1. Lactate Dehydrogenase  A widespread cytosolic enzyme that catalyses the interconversion of lactate and pyruvate.  It converts pyruvate to lactic acid when oxygen is not available, and it does the reverse action in the liver.  Organs whose cells are relatively rich in LDH: heart, kidney, liver, muscle, red blood cells.  Upper limit for adults is in the range of 200 units/liter. Normal LDH levels vary with age, being higher in childhood due to bone growth.

2. Isoenzymes LDH-1 (4H) - in the heart. LDH-2 (3H1M) - in the reticuloendothelial system (predominant form in the serum). LDH-3 (2H2M) - in the lungs. LDH-4 (1H3M) - in the kidneys.

3. High LDH levels (in serum) Hemolysis. MI (peak 3-4 days). Testicular cancer ( in 50-60% of pts. with metastatic nonseminoma; in up to 80% in pts. with seminoma). Tumor marker: lymphoma, leukemia, Ewing's sarcoma. Empyema, Pleural exudates. Bacterial meningitis, viral encephalitis (in CSF). HIV patients: Pneumocystis pneumonia, Histoplasmosis (>600 IU/L). Acute pancreatitis. Acute hepatitis, liver diseases. Megaloblastic anemia (B-12 deficiency). Pulmonary emboli. Hypothyroidism.

4. LDH Deficiency  Rare.  Defect in M subunit.  Symptoms: exercise intolerance, muscle cramps, myoglobinuria on strenuous exercise, erythematous skin eruption, uterine stiffness.