In vitro metabolism of polychlorinated biphenyl congeners by beluga whale (Delphinapterus leucas) and pilot whale (Globicephala melas) and relationship to cytochrome P450 expression

We measured rates of oxidative metabolism of two tetrachlorobiphenyl (TCB) congeners by hepatic microsomes of two marine mammal species, beluga whale and pilot whale, as related to content of selected cytochrome P450 (CYP) fo rms. Beluga liver microsomes oxidized 3,3',4,4'-TCB at rates averaging 21 a nd 5 pmol/min per mg for males and females, respectively, while pilot whale samples oxidized this congener at 0.3 pmol/min per mg or less. However, ra tes of 3,3',4,4'-TCB metabolism correlated with immunodetected CYP1A1 prote in content in liver microsomes of both species. The CYP1A inhibitor alpha-n aphthoflavone inhibited 3,3'.4,4'-TCB metabolism by 40% in beluga, supporti ng a role for a cetacean CYP1A as a catalyst of this activity. Major metabo lites of 3,3',4,4'-TCB generated by beluga liver microsomes were 4-OH-3,3', 4',5-TCB and 5-OH-3,3',4,4'-TCB (98% of total), similar to metabolites form ed by other species CYP1A1, and suggesting a 4,5-epoxide-TCB intermediate. Liver microsomes of both species metabolized 2,2',5,5'-TCB at rates of 0.2- 1.5 pmol/min per mg. Both species also expressed microsomal proteins cross- reactive with antibodies raised against some mammalian CYP2Bs (rabbit; dog) , but not others (rat; scup). Whether CYP2B homologues occur and function i n cetaceans is uncertain. This study demonstrates that PCBs are metabolized to aqueous-soluble products by cetacean liver enzymes, and that in beluga, rates of metabolism of 3,3',4,4'-TCB are substantially greater than those of 2,2',5,5'-TCB. These directly measured rates generally support the view that PCB metabolism plays a role in shaping the distribution patterns of PC B residues found in cetacean tissue. (C) 2000 Elsevier Science Inc. All rig hts reserved.