Abstract

Optimum conditions are described for the assay of cytochrome P-450 reduction by NADPH in rat liver microsomes. The addition in vitro of chemical compounds which produce type I spectral changes markedly accelerated the initial rate of NADPH-linked cytochrome P-450 reduction, while the addition of compounds which elicit type II spectral changes significantly decelerated the rate. These changes occurred in the absence of appreciable changes in the absorbance of the cytochrome P-450—CO complex, suggesting that they are not due simply to differences in the extinction coefficients of the complexes. The acceleration produced by ethylmorphine, a type I compound, was much greater in hepatic microsomes from male rats than in microsomes from female rats; this differential enhancement may play a role in determining sex differences in drug metabolism. None of the type I compounds significantly influenced microsomal cytochrome c reductase activity at concentrations that produced maximal enhancement of cytochrome P-450 reduction. NADPH oxidation, measured in the absence of exogenous electron acceptors, was significantly increased by type I substances. Addition of type II compounds to microsomal suspensions either did not affect or decreased cytochrome c reductase and NADPH oxidase activities.

The data suggest that the complexes formed between cytochrome P-450 and type I compounds are more readily reduced by NADPH than is the endogenous hemoprotein. Possible mechanisms by which type II compounds inhibit the reduction of cytochrome P-450 are discussed.