Specificity in the activation and inhibition by flavonoids of benzo[a]pyrene hydroxylation by cytochrome P-450 isozymes from rabbit liver microsomes

The effect of flavone and 7,8-benzoflavone on the metabolism of benzo[a]pyrene to fluorescent phenols by five cytochrome P-450 isozymes obtained from rabbit liver microsomes was determined. Benzo[a]pyrene metabolism was stimulated more than 5-fold by the addition of 600 microM flavone to a reconstituted monooxygenase system consisting of NADPH, cytochrome P-450 reductase, dilauroylphosphatidylcholine, and cytochrome P-450LM3c or cytochrome P-450LM4. In contrast, an inhibitory effect of flavone on benzo[a]pyrene metabolism was observed when cytochrome P-450LM2, cytochrome P-450LM3b, or cytochrome P-450LM6 was used in the reconstituted system. 7,8-Benzoflavone (50-100 microM) stimulated benzo[a]pyrene metabolism by the reconstituted monooxygenase system about 10-fold when cytochrome P-450LM3c was used, but benzo[a]pyrene hydroxylation was strongly inhibited when 7,8-benzoflavone was added to the cytochrome P-450LM6-dependent system. Smaller effects of 7,8-benzoflavone were observed on the metabolism of benzo[a]pyrene by the cytochrome P-450LM2-, cytochrome P-450LM3b-, and cytochrome P-450LM4-dependent monooxygenase systems. These results demonstrate that the activating and inhibiting effects of flavone and 7,8-benzoflavone on benzo[a]pyrene metabolism depend on the type of cytochrome P-450 used in the reconstituted monooxygenase system.     

Induction of cytochrome P-450 forms in liver microsomes of rats in the early neonatal period after administration of phenobarbital and 3-methylcholanthrene

The activity of cytochrome P-450 dependent monooxygenase system from rat liver microsomes after induction by phenobarbital and 3-methylcholantrene in early neonatal period (3-16 days after birth) was studied. It was found that the total amount of cytochrome P-450 increases after injection of these inducers in neonatal rats of all age groups. In parallel, in the case of 3-methylcholantrene induction the benz(a)pyrene hydroxylase and 7-ethoxyresorufin deethylase activities increase; phenobarbital induction causes a rise in the benzphetamine-N-demethylase and benz(a)pyrene hydroxylase activities. Immunochemical analysis involving the use of antibodies specifically directed against cytochrome P-450 of adult rats revealed that the level of cytochrome P-450 in the case of 3-methylcholantrene induction increases from 5 to 50%, whereas that of cytochrome P-450 upon phenobarbital induction increases from 5 to 40% in liver microsomes of 3- and 16-day-old rats. The mode of inhibition of various substrates metabolism by antibodies in neonatal rat microsomes suggests that the 3-methylcholantrene-induced cytochrome P-448, like in adult rats, participates in the hydroxylation of benz(a)pyrene and O-deethylation of 7-etoxyresorufin. The participation of phenobarbital-induced cytochrome P-450 in the metabolism of benzphetamine and aldrin in neonatal rats is much lower than in the adult ones. The metabolism of benz(a)pyrene in phenobarbital-induced neonatal rat microsomes in all age groups is not inhibited by antibodies. The age-dependent differences in inhibition of metabolism and the increase in the benz(a)pyrene hydroxylase activity in phenobarbital-induced rats suggest that the spectrum of inducible forms of cytochrome P-450 in neonatal rats differ from that in adult animals.     

