Kinetic properties of electrogenic Na+/H+ antiport in membrane vesicles from an alkalophilic Bacillus sp.
The effects of imposed proton motive force on the kinetic properties of the alkalophilic Bacillus sp. strain N-6 Na+/H+ antiport system have been studied by looking at the effect of delta psi (membrane potential, interior negative) and/or delta pH (proton gradient, interior alkaline) on Na+ efflux or H+ influx in right-side-out membrane vesicles. Imposed delta psi increased the Na+ efflux rate (V) linearly, and the slope of V versus delta psi was higher at pH 9 than at pH 8. Kinetic experiments indicated that the delta psi caused a pronounced increase in the Vmax for Na+ efflux, whereas the Km values for Na+ were unaffected by the delta psi. As the internal H+ concentration increased, the Na+ efflux reaction was inhibited. This inhibition resulted in an increase in the apparent Km of the Na+ efflux reaction. These results have also been observed in delta pH-driven Na+ efflux experiments. When Na(+)-loaded membrane vesicles were energized by means of a valinomycin-induced inside-negative K+ diffusion potential, the generated acidic-interior pH gradients could be detected by changes in 9-aminoacridine fluorescence. The results of H+ influx experiments showed a good coincidence with those of Na+ efflux. H+ influx was enhanced by an increase of delta psi or internal Na+ concentration and inhibited by high internal H+ concentration. These results are consistent with our previous contentions that the Na+/H+ antiport system of this strain operates electrogenically and plays a central role in pH homeostasis at the alkaline pH range.
Kitada
M
M
Riken Institute, Saitama, Japan.
Horikoshi
K
K
D000468:Alkalies / Q000378:metabolism; D001407:Bacillus / Q000502:physiology*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000502:physiology; D011522:Protons; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers
D000468:Alkalies; D002352:Carrier Proteins; D011522:Protons; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium