Involvement of cytochrome b(5) in the metabolism of tetrachlorobiphenyls catalyzed by CYP2B1 and CYP1A1

The role of cytochrome b(5) in the cytochrome P450 (CYP)-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted mixed function oxygenase (MFO) system containing purified CYP2B1 or 1A1, and rat liver microsomes. Hydroxylations of 2,2',5,5'- and 3,3',4,4'-TCBs were catalyzed mainly by CYP2B1 and 1A1, respectively, in the reconstituted MFO system and those of 2,3',4',5- and 2,3',4,4'-TCBs were mediated by both cytochrome P450 systems. The activity toward 2,2',5,5'- and 2,3',4',5-TCB was significantly increased 6.5- and 5.5-fold, respectively, by addition of cytochrome b, in the reconstituted MFO system containing of CY2B1. Either hydroxylation activity toward 2,3',4,4'-TCB with the CYP2B1 system was very low or decreased by addition of cytochrome b(5). These results suggest that the involvement of cytochrome b(5) to the hydroxylation of TCBs is dependent on the TCB congener being metabolized, and the cytochrome P450 isoform involved in its metabolism.