ISOZYMES OF LACTIC DEHYDROGENASE IN HUMAN TISSUES*

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Abstract
Hetero- geneity of lactic dehydrogenase (LDH) activity in the electrophoretically separated homogenates of human heart, kidney, liver, skeletal muscle, and extracts of leukocytes and erythrocytes is described. Five peaks of LDH activity were found in liver; 4 in kidney, leukocytes and erythrocytes, and 3 in heart and skeletal muscle. Each tissue exhibited a characteristic distribution of LDH activity liver and skeletal muscle had the greatest activity in peaks 1, 2 and 3; whereas heart, hemolysate and kidney had most activity in peaks 4 and 5. Leukocytes and serum showed highest activity in peak 3. The electro-phoretic patterns of LDH activity in human tissues correlated with alterations in serum activity in disease states. In the serum from a case of hepatitis, a large peak of LDH activity is described which is present only in trace amounts in normal serum. This activity peak had the same mobility as the peak in liver with greatest LDH activity. These observations suggest that assay of the LDH activity in the fractions of electrophoretically separated serum permits a greater specificity in localizing pathology than does assay of whole serum LDH activity. Studies of Michaelis-Menten constants of the 5 peaks of LDH show peaks 1 and 2 to differ from peaks 3, 4 and 5. The possibility that the active centers of peaks 1 and 2 are different from the active centers of peaks 3, 4 and 5 is discussed.