Starch Synthesis in Potato Tubers: Identification of the in vitro and the in vivo Acceptor Molecules of Soluble Starch Synthase Activity

A soluble starch synthase preparation obtained from potato tubers (Solanum tuberosum L.) was used to identify the acceptor molecules of the enzyme in vitro. Maltose was the shortest effective primer molecule, and several straight and branched glucose polymers were also able to serve as primer. Osmotically permeabilized potato tissue slices were used to identify the acceptor molecules of granule-bound and soluble starch synthases under semi in vivo conditions. Specific assay conditions were employed to differentiate between the different starch synthesizing enzyme activities. Both the granule-bound and the primed soluble starch synthases elongated amylose and amylopectin molecules which were already deposited into starch granules. Soluble malto-oligosaccharides also functioned as acceptor molecules. In the presence of 0.5 M sodium citrate, starch synthase activity was enhanced. Both amylose and amylopectin accepted more glucose units than in the absence of sodium citrate, indicating that the citrate stimulated soluble starch synthase elongated both starch components as well. The acceptor molecules of the citrate stimulated reaction are probably partly soluble. The elongation patterns of the different starch synthases are dealt with in view of some of the models explaining the side-by-side synthesis of amylose and amylopectin in higher plants.