NAD+/NADP+-dependent malic enzyme: evidence of a NADP+ preferring activity in human skeletal muscle

The L-malate NAD(P)+ oxidoreductase (decarboxylating) E.C.1.1.1.39 was purified from human skeletal muscle; the specific activity estimated in the presence of NAPD+ as coenzyme was approximately 15 mumol/min/mg. The apparent Vmax values for NAD+ (approximately 8 mumol/min/mg) and NADP+ (approximately 16 mumol/min/mg) show that the enzyme (in this tissue) is more active in the presence of NAPD+. This observation was confirmed by the estimation of enzymatic activity in competition experiments where both NAD+ and NADP+ were used together as coenzymes. The absence of pyruvate carboxylation and of oxalacetate decarboxylation activities demonstrates that the enzyme studied is E.C.1.1.1.39. In addition, the apparent Km values for NAD+ and NADP+ were calculated (15 and 0.05 mM, respectively). This paper provides the first demonstration of a NADP+ preferring activity of the enzyme in human skeletal muscle.