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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.
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