title: Na+ currents generated by the purified (Na+ + K+)-ATPase on planar lipid membranes.
abstract: Purified (Na+ + K+)-ATPase from pig kidney was attached to black lipid membranes and ATP-induced electric currents were measured as described previously by Fendler et al. ((1985) EMBO J. 4, 3079-3085). An ATP concentration jump was produced by an ultraviolet-light flash converting non-hydrolysable caged ATP to ATP. In the presence of Na+ and Mg2+ this resulted in a transient current signal. The pump current was not only ATP dependent, but also was influenced by the ATP/caged ATP ratio. It was concluded that caged ATP binds to the enzyme (and hence inhibits the signal) with a Ki of approx. 30 microM, which was confirmed by enzymatic activity studies. An ATP affinity of approx. 2 microM was determined. The addition of the protonophore 1799 and the Me+/H+ exchanger monensin made the bilayer conductive leading to a stationary pump current. The stationary current was strongly increased by the addition of K+ with a K0.5 of 700 microM. Even in the absence of K+ a stationary current could be measured, which showed two Na+-affinities: a high-affinity (K0.5 less than or equal to 1 mM) and a low-affinity (K0.5 greater than or equal to 0.2 M). In order to explain the sustained electrogenic Na+ transport during the Na+-ATPase activity, it is proposed, that Na+ can replace K+ in dephosphorylating the enzyme, but binds about 1000-times weaker than K+. The ATP requirement of the Na+-ATPase was the same (K0.5 = 2 microM) with regard to the peak currents and the stationary currents. However, for the (Na+ + K+)-ATPase the stationary currents required more ATP. The results are discussed on the basis of the Albers-Post scheme.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D004553:Electric Conductivity; D007668:Kidney / Q000166:cytology / Q000201:enzymology; D008274:Magnesium / Q000378:metabolism; D008563:Membrane Lipids / Q000378:metabolism*; D008958:Models, Molecular; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine; D014466:Ultraviolet Rays
chemical_list: D008563:Membrane Lipids; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium