Lactate dehydrogenase (LDH) catalyzes the reaction below. This reaction provides an important source of NAD⁺ for cells undergoing anaerobic glycolysis.

Pyruvate + NADH + H+  <=> Lactate + NAD⁺

LDH is a tetrameric protein consisting of two types of subunits, called M and H, which have small differences in amino acid sequence. Different molecular forms of an enzyme are called isoenzymes or isozymes. M subunits predominate in skeletal muscle and liver, and H subunits predominate in heart. M and H subunits combine randomly with each other, so that the five major isoenzymes have the compositions M4, M3H, M2H2, MH3, and H4. Because of random subunit reassortment, the isoenzymic composition of a tissue is determined primarily by the activities of the genes specifying the two subunits.