(Na+K)-ATPase in artificial lipid vesicles : influence of the concentration of mono- and divalent cations on the pumping rate

Abstract

(Na,K)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. Transport activity was induced by adding ATP to the external medium. A voltage-sensitive dye was used to detect the ATP-driven potassium extrusion in the presence of valinomycin. The observed substrate-protein interactions of the reconstituted (Na+K)-ATPase largely agree with that from native tissues. An agreement between ATP hydrolysis and transport activity is given for concentration dependences of sodium, potassium, magnesium and calcium ions. The only significant deviations were observed in the influence of pH. Protons were found to have different influence on transport, enzymatic activity and phosphorylation of the enzyme. The transport studies showed a twofold interaction of protons with the protein: (1) competition with sodium at the cytoplasmic ion binding sites, (2) a non competitive inhibition of transport which is not correlated with protein phosphorylation.