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Plant Physiology 69:1027-1030 (1982)
© 1982 American Society of Plant Biologists

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Articles

Kinetic Characterization of Spinach Leaf Sucrose-Phosphate Synthase 1

Jacob Amir2 and Jack Preiss

Department of Biochemistry and Biophysics, University of California, Davis, California 95616

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 Ki 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 Ki 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.

2 Permanent address: The Volcani Institute of Agricultural Research, Gilat Experiment Station, Mobil-Post 2 Negev, Israel.

1 Partially supported by National Science Foundation Grant PCM 78-16127.






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Copyright © 1982 by the American Society of Plant Biologists