title: On the Regulation of Phosphoenolpyruvate Carboxylase Activity from Maize Leaves by L-malate. Effect of pH.
abstract: The effect of L-malate as reversible inhibitor of phosphoenolpyruvate carboxylase purified from maize leaves was studied between pH 7 and 8. The malate concentration required to reach fifty per cent inhibition was pH dependent being higher at pH 8. Complex inhibition kinetics with upwardly curved Dixon plots were obtained. Hill plots showed cooperative effects giving coefficients (n(H)) between 2.0 and 3.9. The K(i) values were 0.8 and 10 mM at pH 7 and 8 respectively. In addition, at lower pH the type of inhibition was mainly competitive with respect to phosphoenolpyruvate while at pH 8 it was non-competitive. The presence of glucose-6-phosphate in the assay medium reduced the competitive inhibition by malate without modifying the non-competitive effect, changing the Hill coefficient values to one at pH 8.0. Moreover, the dissociation constants for phosphoenolpyruvate (plus or minus MgCl(2)) and malate in the presence of glucose-6-phosphate were calculated at pH 7.0, 7.5, and 7.9 from the protection afforded by these compounds against chemical modification of phosphoenolpyruvate carboxylase by phenylglyoxal. The possible existence of competitive as well as non-competitive malate binding sites in phosphoenolpyruvate carboxylase and the physiological meaning of this interaction are discussed.
authors.affiliation: Centro de Estudios Fotosintéticos y Bioquímicos. CEFOBI. Suipacha 531, 2000 Rosario, Argentina; (Consejo Nacional de Investigaciones Científicas y Técnicas, Fundación Miguel Lillo y Universidad Nacional de Rosario).
authors.forename: D H
authors.initials: DH
authors.lastname: Gonzalez
authors.forename: A A
authors.initials: AA
authors.lastname: Iglesias
authors.forename: C S
authors.initials: CS
authors.lastname: Andreo