Molecular Pharmacology, Vol 8, 339-344, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

Hepatic Organelle Interaction

II. Effect of Tricarboxylic Acid Cycle Intermediates on N-Demethylation and Hydroxylation Reactions in Rat Liver

¹ Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510

Hepatocytes in rat liver slices accomplish the slow oxidative metabolism of drugs like aminopyrine and ethylmorphine. Although the enzyme system responsibile for this metabolism resides in the endoplasmic reticulum, mitochondrial Krebs cycle intermediates also affect the process. Succinate and isocitrate each elevate the metabolism of aminopyrine and ethylmorphine about 2-3-fold; none of the other intermediates tested had any effect. NADPH elevated the rates of aminopyrine and ethylmorphine N-demethylation 18- and 28-fold, respectively; NADP was only 50% as effective as NADPH in supporting these oxidations. When NADPH and isocitrate were added together, an additive effect was obtained on aminopyrine and ethylmorphine oxidation; the combination of succinate and NADPH caused a synergistic effect, more marked on the oxidation of aminopyrine than of ethylmorphine. Fumarate, oxalacetate, or malate, when combined with NADPH, was antagonistic to the liver slice oxidation of these two drugs, while malonate, when added with succinate and NADPH, only removed the synergism. The same effects as obtained with liver slices could be demonstrated with whole liver homogenates, but not with isolated liver microsomes. None of the Krebs cycle intermediates had a stimulatory effect on aniline hydroxylation, although oxalacetate decreased the hydroxylation seen with aminopyrine and ethylmorphine. These observations indicate that drug metabolism in the hepatocyte endoplasmic reticulum is controlled by the mitochondria, not merely via a flow of reducing equivalents, since even in the presence of an excess of NADPH succinate exerts a synergistic effect, more than doubling the rate of aminopyrine oxidation and causing a 50% increase in ethylmorphine oxidation.