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Identification, Preliminary Characterization, and Evidence for Regulation of Invertase in Streptococcus mutans

¹ Veterans Administration Hospital and University of Connecticut Health Center, Newington, Connecticut 06111

ABSTRACT

Sucrose dissimilation was studied in five strains of Streptococcus mutans. Glucose-adapted strain SL-1 makes acid more slowly from sucrose than from glucose; glucose-adapted strain SL-1 gives diauxie growth kinetics in broth containing limiting amounts of both glucose and sucrose. Thus, at least part of the sucrose dissimilative system appears inducible. Sucrase activity was identified in the 37,000 x g soluble cell fraction of five strains. Its intracellular location implies the presence of sucrose permease. The specific activity of the sucrase is higher in sucrose-adapted cells than in cells adapted to glucose or other sugars, further suggesting its inducibility. The enzyme from strain SL-1 was partially purified by diethylaminoethyl-cellulose chromatography and shown to be a single molecule with a molecular weight of about 48,000. The partially purified enzyme is specific for sucrose and produces equimolar glucose and fructose. Since it degrades raffinose, but not melezitose or other -glucosides, it is an invertase. The invertase has a relatively high K_m for its substrate and a pH optimum of 5.5 to 6.2. It is activated by inorganic orthophosphate (P_i), P_i functioning as a positive effector. Arsenate can substitute for phosphate. Neither the crude cell-free extract nor the partially purified enzyme preparations has detectable sucrose phosphorylase activity. A possible potent role of the invertase in the regulation of sucrose carbon flow in S. mutans is discussed.

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