Biophys J, August 2000, p. 853-862, Vol. 79, No. 2

Combined Allosteric and Competitive Interaction between Extracellular Na⁺ and K⁺ During Ion Transport by the ₁, ₂, and ₃ Isoforms of the Na, K-ATPase

David M. Balshaw,^* Lauren A. Millette, Katherine Tepperman, and Earl T. Wallick^*

 ^*Department of Pharmacology and Cell Biophysics, College of Medicine, and  Department of Biological Sciences, McMicken College of Arts and Sciences, University of Cincinnati, Cincinnati, Ohio 45267-0575 USA

A combined allosteric and competitive model describes the interaction between extracellular Na⁺ and Rb⁺ during ion transport mediated by the Na, K-ATPase. The model was developed from experiments based on ⁸⁶Rb uptake by whole cells transfected with rat isoforms of the enzyme. In the absence of Na⁺, only a single transport site for extracellular Rb⁺ exists. After the occupation of the Na⁺-specific allosteric site, the Rb⁺ transport pocket opens to allow occupation by an additional Rb⁺ and the subsequent transport of the two Rb⁺ ions into the cells. Na⁺ can also directly compete with Rb⁺ for binding to at least one of the transport sites. While the model derived here applies to each of the three rat isoforms of the Na, K-ATPase expressed in HeLa cells, subtle differences exist among the isoforms. The ₃* isoform has an increased intrinsic affinity for Rb⁺ and a lower affinity for the allosteric Na⁺ site than ₁ or ₂*. The stimulation of uptake observed according to the best-fit model is due to the displacement by Rb⁺ of inhibitory Na⁺ bound to the transport site.

Biophys J, August 2000, p. 853-862, Vol. 79, No. 2
© 2000 by the Biophysical Society   0006-3495/00/08/853/10  $2.00

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