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Metabolism of the aryl hydrocarbon receptor agonist 3,3',4,4'-tetrachlorobiphenyl by the marine fish scup (Stenotomus chrysops) in vivo and in vitro.

The metabolism of the polychlorinated biphenyl congener 3,3',4,4'-tetrachlorobiphenyl (TCB) was examined in vitro and in vivo in the marine fish scup (Stenotomus chrysops). Untreated scup liver microsomes catalyzed metabolism of TCB with an estimated KM of 0.7 microM, at a rate < or = 0.13 pmol/min/mg. Metabolism was NADPH-dependent and inhibited by cytochrome c and CO, indicating cytochrome P450 (CYP) involvement. alpha-Naphthoflavone strongly inhibited microsomal TCB metabolism, and treatment of fish with CYP1A inducers increased the rates by approximately 2-fold, suggesting involvement of CYP1A. Scup were injected intraperitoneally with 0.1 or 5 mg TCB/kg and sampled on days 1-16 after treatment (after 3 days without food at each sampling). Concentrations of unmetabolized TCB in liver peaked on day 5 in low dose fish and on day 12 in high dose fish. In both groups the TCB content in the liver had declined 60% or more by day 16, suggesting depuration or redistribution from the liver. GC and MS revealed TCB and TCB metabolites in bile within 24 hr of treatment. The concentrations of TCB and metabolites in bile peaked at the same time that TCB concentrations peaked in the liver. The major metabolites were 5-hydroxy-3,3'4,4'-TCB (5-OH-TCB) and 4-hydroxy-3,3',5,4'-TCB (4-OH-TCB); 2-hydroxy-3,3',4,4'-TCB and 6-hydroxy-3,3',4,4'-TCB were minor metabolites. Animals given the high dose had much less 5-OH-TCB and much more parent TCB in bile than did fish given the low dose. Amounts of 4-OH-TCB in bile did not differ between doses. The reduced excretion of 5-OH-TCB coincided with a suppression of CYP1A in fish given the high dose, that did not occur in low dose fish, consistent with an involvement of CYP1A in TCB metabolism and particularly in formation of 5-OH-TCB. This study provides the first direct demonstration of 3,3',4,4'-TCB metabolism by fish. Data also indicate that these fish are able to eliminate TCB both as parent compound and as metabolites, despite a very slow rate of metabolism in vitro.