Classification of Someα-Glucosidases andα-Xylosidases on the Basis of Substrate Specificity

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Abstract
Three alpha-glucosidases which passed under the names of transglucosidase (from Aspergillus niger), maltase (from Brewers yeast), and isomaltase (from Bakers yeast) for reasons of their substrate specificities and transfer actions, were purified to electrophoretically pure states. These purified alpha-glucosidases were made uniform in the hydrolyzing activities using p-nitrophenyl alpha-glucopyranoside (alpha-p-NPG) and were reacted with p-nitrophenyl alpha-xylopyranoside (alpha-p-NPX) or isoprimeverose (xylopyranosyl-alpha-1,6-glucopyranose), which are typical substrates of alpha-xylosidase. Only Asp. niger alpha-glucosidase among them hydrolyzed alpha-p-NPX and isoprimeverose. Further the substrate specificities of three alpha-glucosidases and two alpha-xylosidases (I and II from Asp. flavus MO-5) were investigated on maltose, isomaltose, alpha-p-NPG, isoprimeverose, and alpha-p-NPX in detail, and kinetic parameters [Km, Vmax, and molecular activity (Ko)] were estimated and compared with each other. In the comparison of kinetic parameters, Asp. niger alpha-glucosidase showed a broad specificity, that is, containing isoprimeverose in addition to maltose, isomaltose, and alpha-p-NPG. Though this enzyme barely hydrolyzed alpha-p-NPX too, the velocity was very slow. Though both yeast alpha-glucosidases barely hydrolyzed alpha-p-NPX or isoprimeverose too, these substrates were not good for yeast enzymes. On the other hand, two alpha-xylosidases showed narrow specificities, such that the substrates except for alpha-p-NPX and isoprimeverose were not hydrolyzed at all. The action on isoprimerose by Asp. niger alpha-glucosidase was completely the same as that on isomaltose at optimum pH, optimum temperature, inhibition pattern of hydrolyzing activity by 1-deoxynojirimycin, and transfer action pattern. Accordingly, we interpret these results as indicating that the hydrolyzations of isomaltose and isoprimeverose by Asp. niger alpha-glucosidase were catalyzed at the same active site. Asp. niger enzyme that has both alpha-glucosidase activity and alpha-xylosidase activity was shown to be classified in a middle position between alpha-glucosidase and alpha-xylosidase.

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