Calcium/Proton and Sodium/Proton Antiport Systems in Escherichia coli1

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Abstract
The cation/H+ antiport reaction was studied in Escherichia coli. 45Ca2+ transport in isolated membrane vesicles was previously shown to be energized by an artificially imposed H+ gradient (Tsuchiya, T. and Rosen, B.P. (1976) J. Biol. Chem. 251, 962–967). That observation suggested an antiport mechanism for Ca2+/H+ exchange. To evaluate this possibility we tested Ca2+ induced H+ flux by measuring the fluorescence change of 9-aminoacridine and by monitoring the change of pH. Upon supply of a respiratory substrate, generation of a large pH gradient (interior-acidic) was observed in everted membrane vesicles. The addition of Ca2+ to respiring vesicles caused a partial collapse of the pH gradient, indicating that Ca2+ induced an H+ efflux. The addition of Ca2+ to an anaerobic cell suspension induced an acidification of the medium (efflux of H+). These findings strongly support a Ca2+/H+ antiport mechanism in Escherichia coli. Similar results were obtained for Na+/H+ exchange. The Ca2+/H+ antiport reaction seemed to be an electrogenic process, transferring positive charge with Ca2+ movement, while the Na+/H+ antiport reaction seemed to be an electroneutral process. Sr2+ inhibited the Ca2+/H+ antiport. Phosphate had no significant effect on the Ca2+/H+ antiport process, although phosphate greatly stimulated the accumulation of 45Ca2+ in the vesicles when energized either by respiration (or ATP hydrolysis) or by an artificial H+ gradient.