FURTHER STUDIES ON THE METABOLISM OF DRUGS BY SUBFRACTIONS OF HEPATIC MICROSOMES. III. EFFECTS OF THE NADPH-GENERATING SYSTEM

¹ Department of Pharmacology, College of Medicine, University of Iowa, Iowa City, Iowa

In previous work we have found a marked concentration of several drug-metabolizing enzymes in the smooth-surfaced subfraction of hepatic microsomes. In the present work we studied the influence of three different NADPH- generating systems as well as addition of NADPH (reduced nicotinamide adenine dinucleotide phosphate) itself on the apparent distribution of three drug-metabolizing enzymes within microsomal subfractions. The NADPH-generating systems consisted of one of the following : 1) NADP (nicotinamide adenine dinucleotide phosphate), hepatic soluble fraction (105,000 x g supernatant) and glucose-6-phosphate, 2) NADP, yeast glucose-6-phosphate dehydrogenase and glucose-6-phosphate or 3) NADP, isocitrate and isocitrate dehydrogenase.With all three of the generating systems, we observed significantly greater activity in smooth-surfaced microsomes than in rough-surfaced microsomes for the metabolism of hexoharbital, aniline and aminopyrine. The addition of NADPH to incubation mixtures produced less consistent results; with hexobarbital or aminopyrine as substrates, no differences in drug metabolism were observed between smooth-and rough-surfaced microsomes, but with aniline as substrate, NADPH addition resulted in significantly greater metabolism by smooth-surfaced microsomes than by rough-surfaced microsomes. These results further support the concept of inequality of enzyme distribution within hepatic microsomal subfractions and suggest an important difference between enzymically reduced NADP and exogenous NADPH with respect to availability or utilizability by microsomal enzymes.