Tissue-specific versus isoform-specific differences in cation activation kinetics of the Na,K-ATPase.
The experiments described in this report reconcile some of the apparent differences in isoform-specific kinetics of the Na,K-ATPase reported in earlier studies. Thus, tissue-specific differences in Na+ and K+ activation kinetics of Na,K-ATPase activity of the same species (rat) were observed when the same isoform was assayed in different tissues or cells. In the case of alpha1, alpha1-transfected HeLa cell, rat kidney, and axolemma membranes were compared. For alpha3, the ouabain-insensitive alpha3*-transfected HeLa cell (cf. Jewell, E. A., and Lingrel, J. B. (1991) J. Biol. Chem. 266, 16925-16930), pineal gland, and axolemma (mainly alpha3) membranes were compared. The order of apparent affinities for Na+ of alpha1 pumps was axolemma approximately rat alpha1-transfected HeLa > kidney, and for K+, kidney approximately alpha1-transfected HeLa > axolemma. For alpha3, the order of apparent affinities for Na+ was pineal gland approximately axolemma > alpha3*-transfected HeLa, and for K+, alpha3*-transfected HeLa > axolemma approximately pineal gland. In addition, the differences in apparent affinities for Na+ of either kidney alpha1 or HeLa alpha3* as compared to the same isoform in other tissues were even greater when the K+ concentration was increased. A kinetic analysis of the apparent affinities for Na+ as a function of K+ concentration indicates that isoform-specific as well as tissue-specific differences are related to the apparent affinities for both Na+ and K+, the latter acting as a competitive inhibitor at cytoplasmic Na+ activation sites. Although the nature of the tissue-specific modulation of K+/Na+ antagonism remains unknown, an analysis of the nature of the beta isoform associated with alpha1 or alpha3 using isoform-specific immunoprecipitation indicates that the presence of distinct beta subunits does not account for differences of alpha1 of kidney, axolemma, and HeLa, and of alpha3 of axolemma and HeLa; in both instances beta1 is the predominant beta isoform present or associated with either alpha1 or alpha3. However, a kinetic difference in K+/Na+ antagonism due to distinct betas may apply to alpha3 of axolemma (alpha3beta1) and pineal gland ( alpha3beta2).
Therien
A G
AG
Department of Biochemistry, McGill University, Montreal, Canada.
Nestor
N B
NB
Ball
W J
WJ
Blostein
R
R
D000818:Animals; D001665:Binding Sites; D002412:Cations; D003593:Cytoplasm / Q000378:metabolism; D004789:Enzyme Activation; D006367:HeLa Cells; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism*; D007668:Kidney / Q000201:enzymology; D007700:Kinetics; D010870:Pineal Gland / Q000201:enzymology; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D002412:Cations; D007527:Isoenzymes; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium