Effect of Sodium Chloride on the Activity of a Soluble Malate Dehydrogenase from Pea Seeds

An isozyme of malate dehydrogenase (EC 1.1.1.37) from pea seeds has been isolated. The effect of NaCl on the kinetics of the reaction catalyzed by this enzyme has been studied at four pH values, at all of which the results were essentially the same. The activity of the isozyme increases with increasing salt concentrations up to 0.02 m. Higher concentrations are inhibitory. Salt increases the apparent maximal velocity of the reaction and decreases the affinity of oxalacetate for the enzyme. At NaCl concentrations inhibitory to reduction of oxalacetate, the kinetics is that of competitive inhibition. The Km for reduced diphosphopyridine nucleotide is not affected by salt. The effect of sodium ion is nonspecific, since other univalent cations behave similarly.