Inhibition of NADPH oxidation and related drug oxidation in liver microsomes by zinc

Abstract

Rat liver microsomes were incubated in the presence of zinc and the rate of NADPH oxidation and related metabolism of aniline and ethylmorphine by appropriate oxidases were studied. A competitive mechanism of the inhibition of NADPH oxidation by zinc was found, with V max = 10.3 nmoles NADP/min/mg of protein and K i amounting to 7.22 μM zinc. In microsomes dialyzed against EDTA, addition of Mn 2+ but not of Mg 2+ enhanced the rate of NADPH oxidation. A complex relation of Zn 2+ and Mn 2+ in liver microsomes was found, the data not obeying the rigorous treatment for enzyme kinetics. The activity of aniline hydroxylase and ethylmorphine- N-demethylase was inhibited by zinc; 50 per cent inhibition was reached at 60 and 55 μM Zn 2+ respectively. Another microsomal enzyme, glucose 6-phosphatase, independent of NADPH, was not affected by zinc. The content and spectral characteristics of cytochrome P-450 were not affected by zinc. It is concluded that Zn 2+ inhibits oxidation of NADPH and prevents this pyridine nucleotide from functioning in the microsomal electron transport system. The possibility that Zn 2+ may interfere with other ions or enzymes involved in microsomal electron transport cannot be excluded.