Are the states that occlude rubidium obligatory intermediates of the Na(+)/K(+)-ATPase reaction?
In the Albers-Post model, occlusion of K(+) in the E(2) 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.
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.
S B
SB
Kaufman
R M
RM
González-Lebrero
P J
PJ
Schwarzbaum
J G
JG
Nørby
P J
PJ
Garrahan
R C
RC
Rossi
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D006868:Hydrolysis; D017136:Ion Transport; D007668:Kidney / Q000378:metabolism; D012413:Rubidium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013379:Substrate Specificity; D013552:Swine
D000255:Adenosine Triphosphate; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium