IN-VITRO DEGRADATION OF STARCH GRAINS BY PHOSPHORYLASES AND AMYLASES FROM POPLAR WOOD

Three amylases and two isoforms of starch phosphorylase (Type I and Ty pe II) from poplar (Populus x canadensis Moench <<robusta>>) wood were partially purified and the interaction of these enzymes in starch deg radation was investigated with a model system comprising potato starch grains as substrate. The oligo- and disaccharides, which were release d by the action of endo- and exoamylases, were differentially determin ed by digestion with amyloglucosidase and alpha-glucosidase. Alpha-amy lase was more active than beta-amylase in degrading starch grains. A t hird amylase showed no activity with soluble starch whereas it attacke d starch grains with product characteristics of an endoamylase. Synerg istic effects among the starch hydrolases were not detected but the pa ttern of the released glucans shifted from maltooligosaccharides to ma ltose in the presence of beta-amylase. The very low starch grain-degra ding activity of phosphorylase Type II was about eight-fold increased in combination with alpha-amylase, while phosphorylase Type I was able to attack starch grains alone without significant effects of endoamyl ases. If the starch phosphorylases were used in combination with endoa mylases and beta-amylase to digest starch grains, the formation of Glc 1P was suppressed and maltose was the main degradation product. These results are consistent with the assumption that the degradation of sta rch grains was initiated by the attack of endoamylase and that the rel eased oligosaccharides are substrates of phosphorylase and possibly be ta-amylase. The complete inhibition of beta-amylase by maltose at conc entrations that are found in poplar wood during phases of starch degra dation in vivo may therefore be of physiological relevance.