Characterization of the Dextranase Purified from Streptococcus mutans Ingbritt

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We purified dextranase from the culture supernatant of Streptococcus mutans Ingbritt by procedures including ammonium sulfate precipitation, ionexchange chromatography, and gel filtration. The molecular weight of the enzyme was estimated as 78kDa by SDS-PAGE. The enzyme degraded dextran at the optimum pH of 5.5, but not other glucans and fructans at all. Paper chromatographic analysis revealed that the enzyme cleaved dextran by an endo-type mechanism. The enzyme was inhibited by Hg²⁺, Fe³⁺, Zn²⁺, and anionic detergents SDS and deoxycholic acid, but not inhibited by non-ionic detergents Triton X-100, Lubrol PX, Nonidet P-40, and Tween 80. SDS-blue dextran-PAGE analysis of the culture supernatant revealed that the enzyme activity detected in the 96 kDa band shifted gradually to the 78 kDa band during handling the supernatant. This shift was inhibited by phenylmethylsulfonyl fluoride, suggesting that the shift of the molecular size is due to proteolytic degradation of the enzyme by serine protease.