Some kinetic properties of liver pyruvate kinase (type L). II. Effect of pH on its allosteric behavior.

Type L pyruvate kinase from rat liver has been further studied. The allosteric properties of the enzyme are strongly affected by changes in the hydrogen ion concentration of the assay medium. At pH values lower than 7, the enzyme obeys Michaelis-Menten kinetics with respect to one of the substrates (phosphoenolpyruvate (PEP)) and cannot be activated by fructose 1,6-diphosphate (FDP). In addition, a cooperative effect of ATP is observed at a low concentration of this substrate. At pH values higher than 7.2 the enzyme has a sigmoidal response to PEP. This is transformed into a normal hyperbolic relationship in the presence of FDP and the apparent aflinity of the enzyme for this substrate increases. On the other hand, ATP shows cooperative effects at low pH values, at 7.5 in the presence of C.CQ2~ma-FDP, o~~at~high PEP concentration (4 111~). When the enzyme kinetics was studied within a narrow physiological range of pH (6.8 to 7.25), the homotropic effect of PEP was altered, becoming more sigmoidal as the pH was increased. It is concluded that small variations in the intracellular pH could be important in the regulation of pyruvate kinase activity by metabolites in vivo.