Electrogenicity of the Na+-ATPase from the marine microalga Tetraselmis (Platymonas) viridis and associated H+ countertransport.

Sodium accumulation by the Na+-ATPase in the plasma membrane (PM) vesicles isolated from the marine alga Tetraselmis (Platymonas) viridis was shown to be accompanied by deltapsi generation across the vesicle membrane (positive inside) and H+ efflux from the vesicle lumen. Na+ accumulation was assayed with 22Na+; deltapsi generation was detected by recording absorption changes of oxonol VI; H+ efflux was monitored as an increase in fluorescence intensity of the pH indicator pyranine loaded into the vesicles. Both ATP-dependent Na+ uptake and H+ ejection were increased by the H+ ionophore carbonyl cyanide m-chlorophenylhydrazone (CICCP) while deltapsi was collapsed. The lipophilic anion tetraphenylboron ion (TPB-) inhibited H+ ejection from the vesicles and abolished deltapsi. Based on the effects of CICCP and TPB- on H+ ejection and deltapsi generation, the conclusion was drawn that H+ countertransport observed during Na+-ATPase operation is a secondary event energized by the electric potential which is generated in the course of Na+ translocation across the vesicle membrane. Increasing Na+ concentrations stimulated H+ efflux and caused the decrease in the deltapsi observed, thus indicating that Na+ is likely a factor controlling H+ permeability of the vesicle membrane.