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α-Glucosidase with a high regioselectivity for α-1,3-glucosidic linkages for hydrolysis and transglucosylation was purified from culture broth of Acremonium implicatum. The enzyme was a tetrameric protein (M.W. 440,000), of which the monomer (M.W. 103,000; monomeric structure was expected from cDNA sequence) was composed of two polypeptides (M.W. 51,000 and 60,000) formed possibly by posttranslational proteolysis. Nigerose and maltose were hydrolyzed by the enzyme rapidly, but slowly for kojibiose. The k₀/K_m value for nigerose was 2.5-fold higher than that of maltose. Isomaltose was cleaved slightly, and sucrose was not. Maltotriose, maltotetraose, p-nitrophenyl α-maltoside and soluble starch were good substrates. The enzyme showed high affinity for maltooligosaccharides and p-nitrophenyl α-maltoside. The enzyme had the α-1,3- and α-1,4-glucosyl transfer activities to synthesize oligosaccharides …