Respiration-Linked Proton Transport, Changes in External pH, and Membrane Energization in Cells of Escherichia coli

AUTOR(ES)

Gould, J. Michael

RESUMO

The kinetics of respiration-dependent proton efflux and membrane energization have been studied in intact cells of logarithmic-phase Escherichia coli. Parallel measurements of the rate and extent of proton efflux into the external medium (half-time, about 10 s; ratio of H+ to O, about 0.5) and the oxidation of E. coli cytochrome b (half-time, ≤1 s; about 6% oxidized) after a pulse of 5.5 ng-atoms of O indicate that the rate of proton efflux is at least 10 times slower than expected from the time required for the cells to reduce the oxygen added in the pulse. The kinetics of formation and dissipation of the transmembrane electric potential (δψ) after an O2 pulse were estimated from changes in the fluorescence properties of the cell envelope-bound probe N-phenyl-1-naphthylamine. Under anaerobic conditions, a small pulse of oxygen induced a rapid (half-time, ≤1 s) partial decrease in the fluorescence intensity of the probe, followed by a slower relaxation of the fluorescence change to the original intensity. The extent of the initial rapid decrease was linearly dependent upon the amount of oxygen added in the pulse (0 to 11 ng-atoms of O per pulse), whereas the rate of the subsequent relaxation was accelerated by the uncoupler p-trifluoromethoxycarbonylcyanidephenylhydrazone and the K+ ionophore colicin E1. This suggests that the initial fluorescence decrease after an O2 pulse reflects the energization of the membrane, whereas the relaxation of the fluorescence decrease reflects the subsequent deenergization of the membrane arising from counterion redistributions. The fact that the efflux of H+ into the external medium after an O2 pulse was inefficient and much slower (half-time, about 10 s) than the reduction of the added O2 (half-time, ≤1 s) and the energization of the membrane (half-time ≤1 s) suggests that some of the protons translocated across the cytoplasmic membrane during a brief pulse of respiratory activity are accumulated in a region of the cell which is not in rapid equilibrium with the external medium.

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