Na,K-ATPase characterized in artificial membranes. 1. Predominant conformations and ion-fluxes associated with active...

The Na,K-ATPase (NKA) system is the receptor for the cardioactive steroids of plant or animal origin. It is not yet known whether passive ion fluxes traverse the inactivated receptor and thereby contribute to the hormonal, pharmacological or toxic actions of these compounds. To look for putative passive ion-fluxes across the ouabain-NKA complex, we incorporated it into the artificial membrane of liposomes. Since this synthetic membrane is virtually impermeable to Na and K ions, the hypothetical ion-fluxes mediated by the NKA can be determined. E2-forms and E2-ouabain-forms of purified NKA were incorporated, in parallel, into separate liposome preparations and the permeability of the resulting E2-liposomes and E2-ouabain-liposomes to K, Na and Ca ions was compared. The E2-liposomes expressed a typical K-permeability which was not observed in the E2-ouabain-liposomes; the latter showed a slightly higher Na-permeability and a similar Ca-permeability as compared to the former. Thus, ouabain does not induce leaks for K or Ca ions in the NKA molecule.