Cytochrome P3-450 cDNA encodes aflatoxin B1-4-hydroxylase

Aflatoxin B1 (AFB1), a potent hepatocarcinogen and ubiquitous dietary contaminant in some countries, is detoxified to aflatoxin M1 (AFM1) via cytochrome P-450-mediated AFB1-4-hydroxylase. Genetic studies in mice have demonstrated that the expression of AFB1-4-hydroxylase is regulated by the aryl hydrocarbon locus and suggested that different cytochrome P-450 isozymes catalyze AFB1-4-hydroxylase and aryl hydrocarbon hydroxylase activities. We have now examined lysates from mammalian cells infected with recombinant vaccinia viruses containing expressible cytochrome P1-450 or P3-450 cDNAs for their ability to metabolize AFB1 to AFM1. Our results show that cytochrome P3-450 cDNA specifies AFB1-4-hydroxylase. This is the first direct assignment of a specific cytochrome P-450 to an AFB1 detoxification pathway. This finding may have relevance to the dietary modulation of AFB1 hepatocarcinogenesis.     

Aflatoxin B1-4-hydroxylase is associated with cytochrome P3-450 in C57BL/6 mouse liver

Messenger RNA from the livers of Aroclor 1254 treated mice was used to produce a cDNA library. cDNA clones corresponding to cytochromes P1-450 and P3-450 were isolated from this library by screening with a probe for the rat cytochrome P-450c gene. Specific non-cross hybridizing probes for P1-450 and P3-450 were prepared from unique restriction fragments. The radiolabeled probes were hybridized to RNA from mice treated with a low (15 mg/kg) and high (150 mg/kg, 400 mg/kg) doses of beta-naphthoflavone. The low dose of beta-naphthoflavone was found to induce only P3-450 mRNA, whereas higher doses induced both P1-450 and P3-450 mRNA. Similarly, a low dose of beta-naphthoflavone induced aflatoxin B1-4-hydroxylase, whereas higher doses induced both aflatoxin B1-4-hydroxylase and aryl hydrocarbon hydroxylase activities. These results suggest that P3-450 mRNA codes for the cytochrome that is associated with aflatoxin B1-4-hydroxylase activity.     

The murine Ah locus. Comparison of the complete cytochrome P1-450 and P3-450 cDNA nucleotide and amino acid sequences

Mouse cytochrome P1-450 and P3-450 are most closely associated with induced aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.1) and acetanilide 4-hydroxylase activity, respectively. Full-length cDNA clones of P1-450 and P3-450 were generated from mRNA isolated from 3-methylcholanthrene-treated C57BL/6N mouse liver. P1-450 cDNA is 2620 nucleotides in length and has a coding region (base 110 to 1,675) that produces a protein with 521 residues (Mr = 58,914). P3-450 cDNA is 1,894 nucleotides in length and yields a protein with 513 residues (Mr = 58,183). P1-450 mRNA is the first reported example in mouse in which UAG is used as the termination codon. P1-450 and P3-450, both induced by polycyclic hydrocarbons and regulated by the Ah receptor, exhibit overall nucleotide and protein homology of 68, and 73%, respectively. Segments of high homology, interspersed with regions of low homology, support the hypothesis of gene conversion or unequal crossing over as possible mechanisms for divergence of these two genes. Mouse P1-450 and P3-450 cDNAs were compared with previously published data on rat P-450e cDNA and rabbit form 2 protein, corresponding to two P-450 genes from the "phenobarbital inducible" P-450 gene subfamily. Nucleotide homology between a member of either gene subfamily is about 30%, and protein homology is about 15%, suggesting that the Ah locus-associated P-450 gene subfamily diverged from the phenobarbital inducible P-450 subfamily more than 200 million years ago. An N-terminal and a C-terminal cysteinyl fragment corresponding to the regions around P1-450 Cys-158 and Cys-458, respectively, are the only two cysteinyl peptides conserved among all four proteins compared. Because of greater homology in the C-terminal conserved cysteinyl fragment between the two gene subfamilies and a greater hydrophobic pocket in the C-terminal conserved cysteinyl fragment, the data favor this cysteine as the more likely candidate for the thiolate ligand to the heme iron in the P-450 enzyme active-site.     

