The Biosynthesis of Sucrose and Nucleoside Diphosphate Glucoses in Phaseolus aureus

Abstract

Sucrose-phosphate synthetase is detectable only in intact chloroplast preparations of Phaseolus aureus. In contrast, sucrose synthetase and uridine diphosphate glucose (UDP-glucose) pyrophosphorylase activities are low in extracts of photosynthetic tissues of P. aureus but are high in extracts of nonphotosynthetic tissues. Activities for ADP-, dTDP-, CDP-, and GDP-glucose pyrophosphorylases are generally higher in extracts of photosynthetic tissues of P. aureus than in extracts of nonphotosynthetic tissues. The high levels of sucrose synthetase and of UDP-glucose pyrophosphorylase found in dark-grown hypocotyls begin to decline about 4 hours after exposure to light at a rate of 50% every 3 hours.The data suggest that sucrose-phosphate synthetase and sucrose phosphatase are the enzymes responsible for the biosynthesis of sucrose from photosynthetically fixed CO(2), and that the major function of sucrose synthetase is to catalyze the synthesis of UDP-, ADP-, dTDP-, CDP-, and GDP-glucose from translocated sucrose in nonphotosynthetic tissues; in photosynthetic tissues the pyrophosphorylases may replace sucrose synthetase in catalyzing the synthesis of these nucleoside diphosphate glucoses. We offer the suggestion that sucrose synthetase and UDP-glucose pyrophosphorylase play a major role in the uptake and metabolism of sucrose in nonphotosynthetic tissues. Results are presented from preliminary studies on the conversion in vitro of sucrose to glucose 1-phosphate by the coupled reactions of sucrose synthetase and UDP-glucose pyrophosphorylase with highly purified preparations of these enzymes.