Biosynthesis of starch. Formation of a glucoproteic acceptor by a potato non-sedimentable preparation.

1. A non-sedimentable fraction of potato tuber has been found to catalyze [14C]glucose transfer from [14C]glucose 1-phosphate to an endogenous proteic acceptor in the absence of added primer. This transfer is activated by Mn2+. 2. The labeled glucosylated product formed is trichloroacetic acid insoluble and sensitive to proteolytic and amylolytic digestions. It appears to be a glucoprotein with glucosyl chains bound to the peptide portion of the molecule through an unknown linkage. 3. The carbohydrate portion of the glucoprotein can be released by prolonged incubations with the enzymatic preparation, and becomes in turn, trichloroacetic acid soluble and alcohol precipitable. 4. Both products, the glucoprotein as well as the alpha-1,4-glucan that seems to arise from the enzymatic cleavage of the former, can be used as primers by the transglucosylating system with ADP[14C]glucose, UDP[14C]glucose or GDP[14C]glucose as glucosyl donors. The results presented in this paper are the first demonstration of soluble glucosyl transferases with the same glucose donor specificity to that of the particulate starch synthetase. 5. This report presents further evidence in favor of the assumption of a glucoproteic intermediate in alpha-a,4-glucan synthesis initiation.