Lactate dehydrogenase (LDH or LD) is a key enzyme in most cells, catalyzing the reversible conversion of pyruvate to L-lactate. Its contemporaneous main clinical uses are limited primarily to the investigation of hemolysis, serous collections and as a tumor marker.
L-lactate dehydrogenase is an enzyme of the oxidoreductase class, found in the cytosol of almost every single cell in the human body. The enzyme is a tetramer of four subunits, with two main subunits H and M, therefore forming five possible isozymes, LD1 to LD5:
- LD1 (H4): cardiac, renal, red blood cells
- LD2 (H3M1): white cells, lymph nodes, spleen, pulmonary
- LD3 (H2M2): white cells, lymph nodes, spleen, pulmonary
- LD4 (H1M3): white cells, lymph nodes, spleen, pulmonary
- LD5 (M4): hepatic, skeletal muscle cells
LDH has an important role in glycolysis, mediating the metabolism of pyruvate to L-lactate. This reaction is reversible.
Historically lactate dehydrogenase was an important blood test for the delayed diagnosis of myocardial infarction, however troponin has rendered this usage obsolete. It has also been shown that its specificity for cardiac ischemia was poor . It was also previously advocated as a marker for liver function, but markers such as the transaminases are superior in this regard. Its use as a marker for inflammation of the muscle is also discontinued in view of the widespread availability of assays for creatine kinase (CK) which has better specificity than LDH.
Its main clinical uses currently are:
- marker for hemolysis
- tumor marker
- pleural fluid evaluation
NB: lactate dehydrogenase is elevated in many other pathologies (i.e. it has very poor specificity) hence its lack of clinical utility in contemporary clinical practice