In Vitro Degradation of Starch Grains by Phosphorylases and Amylases from Poplar Wood

Abstract

Summary Three amylases and two isoforms of starch phosphorylase (Type I and Type II) from poplar ( Populus x canadensis Moench «robusta» ) wood were partially purified and the interaction of these enzymes in starch degradation was investigated with a model system comprising potato starch grains as substrate. The oligo- and disaccharides, which were released by the action of endo- and exoamylases, were differentially determined by digestion with amyloglucosidase and α-glucosidase. Alpha-amylase was more active than β-amylase in degrading starch grains. A third amylase showed no activity with soluble starch whereas it attacked starch grains with product characteristics of an endoamylase. Synergistic effects among the starch hydrolases were not detected but the pattern of the released glucans shifted from maltooligosaccharides to maltose in the presence of β-amylase. The very low starch grain-degrading activity of phosphorylase Type II was about eight-fold increased in combination with a-amylase, while phosphorylase Type I was able to attack starch grains alone without significant effects of endoamylases. If the starch phosphorylases were used in combination with endoamylases and β-amylase to digest starch grains, the formation of Glc1P was suppressed and maltose was the main degradation product. These results are consistent with the assumption that the degradation of starch grains was initiated by the attack of endoamylase and that the released oligosaccharides are substrates of phosphorylase and possibly β-amylase. The complete inhibition of β-amylase by maltose at concentrations that are found in poplar wood during phases of starch degradation in vivo may therefore be of physiological relevance.