The Influence of Exogenous Nicotinamide Adenine Dinucleotide on the Oxidation of Malate by Jerusalem Artichoke Mitochondria

Abstract

Mitochond$$$a isolated from Jerusalem artichoke tubers oxidized endogenous NADH by both a piericidin A-sensitive and -resistant dehydrogenase if the level of oxaloacetate was kept low. In washed mitochondria the addition of NAD⁺ stimulated respiration in the presence of a variety of NAD⁺ -linked substrates. In mitochondria purified through a sucrose density gradient exogenous NAD⁺ caused a substantial stimulation of respiration only in the presence of malate. The apparent K_m for malate was 20 mM in the absence of NAD⁺ and 2 mM in the presence of exogenous NAD⁺ The products of malate oxidation were altered by the addition of exogenous NAD⁺. Oxaloacetate and pyruvate were produced in equal amounts in the absence of added NAD⁺, but in the presence of exogenous NAD⁺ more pyruvate was formed. The rapid oxidation of malate in the absence of added NAD⁺ required phosphate while the NAD⁺-stimulated component was not affected by the absence of phosphate. Phenylsuccinate inhibited the reduction of exogenous NAD⁺ by malate; this compound was found to inhibit isolated ma!ate dehydrogenase and mahe enzyme. Several lines of evidence suggest that electron flux through one of the NADH dehydrogenase systems may directly affect the flow through the other dehydrogenases.