LDH

LDH, or lactate dehydrogenase, is a cytosolic enzyme that catalyzes the reversible interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD+. The enzyme plays a central role in energy metabolism, particularly during anaerobic glycolysis, by regenerating NAD+ needed for glycolysis to continue under low oxygen conditions. In tissues, lactate produced by muscle and erythrocytes can be transported to the liver for reconversion to glucose via the Cori cycle.

LDH exists as a family of tetrameric isozymes composed of two subunit types. The subunits are encoded by two genes, LDHA and LDHB, producing the H (heart) and M (muscle) subunits. The five major isozymes are LDH-1 through LDH-5, with the subunit compositions: LDH-1 (H4), LDH-2 (H3M), LDH-3 (H2M2), LDH-4 (HM3), and LDH-5 (M4). These isozymes have distinct tissue distributions: LDH-1 is rich in heart and erythrocytes; LDH-2 is common in the reticuloendothelial system; LDH-3 is prominent in the lungs; LDH-4 is found in kidneys and placenta; and LDH-5 is abundant in liver and skeletal muscle. The balance of isozymes in a sample can shift in response to disease or tissue damage.

Clinically, total serum LDH is an indicator of tissue injury but is nonspecific; elevations can occur with myocardial infarction, liver disease, hemolysis, muscle injury, and various cancers. Isoenzyme patterns historically aided differential diagnosis, though modern practice relies more on more specific biomarkers such as troponin. Research interest continues in LDH, particularly LDHA, as a potential target for cancer therapy, given its role in aerobic glycolysis and tumor metabolism.