Degradation of dextrans by an α-1,6-glucan glucohydrolase from Streptococcus mitis

Abstract

A dextran-glucosidase (α-1,6-glucan glucohydrolase) found in cell extracts of several strains of Streptococcus mitis has been purified, and some of its properties have been investigated. Dextran-glucosidase hydrolysed isomaltose and isomaltose saccharides at comparable rates, and a comparison of K m values for isomaltose, isomalto-pentaose and dextran showed that the enzyme had equal affinity for these substrates despite their different chain-length. The action pattern of the enzyme was not completely characteristic of an exo-glucanase or of a glucosidase. Its ability to act on polymers, and its complete specificity for α-(1→6)- D-glucosidic linkages were properties associated with exo-glucanases, whereas its transferring ability, the retention of configuration, and inhibition by nojirimycin supported its classification as a glucosidase. Dextrans were incompletely degraded by the enzyme, the extent of hydrolysis being related to the proportion of α-(1→6)- D-glucosidic linkages. It was concluded that the α-(1→4)- α-(1→3)-, and α-(1→2)- D-glucosidic linkages that occur in bacterial dextrans were resistant to the action of dextran-glucosidase, and arrested the further enzymic degradation of dextran. The use of the enzyme in investigations of the fine structure of dextrans is discussed.