LDH3

LDH3, or lactate dehydrogenase 3, is one of the five human LDH (lactate dehydrogenase) isoenzymes. LDH enzymes catalyze the interconversion of pyruvate and lactate with concomitant transfer of NADH and NAD+. The five isoenzymes are tetramers formed from heart-type H subunits and muscle-type M subunits. LDH-3 corresponds to the H2M2 tetramer, containing two H and two M subunits.

In terms of tissue distribution, LDH-3 is relatively enriched in lung tissue and is also present in other tissues such as the reticuloendothelial system and, to a lesser extent, the brain. Because of this distribution, changes in LDH-3 levels can be informative about pulmonary tissue injury.

Clinical relevance and testing considerations: Total LDH activity is a non-specific marker of tissue damage. Isoenzyme analysis can help localize injury by identifying which LDH fractions are elevated. An increased LDH-3 fraction or activity can be associated with lung-related pathology, including pneumonia, pulmonary embolism, acute lung injury, and other pulmonary diseases. However, LDH-3 elevations are not diagnostic on their own, and patterns must be interpreted alongside clinical and radiographic findings. Isoenzyme testing is typically performed by electrophoresis or other isoenzyme-specific assays, reported as a percentage of total LDH or as absolute activity.

Genetics: The subunits are encoded by LDHB (heart-type H) and LDHA (muscle-type M) genes, with LDH-3 arising from the combination of two H and two M subunits. Preanalytic factors, such as hemolysis, can affect LDH isoenzyme results and should be considered during interpretation.