An aryl hydrocarbon hydroxylating hepatic cytochrome P-450 from the marine fish Stenotomus chrysops

The monooxygenases of birds, reptiles and amphibians

1. Microsomal monooxygenase systems which contain cytochrome P-450 forms as their active centres are found in birds, reptiles and amphibians. Liver provides a rich source of monooxygenases but they are also present in other tissues. 2. In the hepatic microsomes of these species, levels of cytochrome P-450 and, in most cases monooxygenase activities, are lower than are found in hepatic microsomes of mammals. 3. Amongst birds, the lowest hepatic microsomal monooxygenase activities have been reported for specialized predators (fish-eaters and raptors). 4. When birds are dosed with inducers of the 3-methylcholanthrene (MC) type, the pattern of induction is similar to that in mammals. In contrast, phenobarbitone-type inducers are less effective in birds than in mammals--in some cases having no action at all. Prochloraz is a stronger inducer in birds than in the rat; it is an inducer of mixed type in birds. 5. Partial purification of avian cytochromes P-450 indicates substantial differences in properties from those of mammals. Further differences between birds and mammals in regard to the immunochemical properties of isoforms are evident from comparative studies using Western blotting. 6. On the evidence of Western blotting and response to inducers, cytochromes of family II (P450 II) seem less well represented in birds than in mammals. 7. The low monooxygenase activities of certain species of birds may make them relatively susceptible to lipophilic environmental chemicals that are detoxified by this system.

Cytochrome P-450 mediated O-dealkylation of 7-alkoxycoumarins in liver microsomes from rainbow trout (Salmo gairdneri)

Evidence has recently been presented for variation in the inducibility of various 7-alkoxycoumarin-O-dealkylase activities in liver microsomes from a number of mammalian species by β-naphthoflavone (βNF). In the present study we have investigated the inducibility of hepatic microsomal 7-methoxycoumarin-O-demethylase, 7-ethoxycoumarin-O-deethylase, 7-propoxycoumarin-O-depropylase and 7-butoxycoumarin-O-debutylase activities in rainbow trout by βNF. O-demethylase activity was increased approximately 17-fold, O-deethylase and O-depropylase activities approximately 9-fold and O-debutylase activity approximately 25-fold. The kinetics of the various hepatic microsomal 7-alkoxycoumarin-O-dealkylase activities were investigated in control and βNF-treated rainbow trout. The O-demethylase-, O-depropylase- and O-debutylase activities exhibited monophasic Michaelis-Menten kinetics in liver microsomes from both control and βNF-treated rainbow trout, whereas the O-deethylase activity exhibited biphasic Michaelis-Menten kinetics in control liver microsomes and monophasic Michaelis-Menten kinetics in liver microsomes from βNF-treated rainbow trout.

Effects of ortho- and non-ortho-substituted polychlorinated biphenyl congeners on the hepatic monooxygenase system in scup (Stenotomus chrysops)

Polychlorinated biphenyl congeners that are abundant in environmental samples, and known to induce hepatic monooxygenase isozymes in the P450IA gene subfamily in mammals, were examined for their ability to induce hepatic monooxygenase activity in scup, a marine teleost. Scup were dosed ip with 3,3',4,4'-tetrachlorobiphenyl (congener 77), 2,3,3',4,4'-pentachlorobiphenyl (congener 105), 2,3',4,4',5-pentachlorobiphenyl (congener 118), 2,2',3,4,4',5'-hexachlorobiphenyl (congener 138), 2,2',3,3',4,4'-hexachlorobiphenyl (congener 128), or beta-naphthoflavone and examined for increases in ethoxyresorufin O-deethylase (EROD) activity, immunodetectable cytochrome P450E (the EROD catalyst in scup), and in vitro translatable mRNA for P450E. Monooxygenase parameters were significantly induced only by 3,3',4,4'-tetrachlorobiphenyl (TCB). However, while translatable mRNA for P450E was induced at all doses (1, 5, and 10 mg/kg), EROD activity and P450E were decreased at the 5 and 10 mg/kg doses, relative to the response at 1 mg/kg. A strong relationship between residual TCB concentration in the liver and the decreased EROD activity was evident at the higher doses of TCB. Aminopyrine N-demethylase, a monooxygenase activity not catalyzed by P450E, was unaffected by TCB treatment, indicating a specificity in the TCB effect. Analysis in vitro revealed that TCB was a potent competitive inhibitor of EROD activity, with half-maximal inhibition at 0.3 microM, near the Km for ethoxyresorufin, suggesting one mechanism for the in vivo effect of TCB. These results demonstrate that PCB congeners with ortho-chlorine substitution, and which are effective inducers of AHH and EROD activity in mammals, are ineffective, at the doses tested, as inducers in the teleost scup.

