Studies on the pyrophosphorylases of wheat

Adenosine diphosphoglucose and uridine diphosphoglucose pyrophosphorylase activities (class EC 2.7.7.9) were assayed radiochemically in ammonium sulfate fractions prepared from mature and germinating wheat grain. Both enzymes showed similar pH optima and metal ion requirements but differed markedly in their response to activation by d-glycerate 3-phosphate and inhibition by sulfate and phosphate.Their stability characteristics also differed greatly. While UDP-glucose pyrophosphorylase was very stable, ADP-glucose pyrophosphorylase rapidly lost activity in dilute solution and upon desalting on columns of Sephadex and was only partially stabilized when stored as a suspension in ammonium sulfate. Relatively high salt concentrations were required to maintain even short term stability. ADP-glucose and UDP-glucose pyrophosphorylases had apparent K(m) values for alpha-d-glucose 1-phosphate of 4.4 x 10(-5)m and 10(-3)m, respectively. In extracts from dry grain, the number of units of UDP-glucose pyrophosphorylase were about 30-fold greater than the number of units of ADP-glucose pyrophosphorylase. During germination, the activity of UDP-glucose pyrophosphorylase increased markedly in both the embryo and the endosperm, while ADP-glucose pyrophosphorylase fell slightly over the initial 4 days of germination.