Classification of Some α-Glucosidases and α-Xylosidases on the Basis of Substrate Specificity

Abstract

Three α-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 α-glucosidases were made uniform in the hydrolyzing activities using p-nitrophenyl α-glucopyranoside (α-p-NPG) and were reacted with p-nitrophenyl α-xylopyranoside (α-p-NPX) or isoprimeverose (xylopyranosyl-α-1,6-glucopyranose), which are typical substrates of α-xylosidase. Only Asp. niger α-glucosidase among them hydrolyzed α-p-NPX and isoprimeverose. Further the substrate specificities of three α-glucosidases and two α-xylosidases (I and II from Asp.flavus MO-5) were investigated on maltose, isomaltose, α-p-NPG, isoprimeverose, and α-p-NPX in detail, and kinetic parameters [K_m, V_max, and molecular activity (k₀)] were estimated and compared with each other.

In the comparison of kinetic parameters, Asp. niger α-glucosidase showed a broad specificity, that is, containing isoprimeverose in addition to maltose, isomaltose, and α-p-NPG. Though this enzyme barely hydrolyzed α-p-NPX too, the velocity was very slow. Though both yeast α-glucosidases barely hydrolyzed α-p-NPX or isoprimeverose too, these substrates were not good for yeast enzymes. On the other hand, two α-xylosidases showed narrow specificities, such that the substrates except for α-p-NPX and isoprimeverose were not hydrolyzed at all.

The action on isoprimeverose by Asp. niger α-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 α-glucosidase were catalyzed at the same active site. Asp. niger enzyme that has both α-glucosidase activity and α-xylosidase activity was shown to be classified in a middle position between α-glucosidase and α-xylosidase.