A Levansucrase from Streptococcus mutans

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Abstract
One of the predominant bacteria in early dental plaque, Streptococcus mutans, has been shown to synthesize levansucrase. This enzyme was obtained from the supernatant fluid of a glucose-broth culture. Purification of the enzyme was achieved in two steps by adsorption on hydroxylapatite followed by isoelectric focusing and fractionation. This resulted in a 10% yield and a 200-fold purification. Sucrose as well as raffinose could serve as substrate for the enzyme. In addition to levan (MW > 25 × 106), free fructose and oligosaccharides were formed from sucrose. The enzyme had a pH optimum at 6.0, optimal activity around 40° C and isoelectric point at pH 4.2. Iodoacetamide, cysteine, Tris and sodium fluoride in a concentration of 0.05 m had no effect on the enzyme activity. The enzyme was totally inhibited by 0.1 mM Hg++ and to about 40% by 1 mM Pb++, Ag+, Ni++ and Cu++. EDTA in a concentration of 1 mM reduced the over-all activity of the enzyme by 75% and specifically interfered with the synthesis of levan. Most divalent cations could restore the activity of the enzyme in the presence of EDTA. The properties of this streptococcal levansucrase were similar to those previously described for levansucrases from Aerobacter levanicum and Bacillus subtilis. The finding, that EDTA interfered with the synthesis of levan, may give new possibilities in elucidating the mechanism of this enzyme reaction.

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