Immunochemical detection of different isoenzymes of cytochrome P-450 induced in chick hepatocyte cultures

This study investigated whether the same cytochrome P-450 (P-450) isoenzymes were inducible in cultures of chick-embryo hepatocytes as in the liver of chicken embryos. We purified two isoenzymes of cytochrome P-450 from the livers of 17-day-old-chick embryos: one of molecular mass approx. 50 kDa induced in vivo by the phenobarbital-like inducer glutethimide, and the second of approx. 57 kDa induced by 3-methylcholanthrene. Rabbit antiserum against the 50 kDa protein inhibited benzphetamine demethylase activity in hepatic microsomes (microsomal fractions) from glutethimide-treated chick embryo. Antiserum to the 57 kDa protein inhibited ethoxyresorufin de-ethylase activity in hepatic microsomes from methylcholanthrene-treated chick embryo. Cultured chick hepatocytes were treated with chemicals known to induce isoenzymes of P-450 in rodent liver. The induced P-450s were quantified spectrophotometrically and characterized by immunoblotting and enzyme assays. From these studies, chemical inducers were classified into three groups: (i) chemicals that induced a P-450 isoenzyme of 50 kDa and increased benzphetamine demethylase activity: glutethimide, phenobarbital, metyrapone, mephenytoin, ethanol, isopentanol, isobutanol, lindane, lysodren; (ii) chemicals that induced a P-450 isoenzyme of 57 kDa and increased ethoxyresorufin de-ethylase activity: 3-methylcholanthrene and 3,3',4,4'-tetrachlorobiphenyl; and (iii) the mono-alpha-substituted 2,3',4,4',5-pentabromobiphenyl, which induced both proteins and both activities. The immunochemical data showed that chick-embryo hepatocytes in culture retain the inducibility of glutethimide- and methylcholanthrene-induced isoenzymes of P-450 that are inducible in the liver of the chicken embryo.

The temporal relationships between P450E protein content, catalytic activity, and mRNA levels in the teleost Fundulus heteroclitus following treatment with beta-naphthoflavone

P450 induction occurs in some marine organisms following chemical exposure. The mode of 3-methylcholanthrene (MC)-type induction was evaluated by examining hepatic isozyme P450E content, catalytic activity, and mRNA levels in the marine teleost Fundulus heteroclitus after exposure to a single dose of beta-naphthoflavone (BNF). P450E is the major teleost P450 induced by MC-type compounds and is the catalyst for aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities. In a 20-day experiment, EROD activity was elevated in BNF-treated animals from Day 4 through Day 20. Increases in immunodetectable P450E showed the same trend as EROD activity, with consistently low control values and at least a 19-fold increase in the BNF-treated fish. Precipitation of liver RNA in vitro translation products with anti-P450E antibody gave no detectable signal from control fish, while the BNF-treated animals showed incorporation of [3H]leucine in a single 6,000 Mr band. In a shorter term experiment, EROD activity and P450E levels were again coordinately increased in response to BNF treatment, and immunoprecipitation of translation products from these fish showed a clear trend of increased P450E mRNA levels for all time points 6 h or more post-treatment. Hybridization of RNA from BNF-treated Fundulus with a trout P450IA1 cDNA also showed increases in a single band with time. The increases in P450E mRNA preceded increases in P450E protein and enzyme activity by about 25 h. However, P450E mRNA declined rapidly, reaching control levels by 5 days, while protein levels remained elevated for at least 13 days. The results support a hypothesis that transcriptional enhancement is involved in induction of MC-inducible P450s in fish, but indicate that P450E induction is also under other forms of regulatory control.

Further consideration of phenobarbital effects on cytochrome P-450 activity in the killifish, Fundulus heteroclitus

