Downhill sodium efflux from right-side-out E. coli membrane vesicles was found to be stimulated by negative electric potential, as has been reported earlier [Bassilana et al., Biochemistry 23 (1984) 1015-1022], and in agreement with the concept of electrogenic Na+/nH+antiporters with n > 1. However, sodium efflux was much more accelerated by positive electric potential, indicating the operation of another sodium transport system. ΔpH (alkaline inside), created by a pH shift from 8.5 to 6.8 in the medium was found to drive sodium efflux against its concentration gradient, but only when the vesicles had been loaded with both Na+and K+. Efflux of K+against the concentration gradient was also observed under these conditions. When the vesicles were loaded separately with sodium tricine or potassium tricine, no K+efflux and insignificant Na+efflux were observed. We propose that there are at least two different mechanisms responsible for Na+efflux in E. coli vesicles. One is the Na+/nH+antiporter previously described, and the other is a novel Na+,K+/mH+antiporter. © 1995.
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