Sweetpotato α- and β-Amylases: Characterization and Kinetic Studies with Endogenous Inhibitors

Sweetpotato α- and β-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. α-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°, but it was inactivated by heat in the absence of Ca²⁺ at > 63°. The Km for soluble starch was 2.08 mg/mL. The molecular weight was 45000 daltons. α-Amylase activity was reduced up to 70% by 0.2 mM K-ascorbate and moderately by Na-oxalate and Na-phytate. β-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° and it was inactivated at 60°. Km for soluble starch was 3.71 mg/mL. At 0.08 mM, K-ascorbate strongly inhibited β-amylase activity.