Lactate dehydrogenase (LDH) is a ubiquitous enzyme that catalyzes the conversion of lactate to pyruvate and back, resulting in conversion of NAD⁺ to NADH and back. LDH exists in five distinct forms, named LDH-1 through LDH-5, each having differential expression in different tissues:
- LDH-1 is found primarily in heart muscle.
- LDH-2 is the major isozyme of the reticuloendothelial system and red blood cells.
- LDH-3 is highest in the lung.
- LDH-4 is highest in the kidney, placenta, and pancreas.
- LDH-5 is highest in the liver and skeletal muscle.