THE ELONGATION OF AMYLOSE AND AMYLOPECTIN CHAINS IN ISOLATED STARCH GRANULES

The aim of this work was to investigate the conditions required for am ylose synthesis in starch granules. Although the major granule-bound i soform of starch synthase - GBSSI - catalyses the synthesis of amylose in vivo, C-14 from ADP[C-14]glucose was incorporated primarily into a specific subset of amylopectin chains when supplied to starch granule s isolated from pea (Pisum sativum L.) embryos and potato (Solanum tub erosum L.) tubers. Incubation of granules with soluble extracts of the se organs revealed that the extracts contained compounds that increase d the incorporation of C-14 into amylose. These compounds were rendere d inactive by treatment of the extracts with alpha-glucosidase, sugges ting that they were malto-oligosaccharides. Consistent with this idea, provision of pure malto-oligosaccharides to isolated granules resulte d in a dramatic shift in the pattern of incorporation of C-14, from am ylopectin chains to amylose molecules. Comparison of the pattern of in corporation in granules from wild-type peas and lam mutant peas which lack GBSSI showed that this effect of malto oligosaccharides was speci fically on GBSSI. The significance of these results for understanding of the synthesis of amylose and amylopectin in storage organs is discu ssed.