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

Addition of Phenylmethylsulfonyl Fluoride Increases the Working Lifetime of the Trout Liver S9 Substrate Depletion Assay, Resulting in Improved Detection of Low Intrinsic Clearance Rates

The activity of a trout liver S9 substrate depletion assay has been shown to decline over time, presumably due to proteolytic degradation of biotransformation enzymes. To address this problem, assay performance was evaluated following the addition of phenylmethylsulfonyl fluoride (PMSF) or a general-purpose protease inhibitor cocktail to liver homogenization buffers and/or S9 reaction mixtures. Addition of PMSF to liver homogenization buffers and/or S9 reaction mixtures had little or no effect on clearance of phenanthrene, a model cytochrome P450 substrate, in short-term (25 or 30 min) depletion experiments but resulted in significant improvements in retention of this initial activity over time. The protease inhibitor cocktail strongly inhibited initial activity when added to homogenization buffers or reaction mixtures. Taking into consideration potential effects on liver carboxylesterases, the treatment approach determined to be optimal was addition of 10 µM PMSF to the S9 reaction mixture. Addition of 10 µM PMSF to the mixture resulted in significantly higher rates of phenanthrene clearance in 2-h incubations relative to those obtained in the absence of PMSF and a 6-fold increase in the working lifetime of the preparation. The results of a statistical power analysis suggest that by increasing the working lifetime of the assay, addition of PMSF to the reaction mixture could result in substantially improved detection of low in vitro clearance rates when compared to current practice. These findings demonstrate the value of adding PMSF to the trout S9 preparation and may have broad implications for use of this assay to support chemical bioaccumulation assessments for fish. Environ Toxicol Chem 2021;40:148-161. © 2020 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Tissue-specific metabolism of benzo[a]pyrene in rainbow trout (Oncorhynchus mykiss): a comparison between the liver and immune organs

Polycyclic aromatic hydrocarbons (PAHs) are immunotoxicants in fish. In mammals, phase I metabolites are believed to be critically involved in the immunotoxicity of PAHs. This mechanism has been suggested for fish as well. The present study investigates the capacity of immune organs (head kidney, spleen) of rainbow trout, Oncorhynchus mykiss, to metabolize the prototypic PAH, benzo[a]pyrene (BaP). To this end, we analyzed 1) the induction of enzymatic capacity measured as 7-ethoxyresorufin-O-deethylase (EROD) activity in immune organs compared with liver, 2) the organ profiles of BaP metabolites generated in vivo, and 3) rates of microsomal BaP metabolite production in vitro. All measurements were done for control fish and for fish treated with an intraperitoneal injection of 15 mg BaP/kg body weight. In exposed trout, the liver, head kidney, and spleen contained similar levels of BaP, whereas EROD induction differed significantly between the organs, with liver showing the highest induction factor (132.8×), followed by head kidney (38.4×) and spleen (1.4×). Likewise, rates of microsomal metabolite formation experienced the highest induction in the liver of BaP-exposed trout, followed by the head kidney and spleen. Microsomes from control fish displayed tissue-specific differences in metabolite production. In contrast, in BaP-exposed trout, microsomes of all organs produced the potentially immunotoxic BaP-7,8-dihydrodiol as the main metabolite. The findings from this study show that PAHs, like BaP, are distributed into immune organs of fish and provide the first evidence that immune organs possess inducible PAH metabolism leading to in situ production of potentially immunotoxic PAH metabolites.

Multi-species comparison of the mechanism of biotransformation of MeO-BDEs to OH-BDEs in fish

Polybrominated diphenyl ethers (PBDEs) and their methoxylated- (MeO-) and hydroxylated- (OH-) analogs are ubiquitously distributed in the environment worldwide. The OH-BDEs have greater potency than PBDEs and can be produced from the transformation of MeO-BDEs. The objectives of the current study were to (1) identify the enzyme(s) that catalyze biotransformation of 6-MeO-BDE-47 to 6-OH-BDE-47 in livers from rainbow trout, and (2) compare biotransformation of 6-MeO-BDE-47 to 6-OH-BDE-47 among rainbow trout, white sturgeon and goldfish. Cytochrome P450 1A (CYP1A) enzymes did not catalyze the biotransformation reaction. However, biotransformation was significantly inhibited by the CYP inhibitors clotrimazole and 1-benzylimidazole but not gestodene. Therefore, the reaction is likely catalyzed by CYP2 enzymes. When biotransformation was compared among species, concentrations of 6-OH-BDE-47 were significantly 3.4- and 9.1-fold greater in microsomes from rainbow trout compared to goldfish or white sturgeon, respectively. Concentrations of 6-OH-BDE-47 in microsomes from goldfish were non-significantly 2.7-fold greater than in sturgeon. The initial rate of biotransformation in microsomes from livers of rainbow trout was significantly 2.0- and 6.2-fold greater than the initial rate of biotransformation in microsomes from livers of goldfish or sturgeon, respectively, while the initial rate in goldfish was significantly 3.1-fold greater than in sturgeon. It is hypothesized that differences in CYP-mediated biotransformation of MeO-BDEs to OH-BDEs could influence concentrations of OH-BDEs in different species of fish.

Benzo(a)pyrene induces hepatic AKR1A1 mRNA expression in tilapia fish (Oreochromis niloticus)

AKR1A1 or aldehyde reductase is a member of the aldo-keto reductases superfamily that is evolutionarily conserved among species. AKR1A1 is one of the five AKRs (AKR1A1 and 1C1-1C4) implicated in the metabolic benzo(a)pyrene (BaP) activation to reactive BaP 7,8-dione. BaP is a polycyclic aromatic hydrocarbon (PAH) widely distributed in aquatic ecosystems and its metabolic activation is necessary to produce its toxic effects. Although the presence of AKR1A1 in fish has been reported, its tissue distribution in tilapia (Oreochromis niloticus) and AKR1A1 inducibility by BaP are not known yet. Moreover, cytochrome P4501A (CYP1A) mRNA expression in fish has been used as a PAH biomarker of effect. Therefore, BaP effects on AKR1A1 and CYP1A gene expressions in tilapia, a species of commercial interest, were investigated by real-time RT-PCR. A partial AKR1A1 cDNA was identified, sequenced and compared with AKR1A1 reported sequences in the GenBank DNA database. Constitutive AKR1A1 mRNA expression was detected mainly in liver, similarly to that of CYP1A. BaP exposure resulted in statistically significant AKR1A1 and CYP1A mRNA induction in liver (20- and 120-fold, respectively) at 24 h. On the other hand, ethoxyquin (EQ) was used as control inducer for AKR1A1 mRNA. Interestingly, EQ also induced CYP1A mRNA levels in tilapia liver. Our results suggest that teleost AKR1A1, in addition to CYP1A, are inducible by BaP. The mechanism of AKR1A1 induction by BaP and its role in fish susceptibility to BaP toxic effects remains to be elucidated.

PCB77 (3,3',4,4'-tetrachlorobiphenyl) co-exposure prolongs CYP1A induction, and sustains oxidative stress in B(a)P-exposed turbot, Scophthalmus maximus, in a long-term study

Cytochrome P4501A (CYP1A), benzo(a)pyrene (B(a)P) activation and biliary elimination, phase II activities, and peroxisomal and antioxidant activities of turbot (Scophthalmus maximus) were studied in a long-term controlled experiment. Fish were serially exposed in water on day 1 and on completion of months 3, 6 and 9 to 0.1, 0.2, 0.1 and 0.1mg B(a)P/l, respectively, while another group was identically treated with additional PCB77 (3,3',4,4'-tetrachlorobiphenyl) at 1% of concomitant B(a)P (w/w). Temporally persistent responses were obtained by sampling on week 3 and 3 months from each latest exposure. Serial exposure to B(a)P+PCB77 progressively induced liver 7-ethoxyresorufin O-deethylase (EROD) activity and CYP1A protein levels (ELISA, western blotting) towards months 9, 12 and gill EROD activity on month 12. It associated with an apparent increase in liver benzo(a)pyrene diol epoxide (BPDE)-DNA adduct levels (ultrasensitive enzyme radioimmunoassay), and elevated bile B(a)P metabolite levels on month 9 females as compared to males. In contrast, B(a)P alone did not cause (p>0.05) comparable effects on liver EROD, CYP1A, adducts nor on bile metabolites. Both exposed groups demonstrated evidence for lasting oxidative stress as hepatic superoxide dismutase, catalase and glutathione peroxidase activities were significantly altered (p<0.05) with symptomatic pro-oxidant associations among them. Both treatments affected liver somatic index similarly (increase on month 3, decrease on month 9 in males). Continued exposure on month 18 (0.2mg B(a)P/l, 1% PCB77) followed by sampling 6 months later showed sustained induction (p<0.001) of hepatic EROD in B(a)P+PCB77 group, which was not seen in B(a)P alone treatment. Thus, PCB77 co-exposure prolonged CYP1A induction and contributed to a persistent oxidative challenge in B(a)P-exposed turbot. The results indicate synergistic effects of polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) exposure in the aquatic environment.

cDNA-directed expression of a functional zebrafish CYP1A in yeast

A cytochrome P450 1A (CYP1A) cDNA was isolated from an adult zebrafish (Danio rerio) library. The 2580-bp clone (GenBank Accession No. AF210727) contained a 62-bp 5'-unstranslated region (UTR), 1557-bp coding region and 962-bp 3'-UTR. The deduced 519-residue protein (calculated molecular weight 58,556, pI = 7.58) shared 74% identity with rainbow trout CYP1A and 57 and 54% identities with mouse and human CYP1A1s, respectively. The zebrafish CYP1A protein coding region was cloned into the pDONR201 entry vector and then transferred to a yeast expression vector pYES-DEST52. Expression of zebrafish CYP1A in Saccharomyces cerevisiae transformants was induced by galactose to a maximum level of 493 pmol CYP1A per mg microsomal protein or about 8 nmol/l of culture. Recombinant CYP1A protein expressed in yeast was mainly in the denatured P420 form under normal microsomal preparation conditions but when the oxygen concentration was reduced in the buffer by degassing and the yeast cells were maintained at less than 10 degrees C, the integrity of the CYP1A was preserved and it exhibited a characteristic reduced CO-difference spectrum maximum at 448 nm. The recombinant zebrafish CYP1A demonstrated 7-ethoxyresorufin O-deethylase (EROD) activity with an apparent Km (Km(app)) and Vmax values at 30 degrees C of 0.31 +/- 0.04 microM and 0.70 +/- 0.10 nmol/min/nmol CYP, respectively. The recombinant protein also metabolized benzo(a)pyrene with a Km(app) and Vmax values of 5.34 +/- 0.58 microM and 1.16 +/- 0.13 nmol/min/nmol CYP, respectively. These results show the recombinant expression of a functional zebrafish CYP in yeast and validated yeast as a host for heterologous expression of zebrafish CYP1A and potentially for other zebrafish CYPs.

Fish as model in pharmacological and biological research

Fish represent the oldest and most diverse classes of vertebrates, comprising around the 48% of the known member species in the subphylum Vertebrata. There are many scientific fields that use fish as models in research, including respiratory and cardiovascular research, cell culture, ecotoxicology, ageing, pharmacological and genetic studies.

In vitro glucuronidation of xanthohumol, a flavonoid in hop and beer, by rat and human liver microsomes

Xanthohumol (XN) is the major prenylated flavonoid of hop plants and has been detected in beer. Previous studies suggest a variety of potential cancer chemopreventive effects for XN, but there is no information on its metabolism. The aim of this study was to investigate in vitro glucuronidation of XN by rat and human liver microsomes. Using high-performance liquid chromatography, two major glucuronides of XN were found with either rat or human liver microsomes. Release of the aglycone by enzymatic hydrolysis with beta-glucuronidase followed by liquid chromatography/mass spectrometry and nuclear magnetic resonance analysis revealed that these were C-4' and C-4 monoglucuronides of XN.

Induction of 7-ethoxyresorufin-O-deethylase activity by planar chlorinated hydrocarbons and polycyclic aromatic hydrocarbons in cell lines from the rainbow trout pituitary

The induction of 7-ethoxyresorufin-o-deethylase (EROD) activity was examined in three rainbow trout pituitary cell lines: RTP-91E, RTP-91F and RTP-2. RTP-91E and RTP-91F were developed from the pituitary of a male and have epithelial-like and fibroblast-like morphologies, respectively. RTP-2, which was described previously, was developed from the pituitary of a female and has an epithelial-like shape. In all cell lines EROD activity was induced by 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD). Immunoblotting with the polyclonal antibody, anti-trout CYP1A1(277-294)/KLH, confirmed induction of a 58-kDa polypeptide. Potential inhibitors of the aryl hydrocarbon receptor, geldanamycin and alpha-naphthoflavone, inhibited EROD induction by TCDD. Other compounds inducing EROD activity were 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and 3-methylcholanthrene (3MC). When judged by the concentration eliciting 50% of the maximal response (EC50), induction was similar in RTP-2 and RTP-91E, and less effective in RTP-91F. Regardless of the cell line, the rank order from most to least potent inducer on the basis of EC50 value was TCDD> or =PCDD>TCDF>PCB 126>>3MC. When induction potencies were expressed relative to TCDD, the values obtained with the pituitary cell lines were similar to previously published values derived with a rainbow trout liver cell line.

Cytochrome P4501A (CYP1A) in teleostean fishes. A review of immunohistochemical studies

Cytochrome P4501A monooxygenase has an important function in the biotransformation of many xenobiotics, including polynuclear aromatic hydrocarbons, and planar organochlorine compounds. The metabolism can lead to detoxification or activation to reactive intermediates. Exposure of fish leads to a receptor-mediated induction of CYP1A gene expression. The induction response can be quantitatively analysed by means of molecular techniques (RT-PCR, Northern Blotting), immunochemical approaches (ELISA, Western Blotting), and enzymatic methods (7-ethoxyresorufin-O-deethylase, EROD) at the catalytical level. Immunohistochemical studies have provided qualitative information on cell and tissue distribution of CYP1A in teleost fish. The liver is the major organ of CYP1A activity in fish, but the enzyme is additionally expressed in numerous extrahepatic organs, including kidney, alimentary canal, heart, gills, olfactory system, gonads, brain and endocrine tissues. In many tissues, the vascular endothelia show a strong CYP1A immunoreactivity. As indicated from immunohistochemical studies with fish embryos and larvae, the typical cell and tissue distribution of CYP1A is established early during fish ontogeny.

Catalytic properties of CYP1A isoforms in the liver of an agnathan (Lampetra fluviatilis) and two species of teleost (Pleuronectes flesus, Anguilla anguilla)

The catalytic activity of CYP1A isoforms and the effect of mammalian CYP1A-specific inhibitors in liver S9 fractions were studied in an agnathan (River lamprey, Lampetra fluviatilis, 30-33 cm) and in two species of teleost fish (European flounder, Pleuronectes flesus, 11-18 cm and common eel, Anguilla anguilla, 31-48 cm). Ethoxyresorufin O-deethylation (EROD), caffeine N-demethylation/C-oxidation and phenacetin O-deethylation (POD) activity increased 3-4-fold in flounders and 17-46-fold in eels, 5 days after fish were injected (i.p.) with 100 mg kg(-1) benzo(a)pyrene (B[a]P). In lampreys, basal EROD activity was very low and no increase in activity was observed following exposure to B[a]P. While the apparent Michaelis constant (K(m)) for each assay showed only small changes after B[a]P injection, maximum reaction velocity (V(max)) values increased by up to 19- and 84-fold for EROD activity, 4- and 35-fold for caffeine-related metabolism and 4- and 19-fold for POD activity in flounders and eels, respectively. The mammalian CYP1A2 inhibitor furafylline (50 microM-1 mM) reduced activity in the EROD, caffeine and POD assays to 65, 21 and 20% of control values in flounders and to 85, 10 and 5% of control values in eels, respectively. By contrast, low concentrations (0.025-0.050 microM) of the mammalian CYP1A1 inhibitor ellipticine completely abolished EROD activity, but had no effect (up to 1 mM) on caffeine metabolism or POD activity in either species. While the inhibitor studies strongly suggest that two separate enzymes are present in flounders and eels, the monophasic Michaelis-Menten kinetics obtained in all the assays imply that only a single CYP1A protein is present that has substrate and inhibitor specificities characteristic of both mammalian CYP1A1 and CYP1A2 isoforms.

