Bovine Heart Malic Enzyme

The cytoplasmic and mitochondrial malic enzyme isozymes isolated from bovine heart have been studied to determine their capability in catalyzing the reductive carboxylation of pyruvate to malate. The cytoplasmic enzyme carries out this carboxylation at rates that approach one-seventh of the maximal decarboxylation velocity at the same pH. Michaelis constants of 13 mm for bicarbonate and 8 mm for pyruvate have been obtained for this enzyme. The mitochondrial isozyme catalyzes the carboxylation reaction at rates which are only about one-three hundredth that of the maximal rate for decarboxylation. This behavior can be modified by the addition of several C4 dicarboxylate anions, including succinate, d(+) malate, l(-) aspartate, and 2-mercaptosuccinate, the latter being the most active. In the presence of the above mentioned dicarboxylate anions, the carboxylation is enhanced 2- to 3-fold, indicating a possible change in the enzyme conformation. The Michaelis constants for bicarbonate and pyruvate, in the presence of mercaptosuccinate, have been calculated as 16.5 mm and 9 mm, respectively. From experiments conducted at low temperatures, it has been concluded that free CO2, rather than the bicarbonate ion, is the active carboxylating species in the reactions catalyzed by both isozymes.