Effect of Bicarbonate and Oxaloacetate on Malate Oxidation by Spinach Leaf Mitochondria

Mitochondria isolated from spinach leaves oxidized malate by both a NAD+-linked malic enzyme and malate dehydrogenase. In the presence of sodium arsenite the accumuation of oxaloacetate and pyruvate during malate oxidation was strongly dependent on the malate concentration, the pH in the reaction medium and the metabolic state condition. Bicarbonate, especially at alkaline pH, inhibited the decarboxylation of malate by the NAD+-linked malic enzyme in vitro and in vivo. Analysis of the reaction products showed that with 15 mM bicarbonate, spinach leaf mitochondria excreted almost exclusively oxaloacetate. The inhibition by oxaloacetate of malate oxidation by spinach leaf mitochondria was strongly dependent on malate concentration, the pH in the reaction medium and on the metabolic state condition. The data were interpreted as indicating that: (a) the concentration of oxaloacetate on both sides of the inner mitochondrial membrane governed the efflux and influx of oxaloacetate; (b) the NAD+/NADH ratio played an important role in regulating malate oxidation in plant mitochondria; (c) both enzymes (malate dehydrogenase and NAD+-linked malic enzyme) were competing at the level of the pyridine nucleotide pool, and (d) the NAD+-linked malic enzyme provided NADH for the reversal of the reaction catalyzed by the malate dehydrogenase.