Pyrophosphorylases in Solanum tuberosum1: II. CATALYTIC PROPERTIES AND REGULATION OF ADP-GLUCOSE AND UDP-GLUCOSE PYROPHOSPHORYLASE ACTIVITIES IN POTATOES

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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 millimolar) or pyrophosphate (0.30 millimolar) and the Vmax were not influenced by high concentrations (2 millimolar) of these substances. The most efficient activator of ADP-glucose pyrophosphorylase was 3-P-glycerate (A0.5 = 4.5 × 10−6 molar). The S0.5 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 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 millimolar). The most potent inhibitor of ADP-glucose pyrophosphorylase was orthophosphate (I0.5 = 8.8 × 10−5. molar). This inhibition was reversed with 3-P-glycerate (1.2 × 10−4 molar), resulting in an increased I0.5 value of 1.5 × 10−3 molar. Likewise, orthophosphate (7.5 × 10−4 molar) caused a decrease in the activation efficiency of 3-P-glycerate (A0.5 from 4.5 × 10−6 molar to 6.7 × 10−5 molar). The significance of 3-P-glycerate activation and orthophosphate inhibition in the regulation of α-glucan biosynthesis in Solanum tuberosum is discussed.

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