Molecular Pharmacology, Vol 8, 481-489, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

Inhibition by Deuterated Water of the Mixed-Function Oxidases of Hepatic Microsomes of the Male Rat

¹ Clinical Pharmacology Section, Veterans Administration Hospital, Minneapolis, Minnesota 55417; Department of Pharmacology, University of Minnesota, Minneapolis, Minnesota 55455; and National Cancer Institute, Baltimore, Maryland 2121l

Ethylmorphine N-demethylase, aniline hydroxylase, and 3,4-benzo[a]pyrene hydroxylase of hepatic microsomes were inhibited 22-32% by substitution of D₂O for H₂O in the incubation medium. The effect on the first two enzymes was not related to the pH or the time course of incubation. The inhibition was linear with D₂O concentration, occurred immediately, and was rapidly reversible. The inhibition was purely noncompetitive. NADPH-cytochrome c reductase in the intact microsomes was inhibited 15%, while in the solubilized form it was inhibited 51%. Basal NADPH oxidase and NADP—cytochrome P-450 reductase activities were not greatly affected, but were significantly inhibited in the presence of 2 mM ethylmorphine. In both cases the difference between the stimulated and basal activities was stoichiometric with ethylmorphine N-demethylase and exhibited the same degree of inhibition. These findings suggest that, in spite of the lipid nature of the microsomal membrane, changes in hydration of the lipoprotein play a significant role in the mixed-function oxidases.