Cytosolic lactate dehydrogenase catalyzes the freely reversible reaction of lactate and NAD+ to form pyruvate and NADH + H+. In liver parenchymal cells, this reaction allows lactate from red blood cells and exercising muscle to be converted to pyruvate which in turn is typically used for gluconeogenesis which also consumes the NADH from the reaction.
Lactate dehydrogenase is active as a tetramer. Two isoforms of lactate dehydrogenase, A and B, are widely expressed in human tissues, and all five tetramers - A4, A3B, A2B2, AB3, and B4 - are found. A third isoform, C, and its tetramer, C4, are found in testis.