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Pyrophosphorylases in Solanum tuberosum¹

II. CATALYTIC PROPERTIES AND REGULATION OF ADP-GLUCOSE AND UDP-GLUCOSE PYROPHOSPHORYLASE ACTIVITIES IN POTATOES

Department of Horticulture Sciences and Plant Physiology, University of Minnesota, St. Paul, Minnesota 55108

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 -glucan biosynthesis in Solanum tuberosum is discussed.

¹ Agricultural Experiment Station, University of Minnesota Scientific Journal Series No. 11,522.

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