Starch biosynthesis: sucrose as a substrate for the synthesis of a highly branched component found in 12 varieties of starches Original Research Article

d-[14C]Glucose was incorporated into starch when 12 varieties of starch granules were incubated with [14C]sucrose. Digestion of the 14C-labeled starches with porcine pancreatic alpha amylase showed that a high percentage (16.1–84.1%) of the synthesized starch gave a relatively high molecular weight α-limit dextrin. Hydrolysis of the 12 varieties of starch granules by alpha amylase, without sucrose treatment, also gave an α-limit dextrin, ranging in amounts from 0.51% (w/w) for amylomaize-7 starch to 8.47% (w/w) for rice starch. These α-limit dextrins had relatively high molecular weights, 2.47 kDa for amylomaize-7 starch to 5.75 kDa for waxy maize starch, and a high degree of α-(1→6) branching, ranging from15.6% for rice starch to 41.1% for shoti starch. ADPGlc and UDPGlc did not synthesize a significant amount (1–2%) of the branched component, suggesting that sucrose is the probable substrate for the in vivo synthesis of the component and that sucrose is not first converted into a nucleotide-glucose diphosphate intermediate.