NAD+/NADP+-Dependent Malic Enzyme: Evidence of a NADP+ Preferring Activity in Human Skeletal Muscle

Abstract

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 NADP+ as coenzyme was ∼15 μmol/min/mg. The apparent Vmax values for NAD+ (∼8 μmol/min/mg) and NADP+ (∼16 μmol/min/mg) show that the enzyme (in this tissue) is more active in the presence of NADP+. 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.