Enzymic mechanism of starch synthesis in ripening rice grains: II. Adenosine diphosphate glucose pathway

Abstract

In studies of the formation of starch granules in ripening rice grains, it was found that the transfer of glucose to starch occurred rapidly through the ADPG pathway. Although some reaction occurs with UDPG, the process is considerably slower; 80% of the glucosyl units were transferred from ADPG to the starch molecule during a 2-hour incubation period, but only one third of this amount was transferred from UDPG during this time. 4 4 Abbreviations used: ADPG, adenosine diphosphate glucose; and UDPG, uridine diphosphate glucose. When ADPG-pyrophosphorylase was coupled with ADPG-starch transglucosylase, glucose was transferred to the starch molecule. The same result was obtained with ADPG-sucrose transglucosylase. These results suggest the mechanism for the production of ADPG in plant tissues. The corresponding UDPG systems were inefficient in promoting the transfer of glucose. The relative effectiveness of the ADPG and UDPG systems was further substantiated by the fact that in studies with paper chromatographic methods with C 14-labeled glucose donors, a high degree of radioactivity remained in the UDPG zone, while little accumulated in the ADPG zone. It was further shown that the glucose transferred through the ADPG pathway largely went into the amylopectin fraction, while that from UDPG was transferred about equally into the amylose and amylopectin fractions.