1. Ethoxyresorufin O-deethylase (EROD) activity, aldrin epoxidase (AE) activity, cytochrome P-450 content, and levels of cytochrome P-450E (the major BNF-inducible P-450 form and primary EROD catalyst in scup) or its homologues were measured in hepatic microsomes isolated from Fundulus heteroclitus, scup (Stenotomus chrysops) and brook trout (Salvelinus fontinalis) treated with beta-naphthoflavone (BNF) or phenobarbital (PB). 2. In all three teleost species, BNF treatment caused expected increases in P-450 content, EROD activity and P-450E level; but either no change or a slight decrease in AE turnover rate (nmol/min/nmol P-450). 3. Polyclonal antibodies to P-450E did not inhibit AE activity in microsomes from BNF-treated scup, confirming that this major BNF-inducible P-450 form does not catalyze AE activity in fish. 4. In contrast, PB treatment did not affect hepatic AE activity, P-450 content or levels of "P-450E" in F. heteroclitus, but did variably affect EROD activity which was suppressed in one experiment and elevated in another. 5. The results indicate that (i) contrary to previous reports, neither PB nor MC-type inducers increase AE activity in F. heteroclitus, (ii) MC-type inducers do not affect AE activity in the other teleost species examined, and (iii) AE activity is not a reliable indicator of P-450 induction by environmental chemicals. 6. We emphasize the need to establish the mechanism of PB action, and the nature of any fish P-450 forms analogous to PB-inducible forms in mammals in order to conclusively evaluate PB-responses in fish.

Purification and characterization of hepatic steroid hydroxylases from untreated rainbow trout

Purification of cytochrome P450 from liver microsomes of untreated juvenile male rainbow trout yielded five fractions designated LMC1 to LMC5. All fractions, except LMC4 and LMC5, appeared homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and showed minimum molecular weights of 50,000 (LMC1), 54,000 (LMC2), 56,000 (LMC3), 58,000 (LMC4), and 59,000 (LMC5). Specific contents ranged from 2.8 (LMC3) to 14.9 (LMC5) nmol heme/mg protein. The catalytic activity of LMC1, LMC2, and LMC5 toward various substrates was examined. LMC2 exhibited the highest estradiol 2-hydroxylase activity and progesterone 16 alpha-hydroxylase activity. LMC2 also was most active in the metabolic activation of aflatoxin B1 (AFB1). In contrast, LMC5 was most active in catalyzing the 6 beta- and 16 beta-hydroxylation of testosterone and the 6 beta-hydroxylation of progesterone. LMC1 showed the highest lauric acid hydroxylase activity. The three isozymes tested had low activity (for LMC2 and LMC5) or no activity (for LMC1) toward benzphetamine or benzo[a]pyrene. Polyclonal antibodies to all five isozymes were raised in rabbits and the antibodies were used to examine the contribution of the P450s to microsomal enzyme activities. The results of microsomal enzyme inhibition studies with polyclonal antibodies showed that anti-LMC2 IgG significantly inhibited the oxidative metabolism of testosterone, lauric acid, AFB1, and benzphetamine. Anti-LMC5 IgG inhibited the oxidation of progesterone, estradiol, benzo[a]pyrene, and benzphetamine. Anti-LMC1 IgG slightly inhibited the microsomal hydroxylation of lauric acid. Anti-LMC3 and anti-LMC4 IgG did not inhibit any of the measured microsomal enzyme activities. These findings suggest that individual constitutive isozymes of trout cytochrome P450 have well-defined contributions to the microsomal metabolism of steroids, fatty acids, and xenobiotics.

Nitrofurantoin-stimulated superoxide production by channel catfish (Ictalurus punctatus) hepatic microsomal and soluble fractions

Nitroaromatic compounds, which frequently contaminate the environment, are known to be reduced to corresponding aromatic amines by fish as well as mammals under anaerobic conditions. Although amine products are not generally formed aerobically, "nitroreductase"-mediated redox cycling of nitroaromatics may occur under these conditions, leading to enhanced production of a potentially toxic oxygen species, superoxide (O-2). In this study, we have investigated the ability of channel catfish (Ictalurus punctatus) hepatic microsomal and soluble fractions to stimulate O-2) production upon exposure to a model redox cycling nitroaromatic compound, nitrofurantoin (NF). Two assays for O-2 production, cytochrome c reduction and cyanide-insensitive oxygen consumption, were stimulated by NF exposure to both hepatic fractions. These reactions were partially inhibited by superoxide dismutase (SOD), and by SOD and catalase in the oxygen consumption assay, providing specific evidence for the involvement of O-2 in the stimulatory effect by NF. Furthermore, results of cofactor requirement and inhibition studies suggest that NF enhancement of O-2 production was mediated by NADPH-cytochrome P-450 (c) reductase in the microsomal fraction and xanthine oxidase in the soluble fraction. These findings comprehensively suggest that the in vitro stimulation of O-2 production by nitroaromatics as indicated in mammals may also occur in fish and, therefore, suggests a similar potential for oxyradical-mediated toxicities in these species.

