Interaction of hydrolytic and phosphorolytic enzymes of starch metabolism in Kalanchoë daigremontiana.

The degradation of starch by a protein fraction of Kalanchoë daigremontiana Hamet et Perrier, obtained by ammoniumsulfate precipitation (30-70%), was found to be catalyzed by α-and β-amylase (EC 3.2.1.1 and EC 3.2.1.2, respectively) and by starch phosphorylase (EC 2.4.1.1). The activity of these enzymes was determined by chromatographic analysis of the reaction products; separation and identification of α-amylase was accomplished by heat-inactivation of β-amylase and α-glucosidase. When the interaction of amylolytic and phosphorolytic enzymes was comparatively studied, it was found that without inorganic phosphorus in the reaction mixture, (14)C-starch was converted predominantly to maltose and glucose; supplementation with 1-10 mM orthophosphate (Pi) resulted in an increase in glucose-1-phosphate formation and a concomitant reduction of maltose production. Since the total volume of starch degradation remained approximately constant, Pi apparently inhibits β-amylase (Ki about 3 mM Pi). Thus, free Pi in the cell participates in the regulation of starch catabolism, serving as a substrate for starch phosphorylase while simultaneously reducing the production of maltose. With respect to glucan synthesis, adenosinediphosphoglucose-α-1,4-glucosyltransferase (EC 2.4.1.22), maltose phosphorylase and maltoseglucosyltransferase were also found to be active. The last-named enzyme catalyzes an exchange between dextrins and is considered to provide primer carbohydrates for the synthesis of polyglucans.