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

Effects of model aromatizable (17α-methyltestosterone) and non-aromatizable (5α-dihydrotestosterone) androgens on the adult mummichog (Fundulus heteroclitus) in a short-term reproductive endocrine bioassay

Androgens originating from pulp mill processing, sewage treatment facilities and agricultural activities have the potential for discharge into aquatic receiving environments. To assess androgen effects on reproductive endocrine status in fish in estuarine environments, male and female adult northern mummichog (Fundulus heteroclitus macrolepidotus) were exposed to an aromatizable androgen (17α-methyltestosterone; MT) and a non-aromatizable androgen (5α-dihydrotestosterone; DHT) in a short-term reproductive endocrine bioassay. Fish were nominally exposed to 10 μg/L or 100 μg/L DHT, or 0.1 μg/L or 1 μg/L MT for 14 days during gonadal recrudescence. Actual concentrations of androgens, as measured by enzyme immunoassay (EIA), were 10-49% of nominal MT 0.1, 3-6% of nominal MT 1, 5-10% of nominal DHT 10 and 3-25% of nominal DHT 100. Female mummichog were impacted to a greater degree by androgen exposure, with increased plasma testosterone (T) at 100 μg/L DHT, depressed plasma 17β-estradiol (E2) at both DHT concentrations and at 1 μg/L MT, as well as depressed in vitro E2 at both MT concentrations and 100 μg/L DHT. Males had depressed plasma T in the 10 μg/L DHT treatment and depressed in vitro 11-ketotestosterone production for both MT concentrations and 10 μg/L DHT. Ovarian aromatase gene expression was induced in females exposed to 1 μg/L MT. DHT at 100 μg/L increased hepatic vitellogenin-1 (VTG1) expression in males and depressed VTG1 expression in females. The range of responses between sexes and among species provides evidence for modes of actions and potential impacts of androgens in aquatic receiving environments.

Assessment of cytochrome P450 fluorometric substrates with rainbow trout and killifish exposed to dexamethasone, pregnenolone-16alpha-carbonitrile, rifampicin, and beta-naphthoflavone

Cytochrome P450s (CYPs) are important xenobiotic metabolizing proteins. While their functions are well understood in mammals, CYP function in non-mammalian vertebrate systems is much less defined, with function often inferred from mammalian data, assuming similar function across vertebrate species. In this study, we investigate whether in vivo treatment with known mammalian CYP inducers can alter the in vitro catalytic activity of fish microsomes using eleven fluorescent CYP-mediated substrates. We investigate the basal metabolism and induction potential for hepatic CYPs in two fish species, rainbow trout (Oncorhynchus mykiss) and killifish (Fundulus heteroclitus). Species differences were found in the baseline metabolism of these substrates. Killifish have significantly higher metabolic rates for all tested substrates except 7-benzyloxyquinoline and 7-benzyloxy-4-trifluoromethylcoumarin (both mammalian CYP3A substrates); significant differences were also seen between male and female killifish. Treatment with dexamethasone, pregnenolone-16alpha-carbonitrile, and rifampicin did not cause broad, measurable CYP induction in either fish species. In trout, dexamethasone (100 mg kg(-1)) significantly induced 3-cyano-7-ethoxycoumarin metabolism and rifampicin (100 mg kg(-1)) induced the dealkylation of 7-methoxyresorufin, although both were highly variable. Female killifish exposed to pregnenolone-16alpha-carbonitrile (100 mg kg(-1)) showed significantly higher metabolism of 7-pentoxyresorufin. Overall, dexamethasone, pregnenolone-16alpha-carbonitrile and rifampicin did not appear to consistently increase CYP activity in fish. Trout treated with 10 or 50 mg kg(-1) beta-naphthoflavone (BNF), a CYP1A inducer, showed significantly induced activity across almost all substrates tested, exceptions being 7-benzyloxyquinoline, 7-benzyloxy-4-trifluoromethylcoumarin and dibenzylfluorescein. 7-Methoxy-4-(aminomethyl)coumarin, a typical CYP2D substrate in mammals, was not metabolized by untreated fish liver microsomes; however, treatment with BNF significantly induced the metabolism of this substrate in trout. Induced substrate metabolism in BNF-treated microsomes was only correlated across selective substrates, suggesting that BNF induces multiple CYPs in fish liver. These include the known BNF inducible CYP1s plus a number of as yet unidentified fish CYPs. Overall, many of these catalytic assays could be valuable tools for identification of the function of specific CYP subfamilies and individual isoforms in fish.

A compilation of in vitro rate and affinity values for xenobiotic biotransformation in fish, measured under physiological conditions

Scientific literature from the past 25 years was searched to obtain in vitro biotransformation rate and affinity data for fish. To maximize the environmental relevance of this dataset, we focused on studies conducted at multiple substrate concentrations, and established acceptance criteria with respect to assay temperature and pH. Altogether, enzyme rate and affinity parameters are provided for 43 species and 77 compounds. In all but three instances, the reported reactions exhibited saturation at high substrate concentrations and could be used to calculate Michaelis-Menten rate (Vmax) and affinity (Km) constants. Most of this information was obtained using in vitro systems derived from liver tissue. Information from non-hepatic tissues was included, however, to provide a basis for comparisons among tissues. Where possible, in vitro enzyme parameters were examined to compare: (1) hepatic metabolism of a common substrate within a species, (2) hepatic metabolism of common substrates by different species, and (3) metabolism of a common substrate by different tissues of one species. Comparisons within species highlight a number of factors that may substantially influence xenobiotic metabolism in fish including gender, life stage, and acclimation temperature. Limited data suggest that Vmax and Km for the same reaction may vary by up to three orders of magnitude among species.

The cytochrome P450 system of atlantic salmon (Salmo salar): I. Basal properties and induction of P450 1A1 in liver of immature and mature fish

The major components of the cytochrome P450 (P450) system in liver microsomes of Atlantic salmon were studied using spectrophotometric, catalytic and immunochemical techniques. In juvenile fish sampled during the winter season, high basal activities of 7-ethoxyresorufin O-deethylase (EROD) were found. The Km for 7-ethoxyresorufin was 0.4 µM, and Vmax 1.23 nmol/min/mg protein in juvenile fish. In mature fish sampled from the same group of fish in December, EROD activity was barely detectable (20-30 pmol/min/mg protein). Treatment with the P450 1A1 inducer β-naphthoflavone (BNF) resulted in almost 2-fold induction of total P450, and 30-40-fold induction of EROD activity in immature fish. A similar fold increase was seen in mature fish. The differences in EROD activity between untreated and BNF-treated fish, was accompanied by similar differences in a P450 1A1 cross-reacting protein (Mr=58,000 D) in immunochemical studies using rabbit anti-cod P450 1A1 IgG. However, judging from these studies, the levels of P450 1A1-protein in mature salmon far exceeded those accounted for by the measured EROD activity in comparison to immature fish (both before and after BNF-treatment), indicating inhibiting effects of sex steroids on the measured activity. This effect was not seen on 7-ethoxycoumarin O-deethylase activity. A long-term storage experiment indicated that Atlantic salmon liver microsomes can be stored for 2 years at -80°C in 20% glycerol without losing more than 20-40% of its catalytic activity.

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.