page_title: Lactate dehydrogenase
section_title: Isozymes
hierarchical_section_title: Lactate dehydrogenase / Isozymes
caption_attribution_description: English: Crystal structure of B-lactate dehydrogenase
context_page_description: Lactate dehydrogenase is an enzyme found in nearly all living cells. LDH catalyzes the conversion of lactate to pyruvate and back, as it converts NAD⁺ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.
  LDH exists in four distinct enzyme classes. Other LDHs act on D-lactate and/or are dependent on cytochrome c: D-lactate dehydrogenase and L-lactate dehydrogenase.
  LDH is expressed extensively in body tissues, such as blood cells and heart muscle. Because it is released during tissue damage, it is a marker of common injuries and disease such as heart failure.
context_section_description: Lactate dehydrogenase is composed of four subunits (tetramer). The two most common subunits are the LDH-M and LDH-H protein, encoded by the LDHA and LDHB genes, respectively. These two subunits can form five possible tetramers (isoenzymes): 4H, 4M, and the three mixed tetramers (3H1M, 2H2M, 1H3M). These five isoforms are enzymatically similar but show different tissue distribution: 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.
  LDH-1 (4H)—in the heart and in RBC (red blood cells), as well as the brain
  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
  LDH-2 is usually the predominant form in the serum. An 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 The use of this phenomenon to diagnose infarction has been largely superseded by the use of Troponin I or T measurement.
  There are two more mammalian LDH subunits that can be included in LDH tetramers: LDHC and LDHBx. LDHC is a testes-specific LDH protein, that is encoded by the LDHC gene. LDHBx is a peroxisome-specific LDH protein. LDHBx is the readthrough-form of LDHB. LDHBx is generated by translation of the LDHB mRNA, but the stop codon is interpreted as an amino acid-encoding codon. In consequence, translation continues to the next stop codon. This leads to the addition of seven amino acid residues to the normal LDH-H protein. The extension contains a peroxisomal targeting signal, so that LDHBx is imported into the peroxisome.
caption: Crystal structure of B-lactate dehydrogenase. From PDB: 1T2F​.
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