Sodium-Proton Antiport in Isolated Membrane Vesicles of Escherichia coli

Sodium movements are studied in isolated membrane vesicles of Escherichia coli. The addition of d-lactate to membrane vesicles previously equilibrated with 22Na+ induces a rapid efflux of 22Na+ against its electrochemical gradient. This energy-dependent efflux can be detected under conditions (pH 6.6) in which both a pH gradient and an electrical potential are formed across the membrane, or under conditions (pH 7.5) in which only an electrical potential is formed. In the former case, valinomycin at a concentration of 2.5 μM completely abolishes the membrane potential and lowers the total proton electrochemical gradient by 70%; however, it has no effect on the D-lactate-induced 22Na+ efflux. At pH 7.5, valinomycin inhibits the 22Na+ efflux concomitantly with the inhibition of the electrical potential. These results are consistent with a change in the Na+/H+ antiport following an alteration in the external pH. It is proposed that at low pH values, the exchange is electroneutral, whereas at high pH values the exchange is electrogenic. As is to be expected, sodium movements induce proton movements in the opposite direction. When Na+ containing vesicles are diluted into a medium devoid of Na+, acidification of the interior of the vesicle, dependent on the existence of a sodium gradient, can be measured. Upon supply of an energy source, inverted membrane vesicles generate a large pH gradient. Under such conditions the addition of Na+ to the medium evokes a rapid proton efflux. Only Li+ can replace the Na+; potassium, choline, or guanidine cannot. The possible physiological implications of the functioning of a Na+/H+ antiport are discussed.