Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: II. Competition of Various Cations

Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: II. Competition of Various... In the E1 state of the Na,K-ATPase all cations present in the cytoplasm compete for the ion binding sites. The mutual effects of mono-, di- and trivalent cations were investigated by experiments with the electrochromic fluorescent dye RH421. Three sites with significantly different properties could be identified. The most unspecific binding site is able to bind all cations, independent of their valence and size. The large organic cation Br2-Titu3+ is bound with the highest affinity (<μm), among the tested divalent cations Ca2+ binds the strongest, and Na+ binds with about the same equilibrium dissociation constant as Mg2+ (∼0.8 mm). For alkali ions it exhibits binding affinities following the order of Rb+≃ K+ > Na+ > Cs+ > Li+. The second type of binding site is specific for monovalent cations, its binding affinity is higher than that of the first type, for Na+ ions the equilibrium dissociation constant is < 0.01 mm. Since binding to that site is not electrogenic it has to be close to the cytoplasmic surface. The third site is specific for Na+, no other ions were found to bind, the binding is electrogenic and the equilibrium dissociation constant is 0.2 mm. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Membrane Biology Springer Journals

Ion Selectivity of the Cytoplasmic Binding Sites of the Na,K-ATPase: II. Competition of Various Cations

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Publisher
Springer-Verlag
Copyright
Copyright © Inc. by 2001 Springer-Verlag New York
Subject
Life Sciences; Biochemistry, general; Human Physiology
ISSN
0022-2631
eISSN
1432-1424
D.O.I.
10.1007/s002320010051
Publisher site
See Article on Publisher Site

Abstract

In the E1 state of the Na,K-ATPase all cations present in the cytoplasm compete for the ion binding sites. The mutual effects of mono-, di- and trivalent cations were investigated by experiments with the electrochromic fluorescent dye RH421. Three sites with significantly different properties could be identified. The most unspecific binding site is able to bind all cations, independent of their valence and size. The large organic cation Br2-Titu3+ is bound with the highest affinity (<μm), among the tested divalent cations Ca2+ binds the strongest, and Na+ binds with about the same equilibrium dissociation constant as Mg2+ (∼0.8 mm). For alkali ions it exhibits binding affinities following the order of Rb+≃ K+ > Na+ > Cs+ > Li+. The second type of binding site is specific for monovalent cations, its binding affinity is higher than that of the first type, for Na+ ions the equilibrium dissociation constant is < 0.01 mm. Since binding to that site is not electrogenic it has to be close to the cytoplasmic surface. The third site is specific for Na+, no other ions were found to bind, the binding is electrogenic and the equilibrium dissociation constant is 0.2 mm.

Journal

The Journal of Membrane BiologySpringer Journals

Published: Feb 1, 2001

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