The effect of zinc on NADPH oxidation and monooxygenase activity in rat hepatic microsomes

This study confirms that zinc is able to inhibit hepatic microsomal drug metabolism and the related oxidation of NADPH. Zinc activates microsomal pyrophosphatase, a zinc-containing enzyme capable of metabolizing both NADPH and NADH. Although this reaction produces a potent inhibitor of drug metabolism, 2',5'-ADP', the activation of pyrophosphatase by zinc was found not to be solely responsible for the zinc-dependent inhibition of drug metabolism. Zinc was seen to inhibit drug metabolism in the presence of 5'-AMP, which inhibits pyrophosphatase. Incubation of zinc with microsomes prior to the addition of NADPH caused an interaction between zinc and the microsomal enzymes that was not reversed by NADPH. Zinc was found to exhibit noncompetitive inhibition of cytochrome c reduction and mixed inhibition of drug metabolism, with respect to NADPH. Zinc inhibition of drug metabolism was noncompetitive with drug substrate. Zinc was found to interact with cytochrome P-450, decreasing its ability to bind to both drugs and carbon monoxide. Zinc had a far greater effect on the reduction of cytochrome P-450 (90% inhibited) than on the reduction of exogenous cytochrome c (20% inhibited), although the same reductase is responsible for both reactions. It is concluded that zinc inhibits drug metabolism either by alteration of the oxidation-reduction potential of the flavoprotein or by interacting with a flavoprotein/cytochrome P-450 complex.