CHARACTERIZATION OF TRANSMEMBRANE MOVEMENT OF GLUCOSE AND GLUCOSE ANALOGS IN STREPTOCOCCUS-MUTANS INGBRITT

The transmembrane movement of radiolabeled, nonmetabolizable glucose analogs in Streptococcus mutans Ingbritt was studied under conditions of differing transmembrane electrochemical potentials (ΔΨ) and pH gradients (ΔpH). The ΔpH and ΔΨ were determined from the transmembrane equilibration of radiolabeled benzoate and tetraphenylphosphonium ions, respectively. Growth conditions of S. mutans Ingbritt were chosen so that the cells had a low apparent phosphoenolpyruvate (PEP)-dependent glucose:phosphotransferase activity. Cells energized under different conditions produced transmembrane proton potentials ranging from -49 to -103 mV but did not accumulate 6-deoxyglucose intracellularly. An artificial transmembrane proton potential was generated in deenergized cells by creating a ΔΨ with a valinomycin-induced K+ diffusion potential and a ΔpH by rapid acidification of the medium. Artificial transmembrane proton potentials up to -83 mV, although producing proton influx, could not accumulate 6-deoxyglucose in deenergized cells or 2-deoxyglucose or thiomethylgalactoside in deenergized, PEP-depleted cells. The transmembrane diffusion of glucose in PEP-depleted, KF-treated cells did not exhibit saturation kinetics or competitive inhibition by 6-deoxyglucose or 2-deoxyglucose, indicating that diffusion was not facilitated by a membrane carrier. As proton-linked membrane carriers have been shown to facilitate diffusion in the absence of a transmembrane proton potential, the results therefore are not consistent with a proton-linked glucose carrier in S. mutans Ingbritt. This together with the lack of proton-linked transport of the glucose analogs suggests that glucose transmembrane movement in S. mutans Ingbritt is not linked to the transmembrane proton potential.