An isomaltose-hydrolyzing cy-D-glucosidase from the alkalophilic Bacillus designated strain F5 was purified to an electrophoretically homogeneous state. The molecular weight of the purified glucosidase was 60 000 by SDS-poly(acrylamide) gel electrophoresis, and 63 000 by Sephacryl S-200 gel-filtration chromatography. The enzyme was most active for isomaltose at pH 6.0-6.5 and 45", and stable up to 50" at pH 7.0 and in the range of pH 6.0-9.0 at 50" by lo-min incubation. The apparent V,, and K,,, values for isomaltose were 34.5 ~ol.min-l.mg-l of protein, and 3.33mM. Panose and isomaltotriose are the best substrates for this enzyme. The restricted substrate specificity indicated the assignment of the enzyme to be an oligo-1,6-glucosidase (dextrin 6-a-glucanohydrolase; EC 3.2.1. lo), but it was suggested that it could be a new type of oligo-1,6-glucosidase on the basis of its action on a series of (1+4)-a-malto-oligosaccharides. INTRODUCI'ION Three types of enzyme hydrolyzing 1,6-a-D-glucosidic linkages are known, i.e., (a) oligo-1 ,6-D-glucosidase1-6 [EC 3.2.1. lo] hydrolyzes (l-+6)-a-D-glucosidic linkages in isomaltose and dextrins produced from starch and glycogen by alpha amylase, (b) amylo-(l+6)-glucosidase7+r [EC 3.2.1.331 endohydrolyzes (l-+6)-~-~glucosidic linkages at points of branching in chains of (1+4)-linked cr-D-glucosyl residues, and (c) exo-1,6-a-D-glucosidase g~lo [EC 3.2.1.701 successively hydrolyzes D-glucosyl residues from 1,6-cu-D-&cans and derived oligosaccharides. Some kinds of cw_D-glucosidase i1-13 [EC 3.2.1.20] also show only a little activity to (1+6)-CX-Dglucosidic linkages.
Previouslyt4, we reported on the characterization of an alkalophilic soil bacterium which produces intracellular, isomaltose-hydrolyzing a-D-glucosidase in an alkalophilic medium containing 1% of NaHCO,. This crude enzyme showed a strict substrate specificity to the (l-6)-cr-D-glucosidic linkage of disaccharides com-