, and that this action is likely mediated by the classical type II glucocorticoid receptor.

Cytochrome P-450-dependent monooxygenase activities in prenatal and postnatal human livers: comparison of human liver 7-alkoxycoumarin O-dealkylases with rat liver enzymes

Human liver samples from 17 embryos, 5 fetuses, 5 infants and 4 adults were used to investigate human liver cytochrome P-450-dependent 7-alkoxycoumarin O-dealkylase activities, and their drug-metabolizing activities were compared to those of rat livers. The O-dealkylase activities in human embryos and fetuses were very low, although detectable, similar to those in fetal rats. Both male and female rats showed a postnatal increase of hepatic O-dealkylase activities with a maximum at about 30-40 days after birth and then a decline in the activities which was marked in female rats. Adult female rats showed a marked decrease in the hepatic enzyme activity observed in the O-depropylation reaction rather than the O-demethylation and O-deethylation reactions. During the developmental period of human infants, the O-demethylase activity, but not O-depropylase activity, increased gradually. Enzymes in adult human livers metabolize the O-methyl derivative of 7-hydroxycoumarin in preference to the O-ethyl and O-propyl derivatives. The metabolic activities of human adult enzymes for 7-alkoxycoumarin resembled those in adult female rats and were quite different from those in male rats. The study demonstrated that caution must be exercised in extrapolating pharmacological results from animal to man in the field of drug metabolism.

Metabolism of 2-aminofluorene by rat small-intestine fractions: differential effect of intragastric versus intraperitoneal administration of 3-methylcholanthrene

The influence of 3-methylcholanthrene (3-MC) induction on small intestinal enzymes was studied by measuring the enzymatic characteristics of 2-aminofluorene N-hydroxylase and the activation of this aromatic amine into mutagen(s) towards Salmonella typhimurium. When injected intraperitoneally to rats, 3-MC modified neither the N-hydroxylation nor the mutagenic properties of 2-aminofluorene (2-AF) in the presence of postmitochondrial and microsomal fractions from small intestine. On the other hand, the intragastric administration of the inducer slightly increased affinity and significantly induced maximum velocity of N-hydroxylase. Moreover, the ability of the small intestinal fractions to activate 2-AF into mutagenic intermediate(s) was enhanced after intragastric application. The mode of administration seems to be crucial in the induction of small intestinal enzymes required for metabolic and mutagenic activation of carcinogenic aromatic amines such as 2-AF.

Effects of cytochrome b5 on aniline hydroxylation catalyzed by a reconstituted system containing acetone or 2,2'-bipyridine

Cytochrome b5 did not exert any effect on NADPH-dependent aniline hydroxylation in the absence of acetone or 2,2'-bipyridine, whereas cytochrome b5 exhibited a stimulatory effect on the reaction in the presence of acetone or 2,2'-bipyridine. In addition, cytochrome b5 did not have any significant effect on the cumene hydroperoxide-dependent reaction in the presence of acetone or 2,2'-bipyridine.

Metabolic activation of mutagenic heterocyclic aromatic amines from protein pyrolysates

Mutagenic heterocyclic amines are metabolized to mutagens which act directly on Salmonella typhimurium by P-448 forms of cytochrome P-450. These direct mutagens are N-hydroxylated heterocyclic amines, such as N-hydroxy-Trp-P-1, N-hydroxy-Trp-P-2, N-hydroxy-Glu-P-1, N-hydroxy-Glu-P-2, N-hydroxy-IQ, N-hydroxy-2-amino-alpha-carboline (N-hydroxy-A alpha C), and N-hydroxy-2-amino-3-methyl-alpha-carboline (N-hydroxy-MeA alpha C). The treatment of rats with polychlorinated biphenyl stimulated N-hydroxylation of heterocyclic amines about 10- to 260-fold depending on the substrates used. The N-hydroxylation activities of purified cytochrome P-448-H and P-448-L were markedly different. P-448-H, which had very low activity for benzo[a] pyrene metabolic activation, showed high N-hydroxylation activity. The activity ratio P-448-H:P-448-L was markedly different depending on the amines used. This ratio was 45, 22, 3, and 0.02, respectively, for Glu-P-1, IQ, Trp-P-2, and benzo[a] pyrene. On the other hand, N-acetylation of the heterocyclic amines was very low. Although marked species differences in the N-acetylation were observed, the activities of the heterocyclic amines were about 1/100 of that of 2-aminofluorene. N-Hydroxy-Trp-P-2 could react directly to DNA, but N-hydroxy-Glu-P-1 could not. Therefore we need to consider the presence of a further activating system in mammalian and bacterial cells. We observed that N-hydroxy-Trp-P-2 was activated by prolyl-t-RNA synthetase, but N-hydroxy-Glu-P-1 was not activated by the same system. In the bacterial cells, both N-hydroxy-Trp-P-2 and N-hydroxy-Glu-P-1 were not activated by prolyl-t-RNA synthetase. However, both hydroxylamines were activated by the acetyl-CoA-dependent mechanism in mammalian and bacterial cells. These results indicated that the O-acetylation is an important pathway for DNA damage by heterocyclic amines in chemical carcinogenesis.