The nitroxide stable radical tempo prevents metal-induced inhibition of CYP1A1 expression and induction

Heavy metals are known to provoke oxidative stress in fish liver cells. Because H2O2, OH*- and intracellular superoxide are involved in this oxidation, we investigated the effect of nitroxide radical, 2,2,6,6-tetramethylpiperidinyl-N-oxyl (abbreviated as TEMPO), a cell-permeable agent possessing antioxidant properties, on CYP1A expression in trout (Oncorhynchus mykiss) hepatocytes. 3-methylcholanthrene (3-MC) induced the CYP1A-related EROD activity. This induction was inhibited by concomitant exposure to Cd (II), Cu (II), Pb (II) or Zn (II). CYP1A mRNA levels were also reduced. Simultaneous treatment with 3-MC, a heavy metal and TEMPO suppressed both the inhibition of EROD activity and the decrease of CYP1A mRNA expression. These results suggest a working hypothesis that heavy metals produce multiple oxidative effects, including generation of hydroxyl radicals, which could down-regulate CYP1A1 expression. This metal-induced inhibition was prevented by TEMPO, which might protect trout hepatocytes by scavenging free radicals and thus preventing their inhibitory effects on CYP1A induction and expression.

Potency of dietary indole-3-carbinol as a promoter of aflatoxin B1-initiated hepatocarcinogenesis: results from a 9000 animal tumor study

Indole-3-carbinol (I3C), a metabolite of glucobrassicin found in cruciferous vegetables, is documented as acting as a modulator of carcinogenesis and, depending on timing and dose of administration, it may promote hepatocarcinogenesis in some animal models. In this study we demonstrate that, when given post-initiation, dietary I3C promotes aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in the rainbow trout model at levels as low as 500 p.p.m. Trout embryos (approximately 9000) were initiated with 0, 25, 50, 100, 175 or 250 p.p.b. AFB1 by a 30 min immersion. Experimental diets containing 0, 250, 500, 750, 1000 or 1250 p.p.m. I3C were administered starting at 3 months and fish were sampled for liver tumors at 11-13 months. Promotion at the level of tumor incidence was statistically significant for all dietary levels, except 250 p.p.m. Relative potency for promotion markedly increased at dietary levels >750 p.p.m. We propose that more than one mechanism could be involved in promotion and that both estrogenic and Ah receptor-mediated pathways could be active. The estrogenicity of I3C, measured as its ability to induce vitellogenin (an estrogen biomarker in oviparous vertebrates) was evident at the lowest dietary level (250 p.p.m.), whereas CYPIA (a P450 isozyme induced through the Ah receptor pathway) was not induced until dietary levels of 1000 p.p.m. Therefore, at lower dietary levels, promotion by I3C in this model could be explained by estrogenic activities of I3C acid derivatives, as it is known that estrogens promote hepatocarcinogenesis in trout. Much stronger promotion was observed at high dietary I3C levels (1000 and 1250 p.p.m.), at which levels both CYP1A and vitellogenin were induced.

Immunochemical approaches to studies of CYP1A localization and induction by xenobiotics in fish

There is an increasing understanding that polynuclear aromatic hydrocarbons (PAHs) and organochlorine compounds (like polychlorinated biphenyls (PCBs), certain pesticides and dioxins) in the aquatic environment may lead to physiological and pathological effects such as immunological disturbances, effects on reproduction and development, and even neoplasms. Exposure to pollutants may have consequences at all levels in the biological organization, from the cellular level over effects on the individual organism, population, to the entire ecosystem. The cytochrome P450 system (CYP or P450) has an essential function in the biotransformation of endogenous and exogenous compounds. The fact that many different environmental pollutants induce de novo synthesis of cytochrome P450 1A (CYP1A) proteins in fish, gives these enzymes an interesting position in aquatic toxicology. Many investigations concerning the CYP1A system in fish have been performed over the last two decades, demonstrating its usefulness as a biomarker for aquatic pollution. A general overview of the biochemical and toxicological aspects concerning the cytochrome P450 system will be given here, followed by a more detailed description of CYP1A induction responses in fish. Ecotoxicological consequences of CYP1A induction and the use of immunochemical techniques for CYP1A detection as a biomarker in environmental monitoring will be discussed.

Preparation of highly purified cytochrome P4501A1 from leaping mullet (Liza saliens) liver microsomes and its biocatalytic, molecular and immunochemical properties

Cytochrome P4501A1 was purified to electrophoretic homogeneity from the liver microsomes of feral fish leaping mullet (Liza saliens) collected in Izmir Bay, Aegean coast of Turkey. Purification of cytochrome P4501A1 involved anion exchange chromatography of Emulgen 913-cholate solubilized microsomes on first- and second-DEAE-cellulose columns, hydrophobic interaction chromatographies of the partially purified cytochrome P4501A1 on Porapak Q and phenyl-Sepharose CL-4B and further purification on adsorption chromatography on the hydroxylapatite column. Finally, it is further concentrated and purified on the third DEAE-cellulose column. The purified cytochrome P4501A1 was characterized with respect to spectral, electrophoretic, immunochemical and biocatalytic properties. Cytochrome P4501A1, purified 32-fold with a specific content of 15-17 nmoles P450 (mg protein)-1, produced a single band on SDS-polyacrylamide gel electrophoresis having monomer molecular weight of 58,000 +/- 500. Absolute absorption spectrum of the purified cytochrome P4501A1 fractions showed maximal absorption at 417.5 nm and CO-difference spectrum of dithionite-reduced cytochrome P4501A1 gave a peak at 448 nm. Purified P4501A1 was found to be active in the O-deethylation of 7-ethoxyresorufin in the reconstituted system containing purified fish liver cytochrome P450 reductase and synthetic lipid. However, it was unable to catalyze the oxidation of the other monooxygenase substrates such as benzphetamine and aniline known to be specific for the other isozymes. Purified L. saliens liver microsomal cytochrome P4501A1 showed strong cross-reactivity with the antibodies directed against the cytochrome P4501A1 homologues purified from other teleost species such as rainbow trout and scup. Spectral, electrophoretic, immunochemical and biocatalytic properties of the purified cytochrome P4501A1 strongly suggested that it is the CYP1A1 in the L. saliens liver.

Rainbow trout cytochrome P450s: purification, molecular aspects, metabolic activity, induction and role in environmental monitoring

Cytochromes P450 (P450s or CYPs) constitute a superfamily of heme-thiolate proteins that play important roles in oxidative metabolism of endogenous and exogenous compounds. This review provides some limited history but addresses mainly the research progress on the cytochrome P450s in rainbow trout (Oncorhynchus mykiss), their purification, structures at the primary level, role in metabolism, responses to chemicals and environmental pollutants, application to biomonitoring and the effect of various factors on their expression or activities. Information obtained to date suggests that the rainbow trout P450 systems are as complex as those seen in mammals. Fourteen P450s have been purified from liver or trunk kidney to relatively high specific content. cDNAs belonging to seven different P450 families have been documented from trout liver, kidney and ovary. Two CYP1A genes, nine cDNAs containing open reading frames, and a cDNA fragment were entered into GenBank. Among them, CYP2K1, CYP2K3, CYP2K4, CYP2M1, CYP3A27 and CYP4T1 are the most recently described forms. CYP2K1, CYP2M1 and CYP4T1 represent newly identified P450 subfamilies first described in the rainbow trout. In many cases, the cloned rainbow trout P450s have subsequently been expressed in heterologous expressions systems such as COS-7 cells, yeast and baculovirus infected insect cells. Some of the overexpressed P450 isoforms have been partially characterized. Potential future research directions are discussed.

