Pyrophosphorylases in Solanum tuberosum

Pyrophosphorylytic kinetic constants (S(0.5), V(max)) of partially purified UDP-glucose- and ADP-glucose pyrophosphorylases from potato tubers were determined in the presence of various intermediary metabolites. The S(0.5) of UDP-glucose pyrophosphorylase for UDP-glucose (0.17 millimolar) or pyrophosphate (0.30 millimolar) and the V(max) were not influenced by high concentrations (2 millimolar) of these substances. The most efficient activator of ADP-glucose pyrophosphorylase was 3-P-glycerate (A(0.5) = 4.5 x 10(-6) molar). The S(0.5) for ADP-glucose and pyrophosphate was increased 3.5-fold (0.83 to 0.24 millimolar) and 1.8-fold (0.18 to 0.10 millimolar), respectively, with 0.1 millimolar 3-P-glycerate while the V(max) 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 millimolar). The most potent inhibitor of ADP-glucose pyrophosphorylase was orthophosphate (I(0.5) = 8.8 x 10(-5). molar). This inhibition was reversed with 3-P-glycerate (1.2 x 10(-4) molar), resulting in an increased I(0.5) value of 1.5 x 10(-3) molar. Likewise, orthophosphate (7.5 x 10(-4) molar) caused a decrease in the activation efficiency of 3-P-glycerate (A(0.5) from 4.5 x 10(-6) molar to 6.7 x 10(-5) molar). The significance of 3-P-glycerate activation and orthophosphate inhibition in the regulation of alpha-glucan biosynthesis in Solanum tuberosum is discussed.