Pyrophosphorylases in Solanum tuberosum

Abstract

Pyrophosphorylytic kinetic constants (S0.5, Vmax) of partially purified UDP-glucose- and ADP-glucose pyrophosphorylases from potato tubers were determined in the presence of various intermediary metabolites. The S0.5 of UDP-glucose pyrophosphorylase for UDP-glucose (0.17 mM) or PPi (0.30 mM) and the Vmax were not influenced by high concentrations (2 mM) of these substances. The most efficient activator of ADP-glucose pyrophosphorylase was 3-P[phosphate]-glycerate (A0.5 = 4.5 .times. 10-6 M). The S0.5 for ADP-glucose and PPi was increased 3.5-fold (0.83-0.24 mM) and 1.8-fold (0.18-0.10 mM), respectively, with 0.1 mM 3-P-glycerate while the Vmax was increased nearly 4-fold. The magnitude of 3-P-glycerate stimulation was dependent upon the integrity of key sulfhydryl groups (-SH) and pH. Oxidation or blockage of -SH groups resulted in a marked reduction of enzyme activity. Stimulations of 3.1-, 2.9-, 4.8- and 9.5-fold were observed at pH 7.5, 8.0, 8.5, and 9.0, respectively, in the presence of 3-P-glycerate (2 mM). The most potent inhibitor of ADP-glucose pyrophosphorylase was orthophosphate (I0.5 = 8.8 .times. 10-5 .cntdot. M). This inhibition was reversed with 3-P-glycerate (1.2 .times. 10-4 M), resulting in an increased I0.5 value of 1.5 .times. 10-3 M. Pi (7.5 .times. 10-4 M) caused a decrease in the activation efficiency of 3-P-glycerate (A0.5 from 4.5 .times. 10-6 M to 6.7 .times. 10-5 M). The significance of 3-P-glycerate activation and Pi inhibition in the regulation of .alpha.-glucan biosynthesis in S. tuberosum is discussed.