[Studies on pyruvate kinase from pig dental pulp and brain]

Most of the enzymes involved in glycolysis are readily reversible and are also active in gluconeogenesis. However, three reaction steps are irreversible, i.e., those catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase; for in each of these reactions there occurs a large negative free-energy change, and these are reactions thus bypassed by alternate enzyme-catalyzed reactions. Pyruvate kinase (EC 2.7.1.40, PK) plays an important role in controlling glycolysis and gluconeogenesis. To clarify the characteristics of glycolysis in dental pulp, we examined the enzymatic properties of pyruvate kinase from pig dental pulp and compared them with those of the enzyme from pig brain. 1) Pyruvate kinase from dental pulp and brain were purified by use of ammonium sulphate fractionation, phosphocellulose colum chromatography, and isoelectric focusing. The prepared enzymes showed a single protein band on SDS polyacrylamide gel electrophoresis. 2) The subunit molecular weight of dental pulp and brain enzymes was determined to be 63,000 and 59,000, respectively. 3) Substrate inhibition of dental pulp and brain enzymes by phosphoenolpyruvate was not observed, and the relationship between reaction velocity and substrate concentration at pH 7.2 was explained by the Michaelis-Menten equation. Fructose-1,6-diphosphate had no observable effect on either enzyme. 4) Effect of amino acids on dental pulp and brain enzyme activity were examined, and no significant relationship was observed between the side chain structure of amino acids and their potency in inhibiting dental pulp and brain enzyme activity. Glutamic and aspartic acids markedly inhibited dental pulp and brain enzymes at pH 7.2. 5) Oxalate showed inhibitory activity against dental pulp and brain enzymes, and the Ki value was determined to be 50 microM and 80 microM, respectively. The inhibition of dental pulp and brain enzyme activity by oxalate was competitive with respect to phosphoenolpyruvate. 6) Both dental pulp and brain enzymes were clearly inhibited by malate at concentrations higher than 1.0 mM: 50% and 100% inhibition occurred at 2.2-2.3 mM and 3.0 mM malate, respectively.