The relationship between ouabain-sensitive ATPase activity and occluded Rb+ at micromolar ATP-concentrations

R. C. ROSSI; P.J. GARRAHAN; S. B. KAUFMAN; J. G. NØRBY; P.J. SCHWARZBAUM

ANN. N. Y. ACADEMY SCIEN.

The New York Academy of Sciences

Año: 1997 vol. 834 p. 327 - 332

1749-6632

This communication describes an attempt to evaluate the obligatory role of the intermediate with occluded potassium, E2(K2), during K+ transport by the (Na+/K+)-ATPase. Using 86Rb to measure the occluded form at steady state and [ATP] from 0.5-10 micromolar we found that when [Rb+] 5 10 mM, only part of the (Na+/K+)-ATPase activity takes place through the E2(Rb2) and E2(Rb2)ATP forms, so that at these Rb+ concentrations the preparation has "extra" ouabain-inhibitable ATPase activity. The scheme in FIGURE1 represents the currently accepted model for K+ activation and K+ transport by the (Na+/K+)-ATPase. Equation 4 (FIG. 1, legend) predicts that at saturating K+ concentrations, the ratio v/Eoccl will increase with the concentration of ATP along a rectangular hyperbola. However, under the conditions of this study, [ATP] << KATP, so that v/Eoccl approaches a linear function of [ATP]: v/Eoccl = k0 + koo [ATP]/KATP. To assess the competence of the occluded intermediates we measured v and Eoccl and compared the rate coefficients calculated as v/Eoccl with deocclusion rate coefficients determined directly under the same experimental conditions.