Kinetic Characterization of Spinach Leaf Sucrose-Phosphate Synthase

作者：Jacob Amir, Jack J. Preiss · 发表于：PLANT PHYSIOLOGY · 年份：1982 · DOI：10.1104/pp.69.5.1027 · 被引用次数：57 · 研究领域：Plant nutrient uptake and metabolism、Enzyme Production and Characterization、Microbial Metabolites in Food Biotechnology

The spinach (Spinacia oleracea) leaf sucrose-phosphate synthase was partially purified via DEAE-cellulose chromatography, and its kinetic properties were studied. Fructose-6-phosphate saturation curves were sigmoidal, while UDPglucose saturation curves were hyperbolic. At subsaturating concentrations of fructose-6-phosphate, 1,5 anhydroglucitol-6-phosphate had a stimulatory effect on enzyme activity, suggesting multiple and interacting fructose-6-phosphate sites on sucrose-phosphate synthase. The concentrations required for 50% of maximal activity were 3.0 millimolar and 1.3 millimolar, respectively, for fructose-6-phosphate and UDPglucose. The enzyme was not stimulated by divalent cations. Inorganic phosphate proved to be a potent inhibitor, particularly at low concentrations of substrate. Phosphate inhibition was competitive with UDPglucose, and its K(i) was determined to be 1.75 millimolar. Sucrose phosphate, the product of the reaction, was also shown to be a competitive inhibitor towards UDPglucose concentration and had K(i) of 0.4 millimolar. The kinetic results suggest that spinach leaf sucrose-phospahte synthase is a regulatory enzyme and that its activity is modulated by the concentrations of phosphate, fructose-6-phosphate, and UDPglucose occurring in the cytoplasm of the leaf cell.