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

The ability of Streptococcus downeii MFe28 to maintain a transmembrane pH gradient at low extracellular pH values (pH E ) was tested. Carbohydrate-depleted cells were suspended in peptone-glucose-KCI medium buffered with potassium phosphate-citrate to pH values between 4.0 and 7.0. pH E was measured with a pH meter after 30 min equilibration at 20°C and intracellular pH(pH 1 ) calculated from the distribution of 14 C-benzoate using 3 H-inulin as an extracellular marker. Internal volume was determined as the difference in volume occupied by 14 C-carboxyinulin and 3 H-water. Cells maintained a positive ΔpH(H 1 -pH E ) over the pH range 4.0-7.0 with a maximum value when pH E =5.0. Depletion of energy by the omission of glucose collapsed the gradients (ΔpH = 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  pH E =5.5-6.0. It is suggested that fluoride preferentially inhibits the F 0 component of the proton-translocating membrane ATPase which leads to internal acidification and the subsequent inhibition of glycolysis.