Lactic Dehydrogenase Isozymes

Lactic dehydrogenase isozymes (I-lactate, NAD oxidoreductase), which are key enzymes in glycolysis, have been the object of extensive investigation. Five LDH isozymes have been found. The five isozymes represent the five possible tetramaric combinations of the two polypeptides. Thus, the five tetramers range from LDH-l( H4 ) to LDH -5 ( M4 ), the three hybrid enzymes being H3MI, H2 M2, and H j M3• Each isozyme is composed of four subunits which may be one or both of the two polypeptides called subunit H and subunit M. The synthesis of the polypeptides is controlled by different genes. Each subunit confers on the tetramer distinct catalytic, physical, and immunological properties according to its relative position in the individual isozymes. These findings have' led to a better understanding of the physiological significance of isozymes (1, 4, 6, 9, 17 . 18, 23 ). For a number of years research on LDH in cancer has concentrated on the changes which can be demonstrated in body fluids in progressive malignancies. The results indicate that the isozyme activity in malignant tissues shifts towards the cathodic isozymes LDH-5 and LDH-4 and is thought partly to explain the high capacity for anaerobic glycolysis in malignancies (12 , 24. ). Only a few studies have been reported ( 15) on LDH isozymes in leprosy, a chronic granulomatous disease. We reported previously that the amount of lactic acid in the blood of leprosy patients, especially lepromatous cases, increased to more than twice that of normal subjects, and that serum LDH isozymes showed a tendency to an increase in Mcontaining isozymes in contrast to normal serum (22).