Topically applied nitropyrenes are potent inducers of cutaneous and hepatic monooxygenases

The inducibility of skin and liver microsomal cytochrome P-450 dependent aryl hydrocarbon hydroxylase and other monooxygenases by a mixture of nitropyrenes was assessed and compared with the parent non-nitrated compound, pyrene. A single topical application of nitropyrenes to neonatal rats resulted in highly significant induction of aryl hydrocarbon hydroxylase, ethoxycoumarin O-de-ethylase, and ethoxyresorufin O-de-ethylase activities in skin and liver after 24 hours. Inducibility of the skin and liver enzymes was 3.9-5.7 fold and 1.8-10.3 fold respectively. On the other hand, aminopyrine N-demethylase, benzphetamine N-demethylase and epoxide hydrolase activities in the liver were unaffected by topically applied nitropyrenes. Furthermore, treatment with nitropyrenes produced a 1 nm shift to the blue region in the wavelength maximum of hepatic microsomal cytochrome P-450. Topically applied pyrene produced only marginal or no effects on cutaneous and hepatic enzyme activities. Our results suggest that nitration of pyrene, a relatively ineffective enzyme inducer, produces nitropyrenes which are potent inducers of hepatic and cutaneous monooxygenases and they resemble 3-methylcholanthrene in this inducing effect.     

Monoclonal antibody-directed phenotyping of cytochrome P-450-dependent aryl hydrocarbon hydroxylase and 7-ethoxycoumarin deethylase in mammalian tissues

The distribution of cytochromes P-450 that catalyze aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase were studied with monoclonal antibody (MAb) 1-7-1 which completely inhibits these activities of a purified 3-methylcholanthrene-induced rat liver cytochrome P-450. The degree of inhibition by MAb 1-7-1 quantitatively assesses the contribution of different cytochromes P-450 in the liver, lung, and kidney microsomes from untreated, 3-methylcholanthrene- and phenobarbital (PB)-treated rats, mice, guinea pigs, and hamsters. Enzyme sensitivity to MAb 1-7-1 inhibition defines two types of cytochrome P-450 contributing to aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase. The MAb 1-7-1-sensitive cytochrome P-450 is a major contributor to aryl hydrocarbon hydroxylase in rat liver, lung, and kidney of 3-methylcholanthrene-treated rats, C57BL/6 mice, guinea pigs, and hamsters; this type is also present in lesser amounts in the extrahepatic tissues of the control and PB-treated animals, and in the lungs of the relatively "noninducible" DBA/2 mice treated with 3-methylcholanthrene. This form however makes little or no contribution to liver aryl hydrocarbon hydroxylase of control or PB-treated animals. 7-Ethoxycoumarin O-deethylase is also a function of both the MAb 1-7-1-sensitive and insensitive classes of cytochrome P-450. The ratio of the classes contributing to aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase differs in the various tissues and species and after inducer treatment. All of the 7-ethoxycoumarin O-deethylase activity in guinea pigs and hamsters is a function of cytochromes P-450 different than the MAb 1-7-1-sensitive cytochrome P-450 responsible for aryl hydrocarbon hydroxylase activity. Thus, the MAb 1-7-1 antigenically defines the type of cytochromes P-450 contributing to each reaction. Cytochromes P-450 can be viewed as paradigmatic for enzyme systems in which the nature and amount of product is regulated by multiple isoenzymic forms. Analyses using monoclonal antibodies to specific isoenzymes may thus have broad application to a variety of other complex systems which are composed of multiple isoenzymes.     

