an enzyme of the oxidoreductase class; it catalyzes the reversible reduction of pyruvic acid to L-lactic acid with the participation of reduced nicotinamide adenine dinucleotide (NAD·H + H⁺) as a coenzyme:

Lactic dehydrogenase, extracted from the muscle tissue of vertebrates in crystalline form, is composed of four polypeptide sub-units (the molecular weight of one lactic dehydrogenase tetramer is 140,000). The equilibrium of the catalyzed lactic dehydrogenase reaction is strongly shifted toward the formation of lactic acid. The reaction requires the presence of NAD·H + H⁺ and proceeds in most animal tissues under anaerobic conditions. Cancer cells form one exception since a large amount of lactic acid is formed in these cells under aerobic conditions. Five isoenzymes of lactic dehydrogenase are known, which differ in amino acid composition as well as in certain physical, immunological, and catalytic properties. The determination of lactic dehydrogenase activity in blood plasma is of significant diagnostic value. Lactic dehydrogenase has been detected in Lactobacillaceae that catalyze the formation of D-lactic acid with the participation of reduced NAD.