an enzyme that, using either NAD+ or NADP+, catalyzes the dehydrogenation of malate to oxaloacetate or its decarboxylation to pyruvate and CO2. At least six malate dehydrogenases are known, distinguished by their products, use of NAD+ or NADP+, and specificity of substrate (one acts on d-malate, the rest act on l-malate); one of these proteins is an enzyme in the tricarboxylic acid cycle. SYN: malic acid dehydrogenase, malic dehydrogenase, malic enzyme, pyruvic-malic carboxylase.