Isolation and characterization of multiple forms of cytochrome P-450 from liver microsomes of 3-methylcholanthrene-induced Wistar rats

Chromatography on 1.8-diaminooctyl-Sepharose and DEAE-Sephacel resulted in 4 fractions of cytochrome P-450 from liver microsomes of 3-methylcholanthrene-induced Wistar rats. All the four fractions differed in terms of their absorption maxima in the CO-reduced state, Mr and catalytic activity. Only one cytochrome fraction (cytochrome P-450 C) possessed a high activity upon benz(a)pyrene hydroxylation. All cytochrome P-450 forms were characterized by a low rate of aminopyrine N-demethylation. Antibodies against cytochrome P-450 C (P-448) (anti-P-448) were raised. Cytochromes of fractions A, B1 and B2 in the Ouchterlony reaction of double immunodiffusion did not give precipitation bands with anti-P-448. Neither of the four cytochrome P-450 forms interacted with the antibodies raised against cytochrome P-450 isolated from liver microsomes of rats induced with phenobarbital. The procedure developed is applicable to the isolation of multiple forms of cytochrome P-450 from liver microsomes of 3-methylcholanthrene-induced rats. Using rocket immunoelectrophoresis, cytochrome P-450 C possessing a high (as compared to benz(a)pyrene metabolism) activity (18 nmol/min/nmol cytochrome) and a high (60-70%) content in 3-methylcholanthrene-induced rat liver microsomes was shown to give a relatively high yield.     

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.     

Purification, composition, and some properties of rat liver carbamyl phosphate synthetase (ammonia).

Hepatic microsomes prepared from vitamin E deficient and supplemented rats were analyzed for cytochrome P-450 content and drug metabolizing activity. Reduced levels of benzo[α]pyrene hydroxylase and ethylmorphine N-demethylase activities were observed in microsomes derived from rats fed a diet deficient in vitamin E compared to those of control rats. NADPH-mediated destruction of P-450, and pentobarbital and zoxazolamine sleeping times were similar in the two groups. Induction with 3-methylcholanthrene raised the levels of benzo[α]pyrene hydroxylase activity of both supplemented and deficient rats to the same absolute levels. No differences were noted in cytochrome P-450 or P-448 content between control and tocopherol deficient rats, nor did the activity of liver catalase differ between the two dietary groups. Thus, these studies did not demonstrate any impairment of heme protein synthesis in vitamin E deficient rats.     

Effects of the hepatic S9 fraction from aroclor-1254-treated rats on the mutagenicity of benzo[alpha]pyrene and 2-aminoanthracene in the Salmonella/microsome assay.

The mutagenicity of benzo[alpha]pyrene and 2-aminoanthracene for Salmonella typhimurium TA98 in the plate-incorporation test was studied using liver S9 from untreated and aroclor-1254-treated rats. The induction of liver S9 protein, arylhydrocarbon hydroxylase (AHH), and cytochrome P448/450 was followed with time. There was no change in protein concentrations with induction; AHH and cytochrome levels were increased at 1, 3, 5 and 7 days post Aroclor treatment. Benzo[alpha]pyrene mutagenicity was enhanced with Aroclor treatment while 2-aminoanthracene mutagenicity was depressed. The benzo[alpha]pyrene mutagenicity showed a positive correlation with the levels of AHH and cytochrome on the plate; 2-aminoanthracene showed a negative correlation with activity in induced samples.     

Activation of benzo[a]pyrene and 2-aminoanthracene to bacteria mutagens by mussel digestive gland postmitochondrial fraction

The ability of the mussel postmitochondrial fraction (S9) to activate benzo[a]pyrene (BaP) and 2-aminoanthracene (2AA) to mutagenic metabolites towards Salmonella typhimurium strain TA98 was tested. The mechanisms involved in this activation were investigated and mussel cytochrome P-450-dependent monooxygenases and its NADPH cytochrome c reductase were found to contribute to the activation of BaP. This activation was improved by treating the mussel with 4,5,4′,5′-tetrachlorobiphenyl (TCB) (a 3-methylcholanthrene-type inducer of cytochrome P-450-dependent monooxygenase in marine fish) and was inhibited by α-naphthoflavone (ANF), a cytochrome P-450 inhibitor. However, both BaP activation and cytchrome P-450-related metabolic activities are much weaker in mussels than in vertebrates. Mussel S9 activates aromatic amines more effectively than BaP. Pretreatment of mussels with TCB or addition of ANF in the incubation medium has no effect on 2AA activation. As suggested by Kurelec (1985), aromatic amine metabolism may be supported by a flavoprotein mixed-function amine oxidase which is NADPH-dependent.     

