Role of cytochrome b₅ in the oxidative metabolism of polychlorinated biphenyls catalyzed by cytochrome P450

The role of cytochrome b₅ in the cytochrome P450-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted system consisting of CYP2B1 and CYP1A1 and rat liver microsomes. By addition of cytochrome b₅ to the reconstituted system containing CYP2B1, the 3-hydroxylation of 2,5,2,'5'- and 2,5,3',4'-TCB was increased about six-fold, but the 3- and 5-hydroxylation of 2,4,3',4'-TCB was decreased by about 50%. All hydroxylations of 3,4,3',4'-, 2,5,3',4'- and 2,4,3',4'-TCBs were decreased by addition of cytochrome b₅ to the reconstituted system containing CYP1A1. In stoichiometry measurements, changes in NADPH oxidation and coupling efficiency by addition of cytochrome b₅ was observed and these differed according to the position of chlorine atoms of TCBs and cytochrome P450 isoforms used in the systems.