Effect of dextran and ammonium sulphate on the reaction catalysed by a glucosyltransferase complex from Streptococcus mutans.

The highly aggregated proteins precipitated by (NH4)2SO4 from the culture fluid of three strains of Streptococcus mutans gradually released less aggregated glucosyltransferase activities - dextransucrase and mutansucrase - which catalysed the synthesis of water-soluble and insoluble glucans from sucrose. Mutansucrase was eluted from a column of Sepharose 6B before dextransucrase. This activity was lost during subsequent dialysis and gel filtration, but there was a corresponding increase in dextransucrase activity which catalysed the formation of soluble glucan when incubated with sucrose alone, and insoluble glucan when incubated with sucrose and 1.55 M-(NH4)2SO4. Relative rates of synthesis of soluble and insoluble glucan in the presence of 1.55 M-(MH4)2SO4 were dependent upon the enzyme concentration: high concentrations favoured insoluble glucan synthesis. Insoluble glucans synthesized by mutansucrase or by dextransucrase in the presence of 1.55 M-(NH4)2SO4 were more sensitive to hydrolysis by mutanase than by dextranse, but soluble glucans were more extensively hydrolysed by dextranase than by mutanase. Partially purified dextransucrase sedimented through glycerol density gradients as a single symmetrical peak with an apparent molecular weight in the range 100000 to 110000. In the presence of 1.55 M-(NH4)2SO4, part of the activity sedimented rapidly as a high molecular weight aggregate. The results strongly suggest that soluble and insoluble glucans are synthesized by interconvertible forms of the same glucosyltransferase. The aggregated form, mutansucrase, preferentially catalyses (1 leads to 3)-alpha bond formation but dissociates during gel filtration to the dextransucrase form which catalyses (1 leads to 6)-alpha bond formation.