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 Delta Psi generation across the vesicle membrane (positi ve inside) and H+ efflux from the vesicle lumen. Na+ accumulation was assay ed with Na-22(+); Delta Psi generation was detected by recording absorption changes of oxonol VI; H+ efflux was monitored as an increase in fluorescen ce intensity of the pH indicator pyranine loaded into the vesicles. Both AT P-dependent Na+ uptake and H+ ejection were increased by the H+ ionophore c arbonyl cyanide m-chlorophenylhydrazone (CICCP) while Delta Psi was collaps ed. The lipophilic anion tetraphenylboron ion (TPB-) inhibited H+ ejection from the vesicles and abolished Delta Psi. Based on the effects of CICCP an d TPB- on H+ ejection and Delta Psi generation, the conclusion was drawn th at 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+ concentrat ions stimulated H+ efflux and caused the decrease in the Delta Psi observed , thus indicating that Na+ is likely a factor controlling H+ permeability o f the vesicle membrane. (C) 1999 Federation of European Biochemical Societi es.