Effects of the hepatic S9 fraction from aroclor-1254-treated rats on the mutagenicity of benzo[a]pyrene and 2-aminoanthracene in the salmonella/microsome assay

The mutagenicity of benzo[a]pyrene and 2-aminoanthracene for Salmonella typhimurium TA98 in the plate-incorporation test was studied using liver S9 from untreated and Aroclor-1254-treated rats. The induction of liver S9 protein, arylhydrocarbon hydroxylase (AHH), and cytochrome P448/450 was followed with time. There was no change in protein concentrations with induction; AHH and cytochrome levels were increased at 1, 3, 5 and 7 days post Aroclor treatment. Benzo[a]pyrene mutagenicity was enhanced with Aroclor treatment while 2-aminoanthracene mutagenicity was depressed. The benzo[a]pyrene mutagenicity showed a positive correlation with the levels of AHH and cytochrome on the plate; 2-aminoanthracene showed a negative correlation with activity in induced samples.     

Nonadditive effects of combined in vivo hepatic microsomal enzyme inducers in the Wistar rat

Compounds that are known to increase the hepatic microsomal cytochrome P-450 dependent monooxygenases were administered to adult female rats, alone or in combination, to determine whether their effects on certain substrate oxidations were additive. 3-Methylcholanthrene (3-MC) and pregnenolone-16 alpha-carbonitrile (PCN), known to induce different forms of cytochrome P-450, when administered together increased benzo[a]pyrene oxidation to the same level as observed following 3-MC treatment alone. Phenobarbital (Pb) and PCN when administered concomitantly increased benzo[a]pyrene, amino-pyrine, and ethylmorphine metabolism to the same extent as seen following PCN administration alone. Both compounds are known to induce different forms of cytochrome P-450. Nonadditive effects were also observed with Pb and spironolactone, as well as with Pb and trans-stilbene oxide. Treatment of adult male rats with either PCN or 3-MC resulted in significantly smaller increases in benzo[a]pyrene oxidation than observed in adult female rats. These results suggest that oxidative metabolism in hepatic microsomes is not the sum of activities of a number of cytochrome P-450s, but may represent the activity of a single predominant hemeprotein. In addition, it appears that the oxidation of substrate by a particular cytochrome P-450, in intact microsomes, is greatly influenced by the presence of another form.     

Purification and characterization of seven distinct forms of liver microsomal cytochrome P-450 from untreated and inducer-treated male Wistar rats

A total of nine forms of cytochrome P-450 were purified to homogeneity from liver microsomes of male Wistar rats. They were P-451 I and P-451 II from untreated rats, P-450 II and P-450 III from phenobarbital-treated rats, MC-P-448 L and MC-P-448 H from 3-methylcholanthrene-treated rats, and P-452, P-448 L, and P-448 H from 3,4,5,3',4'-pentachlorobiphenyl-treated rats. Among them, MC-P-448 L and MC-P-448 H were indistinguishable from P-448 L and P-448 H, respectively, with regard to electrophoretic, spectral, catalytic and immunochemical properties, and thus seven forms were distinct hemoproteins. The minimal molecular weight of each form was as follows: P-451 I (49,000), P-451 II (52,000), P-450 II (52,000), P-450 III (53,500), P-452 (48,000), P-448 L (56,000), P-448 H (54,000). Judging from the oxidized absolute spectra, P-448 H was a high-spin form and the others were of low-spin type. In a reconstituted system, N-demethylations of benzphetamine and aminopyrine were catalyzed by most of the forms at comparable rates. On the other hand, the activities for the oxidations of benzo[a]pyrene, 7-ethoxycoumarin, biphenyl, and estradiol-17 beta varied greatly among the forms of cytochrome P-450. The most efficient catalysts were as follows: P-448 L and P-451 II for benzo[a]pyrene 3-hydroxylation; P-448 L for 7-ethoxycoumarin O-deethylation; P-448 L, P-451 II, and P-448 H for biphenyl 4-hydroxylation; P-448 L and P-448 H for biphenyl 2-hydroxylation; and P-451 II and P-448 H for estradiol 2-hydroxylation. P-451 I, P-450 II, and P-450 III were somewhat poorer catalysts in metabolizing all the substrates except for benzphetamine and aminopyrine, but their substrate specificities were still distinguishable from one another. Of all the purified cytochrome P-450's, P-452 showed the least ability to metabolize all the substrates. Judging from the properties, it appears that six forms in male Wistar rats correspond to the distinct forms of cytochrome P-450 in Long-Evans and/or Sprague-Dawley rats reported by other workers, but P-451 I is a new constitutive isozyme in Wistar rats.     