CYP2M1: cloning, sequencing, and expression of a new cytochrome P450 from rainbow trout liver with fatty acid (omega-6)-hydroxylation activity

A cDNA clone was isolated from a female rainbow trout liver lambda g tau 11 library using polyclonal antibodies raised against rainbow trout cytochrome P450 LMC1. This 2149-nucleotide clone contained an open reading frame encoding a protein of 499 amino acids with a calculated M(r) of 56,850 Da. On the basis of cytochrome P450 (P450) amino acid sequence comparisons, this rainbow trout P450 was assigned by the P450 Nomenclature Committee to a new P450 subfamily designated as CYP2M1. Northern blot results suggest that the expression of CYP2M1 at the transcriptional level was generally sex, tissue, and age specific. By use of a full-length CYP2M1 cDNA probe, it was observed that this cDNA hybridized strongly to a single 2.2-kb transcript in juvenile female rainbow trout trunk kidney and in liver from juvenile and sexually mature trout from both sexes. Negligible amounts of mRNA hybridizable to CYP2M1 cDNA were found in the juvenile and sexually mature male trunk kidney. cDNA-directed expression in COS-7 cells and of recombinant baculovirus in insect cells produced a protein that was reactive with rabbit anti-trout P450 LMC1 polyclonal antibody and exhibited the unique (omega-6)-hydroxylation toward lauric acid previously observed with rainbow trout P450 LMC1.

Xenobiotics and xenoestrogens in fish: modulation of cytochrome P450 and carcinogenesis

As is the case with mammals, an ever-increasing number of cytochromes P450 (CYPs) are being characterized from fish. The focus of work on fish CYPs has been primarily on environmental induction of CYP1A by pollutants such as the polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and dibenzofurans. This response has been the basis for a sensitive biomonitoring tool of ecosystem health for a number of years. Studies have documented a correlation between CYP1A induction, pollutant levels and tumor incidence, especially in bottom-dwelling species. The rainbow trout has been utilized as a tumor model to document the role of CYP1A modulation in the inhibition or promotion of cancer. Fish are also very responsive to the class of chemicals known as xenoestrogens. Recent evidence is presented documenting the modulation of CYPs by xenoestrogens and their potential role as modulators of the tumor response. In this paper, we summarize the current knowledge concerning the occurrence of CYPs in fish and focus on the role of CYP1A induction in environmental monitoring of various genotoxic carcinogens and in the modulation of cancer in the trout model. Finally, the important class of aquatic pollutants known as xenoestrogens have now been shown to modulate CYP levels perhaps leading to alterations in tumor response or other adverse effects.

Evaluation of chemicals as inhibitors of trout cytochrome P450s

This study examined the ability of several inhibitors of mammalian cytochrome P450s to affect hepatic P450-mediated monooxygenase activities in microsomes from beta-naphthoflavone (BNF)-treated rainbow trout. Three monooxygenase activities, namely, lauric acid (omega-1)-hydroxylase (LA-OH), 7,12-dimethylbenz[a]anthracene hydroxylase (DMBA-OH), and progesterone 6beta-hydroxylase (PROG-OH) activities were used as functional markers for trout hepatic CYP2K1, CYP1A1, and CYP3A27, respectively. The chemicals that were examined for their inhibitory effects were reversible, quasi-irreversible, or irreversible inhibitors of mammalian P450. At 100 microM concentration, the reversible inhibitors (ketoconazole, miconazole, and clotrimazole) were most potent in inhibiting LA-OH activity. These imidazole compounds, as well as ellipticine, parathion, and alpha-naphthoflavone, were the strongest inhibitors of DMBA-OH and PROG-OH activities. In addition, isosafrole, piperonyl butoxide, gestodene, 17alpha-ethynylestradiol, 1-aminobenzotriazole, and 5,8,11,14-eicosatetraynoic acid strongly inhibited PROG-OH activity. The global inhibitors, metyrapone, chloramphenicol, and allylisopropylacetamide, had very little or no inhibitory effect on trout LA-OH and DMBA-OH activities. Triacetyloleandomycin, a CYP3A inhibitor, did not affect PROG-OH activity catalyzed by trout CYP3A27. Diethyldithiocarbamate was an activator of LA-OH activity. None of the above enzyme activities was selectively inhibited by any of the chemicals when used at a concentration of 100 microM. There was no difference in the inhibition of LA-OH activities by representative P450 inhibitors between liver microsomes from untreated trout and BNF-treated trout. The results of this study suggest that inhibition data from mammalian studies could not be directly extrapolated to trout and likely other fish species and that care must be observed when mammalian P450 inhibitors are used to determine the participation of P450s in the metabolism and toxicity of xenobiotics in nonmammalian species.

Beta-naphthoflavone induced CYP1A1 and 1A3 proteins in the liver of rainbow trout (Oncorhynchus mykiss)

1. Two CYP1A proteins, designated HAP 1 and HAP 2, were isolated from the liver of the beta-naphthoflavone (BNF)-treated rainbow trout. The proteins were initially resolved by chromatography on a DEAE sepharose column and were further purified by hydroxyl-apatite chromatography. 2. Both HAP 1 and HAP 2 proteins exhibited high 7-ethoxyresorufin, methoxy-resorufin and phenacetin O-dealkylase activities and were good catalysts for the oxidation of 7,12-dimethylbenz[a]anthracene (DMBA). No qualitative difference was observed between the two proteins in their ability to catalyse the formation of the individual metabolites of DMBA. 3. The two purified proteins showed identical amino acid sequence for the first 13 amino acids. However, the 14th amino acid was valine for HAP 1 protein and alanine for HAP 2 protein. 4. Alignment of the amino acid sequences showed that HAP 1 protein was identical to the deduced protein of the previously reported trout CYP1A2 (renamed CYP1A1) gene for the first 24 amino acids at the N-terminal region. HAP 2 protein corresponded to the deduced protein sequence of CYP1A3 gene for the first 14 amino acids. However, unlike the deduced sequences of CYP1A1 and 1A3 the N-terminal methionine was absent in the purified proteins. 5. We conclude that HAP 1 and HAP 2 are the products of the CYP1A1 and CYP1A3 genes respectively, and are found in the liver of the BNF-treated rainbow trout.

Experimental chemical carcinogenesis in fish

Experimental carcinogenesis using fish species as alternative models is a dynamic field of research. The 1940's expansion of synthetic chemical producing industries coincided with a number of pollution-associated fish neoplasia epizootics, with polycyclic aromatic hydrocarbons as significant components of contaminated sediment in several cases. Epizootics of primarily liver and skin neoplasia in benthic species near coastal urban or industrial areas indicated the sensitivity of fish species to known mammalian carcinogens. Stressing a mechanistic approach, investigators have used data compiled from epizootics as the backbone of current research efforts to define carcinogenesis in fish species. With liver as the focus, patterns of neoplastic development similar to those seen in rodent bioassays have been induced in various fish species by genotoxic carcinogens. Similarities between fish and rodent models include chemical and species-specific responses to exposure and the development of predictable preneoplastic and neoplastic lesions. The expression of molecular molecules related to carcinogenesis is currently under investigation, which includes alterations in certain proteins, enzyme activity, and oncogene/tumor suppressor gene function. The potential for the application of research findings to both human and environmental health issues makes fish species attractive and valuable alternative models in carcinogenesis and toxicity research.