Beta-naphthoflavone induction of a cytochrome P-450 arachidonic acid epoxygenase in chick embryo liver distinct from the aryl hydrocarbon hydroxylase and from phenobarbital-induced arachidonate epoxygenase.

Cytochrome P-450-mediated arachidonic acid metabolism in chick embryo liver microsomes was increased by both Ah receptor-dependent (2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and beta-naphthoflavone) and independent (phenobarbital) P-450 inducers. Arachidonic acid epoxides and monohydroxyeicosatetraenoic acids were increased 9-12-fold. omega-1-OH arachidonic acid was also significantly increased by TCDD and beta-naphthoflavone while omega-OH arachidonic acid, the main metabolite in uninduced livers, was decreased by all three agents. The P-450s catalyzing the enhanced arachidonate metabolism in beta-naphthoflavone- and phenobarbital-treated liver were investigated in reconstituted systems containing wholly or partially purified P-450s. beta-Naphthoflavone induced formation of a 55-kDa P-450 selective for arachidonate metabolism and for epoxygenation in particular. This P-450 was purified (beta NFAA). It was found to be distinct from a 54.5-kDa beta-naphthoflavone-induced P-450 catalyzing aryl hydrocarbon hydroxylase and 7-ethoxyresorufin deethylase (designated NF1). Mean turnover numbers for arachidonate epoxygenase, aryl hydrocarbon hydroxylase, and 7-ethoxyresorufin deethylase were 11.2, 0.56, and 0.04, respectively, for reconstituted beta NFAA and 0.33, 11.8, and 2.4 for NF1. beta NFAA and NF1 also differed in chromatography elution characteristics and N-terminal amino acid sequences. Both were low spin, with carbon monoxide binding peaks at 448 nm. The phenobarbital-induced arachidonate epoxygenation was catalyzed by P-450 fractions containing the main 48- and 49-kDa phenobarbital-induced P-450s; fractions in which the 49-kDa P-450 predominated were the most active. Turnover numbers for arachidonic acid epoxygenation were not correlated with those for aminopyrine demethylation or 7-ethoxycoumarin deethylation for P-450s from phenobarbital-treated livers or with aryl hydrocarbon hydroxylase, 7-ethoxyresorufin deethylase, or 7-ethoxycoumarin deethylase for P-450s from beta-naphthoflavone-treated livers. Also, different P-450s catalyzed the epoxygenation and the omega-hydroxylation of arachidonic acid in both beta-naphthoflavone- and phenobarbital-treated livers. The findings support a physiologic role for P-450-induced arachidonate metabolism and provide a basis for a possible link between TCDD's induction of P-450 and alterations of cellular homeostasis.     

Interaction of benzanthrone with cytochrome P450: altered patterns of hepatic xenobiotic metabolism in rats

Benzanthrone, an anthraquinone dye intermediate, is commonly used for the synthesis of a number of polycyclic vat and disperse dyes. Our prior studies have shown that benzanthrone can be metabolized by rat hepatic microsomal cytochrome P450 (P450) (Biochem. Int., 18, 1989, 1237). In this study, the interaction of benzanthrone with rat hepatic microsomal P-450 and its effect on xenobiotic metabolism have been investigated. Parenteral administration of benzanthrone (40 mg/kg body weight) for 3, 7, or 21 days caused no change in the relative body weight or organ weight of rats. The levels of P450 were found to be reduced (33%-50%) in all the benzanthrone-exposed animals at all the time periods. In vitro addition of benzanthrone caused a spectral change with oxidized P450 and concentration-dependent reduction in the carbon monoxide spectrum of dithionite-reduced P450. The addition of benzanthrone to hepatic microsomes prepared from phenobarbital-treated rats resulted in spectral changes characterized by an absorbance maximum at 397 nm indicative of type I binding. In vitro addition of benzanthrone showed a concentration-dependent inhibition of hepatic aminopyrine N-demethylase (APD) and ethoxyresorufin-O-deethylase (ERD) activities with respective I50 values of 9.5 x 10(-4) and 8.0 x 10(-5) M. However, the inhibition of aryl hydrocarbon hydroxylase (AHH) even at the highest concentration of benzanthrone (10(-2) M), was of the order of only 29%. In vivo administration of benzanthrone also led to the inhibition of APD, AHH, and ERD activities at all treatment times although the magnitude of inhibition was of a lower order.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isosafrole-induced cytochrome P2-450 in DBA/2N mouse liver. Characterization and genetic control of induction