Trout P450IA1: cDNA and deduced protein sequence, expression in liver, and evolutionary significance

The cytochrome P1(450) (P450IA1) cDNA has been isolated and sequenced from liver of 3-methylcholanthrene-treated rainbow trout (Salmo gairdneri). The cDNA hybridizes to a 2.8-kb mRNA that is induced at least 10-fold by 3-methylcholanthrene. Southern blot analysis suggests the presence of a single gene or a very small number of genes. An open reading frame of the 2573-bp cDNA encodes a 522-residue protein (Mr = 59,241) that is more similar to the mammalian P450IA1 than the mammalian P450IA2 proteins. The aromatic hydrocarbon (Ah) receptor, responsible for mammalian P450IA1 and IA2 inducibility, was detected in trout liver cytosol by specific binding to [1,6-3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vitro. Comparison of the fish P450IA1 protein with human, mouse, rat and rabbit P450IA1 and P450IA2 proteins reveals the presence of a remarkably large number of single amino acids and stretches of 2-6 residues in a row that are invariant among these nine P450 proteins. These conserved regions may participate in the binding of the NADPH-P450 oxidoreductase flavoprotein, substrate, or heme.

Effect of preexposure to dietary benzo[a]pyrene (BP) on the first-pass metabolism of BP by the intestine of toadfish (Opsanus tau): in vivo studies using portal vein-catheterized fish

The effect of preexposure of fish to dietary benzo[a]pyrene (BP) on the intestinal metabolism of BP was examined in toadfish (Opsanus tau). The portal veins of toadfish were cannulated following administration of radiolabeled BP to the intestinal lumen. Because these fish lack a lymphatic vessel system, the portal vein is the sole route by which BP and its metabolites enter the circulation. In fish preexposed to dietary BP (10 mg BP/kg food), the radioactivity entering the portal vein was almost entirely (ca. 90%) BP metabolites. In fish fed a laboratory control diet, a smaller percentage (ca. 60%) of the radioactivity entering the portal vein was in the form of BP metabolites. The enhanced efficiency of the intestines of preexposed fish in metabolizing BP appears to be a result of induction of intestinal aryl hydrocarbon hydroxylase (AHH) activity. Intestinal microsomal AHH activities in control and preexposed fish were 0.033 +/- 0.032 and 0.320 +/- 0.060 nmol.min-1.mg-1, respectively. Gel filtration of portal vein plasma indicated differences in the roles of plasma proteins in transporting BP and BP metabolites. Native BP was associated primarily with the high density lipoproteins, whereas organic-soluble BP metabolites were associated primarily with serum albumin fractions. A large percentage of BP metabolites was recovered as water-soluble conjugates. These studies indicate that in fish, the intestine can be an important organ involved in dietary BP metabolism.

Species characteristics of the hepatic xenobiotic and steroid biotransformation systems of two teleost fish, Atlantic cod (Gadus morhua) and rainbow trout (Salmo gairdneri)

Groups of Atlantic cod and rainbow trout were treated (ip) with beta-naphthoflavone (BNF), phenobarbital, or peanut oil (controls), and properties of the hepatic xenobiotic and steroid metabolizing enzyme systems were evaluated. In both species, BNF treatment resulted in significant induction of microsomal 7-ethoxycoumarin O-deethylase, 7-ethoxyresorufin O-deethylase, biphenyl 4-hydroxylase, and phenanthrene oxidation, especially at the 1,2-position. Immunochemical studies with rabbit IgG prepared against the major BNF-inducible cytochrome P-450 in cod, P-450c, revealed increased amounts of immunoreactive protein in liver slices from both species after BNF treatment. The molecular weight of the induced protein was approximately 58,000 Da, as shown by Western blotting. When titrating biphenyl 4-hydroxylation, however, the antibodies distinguished between the two species, inhibiting the activity of BNF-induced cod 90% and that of rainbow trout 40% at 10 mg IgG/nmol P-450. Furthermore, cytochrome b5 content and UDP-glucuronyltransferase activity were significantly induced only in rainbow trout, whereas the specific content of cytochrome P-450 was significantly increased only in cod. Differences between the two species were observed in the levels of constitutive activities, the amount of induction, and in the regioselectivity of phenanthrene oxidation and androstenedione metabolism. Treatment with phenobarbital showed no effect on any of the parameters investigated in either species. The results show that although there are many common features of the hepatic xenobiotic and steroid biotransformation systems of the two teleosts, certain species characteristics exist in constitutive properties and induction responses.

