Lactate Dehydrogenase - Enzyme Isoforms

Enzyme Isoforms

Functional lactate dehydrogenase are homo or hetero tetramers composed of M and H protein subunits encoded by the LDHA and LDHB genes, respectively:

• LDH-1 (4H)—in the heart
• LDH-2 (3H1M)—in the reticuloendothelial system
• LDH-3 (2H2M)—in the lungs
• LDH-4 (1H3M)—in the kidneys, placenta, and pancreas
• LDH-5 (4M)—in the liver and striated muscle

The five isoenzymes that are usually described in the literature each contain four subunits. The major isoenzymes of skeletal muscle and liver, M₄, has four muscle (M) subunits, while H₄ is the main isoenzymes for heart muscle in most species, containing four heart (H) subunits. The other variants contain both types of subunits.

Usually LDH-2 is the predominant form in the serum. A LDH-1 level higher than the LDH-2 level (a "flipped pattern") suggests myocardial infarction (damage to heart tissues releases heart LDH, which is rich in LDH-1, into the bloodstream). The use of this phenomenon to diagnose infarction has been largely superseded by the use of Troponin I or T measurement.