ABSTRACT
Ion-exchange chromatography is the most widely used technique in protein chromatography (1). This is because it is nearly always possible to develop successful ionexchange separations for proteins, and the materials required are relatively inexpensive.
ABSTRACT
Two methods were used to purify the bifunctional extracellular enzyme sucrose: (1-6)- and (1-3)-alpha-D-glucan-6-alpha-D-glucosyltransferase (EC 2.4.1.5; dextransucrase) from continuous cultures of a serotype c strain of Streptococcus mutans. The first method, based on a previously published report, involved Sepharose 6B gel filtration and DEAE cellulose anion exchange chromatography. This resulted in a dextransucrase preparation with an apparent molecular mass of 162 kDa and a specific activity of 125 mg of glucan formed from sucrose h-1 (mg of protein)-1, at 37 degrees C. It was almost homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The ratio of carbohydrate to protein was 0.14 and the recovery was 14% relative to the total glucosyltransferase activity in the original culture fluid. In the subsequently preferred method, hydroxyapatite-Ultrogel was used to purify dextransucrase with a 24% yield. The specific activity, 197 mg of glucan formed h-1 (mg of protein)-1, was the highest yet reported and this preparation contained less than 0.5 glucose-equivalent per subunit of molecular mass 162 kDa. Dextransucrase is therefore not a glycoprotein. Exogenous dextran stimulated activity, but was not essential for activity. The purified protein slowly degraded to multiple lower molecular mass forms during storage at 4 degrees C and 87% of the activity was lost after 20 days. The molecular mass of the most prominent, active degradation product was 140 kDa, similar to that of one of the multiple forms of dextransucrase detected in other laboratories. Preparations in which either the 140-kDa or the 162-kDa species predominated catalyzed the synthesis of a water-soluble glucan with sucrose alone, but catalyzed that of an insoluble glucan with sucrose and a high concentration of either (NH4)2SO4 or polyethylene glycol. The water-insoluble glucan was shown to lack sequences of 1,3-alpha-linked glycosyl residues typical of the insoluble glucan, mutan, which has been implicated in dental caries. We conclude that mutan is synthesized by the concerted action of two independent glucosyltransferases rather than by interconvertible forms of a single enzyme, as was proposed previously.
