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'-tetr achlorobiphenyl (TCB) was examined in vitro and in vivo in the marine fish scup (Stenotomus chrysops). Untreated soup liver microsomes catal yzed metabolism of TCB with an estimated K-M of 0.7 mu M, at a rate le ss than or equal to 0.13 pmol/min/mg. Metabolism was NADPH-dependent a nd inhibited by cytochrome c and CO, indicating cytochrome P450 (CYP) involvement. alpha-Naphthoflavone strongly inhibited microsomal TCB me tabolism, and treatment of fish with CYP1A inducers increased the rate s by similar to 2-fold, suggesting involvement of CYP1A. Soup were inj ected 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). Conce ntrations of unmetabolized TCB in liver peaked on day 5 in low dose fi sh and on day 12 in high dose fish. In both groups the TCB content in liver had declined 60% or more by day 16, suggesting depuration or red istribution from the liver. GC and MS revealed TCB and TCB metabolites in bile within 24 hr of treatment. The concentrations of TCB and meta bolites 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-O H-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 tha n did fish given the low dose, Amounts of 4-OH-TCB in bile did not dif fer 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 i n low dose fish, consistent with an involvement of CYP1A in TCB metabo lism and particularly in formation of 5-OH-TCB. This study provides th e 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 paren t compound and as metabolites, despite a very slow rate of metabolism in vitro.