Regulation of Spinach Leaf Sucrose Phosphate Synthase by Glucose-6-Phosphate, Inorganic Phosphate, and pH

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Abstract
Sucrose phosphate synthase was partially purified from spinach leaves and the effects and interactions among G-6-P, Pi and pH were investigated. G-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 G-6-P. Inorganic phosphate caused a progressive increase in the concentration of G-6-P required for 50% maximal activation from 0.85 mM (minus Pi) to 9.9 mM (20 mM Pi). In the absence of G-6-P, Pi causes partial inhibition of sucrose phosphate synthase activity (.apprx. 65%). The concentration of Pi required for 50% maximal inhibition decreased with a change in pH from 6.5-7.5. When the effect of pH on Pi ionization was taken into account, percent 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; activation by G-6-P was observed over the entire pH range tested. G-6-P and Pi bind to sites distinct from the catalytic site, e.g., allosteric sites, and the interactions of these effectors with pH and concentrations of substrate may be involved in the regulation of sucrose synthesis in vivo.