Structural comparison between the trout and mammalian hydrophilic domain of NADPH-cytochrome P-450 reductase

The isolation of the protease-solubilized NADPH-cytochrome P-450 reductase from trout liver and its properties are described. The sequence of the "hydrophilic domain" [protease-solubilized NADPH-cytochrome P-450 reductase from trout (residues Lys56-Ser678)] is reported. The CNBr fragments of the trout "hydrophilic domain" and their proteolytic subpeptides were sequenced. The CNBr fragments were aligned by homology to the reported sequence of the porcine NADPH-cytochrome P-450 reductase. The structures of the mammalian and the trout NADPH-cytochrome P-450 reductases were compared. Stretches with high exchange rates between the pig and trout reductase were found at the NH2 and the COOH terminal regions of the hydrophilic domain.

Patterns and regulation of estradiol metabolism by hepatic microsomes from two species of marine teleosts

Estradiol (E2) metabolites formed in vitro by microsomes from the marine teleosts winter flounder (Pseudopleuronectes americanus) and scup (Stenotomus chrysops) included at least seven products detected by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The least polar metabolite was shown to be estrone by chromatographic and mass spectrometric identity with authentic estrone. Chromatographic analyses coupled with dual-label experiments also indicated formation of the catecholestrogen 2-hydroxyestradiol (2-OH-E2), which was the most prominent metabolite determined by TLC. Analysis of microsomal E2 2-hydroxylase activity by measuring the specific release of 3H2O from [2-3H]E2 indicated that it is mediated by cytochrome P-450. E2 2-hydroxylase activity normalized to microsomal protein was lower in females than in males for microsomes from both mature scup and winter flounder. Activity normalized to liver weight or body weight in female winter flounder was also lower than that in males. However, activity normalized to cytochrome P-450 content did not show sex differences in either species. E2 2-hydroxylase activity per nanomole cytochrome P-450 was reduced in scup treated with beta-naphthoflavone, which induces the hydrocarbon hydroxylase cytochrome P-450E. Studies employing reconstituted P-450E and microsomes preincubated with polyclonal antibodies against P-450E confirmed that this isozyme does not catalyze E2 2-hydroxylase activity in microsomes. However, preliminary work with scup cytochrome P-450A suggests that it may be an E2 2-hydroxylase. The studies establish that catecholestrogen formation is prominent in fish liver and that it is sexually differentiated, but further investigation is required to define the catalysts as well as the significance and regulation of this function.

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.

Hepatic metabolism of cyclodiene insecticides by constitutive forms of cytochrome P-450 from lower vertebrates

1. Multiple forms of cytochrome P-450 were separated from the hepatic microsomes of untreated male rats, pigeons (Columbia livia), razorbills (Alca torda), puffins (Fratercula arctica), and rainbow trout (Salmo gairdnerii), using anion exchange chromatography and DEAE-cellulose. 2. In some cases cytochrome P-450 forms were further purified on hydroxylapatite and carboxymethyl-sephadex columns. 3. Considerable differences in the distribution of forms between these five species were evident from elution profiles on DEAE cellulose, and on analysis of the cytochrome P-450 containing pools by SDS-PAGE. 4. The metabolism of two organochlorine compounds, aldrin and the dieldrin analogue HCE, were studied in (a) intact microsomes and (b) reconstituted systems containing cytochrome P-450, from each of the five species. 5. In spite of their close structural similarity, significant differences were found between the two substrates in the distribution of catalytic activity between the cytochrome P-450 isozymes of each species.

Characterization of some monooxygenase activities and solubilization of hepatic cytochrome P-450 in two species of freshwater fish, the nase (Chondrostoma nasus) and the roach (Rutilus rutilus)

1. Hepatic monooxygenase activities were studied in microsomal fractions from two species of freshwater fish, the nase (Chondrostoma nasus) and the European roach (Rutilus rutilus). 2. These activities were determined by using four substrates, 7-ethoxycoumarin, 7-ethoxyresorufin, benzo(a)pyrene, and 2,5-diphenyloxazole and were characterized according to pH, temperature, cofactors, and the differential effects of two inhibitors, metyrapone and alpha-naphthoflavone. 3. Solubilization of microsomes was achieved by the use of detergents, with a good recovery of the cytochrome P-450.