Mouse "cytochrome P2-450" is defined as that form of isosafrole-induced P-450 in DBA/2N liver most specifically correlated with isosafrole metabolism. Isosafrole pretreatment does not induce aryl hydrocarbon hydroxylase activity ("cytochrome P1-450") in C57BL/6N or DBA/2N mice, induces acetanilide 4-hydroxylase activity ("cytochrome P3-450") more than 3-fold in C57BL/6N but not in DBA/2N mice, and induces isosafrole metabolite formation more than 3-fold in both C57BL/6N and DBA/2N mice. P2-450 was, therefore, purified from isosafrole-treated DBA/2N liver microsomes having negligible amounts of contaminating P1-450 and P3-450. The apparent molecular weight of P2-450 is 55,000, and the protein appears homogeneous on sodium dodecyl sulfate-polyacrylamide gels. The Soret peak of the reduced purified cytochrome X CO complex is 448 nm. Purified P2-450, reconstituted in vitro, metabolizes acetanilide poorly and benzo[a]pyrene hardly at all. Anti-(P2-450) inhibits (90 to 100%) liver microsomal isosafrole metabolite formation, yet has no effect on aryl hydrocarbon hydroxylase, acetanilide 4-hydroxylase, biphenyl 2- or 4-hydroxylase, or 7-ethoxycoumarin O-de-ethylase activities. 3-Methylcholanthrene induces anti-(P2-450)-precipitable protein about 12-fold in C57BL/6N and 2-fold in DBA/2N liver; 2,3,7,8-tetrachlorodibenzo-p-dioxin (10 micrograms/kg), about 12-fold in both C57BL/6N and DBA/2N liver; isosafrole, more than 3-fold in both C57BL/6N and DBA/2N. Benzo[a]anthracene at maximal doses induces anti-(P2-450)-precipitable protein in C57BL/6N liver no more than 2-fold, yet is known to be a highly potent inducer of P1-450 mRNA in C57BL/6N liver. The sensitivity of the P2-450 induction process to isosafrole is inherited as an autosomal additive trait; studies of offspring from the C57BL/6N(DBA/N)F1 X DBA/2N backcross confirm involvement of the Ah locus or s closely segregating gene. In contrast, among crosses between C57BL/6N and DBA/2N, sensitivity of the P1-450 and P3-450 induction process to 3-methylcholanthrene or 2,3,7,8-tetrachlorodibenzo-p-dioxin is inherited as an autosomal dominant trait. These data suggest that, although P1-450, P2-450, and P3-450 proteins are controlled by the Ah locus, either a P-450 protein polymorphism exists between C57BL/6N and DBA/2N mice or subtle differences may exist in the interaction of various inducers with Ah receptor.     

Hepatic microsomal warfarin metabolism in warfarin-resistant and susceptible mouse strains: influence of pretreatment with cytochrome P-450 inducers

