Purification and substrate specificity of an endo-dextranase of Streptococcus mutans K1-R |
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Authors: | Alexandra Pulkownik Gwen J Walker |
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Institution: | Institute of Dental Research, 2 Chalmers Street, Sydney Australia 2010 |
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Abstract: | An extracellular endo-dextranase has been isolated from Streptococcus mutans K1-R. Incubation of cell-free culture fluid with sucrose permitted the removal of a large proportion of the extracellular d-glucosyltransferases by irreversible adsorption onto the insoluble glucans that these enzymes synthesize from sucrose. The remaining d-glucosyltransferases were separated from dextranase by precipitation with ammonium sulphate, chromatography on hydroxylapatite and DEAE-cellulose, followed by filtration on Ultrogel. The major products of action of the purified dextranase on (1→6)-α-d-glucans were isomaltotriose (IM3), isomaltotetraose (IM4), and isomaltopentaose (IM5). Further hydrolysis of IM4 and IM5 occurred after prolonged incubation with excess of enzyme, to give d-glucose, IM2, and IM3. The relative rate of hydrolysis of isomaltose saccharides fell sharply with decreasing chainlength from IM12 to IM5. The hydrolysis of dextrans containing 96% or more of (1→6)-α-d-glucosidic linkages, expressed as apparent conversion into IM3, was virtually complete, and substrates such as Streptococcus sanguis glucan, containing sequences of (1→6)-α-d-glucosidic linkages, were also effectively hydrolyzed. Dextranase activity towards the soluble glucan of Streptococcus mutans was limited, and there was no action on the insoluble glucan synthesized by S. mutans sucrose 3-d-glucosyltransferase. |
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