Regioselective metabolism of phenanthrene in Atlantic cod (Gadus morhua): studies on the effects of monooxygenase inducers and role of cytochromes P-450

The polycyclic aromatic hydrocarbon phenanthrene was converted mainly (greater than 90%) to the 1,2-dihydrodiol when metabolized in vivo by the marine teleost cod. This is also found in other bony fishes, but contrary to what is known from cartilaginous fish, crustaceans and mammals, where the K-region 9,10-dihydrodiol is the main metabolite. When liver microsomal preparations from differently pretreated cod were incubated with phenanthrene in vitro, the metabolic profile was dramatically different from the in vivo pattern, as shown by gas chromatography-mass spectrometry. The microsomes from untreated, phenanthrene, phenobarbital and pregnenolone-16 alpha-carbonitrile-treated cod converted phenanthrene mainly, but to a varying extent, to the 9,10-dihydrodiol. Treatment with beta-naphthoflavone (BNF), however, resulted in a large increase in the oxidation at the 1,2-position, along with a four- to seven-fold increase in specific activity. The major cytochrome P-450 isozyme purified from BNF-treated cod liver (P-450c) showed highest activity with phenanthrene (a turnover of 0.18 nmol/min per nmol P-450), but with about equal selectivity for the 1,2- and 9,10-region of the substrate in a reconstituted system with phospholipid and NADPH-cytochrome P-450 reductase. The low regioselectivity was also observed as a lack of regioselective inhibition of microsomal phenanthrene metabolism with antiserum to cod P-450c. Two of the minor isozymes, cod cytochromes P-450b and d, showed a similar turnover to P-450c, but with a stronger selectivity for the 1,2-position (55-60%). The results indicate that other control systems, in addition to the content of individual P-450-forms in the regulatory systems, in addition to the content of individual P-450-forms in the endoplasmic reticulum, are involved in the in vivo transformation of phenanthrene by cod to the 1,2-dihydrodiol metabolite.

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.

Sex differences in cytochrome P-450 isozyme composition and activity in kidney microsomes of mature rainbow trout

Kidney microsomes from sexually mature male, as opposed to female, rainbow trout displayed an approximately 20-fold higher cytochrome P-450 specific content, NADPH-cytochrome c reductase activity, and rates of hydroxylation of lauric acid, testosterone, progesterone and aflatoxin B1. Little or no sex difference in metabolism was observed with benzo[a]pyrene or benzphetamine as substrates. A similar pattern was observed in hepatic microsomes from these fish, but the difference was much less striking (approximately 2-fold higher activity in males). Juvenile trout (both sexes) possessed activities intermediate between mature males and females. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of kidney and liver microsomes of juvenile and sexually mature male and female trout suggested that the striking sex difference in kidney could be due to the high amount of trout P-450 isozyme LM2 in sexually mature males. Immunoquantitation of LM2, performed by Western Blotting and immunostaining with rabbit anti-trout LM2-IgG, confirmed that mature male kidney contained much higher levels of P-450 LM2 than juvenile or female kidney, or even of liver microsomes of all three groups. The amount of P-450 LM2 in mature female kidney microsomes was barely detectable. The high amount of LM2 in male trout kidney is consistent with the high activity of these microsomes towards lauric acid and aflatoxin B1, which have been shown previously to be preferentially metabolized by trout P-450 LM2. It is suggested that rainbow trout may serve as an alternative to the rat as an animal model for the study of sex-dependent differences in cytochromes P-450.

Monooxygenase Induction and Chlorobiphenyls in the Deep-Sea Fish Coryphaenoides armatus

Inhibition of liver microsomal ethoxyresorufin O-deethylase and aryl hydrocarbon hydroxylase activities by alpha-naphthoflavone and by polyclonal antibodies to hydrocarbon-inducible cytochrome P-450E from teleost liver indicated a xenobiotic-induced origin of these activities in the deep-sea fish Coryphaenoides armatus. Specific recognition of a protein by antibodies to P-450E in an immunoblot assay further indicated xenobiotic-induced cytochrome P-450 in these animals. Levels of apparently induced cytochrome P-450 and monooxygenase activity correlated positively with the tissue content of chlorobiphenyls of known inducing activity, implicating such compounds in biochemical effects occurring in the deep ocean.

