Enzymic mechanism of starch synthesis in ripening rice grains: III. Mechanism of the sucrose-starch conversion

Abstract

In a coupling system of sucrose-synthetase and starch-synthetase, both isolated from the ripening rice grains, the more efficient transfer of glucose-C 14 from sucrose-C 14 to starch occurred in the presence of ADP as compared to UDP, indicating the predominant role of ADPG in the process. However, UDP was found to inhibit profoundly the ADPG-mediated glucose-C 14 transfer to starch from sucrose-C 14, which was based on its specific inhibition of the ADPG-sucrose transglucosylation reaction. The Km value of sucrose synthetase was determined to be 1.5 × 10 −3 M (ADP) and 1.1 × 10 −4 M (UDP), respectively. These findings may support a view that the sucrose breakdown proceeds through the reversal of UDPG-sucrose transglucosylation rather than directly by way of the ADPG-sucrose transglucosylation. By feeding the rice grains at the mid-milky stage with sucrose-C 14, incorporation of the radioactivity into the starch fraction was observed, although most of the radioactivity in the acid-soluble fraction was recovered as sucrose. It was noted also that a relatively high radioactivity was detected in glucose and fructose. The formation of a prominent radioactive compound(s) emerging around the fraction of glucose-6-phosphate was demonstrated in a Dowex-1 anion-exchange column chromatography; neither its chemical nature nor the role in the sucrose-starch conversion has been revealed. Both ADPG and UDPG became labeled to a much lesser extent than the above compounds under the experimental conditions employed. Attempts to distinguish their predominant role in the biosynthetic conversion of sucrose to starch were unsuccessful, although the specific radioactivity of ADPG was about twice as high as that of UDPG. Based on the experimental results obtained, possible enzymic mechanisms underlying the sucrose-starch conversion in developing rice grains are discussed.