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Kinetic properties of rat liver pyruvate kinase at cellular concentrations of enzyme, substrates and modifiers.

ABSTRACT: Kinetic properties of rat liver pyruvate kinase type I at pH7.5 and 6.5 were studied with physiological ranges of substrates, modifiers and Mg(2+) concentrations at increasing enzyme concentrations, including the estimated cellular concentrations (approx. 0.1mg/ml). Enzyme properties appear unaffected by increased enzyme concentration if phosphoenolpyruvate, fructose 1,6-diphosphate and inhibitors are incubated with enzyme before starting the reaction with ADP. Our data suggest that minimum cellular concentrations of MgATP and l-alanine provide virtually complete inhibition of pyruvate kinase I at pH7.5. The most likely cellular control of existing pyruvate kinase I results from the strong restoration of enzyme activity by the small physiological amounts of fructose 1,6-diphosphate. Decreasing the pH to 6.5 also restores pyruvate kinase activity, but to only about one-third of its activity in the presence of fructose 1,6-diphosphate. Neither pyruvate nor 2-phosphoglycerate at cellular concentrations inhibit the enzyme significantly.

SUBMITTER: Flory W

PROVIDER: S-EPMC1168057 | biostudies-other | 1974 Jul

REPOSITORIES: biostudies-other

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Project description:The kinetic and regulatory properties of cytosolic pyruvate kinase (PKc) isolated from endosperm of germinating castor oil seeds (Ricinus communis L.) have been studied. Optimal efficiency in substrate utilization (in terms of Vmax/Km for phosphoenolpyruvate or ADP) occurred between pH 6.7 and 7.4. Enzyme activity was absolutely dependent on the presence of a bivalent and a univalent metal cation, with Mg2+ and K+ fulfilling this requirement. Mg2+ binding showed positive and negative co-operativity at pH 6.5 (h = 1.6) and pH 7.2 (h = 0.69) respectively. Hyperbolic saturation kinetics were observed with phosphoenolpyruvate (PEP) and K+, whereas ADP acted as a mixed-type inhibitor over 1 mM. Glycerol (10%, v/v) increased the S0.5(ADP) 2.3-fold and altered the pattern of nucleotide binding from hyperbolic (h = 1.0) to sigmoidal (h = 1.79) without modifying PEP saturation kinetics. No activators were identified. ATP, AMP, isocitrate, 2-oxoglutarate, malate, 2-phosphoglycerate, 2,3-bisphosphoglycerate, 3-phosphoglycerate, glycerol 3-phosphate and phosphoglycolate were the most effective inhibitors. These metabolites yielded additive inhibition when tested in pairs. ATP and 3-phosphoglycerate were mixed-type inhibitors with respect to PEP, whereas competitive inhibition was observed for other inhibitors. Inhibition by malate, 2-oxoglutarate, phosphorylated triose sugars or phosphoglycolate was far more pronounced at pH 7.2 than at pH 6.5. Although 32P-labelling studies revealed that extensive phosphorylation in vivo of soluble endosperm proteins occurred between days 3 and 5 of seed germination, no alteration in the 32P-labelling pattern of 5-day-germinated endosperm was observed after 30 min of anaerobiosis. Moreover, no evidence was obtained that PKc was a phosphoprotein in aerobic or anoxic endosperms. It is proposed that endosperm PKc activity of germinating castor seeds is enhanced after anaerobiosis through concerted decreases in ATP levels, cytosolic pH and concentrations of several key inhibitors.

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Kinetic properties of rat liver pyruvate kinase at cellular concentrations of enzyme, substrates and modifiers.

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