Induction of cytochrome P-450-dependent monooxygenase systems in embryos and eleutheroembryos of the killfish Fundulus heteroclitus

Microsomal aryl hydrocarbon hydroxylase (AHH) activity was highly inducible by polychlorinated biphenyls (PCBs) in Fundulus embryos, and stages prior to the appearance of the liver rudiment were competent to respond to these inducers. Consistent with previous observations, basal AHH activity in whole eleutheroembryo microsomes was shown to increase about 9-fold within 24 h of hatching. Aminopyrine N-demethylase (APD) activity also increased with time after hatching. However, the increase in APD activity was much less than that of AHH activity, suggesting a post-hatching change in basal cytochrome P-450 isozyme composition. Also associated with hatching was an increase in the sensitivity to PCBs as inducers of AHH activity. The ED50 for induction of AHH activity in eleutheroembryos was estimated to be only one-third to one-fourth that in embryos. This developmental increase in the sensitivity to PCBs was not due to a redistribution of PCBs between the yolk and tissues with yolk absorption, and was not simply age-dependent, but appeared to require hatching. An additional change in the monooxygenase system associated with hatching was that microsomal NADPH-cytochrome c reductase activity was not inducible by PCBs prior to hatching, but was modestly inducible after hatching. High performance liquid chromatographic (HPLC) analysis of benzo[a]-pyrene (BP) metabolites formed by microsomes from control and PCB-treated eleutheroembryos demonstrated production of dihydrodiols in the 7,8- and 9,10-positions of the benzo-ring. The formation of these metabolites was completely inhibited by the epoxide hydrolase (EH) inhibitor, trichloropropene oxide, indicating the presence of EH in Fundulus eleutheroembryos. Furthermore, these results indicate the Fundulus eleutheroembryos probably can activate BP to its ultimate carcinogenic form, the 7,8-dihydrodiol-9,10-epoxide, and induction of AHH activity by PCBs is likely to increase the rate of formation of activated metabolites from BP and related compounds. However, during the most active period of organogenesis, prior to hatching, basal AHH activity was low, and prehatching stages were relatively insensitive to cytochrome P-450 inducers. The combination of these effects may help to protect these stages from damage from activated metabolites.

Species variations in the metabolism of liposoluble organochlorine compounds by hepatic microsomal monooxygenase: comparative kinetics in four vertebrate species

Liposoluble organochlorine compounds were used as substrates in a kinetic study of the hepatic microsomal monooxygenases of the male rat, feral pigeon (Columbia livia), frog (Rana pipiens) and rainbow trout (Salmo gairdnerii). Substrate concentrations were taken down to environmentally realistic levels. Lineweaver-Burk plots gave straight lines for both aldrin (HHDN) and the dieldrin analogue HCE. For both substrates activities followed the order rat greater than feral pigeon greater than trout over the entire concentration range. The metabolism of PCB isomers in microsomes from these uninduced animals was very slow. These results give evidence of cytochrome P-450 forms which can metabolize organochlorine compounds at low substrate concentrations.

Benzo[a]pyrene-hydroxylase catalyzed by purified isozymes of cytochrome P-450 from beta-naphthoflavone-fed rainbow trout

We have purified five isozymes of liver microsomal (LM) P-450 from beta-naphthoflavone-fed rainbow trout. Four forms (LM3, LM1, LM4a and LMx) were resolved on DEAE-Sepharose. Chromatography on hydroxylapatite further resolved LMx into two components, LM2 and LM4b. This latter form, obtained in highest yield (5%), had an apparent minimum molecular weight (Mr), as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), of 58,000, a specific content of 11.9 nmoles/mg, a lambda max in the carbon monoxide-ligated, reduced difference spectrum of 447.0 nm, and was active towards benzo[a]pyrene in a reconstituted system. A second form, LM4a, obtained in a final yield of 2%, had a specific content of 10.3 and was indistinguishable from Lm4b by Mr, lambda max, or activity towards benzo[a]pyrene. Form LM2 (2% yield) had a specific content of 10.8, a Mr of 54,000, a lambda max of 449.5 nm, and was not effective in reconstitution of benzo[a]pyrene-hydroxylase. In addition, two other forms with lower specific contents were obtained, LM1 and LM3. Neither LM1 nor LM3 was active towards benzo[a]pyrene. The properties of LM2, LM4a and LM4b were further examined with the aid of antibodies prepared from rabbits. Antibodies to LM4a and LM4b each cross-reacted with the other antigen and formed lines of identity on Ouchterlony plates, and both IgGs exhibited some cross-reaction to P-448 from rat. Neither antibody cross-reacted with trout LM2, and LM2-IgG did not cross-react with any other purified P-450. Benzo[a]pyrene-hydroxylase, catalyzed by either LM4a or LM4b, was inhibited by LM4b-IgG but not by LM4a-IgG, suggesting that these antibodies recognize different antigenic sites. Further comparison of LM4a and LM4b by amino acid composition, peptide mapping, kinetic properties, sensitivity to alpha-naphthoflavone, and regioselectivity towards benzo[a]pyrene-dihydrodiol formation indicates that these forms are highly similar in structure and function.

