Abstract
In order to obtain definite evidence for the participation of cytochrome P-448 in the reduced nicotinamide adenine dinucleotide (NADH)-dependent O-deethylation of p-nitrophenetole and to elucidate the role of the NADH-coupled electron transport system in drug hydroxylation systems, in vitro studies were conducted using rat liver microsomes, and the following results were obtained. 1) The NADH-dependent O-deethylation activities in liver microsomes of rats pretreated with 3-methylcholanthrene (3-MC), benzo-[a] pyrene (BP) and 3, 4, 5, 3', 4', 5'-hexachlorobiphenyl (HCB) were markedly increased (about 20 times), but phenobarbital (PB) pretreatment had a much smaller effect (about 1.5-fold increase). 2) When cytochrome P-448 highly purified from HCB-pretreated rat liver microsomes was added to an incubation mixture for NADH-dependent O-deethylation of p-nitrophenetole, the activity was increased. 3) The synergistic effect of NADH on NADPH-dependent O-deethylation of p-nitrophenetole occurred at the initial stage with untreated microsomes, but not with 3-MC- and HCB-pretreated liver microsomes. On the basis of these results, the role of cytochrome P-448 in NADH-dependent oxidation by liver microsomes and the mechanism of the synergistic effect of NADH are discussed.
