1. Serum Lactate dehydrogenase

2. Description
Lactate dehydrogenase (NAD- dependent enzyme) belongs oxidoreductase class of enzymes acting on L-lactate (EC ).

3. Catalytic function of LDH
Lactate dehydrogenase catalyses the interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD+.
It converts pyruvate, the final product of glycolysis to lactate when oxygen is absent or in short supply.
It performs the reverse reaction during the cori cycle in the liver.

5. Enzyme isoforms LDH-1 (4H) - in the heart LDH-2 (3H1M) -
LDH-3 (2H2M) - in the lungs
LDH-4 (1H3M) - in the kidneys
LDH-5 (4M) - in the liver and striated muscle

7. The major isozymes of skeletal muscle and liver, M5, has four muscle (M) subunits; while H4 is the main isozymes for heart muscle.
Usually LDH-2 is the predominant form in the serum.
A LDH-1 level higher than the LDH-2 level , suggests myocardial infarction (damage to heart tissues releases heart LDH, which is rich in LDH-1, into the bloodstream.

8. Diagnostic use
Tissue breakdown elevates levels of LDH, and therefore a measure of it indicates:
Myocardial infarction: Significant elevation of LDH1 (LDH1LDH2).
Hemolysis: LDH is abundant in red blood cells.
Cancer: Predominant elevation of LDH2 and LDH3 occur in leukaemia.

9. Lactate Dehydrogenase activity: is elevated in plasma following an infarction, peaking 36 to 40 hours after the onset of symptoms remain elevated for up to 10 days.
LDH activity is of diagnostic value in patients admitted more than 48 hours after the time when plasma CK2 may provide equivocal results.
Normal range at 37°C
U/l

11. Enzyme Activity Assay Spectrophotometry
Increase absorption of UV maximum at 340 nm.

12. Separation of different LDH isoenzymes by electrophoresis
Serum LDH-isoenzyme patterns are determined by the agar gel electrophoretic and quantitative determination by densitometry.

13. LDH1 is the fastest moving fraction towards the anode and LDH5 is the slowest moving isoenzyme of LDH.