lactaatdehydrogenase

Lactaatdehydrogenase, often abbreviated LDH, is a cytosolic enzyme that catalyzes the reversible interconversion of pyruvate and lactate, using NADH as a cofactor. In glycolysis, LDH helps regenerate NAD+ by reducing pyruvate to lactate under low oxygen availability, allowing glycolysis to continue. The enzyme is abundant in many tissues and is released into the bloodstream when cells are damaged.

LDH is a tetramer composed of two types of subunits, designated H (heart) and M (muscle). The combination of these subunits gives rise to five isoenzymes: LDH-1 (H4), LDH-2 (H3M), LDH-3 (H2M2), LDH-4 (HM3), and LDH-5 (M4). The distribution of these isoenzymes varies by tissue; for example, LDH-1 is relatively abundant in heart and red blood cells, while LDH-5 is more common in liver and skeletal muscle. Tissue-specific patterns can be used diagnostically to infer the source of elevated LDH levels.

Physiologically, LDH participates in the cori cycle and other metabolic pathways where lactate is produced or consumed. In tissues with high glycolytic flux, lactate can be exported and later converted back to pyruvate for entry into gluconeogenesis or oxidation, linking anaerobic metabolism to broader energy production.

Clinically, elevated serum LDH is a non-specific marker of tissue damage and can occur in a wide range of conditions, including myocardial infarction, liver disease, hemolysis, muscle injury, and certain malignancies. Isoenzyme analysis can help identify the tissue of origin. LDH activity is typically measured spectrophotometrically by assessing the rate of NADH consumption or production in a pyruvate–lactate reaction.