Acid-base titration across the plasma membrane ofMicrococcus denitrificans: Factors affecting the effective proton conductance and the respiratory rate

The object of this work was to measure the effective proton conductance of the plasma membrane ofMicrococcus denitrificans under various conditions and to investigate possible connections between respiration and proton translocation. 1. Pulsed acid-base titrations of suspensions ofM. denitrificans in a medium containing the permeant thiocyanate ion, or when K+ ion permeability was induced by valinomycin in a KCl medium, showed that the normal effective proton conductance of the membrane system was less than 1 μmho/cm2. 2. A pH-overshoot artefact was suppressed by adding carbonic anhydrase. 3. The effective proton conductance was increased by the uncoupler FCCP in the same concentration range as was required to stimulate respiration. Concentrations of FCCP above 1·5 μM inhibited respiration after an initial stimulation. 4. The effective proton conductance in presence of 2 μM FCCP was at least 17 μmho/cm2. 5. The quantitative relationships between the respiratory rate, the stoichiometry of respiration-driven proton translocation, and the effective proton conductance of the membrane of the cells are compatible with the suggestion that stimulation of respiration by FCCP is due to a release of back-pressure exerted by a protonmotive potential on the respiratory chain system in the membrane. Only one amongst other possible explanations of the stimulation of respiration by FCCP is, however, excluded.