title: In-vitro degradation of starch granules isolated from spinach chloroplasts.
abstract: The initial reactions of transitory starch degradation in Spinacia oleracea L. were investigated using an in-vitro system composed of native chloroplast starch granules, purified chloroplast and non-chloroplast forms of phosphorylase (EC 2.4.1.1) from spinach leaves, and α-amylase (EC 3.2.1.1) isolated from Bacillus subtilis. Starch degradation was followed by measuring the release of soluble glucans, by determining phosphorylase activity, and by an electron-microscopic evaluation following deep-etching of the starch granules. Starch granules were readily degraded by α-amylase but were not a substrate for the chloroplast phosphorylase. Phosphorolysis and glucan synthesis by this enzyme form were strictly dependent upon a preceding amylolytic attack on the starch granules. In contrast, the non-chloroplast phosphorylase was capable of using starch-granule preparations as substrate. Hydrolytic degradation of the starch granules was initiated at the entire particle surface, independently of its size. As a result of amylolysis, soluble glucans were released with a low degree of polymerization. When assayed with these glucans as substrate, the chloroplast phosphorylase form exhibited a higher apparent affinity and a higher reaction velocity compared with the non-chloroplast phosphorylase form. It is proposed that transitory starch degradation in vivo is initiated by hydrolysis; phosphorolysis is most likely restricted to a pool of soluble glucan intermediates.
authors.affiliation: Botanisches Institut, Westfälische Wilhelms-Universität, Schlossgarten 3, D-4400, Münster.
authors.forename: M
authors.initials: M
authors.lastname: Steup
authors.forename: H
authors.initials: H
authors.lastname: Robenek
authors.forename: M
authors.initials: M
authors.lastname: Melkonian