Mixtures of polycyclic aromatic compounds inhibit mutagenesis in the Salmonella/microsome assay by inhibition of metabolic activation

We observed that complex mixtures of aromatic compounds isolated from a coal-derived oil suppressed the mutagenic activity of the indirect mutagens benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, 2-aminofluorene, and 2-acetylaminofluorene as measured in the Salmonella/microsome mutagenicity assay, using strain TA98 and metabolic activation with Aroclor-induced rat-liver S9 or microsomes. The mixture also inhibited S9-dependent benzo[a]pyrene metabolism and covalent binding to DNA in a cell-free system. The mixture did not suppress the activity of either the direct acting mutagens 2-nitrofluorene and benzo[a]pyrene diol-epoxide, or of the indirect mutagen N-hydroxy-2-acetylaminofluorene which requires a microsomal deacetylase for metabolic activation. Spectrophotometric measurements showed that components of the mixture bound to microsomal cytochrome P-450. The mixture did not inhibit microsomal NADPH-cytochrome c (P-450) reductase. These observations show that the mixtures inhibited metabolic activation by the microsomal monooxygenase system, probably by binding of unidentified components to cytochrome P-450. The resulting inhibition of mutagenesis may have implications for risk estimates for the mixtures we examined as well as for other types of complex mixtures for which similar inhibitory effects have been observed.     

Catalytic activity of isolated cytochromes P-450d and P-450c

Cytochrome P-450d was isolated from isosafrol-induced rat liver microsomes by affinity chromatography on 1.8-diaminooctyl-Sepharose 4B and chromatography on hydroxylapatite using a linear potassium phosphate gradient (45-250 mM). The enzyme has a molecular mass of 54 kDa, CO-maximum 448 nm is characterized by a high spin state; the rate of 4-aminobiphenyl hydroxylation is 54 nmol/min/nmol of cytochrome P-450d (37 degrees C), those, of 7-ethoxyresorufin O-deethylation and benz (a) pyrene oxidation are 1 nmol/min/nmol of cytochrome P-450d (22 degrees C) and 2 nmol/min/nmol of cytochrome P-450d (37 degrees C), respectively. The properties of cytochrome P-450d were compared to those of cytochrome P-450c isolated from 3-methylcholanthrene-induced rats. The yield of these cytochromes under the conditions used (10% P-450d from isosafrol-induced microsomes and 15% P-450c from 3-methylcholanthrene-induced microsomes) was relatively high. Antibodies to cytochromes P-450d and P-450c were obtained. Using rocket immunoelectrophoresis the percentage of these hemoprotein forms in 3-methylcholanthrene-induced (P-450d-20%, P-450c-70%) and isosafrol-induced rat liver microsomes (P-450d-50%, P-450c-15%) was determined.     

