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

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Abstract
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 Mr of GTF-I was 155000 by SDS-PAGE and the isoelectric point was pH 4.9. The specific activity, the optimum pH and the Km 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.alpha.-linked glucose, 23.6 mol% 1,6-.alpha.-linked glucose, 2.6 mol% 1,3,6-.alpha.-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-.alpha.-linked glucose. The Mr and pI values of purified GTF-I were identical with those of the enzyme in the culture supernatant.