A NOVEL ANTIPORTER ACTIVITY CATALYZING SODIUM AND POTASSIUM-TRANSPORT FROM RIGHT-SIDE-OUT VESICLES OF ESCHERICHIA-COLI

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, Delta pH (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.