Characterization and Subcellular Localization of Debranching Enzyme and Endoamylase from Leaves of Sugar Beet

Abstract

Sugar beet leaves (Beta vulgaris L.) contained up to five endoamylases, two exoamylases, and a single debranching enzyme. Four of the endoamylases and the debranching enzyme were present in the chloroplast. The chloroplastic starch-debranching enzyme and an apoplastic endoamylase were copurified from mature leaves of sugar beet by 35 to 50% ammonium sulfate precipitation and chromatography on diethylaminoethyl-Sephacryl, β-cyclodextrin Sepharose 6B, and Sephadex G-150. The debranching enzyme, which was purified to homogeneity, had a molecular mass of 100 kilodaltons and a pH optimum of 5.5. It showed a high activity with pullulan as a substrate, low activity with soluble starch and amylopectin, and no activity with native starch grains isolated from sugar beet leaves. The endoamylase, which was partially purified, had a molecular mass of 43,000 kilodaltons, a pH optimum of 6.5, required calcium for activity and thermal stability, and showed an ability to hydrolyze native starch grains.