An alternative 7-ethoxyresorufin O-deethylase activity assay: a continuous visible spectrophotometric method for measurement of cytochrome P-450 monooxygenase activity

A procedure to directly measure the cytochrome P-450-dependent 7-ethoxyresorufin O-deethylase activity with a visible spectrophotometer is described and compared to the standard fluorometric method. The two assays yielded identical results with both beta-naphthoflavone-treated mammalian (rat) and fish (scup, Stenotomus chrysops) liver microsomes. The assay takes advantage of a clean distinction in visible absorption spectra obtained for highly purified 7-ethoxyresorufin (substrate) and resorufin (enzymatic product). The purification and characterization of resorufin, the enzymatic product, are detailed, and its extinction coefficient (epsilon 572 = 73 mM-1 cm-1) provides for an accurate quantitation of enzyme activity. The large visible extinction coefficient of the product chromophore provides a high sensitivity for low-activity samples. The application of this enzyme assay in a visible spectrophotometer, along with the considerable evidence that a single aromatic hydrocarbon-inducible cytochrome P-450 isozyme is responsible for the catalysis, enhances the utility of this substrate in microsomal monooxygenase assays. The utility of the visible assay is further demonstrated by the simple determinations of the coupling ratio for 7-ethoxyresorufin oxidation in scup liver microsomes and the K1 for 7,8-benzoflavone and phenylimidazole inhibition of the enzymatic reaction.

Regiospecific hydroxylation of lauric acid at the (omega-1) position by hepatic and kidney microsomal cytochromes P-450 from rainbow trout

Microsomes from liver or kidney of untreated rainbow trout hydroxylated lauric acid specifically at the (omega-1) position. Turnover numbers for liver (2.72 min-1) and kidney (14.1 min-1) were decreased seven- and twofold, respectively, following treatment with beta-naphthoflavone. Laurate hydroxylation activity from untreated trout hepatic microsomes was sensitive to inhibition by SKF-525A, but was not sensitive to metyrapone and only partially inhibited by alpha-naphthoflavone. The temperature optimum of laurate (omega-1) hydroxylation in trout liver microsomes was 25-30 degrees C. The Km and Vmax for (omega-1)- hydroxylaurate formation was 50 microM and 1.63 nmol min-1 mg-1, respectively, in liver and 20 microM and 3.95 nmol min-1 mg-1, respectively, in kidney from untreated trout microsomes. (omega-1) Hydroxylation of laurate, in both liver and kidney microsomes, was sensitive to an antibody raised against a previously purified cytochrome P-450 isozyme (LM2) of trout liver microsomes, which has been shown to be active towards aflatoxin B1. Antibody to the major isozyme of cytochrome P-450 ( LM4b , active towards benzo(a)pyrene) induced by beta-naphthoflavone did not inhibit (omega-1) hydroxylation of laurate in microsomes from untreated or beta-naphthoflavone-treated trout.

Microsomal electron transport and xenobiotic monooxygenase activities during the embryonic period of development in the killifish, Fundulus heteroclitus

The developmental patterns of aryl hydrocarbon hydroxylase (AHH) activity and NADPH-cytochrome c reductase activity were followed during embryonic development in Fundulus. AHH activity was localized in microsomal fractions prepared from whole Fundulus embryos and eleutheroembryos. On the basis of this subcellular localization, the requirements of O2 and NADPH for activity, and sensitivity to carbon monoxide and cytochrome c inhibition, the AHH activity in Fundulus embryos and eleutheroembryos appeared to be cytochrome P-450 dependent. AHH activity was measurable in stages prior to the appearance of the liver rudiment, and during subsequent embryonic development the extrahepatic tissues were likely to have contributed substantially to the AHH activity measured. At all stages assayed before hatching, microsomal AHH specific activity remained uniformly low, but within 24 hr of hatching, AHH specific activity increased about ninefold. This posthatching increase in AHH activity was not age dependent, nor developmental stage dependent, but rather required hatching, and was not due to the presence of endogenous inhibitors in prehatching stages. The levels of NADPH-cytochrome c reductase activity and AHH activity were not closely correlated in whole embryo and eleutheroembryo microsomes, but the AHH activity in these preparations apparently was not limited by the levels of the NADPH-cytochrome c (P-450) reductase. The presence of AHH activity in Fundulus embryos during the period of active organogenesis, prior to hatching, indicates that this species is likely to be susceptible to a variety of teratogens requiring metabolic activation, and this may be the case for other species of fish as well.