Induction of hepatic CYP1A by indole-3-carbinol in protection against aflatoxin B1 hepatocarcinogenesis in rainbow trout

This study examined the significance of hepatic cytochrome P4501A (CYP1A) induction in the inhibition of aflatoxin B1 (AFB1)-DNA adduction by indole-3-carbinol (I3C) in rainbow trout. I3C, fed prior to [3H]AFB1 exposure, provided dose-dependent inhibition of hepatic AFB1-DNA binding, which appeared to vary inversely with hepatic CYP1A-mediated ethoxyresorufin O-deethylase (EROD) activity (r = -0.81, P = 0.051). However, 1000 ppm dietary 13C inhibited AFB1-DNA adduction without detectably inducing CYP1A protein or EROD activity. Dietary I3C was found to inhibit AFB1-DNA adduction by approximately 50%, whether [3H]AFB1 was injected ip 1, 2, 3, 5 or 7 days after the onset of I3C feeding, yet hepatic EROD activity was only transiently induced over this period and was not correlated with AFB1-DNA inhibition. Microsome-catalysed AFB1-DNA binding in vitro did correlate inversely with EROD activity in microsomes from control- and I3C-treated trout (r = -0.955, P = 0.01), but data obtained using microsomes from beta-naphthoflavone-treated trout suggest that this observation may not be indicative of a cause-and-effect relationship. I3C-mediated reduction in covalent binding was not due to I3C derivatives in the microsomal preparation or to reduced CYP protein levels, but may reflect a lower microsomal catalytic capacity for AFB1 epoxidation as a result of enzyme inactivation. In addition, the major I3C derivative found in liver, 3,3'-diindolylmethane, has been shown to be a non-competitive inhibitor of EROD, and of enzymes that catalyse AFB1 epoxidation. These findings indicate little, if any, role for CYP1A induction in the inhibition of AFB1 carcinogenicity in rainbow trout by levels of I3C likely to be encountered in cruciferous vegetables.

The anticarcinogen 3,3'-diindolylmethane is an inhibitor of cytochrome P-450

Dietary indole-3-carbinol inhibits carcinogenesis in rodents and trout. Several mechanisms of inhibition may exist. We reported previously that 3,3'-diindolylmethane, an in vivo derivative of indole-3-carbinol, is a potent noncompetitive inhibitor of trout cytochrome P450 (CYP) 1A-dependent ethoxyresorufin O-deethylase with Ki values in the low micromolar range. We now report a similar potent inhibition by 3,3'-diindolylmethane of rat and human CYP1A1, human CYP1A2, and rat CYP2B1 using various CYP-specific or preferential activity assays. 3,3'-Diindolylmethane also inhibited in vitro CYP-mediated metabolism of the ubiquitous food contaminant and potent hepatocarcinogen, aflatoxin B1. There was no inhibition of cytochrome c reductase. In addition, we found 3,3'-diindolylmethane to be a substrate for rat hepatic microsomal monooxygenase(s) and tentatively identified a monohydroxylated metabolite. These observations indicate that 3,3'-diindolylmethane can inhibit the catalytic activities of a range of CYP isoforms from lower and higher vertebrates in vitro. This broadly based inhibition of CYP-mediated activation of procarcinogens may be an indole-3-carbinol anticarcinogenic mechanism applicable to all species, including humans.

Regulation of hepatic cytochrome P4501A by indole-3-carbinol: transient induction with continuous feeding in rainbow trout

This study investigated the kinetics of hepatic cytochrome P-4501A (CYP1A) induction in rainbow trout by indole-3-carbinol (I3C), a natural tumour modulator from cruciferous vegetables, and its low pH reaction products 3,3'-diindolylmethane (I33'), 5,6,11,12,17,18-hexahydrocyclononal[1,2-b:4,5-b':7,8-b"]triindo le cyclic trimer (CT), and the unresolved I3C acid reaction mixture (RXM). RXM, CT and I33' were potent inducers of total embryonic CYP1A following direct microinjection, and of fingerling hepatic CYP1A following ip exposure, whereas I3C itself produced only a transient and relatively weak induction. It is also reported for the first time that dietary I3C induced hepatic CYP1A and its associated ethoxyresorufin O-deethylase (EROD) activity in trout but, again, the induction was weak and transient even with continuous I3C feeding. Mechanism studies and mixed exposures with the Ah agonist beta-naphthoflavone indicated that transient induction by I3C was not due to diet ageing, but appears to involve inactivation of the Ah inductive pathway and irreversible inactivation of CYP1A-mediated EROD activity by I3C-derived metabolites. Thus, I3C derivatives exhibit dual capacities for CYP1A induction and inhibition in trout.

Use of 7-alkoxyphenoxazones, 7-alkoxycoumarins and 7-alkoxyquinolines as fluorescent substrates for rainbow trout hepatic microsomes after treatment with various inducers

Various fluorescent substrates have been used as specific indicators of induction or activity of different cytochrome P450 isozymes in both fish and mammalian species. In an attempt to identify additional definitive fluorescent substrates for use in fish, we examined a series of 7-alkoxyphenoxazones, 7-alkoxycoumarins and 7-alkoxyquinolines as substrates in O-dealkylation assays with hepatic microsomes from rainbow trout (Oncorhynchus mykiss). Microsomes were prepared after 48 hr of treatment with beta-naphthoflavone (beta-NF), pregnenolone-16 alpha-carbonitrile (PCN), phenobarbital (PB), isosafrole (ISF), or dexamethasone (DEX). Total P450 spectra were obtained, and spectral binding studies were performed. Microsomal O-dealkylation rates were greater after ISF treatment than after beta-NF treatment for 7-methoxy-, 7-ethoxy-, 7-propoxy- and 7-benzyloxyphenoxazones but not for 7-butoxyphenoxazone. DEX treatment resulted in a significant elevation of pentoxyphenoxazone metabolism (about a 144-fold increase) compared with microsomes induced by beta-NF (11-fold) and ISF (37-fold). The rates of dealkylation of the alkoxyphenoxazones by ISF-treated microsomes occurred in the following order: methoxy > ethoxy > propoxy > benzxyloxy > butoxy > pentoxy. When beta-NF-treated microsomes were used, the 7-alkoxyphenoxazones were metabolized as follows: methoxy > ethoxy > propoxy > butoxy > benzyloxy = pentoxy, while the order of metabolism of the 7-alkoxycoumarins was: ethoxy >> butoxy > propoxy = methoxy > benzyloxy > pentoxy. None of the other treatments significantly increased the rate of metabolism of any of the alkoxycoumarins. Treatment with beta-NF did not significantly elevate the rate of metabolism of any of the alkoxyquinolines. DEX treatment produced significant elevations in the rate of metabolism of benzyloxy-, ethoxy-, and butoxy- = pentoxy- = propoxyquinoline, in that order. ISF treatment significantly elevated the rate of metabolism of benzyloxy-, methoxy- and butoxyquinoline, in that order. These results suggest that some of these new fluorescent substrates can be used to characterize induction of rainbow trout hepatic microsomal monooxygenase activity by ISF and DEX, in addition to the commonly used ethoxyphenoxazone and ethoxycoumarin for the characterization of induction by beta-NF or other 3-methylcholanthrene-type P450 inducers. Distinction between ISF-type and beta-NF-type inducers in rainbow trout hepatic microsomes may best be made using 7-methoxycoumarin as a substrate. Distinction between ISF-type and DEX-type inducers and between beta-NF-type and DEX-type inducers may best be made using 7-methoxyphenoxazone as a substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo effects of the anesthetic, benzocaine, on liver microsomal cytochrome P450 and mixed-function oxidase activities of gilthead seabream (Sparus aurata)

Gilthead seabreams were exposed to benzocaine, 4-aminobenzoic acid ethyl ester, 57 mg/l in sea water for 3 min, daily, for 2 or 3 consecutive days. The fish were killed 20 hr after the last treatment. Benzocaine treatment for 2 or 3 days resulted in 57% and 67% inhibition of liver microsomal aniline 4-hydroxylase and ethylmorphine N-demethylase activities, respectively. The total cytochrome P450 content of fish liver microsomes was unaltered following the 2-day benzocaine treatment. However, additional 3 min benzocaine treatment on day 3 reduced cytochrome P450 level by 50%. Benzocaine produced type II difference spectra with rabbit liver microsomes. Difference spectra of fish liver microsomes elicited by benzocaine were complex. The position of peak and intensity were greatly influenced by the concentration of benzocaine.

Effects of in vivo benzo(a)pyrene treatment on liver microsomal mixed-function oxidase activities of gilthead seabream (Sparus aurata)

Benzo(a)pyrene [B(a)P] treatment of gilthead seabream, 25 mg/kg, i.p. for 5 consecutive days, did not cause any significant changes in ethylmorphine N-demethylase and aniline 4-hydroxylase activities of liver microsomes. The same treatment did not alter the liver microsomal cytochrome b5 content, NADH-cytochrome b5 reductase and NADPH-cytochrome P450 reductase activities. However, benzo(a)pyrene treatment caused a 2-3-fold increase in 7-ethoxyresorufin O-deethylase (7-EROD) activity of gilthead seabream liver microsomes. Although, upon treatment, total cytochrome P450 content of liver microsomes increased about 1.7-fold in 1990 fall, no such increase was observed in spring 1991. However, a new cytochrome P450 with an apparent M(r) of 58,000 was observed on SDS-PAGE of liver microsomes obtained from benzo(a)pyrene treated gilthead seabream. Besides, in vitro addition of 0.2 x 10(-6) M benzo(a)pyrene to the incubation mixture inhibited 7-ethoxyresorufin O-deethylase activity by 93%. Gilthead seabream liver microsomal 4-ethoxyresorufin O-deethylase activity was characterized with respect to substrate concentration, amount of enzyme, type of buffer used, incubation period and temperature.

Immunological characterization of flavin-containing monooxygenases from the liver of rainbow trout (Oncorhynchus mykiss): sexual- and age-dependent differences and the effect of trimethylamine on enzyme regulation

Polyclonal antibodies raised against flavin-containing monooxygenase (FMO) enzymes purified from pig liver and rabbit lung were used in conjunction with N,N-dimethylaniline (DMA) N-oxidase to better characterize FMO from the liver of rainbow trout (Oncorhynchus mykiss). Two proteins reacted with polyclonal antibodies raised against pig liver FMO (PL-1 and PL-2) and anti-rabbit lung FMO (RL-1 and RL-2). Although there was no difference in DMA N-oxidase observed between sexually mature male and female trout liver microsomes, RL-2 and PL-2 were significantly less than RL-1 and PL-1, respectively, in sexually mature females. FMO activity and protein content increased as fish aged. DMA oxidase and FMO isozymes were unaltered after pretreatment with the endogenous substrate trimethylamine. Since antibodies to the purified mammalian enzymes react with proteins of similar MW in trout, some forms of FMO appear to be structurally conserved through evolution.

Hepatocarcinogenic potency of mixed and pure enantiomers of trans-7,8-dihydrobenzo[a]pyrene-7,8-diol in trout

The hepatocarcinogenic potency of pure and racemic trans-7,8-dihydrobenzo[a]pyrene-7,8-diol was investigated in embryos and sac-fry rainbow trout. Embryos microinjected with (+/-)-trans-7,8-dihydrobenzo[a]pyrene-7,8-diol ((+/-) BP-7,8-DHD) developed liver tumors 9 months after hatching. However, this exposure protocol resulted in high mortalities. Microinjection of newly hatched sac-fry with 0.01-1.0 microgram of (+/-) BP-7,8-DHD resulted in a dose-dependent production of liver tumors (0-13%) similar to the results with embryos but without the problem of high mortalities. Co-injection of sac-fry with (+/-) BP-7,8-DHD and either beta-naphthoflavone or carbon tetrachloride significantly enhanced the tumor response (approx. 3-fold). The relative carcinogenic potencies of the pure (+) and (-) enantiomers of BP-7,8-DHD were evaluated by microinjection into sac-fry at doses of 0.5-5.0 micrograms. The results demonstrated that the (-) enantiomer was 4-18 times more potent than the (+). Mixed carcinomas were the most prevalent liver tumors observed. These results demonstrate that trout embryos and sac-fry are both responsive to hepatocarcinogenesis initiation by injection with BP-7,8-DHD. The marked enhancement seen with co-injection of sac-fry with beta-naphthoflavone or carbon tetrachloride suggests that both cytochrome P-450-dependent and lipid peroxidation-dependent pathways could be involved in bioactivation of this compound, presumably through epoxidation at the 9,10-position. As is the case with mammals, the (-) enantiomer of BP-7,8-DHD is a more potent carcinogen than the (+) enantiomer.

Alterations in lipid peroxidation, antioxidant enzymes, and carcinogen metabolism in liver microsomes of vitamin E-deficient trout and rat

Feeding rainbow trout for 16 weeks a diet in which the levels of vitamin E were reduced 70-fold resulted in marked depletion (18-fold) of vitamin E levels in liver microsomes from these fish. The susceptibility of hepatic microsomes to lipid peroxidation in vitro and the levels of plasma and liver microsomal lipid hydroperoxides generated in vivo were markedly elevated in vitamin E-depleted trout. No appreciable alterations were observed in the liver microsomal cytochrome P450-dependent mixed-function oxidase system or in the fatty acid composition of trout liver microsomal membranes. Livers from rats fed a vitamin E-deficient diet for 10 weeks also had significantly lower levels of microsomal vitamin E. In addition, total cytochrome P450 levels were depressed (15%) and cytosolic glutathione was enhanced (40%) in livers from rats fed the vitamin E-depleted diet. Covalent binding of [3H]-(+)-benzo[a]pyrene-7,8-dihydrodiol to exogenous DNA in vitro was enhanced with liver microsomes from vitamin E-deficient trout and these fish were much more sensitive to the acute toxicity of this carcinogenic polycyclic aromatic hydrocarbon. These results indicate that trout may be a useful model for studying the significance of peroxidative pathways in carcinogenesis and their manipulation by dietary antioxidants.

Catalytic activity and immunochemical quantification of hepatic cytochrome P-450 in β-naphthoflavone and isosafrol treated rainbow trout (Oncorhynchus mykiss)

In addition to catalytical assays, immunochemical techniques have recently been employed to measure induction of the cytochrome P-450 (P450) monooxygenase system in fish with polyaromatic hydrocarbons (PAH). In the present study, polyclonal antibodies were raised against rainbow trout P450IA1. Levels of rainbow trout P450IA1 determined using protein blotting- and ELISA procedures were compared with levels of 7-ethoxyresorufin-O-deethylase (7-EROD) activity in liver microsomes from rainbow trout. These comparisons showed that values of P450A1 were positively correlated (r=0.99 and r=0.97) with 7-EROD activities. In addition, the effects of isosafrol (ISF) or β-naphthoflavone (βNF) treatments on P450 levels in rainbow trout liver were investigated using immunochemical and catalytical methods. ISF treatment induced 7-EROD activity as well as 7-methoxycoumarin-O-demethylase-, 7-ethoxycoumarin-O-deethylase-, 7-propoxy-coumarin-O-depropylase and 7-butoxycoumarin-O-debutylase activities, although to a lesser extent, compared with the βNF treatment. In contrast, immunochemical quantification of rainbow trout P450IA1 protein revealed a slightly different pattern. ISF appeared to be a weak inducer of P450IA1 in rainbow trout compared with βNF. In addition, the degree of inhibition of 7-alkoxycoumarin-O-dealkylase activities in ISF microsomes differed from that measured in control- and βNF microsomes. The discrepancies between catalytic and immunochemical estimates of rainbow trout P450IA1 in ISF treated fish in addition to differencs between specific inhibitory pattern by specific polyclonal antibodies raised against rainbow trout P450IA1, indicate that important differences exists between the responses induced by βNF- and ISF treatments in the rainbow trout liver.

