Lactate dehydrogenase is an enzyme that catalyzes the conversion of lactate to pyruvate. It is not tissue-specific, being found in a variety of tissues, including liver, heart and skeletal muscle. The enzyme is tetrameric and is composed of four subunits of two molecules, M (muscle) and H (heart). Various combinations of these two molecules result in 5 different isoenzymes.
Elevated LDH levels
- LDH1: This is composed of four H subunits (H4) and is found mostly in cardiac muscle and erythrocytes in small animals, whereas this isoenzyme is found mostly in the kidney and liver of sheep and cattle.
- LDH2: This is composed of three H subunits and one M subunit (MH3). LDH2, LDH3 and LDH4 are found in all tissues.
- LDH3: This is composed of two H and two M subunits (M2H2).
- LDH4: This is composed of one H and three M subunits (M3H).
- LDH5: This is composed of four M subunits (M4) and is found in skeletal muscle in all species and the liver in horses and small animals.
Because LDH is so non-specific and isoenzyme measurement is not routinely available, its measurement does not confer any additional information about skeletal muscle or hepatic disease in domestic animals, than that provided by enzyme assays routinely used for this purpose (i.e. CK for muscle and SDH and ALT for liver).
- Artifact: Hemolysis (LDH is high in erythrocytes in dogs and cats); serum has higher levels than plasma as the enzyme is released from cells during clotting.
- Physiologic: Exercise.
- Liver disease: Increased LDH1 and LDH2 in cattle and sheep, increased LDH5 in horses and small animals.
- Muscle disease: Increased LDH5 in sheep, cattle and horses, e.g. selenium and vitamin E deficient myopathy in cattle and sheep, exertional rhabdomyolysis in horses.
- Neoplasia: e.g. bovine lymphoma.