Lactate Dehydrogenase Subunits in Normal and Neoplastic Tissues of the RaP

A kinetic procedure, based on differences in affinity of lactate dehydrogenase (LDH) subunits for L(+)-lactate, was employed to measure the M and H subunit activities of this enzyme in a variety of normal and neoplastic tissues of the rat. Total LDH activity was highest in liver (both normal and regenerating) at about 150 units per gm fresh tissue and somewhat lower in heart and femoral muscle at about 100 units per gm. All other tissues had less than 50 units per gm and decreased in the order: uterus during pregnancy, spleen, kidney, lactating mammary gland, brain, diaphragm, uterus, thymus, lung, testes, and mammary gland during pregnancy. The proportion of the M form was highest in both normal and regenerating liver at >90% and was also predominant in femoral muscle, spleen, uterus, diaphragm, thymus, and lung. The lactating mammary gland had about 10 times as much M form but the same H form activity as the gland of pregnancy; the uterus of pregnancy had about 3 times the M form but about the same H form activity as the normal uterus. The H form was predominant in heart, kidney, brain, and testes but made up no more than about 60% of the total. Highly differentiated, slow-growing, transplantable hepatomas had relatively low total LDH activities, at about 50 units per gm, whereas poorly differentiated, rapid-growing tumors had activities between 125 and 160 units per gm. Well-differentiated tumors of intermediate growth rates had intermediate total LDH levels. Although all hepatic tumors displayed predominant M form activity, ranging from 69 to 100%, there was no consistent pattern in relation to growth rate or degree of differentiation. Three transplantable kidney tumors had two to four times as much M form activity but about the same H form activity as normal kidney; 2,6-dimethyl-