Malate dehydrogenase; catalyzes a reversible NAD-dependent dehydrogenase reaction involved in central metabolism and redox homeostasis. (S)-malate + NAD(+) = H(+) + NADH + oxaloacetate Optimum pH is 7.41 for the native enzyme. Monomer. Expressed constitutively in roots. Belongs to the LDH/MDH superfamily. MDH type 2 family.