Host-mediated mutagenicity experiments with benzo[a]pyrene and two of its metabolites

Benzo[a]pyrene (BP) and two of its major metabolites, the ultimate mutagen BP-4,5-oxide and the proximate mutagen trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-diol) were investigated for mutagenicity in Salmonella typhimurium TA1538, TA98 and TA100 using an intrasanguineous host-mediated assay. BP and BP-4,5-oxide were not mutagenic under any experimental conditions. BP-7,8-diol was inactive with the strain TA1538 but was mutagenic with the strains TA98 and TA100. The effect was potentiated by pretreatment of the host mice with the cytochrome P-450 inducer 5,6-benzoflavone. We conclude: (i) one of the reasons for the observed insensitivity of the intrasanguineous host-mediated assay towards BP is that BP-4,5-oxide, which contributes to the microsome-mediated mutagenicity of BP, is inactive in the host-mediated assay; (ii) the finding that BP-7,8-diol is mutagenic in the host-mediated assay demonstrates that the lack of mutagenicity of BP is not intrinsic; (iii) the potentiated mutagenicity after treatment of the hosts with 5,6-benzoflavone suggests that cytochrome P-450 is more important in the activation of BP-7,8-diol in this system than other enzymes (e.g. prostaglandin synthase) that can also activate this compound in vitro.     

Electron paramagnetic resonance studies on hepatic microsomal cytochrome P-450 from a marine teleost fish

Hepatic microsomal cytochrome P-450 from fish (Stenotomus versicolor), untreated or treated with 3-methylcholanthrene, 5, 6-benzoflavone, or tricaine methanesulfonate, exhibited an absorption maximum at 450 nm when reduced and ligated to CO. Microsomes from all groups exhibited EPR spectra with g values near 2.4, 2.24 and 1.9, yielding crystal field parameters similar to those for cytochrome P-450 from a variety of other sources. Treatment with 3-methylcholanthrene or 5, 6-benzoflavone resulted in elevated levels of aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity yet produced no apparent change in the levels or optical properties of CO-ligated cytochrome P-450. Tricaine methanesulfonate, a common fish anaesthetic, caused a decrease in the levels of fish cytochrome P-450.     

Purification and properties of cytochrome P-450 generally acting as a catalyst on benzo[a]pyrene hydroxylation from liver microsomes of untreated rats

A form of cytochrome P-450 generally catalyzing benzo[a]pyrene (B[a]P) hydroxylation was purified from liver microsomes of untreated rats on the basis of the catalytic activity. The purification procedures consisted of cholate solubilization and chromatography in 3 steps, on DEAE-Toyopearl (at room temperature), hydroxylapatite, and CM-Toyopearl columns. Cytochrome P-450 purified in this way (named P-450/B[a]P) was homogeneous on SDS-polyacrylamide gel electrophoresis, and the molecular weight was estimated to be 51,000. The absorption spectra of the oxidized form of P-450/B[a]P showed a Soret peak at 417 nm, characteristic of low-spin hemoprotein, and the Soret peak of the reduced cytochrome P-450-CO complex was at 451 nm. Immunochemical analysis of P-450/B[a]P indicated that P-450/B[a]P is immunologically distinct from P-450b (a major phenobarbital-inducible form of P-450) and P-450c (a major 3-methylcholanthrene-inducible form of P-450, which highly catalyzes the hydroxylation of B[a]P). B[a]P hydroxylase activity in liver microsomes of untreated rats was inhibited to about 20% by the P-450/B[a]P antibody. These results demonstrate that P-450/B[a]P is a different form of P-450 from P-450b and P-450c, and generally catalyzes B[a]P hydroxylation in liver microsomes of untreated rats.     

Immune response to benzo(a)pyrene in rabbits immunized with a conjugate of benzo(a)pyrene-albumin, synthesized in the microsomal monooxygenase system of the liver

The feasibility of obtaining a conjugated benz(a)pyrene-protein antigen in the liver cytochrome P-450 system was studied. Covalent binding of benz(a)pyrene (BP) to albumin was performed with the use of liver microsomal fractions of 3-methylcholanthrene-induced rabbits. It was demonstrated that BP oxidation in liver microsomes is accompanied by covalent binding of [14C]BP to exogenous rabbit albumin. Immunization of rabbits with the obtained conjugate results in the development of a specific immune response to BP, and the appearance of specific antibodies and lymphocytes specifically binding [14C]BP in the blood.     

