Purification and Substrate Specificity of Sweet Corn α-Glucosidase

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An α-glucosidase was purified from sweet corn seeds by fractionation with ammonium sulfate, chromatographies on CM-Sepharose and Sepharose 4B, and gel filtrations on Sephadex G-100. The enzyme was homogeneous in disc electrophoretic analysis. The molecular weight was estimated to be about 9.6×104 by SDS-disc electrophoresis. The enzyme showed high activities toward maltose, nigerose, phenyl-α-maltoside, and maltooligosaccharides. The ratios of maximum velocity for maltose, nigerose, kojibiose, isomaltose, phenyl-α-glucoside, phenyl-α-maltoside, panose, turanose, and soluble starch were estimated to be 100: 78 : 17: 11 : 28 : 100 : 31 : 3.4 : 126, and the Km values for these substrates, 1.5 mM, 1.4 mM, 0.48 mM, 14 mM, 4.2 mM, 1.1 mM, 5.0 mM, 0.28 mM and 52 mg/ml, respectively. The maximum velocity for soluble starch was high, but this α-glucan was not a favorable substrate because the Km value was also very high. The Vmax for maltooligosaccharides were somewhat dependent on the degree of polymerization (n). The Km values for substrates having four or more glucose units increased with the increase in n.
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