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Comparative Study of Invertases of Streptococcus mutans

Sucrase activity was studied in 13 strains of Streptococcus mutans representing the five Bratthall serotypes. Sucrose-adapted cells have sucrase activity in the 37,000 × g-soluble fraction of all strains. The enzyme was identified as invertase (β-d-fructofuranoside fructohydrolase; EC 3.2.1.26) because it hydrolyzed the β-fructofuranoside trisaccharide raffinose, giving fructose and melibiose as its products, and because it hydrolyzed the β-fructofuranoside dissacharide sucrose, giving equimolar glucose and fructose as its products. Invertases of c and e strains exhibit two activity peaks by molecular exclusion chromatography with molecular weights of 45,000 to 50,000 and about 180,000; those of serotypes a, b, and d strains exhibit only a single component of 45,000 to 50,000 molecular weight. The electrophoretic mobility of invertases is different between the serotypes and the same within them. Inorganic orthophosphate (Pi) has a weak positive effect on the Vmax of invertases of serotypes c and e cells but a strong positive effect on the invertases of serotype b cells; Pi has a strong positive effect on the apparent Km of the invertases of serotype d cells, but has no effect on the Vmax; Pi has a strong positive effect on both the apparent Km and Vmax of the invertases of serotype a cells. Thus, the invertases were different between all of the serotypes but similar within the serotypes. These findings support the taxonomic schemes of Coykendall and of Bratthall. It was additionally noted that 37,000 × g-soluble fractions of only serotypes b and c but not serotypes a, d, and e cells have melibiase activity, and it could be deduced that serotype d cells lack an intact raffinose permease system.