SWEET-POTATO ALPHA-AMYLASE AND BETA-AMYLASE - CHARACTERIZATION AND KINETIC-STUDIES WITH ENDOGENOUS INHIBITORS

Sweetpotato alpha- and beta-amylases were characterized to assist optimization of direct hydrolysis of starch by endogenous amylases. In kinetic studies purified starch was substrate, and ascorbate, oxalate, phenolics,phytate and sweetpotato extracts were assayed for inhibitory activity. alpha-Amylase had optimum pH between 5.8 and 6.4 and was stable from pH 5.0 to 9.0. Optimum activity occurred at 71.5 degrees C, but it was inactivated by heat in the absence of Ca2+ at > 63 degrees C. The Km for soluble starch was 2.08 mg/mL. The molecular weight was 45000 daltons. alpha-Amylase activity was reduced up to 70% by 0.2 mM K-ascorbate and moderately by Na-oxalate and Na-phytate. beta-Amylase had optimum pH between 5.3 and 5.8, and was stable from pH 4.0 to 8.0. Its maximum activity was at 53 degrees C and it was inactivated at 60 degrees C. Km for soluble starch was 3.71 mg/mL. At 0.08 mM, K-ascorbate strongly inhibited beta-amylase activity.