Enzymic Mechanism of Starch Synthesis in Ripening Rice Grains. Ii. Adenosine Diphosphate Glucose Pathway.

It has long been believed that phosphorylase is involved in the synthesis of starch, which constitutes 70 to 80% of the mature rice grain. Aimi and his associates (1, 2, 3) found that during the ripening process a parallel relationship exists between starch synthesis and phosphorylase activity in rice grains. But we can not exclude the possible existence of mechanisms other than phosphorylase reaction for starch formation. The high ratio of inorganic phosphate to sugar phosphate in plants, which is unfavorable to starch formation (1, 10, 31), raises doubts about the role of phosphorylase in polysaccharide synthesis. An important development in the field of polysaccharide biochemistry has been the recent discovery of starch synthetase by Leloir's group (8, 14), which proposed the following enzymic reaction: UDPG + acceptor (G)n -> UDP + starch (G)n+1. I