Genes for cytochrome P-450 and their regulation

The capacity of the liver microsomal mixed-function oxidase system to metabolize a wide variety of exogenous as well as endogenous compounds reflects the participation of multiple forms of the terminal oxidase, cytochrome P-450, which have different broad, but overlapping, substrate specificities. Several of these isozymes accumulate in the liver after exposure of animals to specific inducing agents. Recent studies employing recombinant DNA techniques to investigate the genetic and evolutionary relatedness of various cytochrome P-450 isozymes as well as the molecular basis for the induction phenomenon are described. The conclusions from these investigations are presented in the context of the substantial body of data obtained from the characterization of specific cytochrome P-450 isozymes and from studies on the induction of specific isozymes or enzymatic activities during development or after treatment of animals with various inducing agents.

Aging affects the drug metabolism systems of rat liver, kidney, colon and lung in a differential fashion

Microsomes prepared from the liver, lungs, colon and kidney cortex of Sprague Dawley rats of ages 2, 4, 10, 24 and 78 weeks were assessed for hydroxylation activity with the substrate benzo[alpha]pyrene. Liver microsomal activity declined after reaching a peak of activity at 10 weeks. The hydroxylation of benzo[alpha]pyrene by colon, kidney and lung microsomes, however, either remained the same or decreased only slightly. During the age range examined inducibility of hydroxylation activity by beta-naphthoflavone decreased with age in liver but actually increased with age in the extrahepatic tissues. Although phenobarbital did not elicit any increases in liver, kidney or lung, it increased substantially benzo[alpha]pyrene hydroxylation activity in colon microsomes of 78 week old rats. Total cytochrome P-450 content was induced at all age groups in all tissues by beta-naphthoflavone and in all tissues except lung by phenobarbital. Induction of cytochrome P-450 in kidney by phenobarbital was only observed in 24 and 78 week old rats. These data suggest an increased role for extrahepatic activation of benzo[alpha]pyrene with aging. In contrast to total content of cytochrome P-450, the beta-naphthoflavone inducible amount of Form 5 which has a high turnover number for benzo[alpha]pyrene, declined by 55% in liver between 2 weeks and 78 weeks while it increases dramatically in all extrahepatic tissues (from 80 to 138%).

Aging modifies the expression of hepatic microsomal cytochromes P-450 after pretreatment of rats with beta-naphthoflavone or phenobarbital

The various forms of hepatic cytochrome P-450 respond differentially to aging and induction. We examined the levels of six forms of cytochrome P-450, designated as Forms 1 through 5 and Form b, as a function of age and induction. Radial immunodiffusion analysis of rat liver microsomes indicate that cytochrome P-450 Forms 1 and 2 respond to induction by beta-naphthoflavone or phenobarbital less well in aging rats than in young rats. beta-naphthoflavone is less effective in inducing Forms 3, 4, and 5 in aging rats than in young rats. Phenobarbital, however, is more effective in inducing Forms 3 and 4 in aging rats than in young rats but does not induce Form 5 in either young or aging rats. Although Form b is induced predominantly by phenobarbital, beta-naphthoflavone induces Form b moderately in aging rats. Phenobarbital induces Form b to approximately the same extent in aging rats and in young rats. In untreated rats Form 2 is the predominant form, while Forms 1 and 3 are present in moderate amounts. The results of the immunodiffusion analysis were confirmed by the resolution and partial purification of cytochromes P-450 from microsomes of aging and young rats pretreated with beta-naphthoflavone or phenobarbital. These results identify changes with age in specific forms of cytochrome P-450 as a function of the aging process in rats.