A CONFORMATION OF NA-K+ PUMP IS PERMEABLE TO PROTON()

The Na+-K+ pump is thought to operate through a two-conformation (E1-E 2) transport cycle in which the cation binding sites are accessible on ly from one side at a time. Using Na+-loaded Xenopus oocytes in which Na+-K+ pumps were overexpressed by injection of cRNA of the Xenopus Na +-K+ pump alpha- and beta-sub units, we observed a Na+-K+ pump-mediate d (ouabain-sensitive) inward current in the absence of other transport able cations, except H+, in the external solution. This inward current was strongly inwardly rectifying, pH dependent, and larger at acid pH . Under conditions favoring a large ouabain-sensitive inward current, we observed a ouabain-sensitive intracellular acidification, and the a mplitude of the acidification was significantly related to the ouabain -sensitive current, indicating that this current was carried by proton s. The reversal potential of the ouabain-sensitive current was depende nt on external pH as expected for a proton-conductive pathway. We conc lude that in the absence of external K+ the Na+-K+ pump can mediate a large inward electrogenic transport of proton. This is most easily exp lained by the hypothesis that the E2 conformation of the Na+-K+ pump w ith cation binding sites exposed to the outside is accessible to proto ns from both sides and thus provides a channellike pathway for protons .