In the present paper, the heterogeneity of hepatic cytochrome P-450 isoenzymes in the mouse has been probed, using warfarin as the substrate. Both sex and strain differences in the in vitro microsomal metabolism of warfarin have been investigated in male and female warfarin-resistant HC and warfarin-susceptible LAC-grey mouse strains. Animals were either untreated or treated with the cytochrome P-450 inducers phenobarbitone, beta-napthoflavone or clofibrate. In both sexes and strains of mice, metabolism of warfarin was stereoselective in favour of the R(+) enantiomer. However, regioselectively was different in both strains and sexes of untreated animals. After pretreatment with phenobarbitone, increases in the rate of formation of 4' and 7-hydroxy R(+) and S(-) warfarin metabolites in HC mice were observed, compared with untreated animals. In LAC-grey mice increases in 4'-, 6-, 7- and 8-hydroxy R(+) and S(-) warfarin metabolites were noted, compared with untreated animals. This data indicated that different amounts or forms of cytochrome P-450s were responsible for warfarin metabolism after phenobarbitone treatment in the two strains. Pretreatment of animals with beta-napthoflavone resulted in significant decreases in the rat of R(+) warfarin metabolism in both strains and sexes of mice indicating that the beta-naphthoflavone-inducible cytochrome P-450 isoenzymes were less active in the metabolism of warfarin, as compared to the uninduced isoenzymes. In addition, the cytochrome P-450 isoenzyme composition in the two mouse strains was different after clofibrate pretreatment, as reflected in reduced levels of some warfarin metabolites and a reduced total metabolism of warfarin, consistent with the narrow substrate specificity of clofibrate-induced cytochrome P450IVA1 for fatty acid hydroxylation. Accordingly, it is clear that both the basal and xenobiotic inducible hepatic cytochrome P-450 isoenzymes in warfarin-resistant and susceptible mice are different and therefore have implications for the in vivo disposition of warfarin.     

Isolation and characterization of full-length mouse cDNA and genomic clones of 3-methylcholanthrene-inducible cytochrome P1-450 and P3-450

Polysome immuno-adsorption, with immunoglobulin G directed against two 3-methylcholanthrene-induced mouse liver cytochrome P-450 proteins, was used to enrich mRNA from 3-methylcholanthrene-treated C57BL/6N mouse liver. cDNA transcribed from the P-450-enriched mRNA was then cloned into the Okayama-Berg vector. Two cDNA classes were detected upon differential screening of the clone bank with [32P]cDNA derived from 3-methylcholanthrene-induced immuno-enriched versus control mRNA. Several representatives of these two classes were judged to be near full length by comparison with their corresponding mRNA mobilities on denaturing agarose gels. A continuous reading-frame near the 5' end of one cDNA class (P1-450) corresponds to a protein having 15 of 17 residues the same as the published N-terminal sequence of rat P-450c. A continuous reading frame near the 5' end of the other class (P3-450) corresponds exactly to the first 25 amino acids of the published N-terminal sequence of rat P-450d. The P1-450 cDNA is at least 700 bp longer than the P3-450 cDNA. Heteroduplex analysis and Southern blot hybridization demonstrate that these mRNAs share approx. 1100 bp of sequence homology. Genomic P1-450 and P3-450 clones were isolated from a gene library constructed from C57BL/6N mouse liver DNA. By heteroduplex analysis with the corresponding cDNA, the P1-450 gene spans about 6 kb and the P3-450 gene about 7 kb. The intron-exon patterns are very similar, with the second and seventh exons being much larger than the other five. The 3' terminal exon of P1-450 is about 500 bp longer than that of P3-450. These data suggest that both P1-450 and P3-450 have diverged from a common ancestral gene.     

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.     

Differential expression and function of three closely related phenobarbital-inducible cytochrome P-450 isozymes in untreated rat liver

The levels of expression of cytochromes P-450b and P-450e (both inducible by phenobarbital (PB) and differing by only 14 of 491 amino acids) in liver microsomes from untreated male rats were separately quantitated by Western blotting with a polyclonal antibody raised against P-450b that is equally effective against P-450e (anti P-450b/e). A protein with mobility identical to P-450e was detected in all microsomal samples. Microsomes from uninduced livers of individual male rats from five different strains exhibited only minor interstrain and interindividual variability in the expression of P-450e (17 +/- 5 pmol P-450e/mg microsomal protein) with the exception of the Brown Norway strain (8.5 +/- 0.5 pmol P-450e/mg). Expression of P-450b varied widely from undetectable levels (less than 2 pmol/mg) in most Sprague-Dawley rats to about 50% of P-450e levels in Fischer and Brown Norway strains. Anti P-450b/e inhibited total metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) by uninduced microsomes, to an extent dependent on rat strain (15-30%), predominantly through inhibition of formation of 12-hydroxymethyl-7-methyl BA (12HOMMBA) (65-85%), the major metabolite of purified P-450e. A specific activity for P-450e-dependent DMBA metabolism was calculated from four sets of microsomes where the P-450b content was either undetectable or very low (0.7-1.0 nmol/nmol P-450e/min-1). Comparable calculated activities were, however, obtained from other untreated rat liver microsomes where P-450b levels were significant. Polymorphism in P-450b was detected but did not affect total P-450b expression or the sensitivity of DMBA metabolism to anti P-450b/e. A fourth band of greater mobility than P-450b (apparent Mr less than 50,000), was also recognized by anti P-450b/e. The intensity of this band did not vary among individual rats or among the different strains and therefore did not correlate with the sensitivity of microsomal DMBA metabolism to anti P-450b/e. A monoclonal antibody (MAb) against P-450b (2-66-3) recognized P-450's b, b2, and e on Western blots but did not react with this higher mobility band. MAb 2-66-3 and two other MAbs produced against P-450b inhibited 12-methylhydroxylation of DMBA by untreated rat liver microsomes to the same extent as anti P-450b/e. Following PB induction, P-450b was induced to about double the level of P-450e in most rat strains examined.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanism of cytotoxicity of aflatoxin B1: role of cytochrome P1-450

A mouse hepatoma cell line, Hepa-1, is highly sensitive to the toxic effects of Aflatoxin B1 (AFB1). Half maximal survival (LD50) of cells occurs at 0.068 ug AFB1/ml. Benzo(a)anthracene, which induces aryl hydrocarbon hydroxylase and cytochrome P1-450 in Hepa-1, causes a slight increase in the toxicity of AFB1 (LD50 = 0.034 ug/ml). An aryl hydrocarbon hydroxylase- and cytochrome P1-450-deficient mutant of Hepa-1 is, however, over 100 times more resistant to AFB1 than Hepa-1. Almost no decline in survival is observed at 5 ug AFB1/ml. Cytochrome P1-450 thus effects strongly on the cytotoxicity of AFB1 in these cells. The basal activity in Hepa-1 is enough to elicit an almost full toxic effect. AFB1, although a substrate for cytochrome P1-450, does not act as an inducer of aryl hydrocarbon hydroxylase.     

Transduction of cytochrome P3-450 by retroviruses: constitutive expression of enzymatically active microsomal hemoprotein in animal cells

Cell lines were established which produce replication-defective ecotropic and amphotropic host range recombinant retroviruses containing the cDNA for mouse cytochrome P3-450 as well as the bacterial Neo gene for G418 resistance. The G418-resistant clones derived from virus-infected cultures were analyzed for the expression, subcellular localization, and catalytic activities of the cytochrome P3-450. Southern blot analysis of the genomic DNAs indicates that the viral DNA was stably integrated into the cellular DNA. Western blot analysis of the proteins showed that the size of the constitutively expressed product was Mr 54,000, indistinguishable from the cytochrome P3-450 found in mouse liver microsomes. Spectral characterization of the P3-450 proteins indicates that the newly synthesized apoprotein incorporated heme and integrated into the microsomes. Enzymatic analysis of the cell homogenates in vitro and of the dividing cells in situ showed very high acetanilide hydroxylase activity and very low aryl hydrocarbon hydroxylase activity, a diagnostic feature of the cytochrome P3-450. The precise transmission of the recombinant retroviral sequences into the target cells and the exceptional fidelity of expression of the enzyme in cells will allow the analysis of an increasing number of cloned genes of cytochrome P-450s by defining the individual enzyme specificities, their physiological role in cells, and consequences of their functional expression, such as in toxicity, mutagenesis, and carcinogenesis.     

