Are the states that occlude rubidium obligatory intermediates of the Na+/K+ ATPase reaction?

SERGIO BENJAMÍN, KAUFMAN; RODOLFO MARTIN, GONZALEZ LEBRERO; PABLO JOSÉ, SCHWARZBAUM; JENS G, NOERBY; PATRICIO J, GARRAHAN; ROLANDO CARLOS, ROSSI; RODOLFO MARTIN GONZALEZ LEBRERO

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Lugar: Bethesda, Maryland; Año: 1999 vol. 274 p. 20779 - 20790

0021-9258

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.