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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 This method has been largely superseded by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.
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Hemolysis
In medicine, LDH is often used as a marker of tissue breakdown. As LDH
is abundant in red blood cells, it can function as a marker for
hemolysis. A blood sample that has been handled incorrectly can show
false-positively high levels of LDH due to erythrocyte damage.
Tissue turnover
Other uses are assessment of tissue breakdown in general; this is
possible when there are no other indicators of hemolysis. It is used
to follow-up cancer patients, as cancer cells have a high rate of
turnover, with destroyed cells leading to an elevated LDH activity.
Exudates and transudates
Measuring LDH in pleural effusion (or pericardial fluid) can help in
the distinction between exudates (actively secreted fluid, e.g. due to
inflammation) or transudates (passively secreted fluid, due to a high
hydrostatic pressure or a low oncotic pressure). LDH is elevated (>200
U/l) in an exsudate and low in a transudate. In empyema, the LDH
levels generally exceed 1000 U/l.
Meningitis and encephalitis
The enzyme is also found in cerebrospinal fluid where high levels of
lactate dehydrogenase in cerebrospinal fluid are often associated with
bacterial meningitis. High levels of the enzyme can also be found in
cases of viral meningitis, generally indicating the presence of
encephalitis and poor prognosis.
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