Changes in the ratio of microsomal electron carriers in reconstituted microsomal membranes

The reconstitution of microsomal membrane monooxygenase system with variable contents of the hydroxylating chain enzymatic components was carried out. It was found that during self-assembly of microsomal membranes solubilized with 4% sodium cholate and gel filtration through Sephadex LH-20 in the presence of isolated microsomal enzymes, two forms of cytochrome P-450, i. e. phenobarbital- and 3-methylcholantrene-induced ones, and NADPH-cytochrome P-450 reductase, the exogenous enzymes are incorporated into the microsomal membrane matrices of control and methyl-cholantrene-treated animals. In the membranes reconstituted from the microsomes of the methylcholantrene-induced animals the catalytic activity of cytochrome P-448 in the metabolism of benz(a)pyrene at varying cytochrome P-448 and NADPH-cytochrome P-450 reductase contents were investigated.     

Preferential activation of 6-aminochrysene and 2-aminoanthracene to mutagenic moieties by different forms of cytochrome P450 in hepatic 9000 X g supernatants from the rat

6-Aminochrysene and 2-aminoanthracene were activated to metabolites which were mutagenic to Salmonella typhimurium TA98 by hepatocytes or hepatic 9000 X g supernatants (S9s) from control or xenobiotic-treated rats. Hepatocytes from Aroclor-1254-treated rats were more efficient than hepatocytes from untreated rats at activating these aromatic amines. When plate-incorporation and liquid-incubation bacterial mutagenesis assays were performed in the presence of limiting amounts of rat hepatic S9, 2-aminoanthracene was activated to a greater extent in both cases, as judged by his+ revertant formation, by 3-methylcholanthrene-induced hepatic S9 than by phenobarbital-induced or control S9s. In contrast, 6-aminochrysene was activated more efficiently by phenobarbital-induced S9 than by 3-methylcholanthrene-induced or control S9s. This unexpected finding was confirmed employing polyclonal antibodies directed against specific forms of rat cytochrome P450. Thus, when employing Aroclor-1254-induced S9 as a source of metabolic activation, antibody directed against cytochrome P450IA1 inhibited the activation of 2-aminoanthracene but not of 6-aminochrysene. In contrast, antibody directed against cytochrome P450IIB1 inhibited the activation of 6-aminochrysene but not of 2-aminoanthracene. These results suggest that under conditions in which the amounts of S9 added are rate-limiting, the two aromatic amines are preferentially activated by different induced forms of cytochrome P-450.     

Drosophila wing-spot test: improved detectability of genotoxicity of polycyclic aromatic hydrocarbons

The somatic mutation and recombination tests (SMART) using eyes or wings in the fruit fly Drosophila melanogaster are flexible and sensitive systems for the detection of genotoxicity of individual chemical compounds and complex mixtures. It is of special interest that adults and larvae possess cytochrome P-450-dependent activation systems able to metabolize most promutagens, e.g., nitrosamines, aflatoxins, pyrrolizidine alkaloids, safrole, etc. The polycyclic aromatic hydrocarbons (PAHs) represent a class of promutagens poorly detectable in Drosophila genotoxicity systems. Therefore, new tester strains for the wing-spot test were constructed by introducing chromosomes 1 and 2 from a wild-type strain with increased cytochrome P-450-dependent metabolism linked to a gene on chromosome 2. Previous investigations with the new strains showed their increased detection capability for diethylnitrosamine. Comparative tests with the 3 PAHs benzo[a]pyrene, benz[a]anthracene and 7,12-dimethylbenz[a]anthracene demonstrate, in a reproducible way, that with the new strains all 3 can be detected as active genotoxic compounds. The dose-response curves for all compounds show a plateau with higher exposures. This is interpreted as indicative of a saturation of the cytochrome P-450-dependent activation systems.     

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

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