Alteration of NAD-linked malic enzyme-catalyzed reactions by sulfhydryl group modification*

NAD-linked malic enzyme [EC 1.1.1.38] of Escherichia coli has been found to catalyze oxaloacetate reduction (reaction III) and pyruvate reduction (reaction IV), in addition to previously reported activities for oxidative decarboxylation of L-malate (reaction I) and oxaloacetate decarboxylation (reaction II). Chemical modification of the enzyme with N-ethylmaleimide (NEM) brought about a complete inactivation for reactions I and II and a 10-fold enhancement for reactions III and IV. Binding studies with 14 C-NEM revealed that 8 sulfhydryl groups were modified per tetrameric enzyme molecule concomitant with an alteration of the activities. Tartronate, a substrate analogue, NAD, and divalent metal ions protected the enzyme against the NEM modification.