Involvement of cytochrome b_s in the metabolism of tetrachlorobiphenyls catalyzed by CYP2B1 and CYP1A1

The role of cytochrome b₅ 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 b₅ in the reconstituted MFO system containing of CYP2B1. Either hydroxylation activity toward 2,3′,4,4′-TCB with the CYP2B1 system was very low or decreased by addition of cytochrome b₅. These results suggest that the involvement of cytochrome b₅ to the hydroxylation of TCBs is dependent on the TCB congener being metabolized, and the cytochrome P450 isoform involved in its metabolism.