Lactate Dehydrogenase (Lactic Dehydrogenase/LDH/ LD)

EC 1.1.1.27

Lactate dehydrogenase catalyzes the reversible oxidation of lactate to pyruvate using the cofactor NAD+. The reverse reaction is actually the thermodynamically favored one under physiological conditions. One of its primary purposes is to replenish the pool of NAD+ under conditions of oxygen insufficiency in order to allow glycolysis to continue unabated¹.

LDH is a tetrameric enzyme, and analogously to CK, there are two common gene products for the subunits. One is named for its predominance in muscle tissue and therefore known as the subunit. The other is named for its predominance in heart muscle and so is called the H subunit. This results in five permutations. The H₄ enzyme is referred to as LDH-1 (or LD1/LD-1), the H₃M₁ as LDH-2 (LD2/LD-2), the H₂M₂ as LDH-3 (LD3/LD-3), the H₁M₃ as LDH-4 (LD4/LD-4) and finally the M₄ enzyme as LDH-5 (LD5/LD-5)². In reality these enzymes are much more widely distributed than this would suggest. To cite just two examples, liver tissue is a rich source of the LDH isoenzymes primarily containing M-subunits, while erythrocytes are a rich source of the LDH isoenzymes primarily containing H-subunits³. Thus in certain tissues such as liver and muscle the pattern observed is LDH-5 > LDH-4 > LDH-3 > LDH-2 > LDH-1. In other tissues such as heart and erythrocytes the pattern is the reverse: LDH-1 > LDH-2 > LDH-3 > LDH-4 > LDH-5.

Because LDH is so widely distributed, any significant elevation in serum LDH level could be due to many reasons, and by itself is indicative of no specific pathology. However LDH isoenzyme analysis can yield much more informative results. The result of the variable isoenzyme distribution is that typically the most abundant LDH isoenzyme in serum from healthy individuals is not LDH-1 nor LDH-5 but rather LDH-2⁴. However since LDH-1 is the most abundant isoenzyme from cardiac muscle, this fact could be utilized to aid in the diagnosis of acute myocardial infarction (AMI). This phenomenon has been referred to as the LD1-2 flip. In other words, when the serum level of LDH-1 exceeded that of LDH-2 (since the opposite is generally true in serum from healthy individuals), this would be strongly indicative of damage to cardiac tissue and possibly due to AMI^5,6. Surprisingly, the fact that LDH in general, and LDH-1 in particular, is so widely distributed did not negate the utility of this technique nearly as much as one might think⁷. Nevertheless the measurement of LDH-1 levels was considered to be of the most diagnostic utility when measured in conjunction with another cardiac marker such as CKMB^8,9.

Total LDH activity can be easily measured in either direction by monitoring either the reduction of NAD+ (oxidation of lactate to pyruvate) or the oxidation of NADH (reduction of pyruvate to lactate) by the change in absorbance at 340 nm. In practice both methods are used, even though the latter (sometimes referred to as the P → L method) is the thermodynamically favored reaction. Measuring LDH isoenzymes is most reliably accomplished by electrophoresis using a chromogenic LDH activity stain. However analogously to CKMB, immunoinhibition assays were developed using an anti-M subunit inhibiting antibody. This gives a more complex result in that it would entirely inhibit only LDH-5, and its inhibitory ability should be somewhat less against LDH-4, even less against LDH-3, etc. Thus in no way is it ever measuring only LDH-1, nevertheless it has proved its utility in diagnosing AMI⁶. The main advantage of such a test over electrophoresis is that it’s readily automated and is much more adaptable to high-throughput systems. Even though various antibodies have been developed against the different LDH isoenzymes, unlike with CKMB, no classical LDH-1 immunoassay has gained commercial acceptance. Although CKMB and the cardiac troponins have generally supplanted LD isoenzymes in the diagnosis of cardiac injury, they have not altogether disappeared and total LDH is still part of a standard general blood chemistry panel.

REFERENCES

1. 1. Guyton, A. Textbook of Medical Physiology, W.B. Saunders Company, 1992.
2. 2. Burtis, C.A., Ashwood, E.R. and Bruns, E.R. Tietz Textbook of Clinical Chemistry, 4th Edition, W.B. Saunders Company, 2006.
3. 3. Nisselbaum, J.S. and Bodansky, O. Reactions of Human Tissue Lactic Dehydrogenase with Antisera to Human Heart and Liver Lactic Dehydrogenases. Journal of Biological Chemistry, 236, 401-404, 1961.
4. 4. Wroblewski, F. and Gregory, K.F. Lactic Dehydrogenase Isozymes and their Distribution in Normal Tissues and Plasma and in Disease States. Ann N Y Acad Sci, 94, 912-932, 1961.
5. 5. Kaman, R.L., et. al. The Effect of Near Maximum Exercise on Serum Enzymes: The Exercise Profile Versus the Cardiac Profile. Clinica Chimica Acta, 81, 145-152, 1977.
6. 6. McCoy, M.T., et. al. Lactate Dehydrogenase Isoenzyme 1: A Rapid Assay for Ischemic Myocardial Disease. Archives of Internal Medicine, 143, 434-436, 1983.
7. 7. Bruns, D.E., et. al. Lactate Dehydrogenase Isoenzyme 1: Changes During the First Day after Acute Myocardial Infarction. Clinical Chemistry, 27, 1821-1823, 1981.
8. 8. Galen, R.S., et. al. Diagnosis of Acute Myocardial Infarction. Journal of the American Medical Association, 232, 145-147, 1975.
9. 9. Lott, J.A. Serum Enzyme Determinations in the Diagnosis of Acute Myocardial Infarction: An Update. Human Pathology, 15, 706-716, 1984.
10. 10. Burtis, C.A. and Ashwood, E.R. Tietz Textbook of Clinical Chemistry 2nd Edition, W.B. Saunders Company, 1994.