Total lactate dehydrogenase activity and the fractions of its M and H subunits were measured in rat tissues and tumors. Less complete measurements by two other methods agreed well, confirming these to be reproducible assays of the relative enzyme and subunit concentrations in tissues. Highest total activities were found in skeletal and heart muscles and in liver. Second only to these were the high values found in tumors. Immature and undifferentiated tissues contained only small fractions of the H subunit,which increased with age to high fractions in the differentiated tissues of heart, brain,thymus and kidney. Tumors were among the tissues with the lowest fractions of H subunit. The fraction of H subunit in all tumors was substantially lower than that in the parent normal tissue (or it was low in both, as in skeletal muscle and rhabdomyosarcoma). A commonality among the four different kinds of tumors examined was this increased expression of the M gene, normally most active in immature or undifferentiated tissues.

Keywords:
Lactate dehydrogenase, Isozymes, Subunits, Tissue distribution, Tumors
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1972
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