The Effects of Potassium Fluoride and Potassium Laurate on pH Gradients inStreptococcus downeii

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Abstract
The ability of Streptococcus downeii MFe28 to maintain a transmembrane pH gradient at low extracellular pH values (pHE) was tested. Carbohydrate-depleted cells were suspended in peptone-glucose-KCl medium buffered with potassium phosphate-citrate to pH values between 4·0 and 7·0. pHE was measured with a pH meter after 30 min equilibration at 20°C and intracellular pH(pH) calculated from the distribution of 14C-benzoate using 3H-inulin as an extracellular marker. Internal volume was determined as the difference in volume occupied by 14C-carboxyinulin and 3H-water. Cells maintained a positive ΔpH(pH,—pHE) over the pH range 4·0–7·0 with a maximum value when pHE = 5·0. Depletion of energy by the omission of glucose collapsed the gradients (Δ pH = 0), as did the addition of an uncoupling agent (carbonyl cyanide-m-chlorophenyl hydrazone) or a weak acid (potassium laurate). In contrast, potassium fluoride produced a negative ΔpH when pHE = 5·5–6·0. It is suggested that fluoride preferentially inhibits the F0 component of the proton-translocating membrane ATPase which leads to internal acidification and the subsequent inhibition of glycolysis.