Summary. (1) Three isozymes of pyruvate kinase found in human tissues differ in chromatographic, electrophoretic, ammonium sulfate solubility, antigenic and kinetic behavior. (2) Isozyme I is found in erythrocytes and liver, isozyme II in kidney, and isozyme III in liver, kidney, leukocytes, skeletal muscle and cardiac muscle. (3) The possible structural relationship of these isozymes to one another and their probably differing roles in regulating glucose metabolism are discussed.

The activities of three human pyruvate kinases are affected to different degrees by P-enolpyruvate, fructose-1,6-diphosphate, ATP, and alanine. PK I and PK II are found in liver, kidney, and erythrocytes. Modulation of these activities may promote glycolysis, or impede glycolysis to allow for efficient gluconeogenesis. PK III, found in all human tissues tested except erythrocytes, can support active glycolysis at all times, regardless of the levels of the ligands studied.