Substrate Specificity ofα-Glucosidase II in Rice Seed

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Abstract
The substrate specificity of rice α-glucosidase II was studied. The enzyme was active especially on nigerose, phenyl-α-maltoside and maltooligosaccharides. The actions on isomaltose and phenyl-α-glucoside were weak, and on sucrose and methyl-α-glucoside, negligible. The α-glucans, such as soluble starch, amylopectin, β-limit dextrin, glycogen and amylose, were also hydrolyzed. The ratio of the maximum velocities for hydrolyses of maltose (G2), nigerose (N), kojibiose (K), isomaltose (I), phenyl-α-maltoside (ϕM) and soluble starch (SS) was estimated to be 100: 94.4: 14.2: 7.1: 89.5: 103.1 in this order, and that for hydrolyses of malto-triose (G3), -tetraose (G4), -pentaose (G5), -hexaose (G6), -heptaose (G7), -octaose (G8), and amyloses ( and ), 113: 113: 113: 106: 113: 100: 106: 106. The Km values for N, K, I, ϕM and SS were 2.4 mm, 0.58 mm, 20 mm, 1.6 mm and 5.0 mg/ml, respectively; those for G2, G3, G4, G5, G6, G7, G8, and , 2.4 mm, 2.2 mm, 2.1 mm, 1.5 mm, 1.0 mm, 1.1 mm, 0.95 mm, 1.5 mm and 1.1 mm. Rice α-glucosidase II is considered an enzyme with a preferential activity on maltooligosaccharides.

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