Catalyzes the reversible oxidation of malate to oxaloacetate. Exhibits higher catalytic efficiency for oxaloacetate reduction than for malate oxidation in vitro. Almost equally active both for NADH and NADPH on the bases of the kcat values at pH 6.5, but catalytic efficiency for oxaloacetate reduction is 50-fold higher with NADH. (S)-malate + NAD(+) = H(+) + NADH + oxaloacetate Citrate activates the enzyme in the oxidation of malate to oxaloacetate and inhibits it in the reverse reaction. kcat is 360 sec(-1) for NADH-dependent reduction of oxaloacetate. kcat is 150 sec(-1) for NADPH-dependent reduction of oxaloacetate. kcat is 140 sec(-1) for NAD(+)-dependent oxidation of malate. Homotetramer. Belongs to the LDH/MDH superfamily. MDH type 2 family.