Purification and Characterization of Extracellular Glucosyltransferase Synthesizing Water-insoluble Glucan from Streptococcus rattus

An extracellular glucosyltransferase synthesizing water-insoluble glucan (GTF-I) was purified from the culture supernatant of Streptococcus rattus strain BHT (mutans serotype b) by hydroxylapatite chromatography, DEAE-Toyopearl chromatography and preparative isoelectric focusing. The M r of GTF-I was 155000 by SDS-PAGE and the isoelectric point was pH 4.9. The specific activity, the optimum pH and the K m value for sucrose were 10.0 i.u. (mg protein)-1, 6.5 and 2.4 mm, respectively. The enzyme synthesized a water-insoluble glucan consisting of 69.4 mol% 1,3-α-linked glucose, 23.6 mol% 1,6-α-linked glucose, 2.4 mol% 1,3,6-α-branched glucose and 4.4 mol% non-reducing terminal glucose, and also a small amount (3% of the total glucan) of soluble glucan with 82.4 mol% 1,6-α-linked glucose. The M r and pI values of purified GTF-I were identical with those of the enzyme in the culture supernatant.