Comparison of rainbow trout and mammalian cytochrome P450 enzymes: evidence for structural similarity between trout P450 LMC5 and human P450IIIA4

Studies were undertaken to determine the immunochemical relationship between constitutive trout cytochrome P450s and mammalian cytochrome P450IIIA enzymes. Polyclonal antibodies (IgG) generated against trout P450 LMC5 reacted strongly with P450IIIA1 in dexamethasone-induced rat liver microsomes and with P450IIIA4 in human liver microsomes in immunoblots. In contrast, rabbit anti-P450 LMC1 IgG did not recognize these proteins in rat and human liver microsomes. Reciprocal immunoblots using anti-rat P450IIIA1 showed that this antibody does not recognize trout P450 LMC1 or LMC5. However, anti-human P450IIIA4 IgG was found to cross react strongly with P450 LMC1 and LMC5. Progesterone 6 beta-hydroxylase activity of trout liver microsomes, a reaction catalyzed by P450 LMC5, was markedly inhibited by anti-P450IIIA4 and by gestodene, a mechanism-based inactivator of P450IIIA4. These results provide evidence for a close structural similarity between trout P450 LMC5 and human P450IIIA4.

Characterization and induction of xenobiotic metabolizing enzyme activities in a primary culture of rainbow trout hepatocytes

1. A primary cell culture from rainbow trout (Oncorhynchus mykiss) liver was prepared and evaluated for biotransformation of xenobiotics. 2. The hepatocytes maintained cytochrome P-450 content, as well as their cytochrome P-450-dependent activities, stable for 5-6 days in serum-free medium. Protein and glutathione levels, as well as other enzyme activities important for biotransformation, were close to their fresh cell levels throughout the culture period. 3. The cells were also responsive to cytochrome P-450 inducers. Both beta-naphthoflavone (BNF) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused an increase in ethoxyresorufin O-deethylase (EROD) activity, which was dose-dependent over the concentration ranges of 3.6-360 nM and 2.5-100 pM, respectively. The induced activities in BNF exposed cells returned to basal levels within 48 h after replacing the medium with a BNF-free medium. Exposure of cells to TCDD (100 pM) for 48 h induced EROD activity which, in contrast to response of BNF-exposed cells, continued to increase after the medium had been replaced with TCDD-free medium. 4. The results show that trout hepatocytes in primary culture afford a reliable in vitro method for studying the regulation and functions of xenobiotic biotransformation enzymes, and for defining toxic effects of aquatic pollutants in cells.

Immunochemical cross-reactivity ofβ-naphthoflavone-inducible cytochrome P450 (P450IA) in liver microsomes from different fish species and rat

Antibodies prepared against the major β-naphthoflavone (BNF)-inducible cytochrome P450 (P450) forms from three species of fish (rainbow trout, Atlantic cod, and scup) well separated in teleost phylogeny, were used to investigate the immunochemical relatedness of liver microsomal P450 in different species of BNF-treated fish and rat. Rabbit polyclonal IgG against all three P450s and mouse monoclonal antibodies prepared against scup P450E were employed in this study. Liver microsomes were prepared from BNF-treated specimens of hagfish, herring, rainbow trout, cod, scup, perch, plaice and rat. With Western blotting it was shown that the various antibodies cross-reacted with a protein band in liver microsomes in the P450-region of each of the BNF-treated fish species. The apparent molecular weight of the cross-reacting proteins showed differences within the range 54,000-59,000 daltons. The effects of the different antibodies on the microsomal BNF-inducible 7-ethoxyresorufin O-deethylase (EROD) activity gave inhibition patterns that reflected to a certain extent the phylogenetic relationship of the species investigated. In rat microsomes a protein band of relative molecular mass similar to rat P450c (Mr=54,000) was recognized by all antibodies. In addition, a second band of lower molecular mass was strongly recognized by anti-cod P450c antibodies, and faintly stained with anti-rainbow trout P450LM4b IgG and anti-scup P450E MAb 1-12-3. This band could correspond to rat P450d, the isosafrole-inducible rat isoenzyme. Considering the long separate evolutionary history of some of these fishes (50-200 million years), the results demonstrate that certain antigenic epitopes in the BNF-inducible P450 isoenzymes have been strongly conserved during the evolution of fish species. These conserved epitopes seem however not to be directly involved in the measured EROD activities. Furthermore, the results suggest that the BNF-inducible P450s in fish contain regions with structural similarity to the homologous counterpart that has evolved through gene duplication into a P450 family in mammals containing at least two gene products (the P450IA gene family).

Initial purification and characterization of hepatic microsomal cytochrome P-450 from BNF-treated perch (Perca fluviatilis)

1. A procedure was developed for isolating and purifying cytochrome P-450 from hepatic microsomes of BNF-treated perch, using modified versions of the methods of Williams and Buhler (1982. Biochim. biophys. Acta 717, 398-404) and Goksøyr (1985. Biochim. biophys. Acta 850, 409-417). 2. Following chromatography on phenyl-Sepharose CL 4B and DEAE-Sepharose CL-6B, the major peaks, fractions b and c, were resolved into five fractions, possibly representing different isoenzymes, by a FPLC with a strong anion exchange column (Mono Q). 3. These fractions have been characterized on the basis of their spectral, electrophoretic and immunological properties. 4. The purified form of cytochrome P-450 in fraction V from perch liver showed a number of similarities to cytochrome P-450c, the major BNF-inducible cytochrome P-450 in cod liver. 5. Therefore we suggest that this purified form of cytochrome P-450 is a BNF-induced form in perch and that it is closely related to the gene subfamily cytochrome P-450 IA1.

Interaction of carbon tetrachloride with beta-naphthoflavone-mediated cytochrome P450 induction in winter flounder (Pseudopleuronectes americanus)

The interaction between beta-naphthoflavone induction (BNF: 100 mg/kg) and carbon tetrachloride (CCl4; 1 ml/kg) hepatotoxicity was examined in the flounder. Treatment groups composed of control, BNF, CCl4, and BNF/CCl4 were compared in terms of cytochrome P450 isozyme content (LM4b; LM2), catalytic activity, isozyme distribution. SGOT-SGPT levels, and pathology. CCl4 administration resulted in significant reductions in both the constitutive P450 (LM2) and the BNF-inducible isozyme (LM4b) as well as elevations in SGPT and SGOT levels. The decline in LM4b isozyme content was reflected by stoichiometric decreases in ethoxyresorufin-O-deethylase activities. BNF/CCl4 coadministration was protective in part against CCl4 hepatotoxicity. Immunohistochemistry indicated that LM4b was diffusely distributed throughout the liver. These interactions have demonstrated a multiple P450 isozyme involvement, the protective nature of BNF against CCl4 hepatotoxicity in the flounder, the ability to maintain an inductive response in face of CCl4 coadministration, and the diffuse distributional pattern of LM4b in the flounder liver.

Regional distribution of microsomal xenobiotic and steroid metabolism in kidney microsomes from rainbow trout

The regional distribution of microsomal cytochrome P-450-mediated reactions with exogenous and endogenous substrates in the kidney of rainbow trout was studied. The cytochrome P-450-dependent 7-ethoxyresorufin- and 7-ethoxycoumarin-0-deethylase activities were significantly higher (3-4 and 10-14 fold, respectively) in the trunk kidney than in the head kidney, whereas ethylmorphine-N-demethylase activity was evenly distributed along the kidney. The microsomal cytochrome P-450-dependent steroid hydroxylases and steroid reducing enzymes when using androstenedione as substrate also exhibited a regional distribution in trout kidney. The 6β- and 16-hydroxylase activities as well as the 5α-reductase and 17 hydroxysteroid oxidoreductase activities were higher in the anterior part of the trunk kidney than in the head kidney and posterior trunk kidney.

Immunological characterization of constitutive isozymes of cytochrome P-450 from rainbow trout. Evidence for homology with phenobarbital-induced rat P-450s

Immunoglobulin G fractions (IgGs), isolated from rabbits immunized against hepatic cytochrome P-450 isozymes were used to investigate the immunochemical homology among trout P-450s and between trout and rat P-450s. The antigens used for immunization were five constitutive trout P-450s (LMC1 to LMC5), one beta-naphthoflavone (BNF)-inducible trout P-450 (LM4b), and one phenobarbital-induced rat P4500IIB1 (PB-B). In the enzyme-linked immunosorbent assay (ELISA), strong cross-reactivity was observed between anti-LMC2 IgG and P-450 LMC1, and between anti-LMC3 IgG and P-450 LMC4. There was little or no cross-reactivity of anti-LMC5 IgG with other trout P-450s. Trout P-450 LM4b was not recognized by any of the antibodies against constitutive trout P-450s. Antibodies to P-450 LMC1 and P450 LMC2 cross-reacted strongly with rat P450IIB1 and with proteins of PB-induced rat liver microsomes. Rat P450IA1 (BNF-B) did not cross-react with anti-LMC1 or anti-LMC2 IgG. These cross-reactions were essentially confirmed by immunoblot (Western blot) analysis. Western blots of PB-induced rat liver microsomes probed with anti LMC1 revealed two major immunoreactive proteins in the P-450 region, one of which co-migrated with rat P450IIB1. P450IIB1 itself cross-reacted strongly with anti-LMC1 IgG. In control rats, a single protein band cross-reacted poorly with anti-LMC1 IgG. Antibodies to LMC1 and LMC2 did not cross-react with rat P450IA1 in Western blots. The antigenic epitopes in rat P450IIB1 recognized by anti-LMC1 IgG and anti-LMC2 IgG are probably not located at or near the active site of the enzyme since these antibodies did not inhibit benzphetamine N-demethylase activity of P450IIB1 or of PB-induced rat liver microsomes. In general, our results demonstrate: (1) the presence of a significant homology between LMC1 and LMC2, and between constitutive trout P-450 (LMC1) and PB-induced rat P-450 (P450IIB1); and (2) distant homology between constitutive trout P-450s and constitutive rat P-450s or BNF-induced rat P-450s.

A comparison of hepatic P450 induction in rat and trout (Oncorhynchus mykiss): delineation of the site of resistance of fish to phenobarbital-type inducers

1. A comprehensive approach was taken to delineate the site of refractivity of trout to phenobarbital-type (PB-type) hepatic monoxygenase (MO) inducers. 2. Model inducers beta-naphthoflavone (BNF; 3-MC-type), and PB as well as the polychlorinated biphenyl isomers, 3,4,5,3',4',5'-hexachlorobiphenyl (3,4,5-HCB; 3-MC-type) and 2,4,5,2',4',5'-hexachlorobiphenyl (2,4,5-HCB; PB-type) were used to assess MO activities, total cytochromes P450, and [35S]-methionine incorporation into de novo synthesized microsomal protein in both trout and rats. 3. In rainbow trout immunodetection of P450 isozymes and nucleic acid hybridization of rainbow trout P(1)450 mRNA using pfP(1)450-3' (trout 3-MC-inducible, P450IA1 gene) and genomic DNA using pfP(1)450-3' or pSP450-oligo (rat PB-inducible, P450IIB1 gene) cDNAs were carried out. 4. In rainbow trout, PB and 2,4,5-HCB do not increase hepatic MO activities, total cytochromes P450, de novo synthesis of microsomal protein, levels of P450 isozymes, or levels of P(1)450 mRNA. 5. Rainbow trout have, within their genome, DNA with sequence(s) similar to rat P450IIB1, but inducibility of this P450 in trout by PB-type inducers is lacking.

The effect of thermal acclimation on the activity of arylhydrocarbon hydroxylase in rainbow trout (Oncorhynchus mykiss)

1. The possibility that temperature acclimation (to 10 or 18 degrees C for 28 days) would alter the cytochromes P-450 of rainbow trout was addressed. 2. The specific content of LM4b (P-450 IA1), the trout isozyme responsible for activation of polynuclear aromatic hydrocarbons, was lower in 18 degrees C fish than it was in 10 degrees C fish. 3. Kinetic analysis of aryl hydrocarbon hydroxylase indicated that, while thermal acclimation caused no change in Vmax, it lowered the apparent Km of this enzyme for benzo[a]pyrene when assayed at acutely shifted temperatures. 4. Thermal acclimation of fish may have significance when feral populations are subjected to acute temperature shifts.

Mechanisms of anti-carcinogenesis by indole-3-carbinol. Studies of enzyme induction, electrophile-scavenging, and inhibition of aflatoxin B1 activation

The induction of oxidation and conjugation enzymes, the scavenging of carcinogen electrophiles, and the inhibition of aflatoxin B1 (AFB1) activation were examined as possible mechanisms of anti-carcinogenesis by indole-3-carbinol (I3C). Liver microsomal 7-ethoxycoumarin O-deethylase and 7-ethoxyresorufin O-deethylase activities were not induced significantly in rainbow trout fed diets containing 500-2000 ppm I3C for 8 days compared to trout fed the control diet. Furthermore, no detectable changes in the specific contents of cytochrome P-450 isozymes LM2 and LM4b, as measured by Western-blotting and immunoquantitation, were found in liver microsomes following dietary I3C administration. Dietary I3C had no significant effect on liver microsomal uridine diphosphate-glucuronyl-transferase activity, measured using the substrates 1-naphthol and testosterone, or on cytosolic glutathione S-transferase activity, measured using the substrate styrene oxide. The ability of I3C or its acid reaction products (RXM; generated by the reaction of I3C with HCl) to act as scavengers for the direct alkylating agent AFB1-8,9-Cl2 was examined. Addition of I3C or RXM to in vitro incubations did not inhibit the covalent binding of AFB1-8,9-Cl2 to calf thymus DNA. Kinetic analyses of microsome-mediated binding of AFB1 to DNA in vitro indicated that RXM inhibited the metabolic activation of AFB1. RXM increased the apparent Km for the AFB1-DNA binding reaction without changing the associated Vmax; the apparent Km values at 0, 3.5, 35, and 350 microM RXM were 35, 38, 66, and 86 microM for trout liver microsomes. RXM also inhibited the activation of AFB1 by rat liver microsomes, but I3C was not an effective inhibitor against AFB1-DNA binding mediated by either rat or trout liver microsomes. The results of the present study indicate that inhibition of microsome-activated AFB1 binding to DNA by I3C products may be of significant importance in I3C inhibition of hepatocarcinogenesis in trout and other species. The inhibition of carcinogen activation by I3C is contrasted with the mechanism of anti-carcinogenesis by beta-naphthoflavone, which involves induction of xenobiotic metabolizing enzymes.

Cytochromes P-450 of sea birds: cross-reactivity studies with purified rat cytochromes

1. Polyclonal antibodies against rat cytochrome P-450c (IA1), P-450d (IA2), P-450b (IIB1), P-450h (IIC11) and P-450j (IIE1), were used to probe liver microsomes prepared from six sea bird species collected from the Irish Sea between 1978 and 1988. 2. Significant cross-reactivity in all the sea bird species was seen only with antibodies to P450 IA1. Expression of cross-reactivity proteins was highly variable between individual birds, which show evidence of environmental induction. 3. Shared epitopes to P450 IA1 and IA2 were seen on a single protein expressed in liver microsomes from the cormorant (Phalacrocorax carbo). 4. Antibodies against members of rat P450 gene family II showed a small degree of cross-reactivity with sea bird microsomes. Antibodies against P450 IIB1 and IIC11 showed weak cross-reactivity in all species with little inter-individual variation. Antibodies to P450 IIE1 showed no cross-reactivity in any bird species. 5. P450 gene family I appears to be well represented in sea birds while P450 gene family II is not well developed in this group of lower vertebrates.

Determination of multiple forms of cytochrome P-450 in microsomes from the digestive gland of Cryptochiton stelleri

Polyclonal antibodies raised against purified trout cytochromes P-450 (P-450) LM2 (anti-LM2) and LM4b (anti-LM4b) were used in Western blot analyses with digestive gland microsomes from control and beta-naphthoflavone (BNF)-treated gumboot chitons Cryptochiton stelleri. An increase and decrease in staining intensity subsequent to treatment with anti-LM4b and anti-LM2, respectively, was observed in digestive gland microsomes from BNF-treated chiton. Thus, there appears to be at least two forms of P-450 in microsomes from the digestive gland of Cryptochiton; one of which is induced by BNF and perhaps is involved in benzo(a)pyrene (BP) biotransformation, and another form which is inhibited by BNF.

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.