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J Biol Chem, Vol. 274, Issue 30, 20779-20790, July 23, 1999

Are the States That Occlude Rubidium Obligatory Intermediates of the Na⁺/K⁺-ATPase Reaction?

Sergio B. Kaufman, Rodolfo M. González-Lebrero, Pablo J. Schwarzbaum, Jens G. Nørby^¶, Patricio J. Garrahan, and Rolando C. Rossi

From the  Instituto de Química y Fisicoquímica Biológicas and Departamento de Química Biológica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956, 1113 Buenos Aires, Argentina and the ^¶ Institute of Biophysics, University of Aarhus, Ole Worms Allé 185, DK-8000 Aarhus, Denmark

In the Albers-Post model, occlusion of K⁺ in the E₂ conformer of the enzyme (E) is an obligatory step of Na⁺/K⁺-ATPase reaction. If this were so the ratio (Na⁺/K⁺-ATPase activity)/(concentration of occluded species) should be equal to the rate constant for deocclusion. We tested this prediction in a partially purified Na⁺/K⁺-ATPase from pig kidney by means of rapid filtration to measure the occlusion using the K⁺ congener Rb⁺. Assuming that always two Rb⁺ are occluded per enzyme, the steady-state levels of occluded forms and the kinetics of deocclusion were adequately described by the Albers-Post model over a very wide range of [ATP] and [Rb⁺]. The same happened with the kinetics of ATP hydrolysis. However, the value of the parameters that gave best fit differed from those for occlusion in such a way that the ratio (Na⁺/K⁺-ATPase activity)/(concentration of occluded species) became much larger than the rate constant for deocclusion when [Rb⁺] <10 mM. This points to the presence of an extra ATP hydrolysis that is not Na⁺-ATPase activity and that does not involve occlusion. A possible way of explaining this is to posit that the binding of a single Rb⁺ increases ATP hydrolysis without occlusion.

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