Abstract
A treatment of buckwheat α-glucosidase with N-acetylimidazole brought about the acetylation of 6.4 tyrosyl residues, 0.38 free sulfhydryl groups and about 50% of free amino groups, and the decrease in the hydrolytic activities toward maltooligosaccharides (G2_??_G8, G_??_13 ) and soluble starch. The affinities for the substrate other than maltose were diminished by the modification and the extent of the reduction of the affinities was apparently dependent on the degree of polymerization of maltooligosaccharides, while the affinity for phenyl a-maltoside was increased. The treatment of the acetylated enzyme with hydroxylamine resulted in the complete restration of the affinities for all substrates tested. It seems that these facts were due to the acetylation of several tyrosyl residues located in or near certain subsites of the enzyme. About 25% of the hydrolytic activity remained inert in spite of the deacetylation with hydroxylamine, which was assumed to be attributed to the partial modification of free sulfhydryl group localized closely near the catalytic site of the enzyme.
