LDHB

Lactate dehydrogenase B (LDHB) is a gene that encodes the B-type subunit of the lactate dehydrogenase (LDH) enzyme. LDH is a tetramer composed of combinations of H- and M-subunits, encoded by LDHB and LDHA, respectively. The five major LDH isoenzymes are LDH-1 (H4), LDH-2 (H3M), LDH-3 (H2M2), LDH-4 (HM3), and LDH-5 (M4). The subunit composition influences enzyme kinetics and tissue distribution, with H-subunit–containing isoenzymes enriched in heart and red blood cells, and M-subunit–containing isoenzymes more common in skeletal muscle and liver.

Expression of LDHB is highest in the heart and brain, and the enzyme contributes to cytosolic LDH activity in various tissues, including red blood cells where glycolysis remains active in the absence of mitochondria. The LDHB and LDHA genes together encode the LDH tetramer's catalytic subunits, allowing the enzyme to operate in multiple directions depending on cellular conditions.

Function and direction: LDH catalyzes the reversible conversion of pyruvate to lactate with concomitant interconversion of NADH and NAD+. The direction of net flux depends on the cellular redox state and energy requirements; in erythrocytes, LDH activity favors lactate formation to regenerate NAD+, while in aerobic tissues such as the heart, lactate can be converted back to pyruvate to feed mitochondrial energy production.

Clinical relevance: LDH isoenzyme patterns, including those containing LDHB, are used clinically to help assess tissue injury and disease processes. Altered LDHB expression or LDH isoenzyme distributions have been observed in various cancers and in response to hypoxic stress, and are subjects of ongoing research in metabolic reprogramming.

See also: LDHA, lactate dehydrogenase, isoenzymes, metabolism.