Effect of a zwitterionic detergent on the state of aggregation and catalytic activity of cytochrome P-450LM2 and NADPH-cytochrome P-450 reductase

The zwitterionic detergent 3-(3-cholamidopropyl)-dimethylammonio-1-propanesulfonate (CHAPS) supports reconstituted cyclohexane hydroxylase activity of cytochrome P-450LM2 and NADPH-cytochrome reductase purified from phenobarbital-induced rabbit liver. Maximum activity (approximately 50% of that with phospholipid) was observed at 2 mM CHAPS. Inhibition took place at higher CHAPS, until at 20 mM CHAPS, no cyclohexane hydroxylase activity was observed. There was little denaturation of the two enzymes under these conditions. At 2 mM CHAPS, P-450LM2 was pentameric (Mr = 250,000) and reductase was dimeric (Mr = 139,500) by sedimentation equilibrium. P-450 was monomeric in 20 mM CHAPS. In addition, a stable complex between the two enzymes was not detected under conditions of maximum activity, even in the presence of saturating substrate. This confirms our previous conclusion that a stable complex between cytochrome P-450LM2 and NADPH-cytochrome P-450 reductase is not a prerequisite for reconstituted xenobiotic hydroxylation (Dean, W. L., and Gray, R. D. (1982) J. Biol. Chem. 257, 14679-14685). Difference spectra of ferric P-450LM2 revealed that below 5 mM CHAPS, the high spin form of the cytochrome was slightly stabilized, while higher CHAPS levels stabilized the low spin form. Monomeric P-450LM2 formed with 20 mM CHAPS catalyzed the hydroxylation of toluene by cumene hydroperoxide. Thus, the reason that monomeric cytochrome P-450LM2 was inactive in NADPH-supported hydroxylation may either be because the bound detergent blocked productive interaction of the cytochrome with reductase or the monomer may be intrinsically incapable of interaction with reductase.     

Expression of cytochrome P-450 isozymes in the liver of hypophysectomized rats. Evidence for different regulation mechanisms concerning P450IIB and P450IIIA subfamilies

1. Monooxygenase activities have been examined in rat liver to determine the effects of castration and hypophysectomy on cytochrome P-450 species. In adult males, hypophysectomy caused a decrease of total P-450 concentration, aniline hydroxylase, benzopyrene hydroxylase, benzphetamine demethylase, testosterone hydroxylase and imipramine hydroxylase and demethylase activities. The treatment of hypophysectomized animals with human growth hormone or testosterone did not restore the full activity. 2. When probed with antibodies, microsomes from hypophysectomized males and females exhibited an intense reaction with a polyclonal anti-(phenobarbital-induced P-450) which was not observed with a monoclonal antibody of anti-(phenobarbital-induced P-450). 3. These microsomal preparations also reacted with an antibody raised against a developmentally regulated P-450. No sex difference could be detected with this antibody. Furthermore, administration of human growth hormone to hypophysectomized males prevented this immunoreaction. 4. Total RNA has been prepared from the same liver; when probed with cDNAs, no changes occurred in the content in P-450 b/e, PB 24 (a constitutive member of the phenobarbital subfamily) and phenobarbital-inducible mRNA for UDP-glucuronosyltransferase. 5. In contrast, P-450 mRNA induced by pregnenolone 16 alpha-carbonitrile was modulated by hormonal manipulations: lower in females and castrated males than in intact males, increased in both sexes after hypophysectomy. Treatment of hypophysectomized males with human growth hormone abolished this rise in pregnenolone-16 alpha-carbonitrile-induced P-450 mRNA accumulation. Data collected in this study support the assumption that hypophysectomy acts differently on the regulation of various P-450 isozymes and that this regulation clearly does not involve the phenobarbital subfamily of P-450s.