Binding of a single Rb+ increases Na+/K+-ATPase, activating dephosphorylation without stoichiometric occlusion.
We used partially purified Na+/K+-ATPase from pig kidney to study dephosphorylation, occlusion, and ATPase activity in the same enzyme preparation and in media of identical composition containing 10 microM ATP and different concentrations of Rb+, used as a K+ congener. The experiments were performed using a rapid-mixing apparatus with a time resolution of 3.5 ms. The main findings were as follows. (i) At sufficiently low Rb+ concentration the initial rate of dephosphorylation was higher than that of occlusion, (ii) as [Rb+] tended to zero the slope of the time course of occlusion but not that of the time course of dephosphorylation approached zero and, (iii) as Rb+ concentration increased, ATPase activity first increased and, after passing through a maximum, tended to a value that was lower than that observed in media without Rb+. None of these results is compatible with the currently held idea that binding of a single Rb+ to the E2P conformer of the ATPase does not modify the rate of dephosphorylation and strongly suggest that a single Rb+ does promote dephosphorylation through a mechanism that is not stoichiometrically coupled to Rb+ occlusion. If this mechanism is included in the currently accepted scheme for ATP hydrolysis by the Na+/K+-ATPase, a reasonable prediction of the experimental results is obtained.
Kaufman
Sergio B
SB
Instituto de Química y Fisicoquímica Biológicas y Departamento de Química Biológica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956, C1113AAD Buenos Aires, Argentina.
González-Lebrero
Rodolfo M
RM
Rossi
Rolando C
RC
Garrahan
Patricio J
PJ
D000818:Animals; D007668:Kidney / Q000201:enzymology; D007700:Kinetics; D010766:Phosphorylation; D011485:Protein Binding; D012413:Rubidium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine
D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium