Characterization of a dextranase produced by an oral strain of Actinomyces israelii.

A dextranase-producing, gram-positive, anaerobic, rod-shaped bacterium isolated from human dental plaque was identified as Actinomyces israeli. Although the extracellular dextranase (EC 3.2.1.11) formed by this microbe appeared to be constitutively produced, the bacterium did not utilize the reaction products as a carbon source during growth. A striking feature of the dextranase was the formation of two distinct groups of oligosaccharide end products. The two groups presumably correspond to the limit dextran and the released reaction product which appeared to be cleaved from the end(s) of larger dextran molecules. Low levels of dextranase activity were measured by [3H]NaBH4 reduction and alcohol fixation of the large, tritiated end products on filter paper disks. Of the carbohydrate substrates tested, only alpha-1,6-linked glucans were cleaved. The enzyme did not exhibit any metal ion requirements, and its pH optimum was 6.3. It is suggested that the A. israelii dextranase may function as a regulatory factor during extracellular in vivo glucan synthesis from sucrose by various plaque microbes.