Lactate dehydrogenase
Lactate dehydrogenase (LDH) is a ubiquitous enzyme present in both plants and animals. It catalyses the reaction between pyruvate and lactate and vice versa, dependent on the abundance of either. As it can also dehydrogenate hydroxybutyrate, it is occasionally called Hydroxybutyrate Dehydrogenase (HBD). LDH requires NAD+ (Nicotinamide adenine dinucleotide) as a hydrogen acceptor.
Enzyme isoforms
Every enzyme is a tetramer of four subunits, where subunits are either H and M (based on their electrophoretic properties.) There are, therefore, five LDH isotypes:
- 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
- LDH-5 (4M) - in the liver and striated muscle
Usually LDH-2 is the predominant form in the serum: if an LDH-1 level is higher than the LDH-2 level (a "flipped pattern"), myocardial infarction is suggested. This method has been largely superseded by the use of Troponin I or T measurement.