Cytosolic lactate dehydrogenase catalyzes the reversible reaction of lactate and NAD+ to form pyruvate and NADH + H+. There are two isoforms of lactate dehydrogenase encoded by two different genes whose expression levels vary from tissue to tissue and as a function of developmental stage. The active form of the enzyme is a tetramer, whose composition in a tissue appears to be determined by the relative abundances of the two monomer isoforms. All five possible lactate dehydrogenase isoforms have been described; no functional differences among them have been found (Lindsay 1963; Hirota et al. 1990).