Plant Physiology 73:989-994 (1983)
© 1983 American Society of Plant Biologists
Articles
Regulation of Spinach Leaf Sucrose Phosphate Synthase by Glucose-6-Phosphate, Inorganic Phosphate, and pH 1
Douglas C. Doehlert and
Steven C. Huber
U. S. Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27650,
Department of Botany, North Carolina State University, Raleigh, North Carolina 27650,
Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27650
Sucrose phosphate synthase was partially purified from spinach leaves and the effects and interactions among glucose-6-P, inorganic phosphate (Pi), and pH were investigated. Glucose-6-P activated sucrose phosphate synthase and the concentration required for 50% of maximal activation increased as the concentration of fructose-6-P was decreased. Inorganic phosphate inhibited sucrose phosphate synthase activity and antagonized the activation by glucose-6-P. Inorganic phosphate caused a progressive increase in the concentration of glucose-6-P required for 50% maximal activation from 0.85 mM (minus Pi) to 9.9 mM (20 mM Pi). In the absence of glucose-6-P, Pi caused partial inhibition of sucrose phosphate synthase activity (about 65%). The concentration of Pi required for 50% maximal inhibition decreased with a change in pH from 6.5 to 7.5. When the effect of pH on Pi ionization was taken into account, it was found that per cent inhibition increased hyperbolically with increasing dibasic phosphate concentration independent of the pH. Sucrose phosphate synthase had a relatively broad pH optimum centered at pH 7.5. Inhibition by Pi was absent at pH 5.5, but became more pronounced at alkaline pH, whereas activation by glucose-6-P was observed over the entire pH range tested. The results suggested that glucose-6-P and Pi bind to sites distinct from the catalytic site, e.g. allosteric sites, and that the interactions of these effectors with pH and concentrations of substrate may be involved in the regulation of sucrose synthesis in vivo.
1 Cooperative investigations of the Agricultural Research Service, United States Department of Agriculture and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 8907 of the Journal series of the North Carolina Agricultural Research Service Raleigh, NC.
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