Kinetic Comparisons of Heart and Kidney Na+,K+-ATPases

Most kinetic measurements of the partial reactions of Na+,K+-ATPase have been conducted on enzyme from mammalian kidney. Here we present a kinetic model that is based on the available equilibrium and kinetic parameters of purified kidney enzyme, and allows predictions of its steady-state turnover and pump current in intact cells as a function of ion and ATP concentrations and the membrane voltage. Using this model, we calculated the expected dependence of the pump current on voltage and extracellular Na+ concentration. The simulations indicate a lower voltage dependence at negative potentials of the kidney enzyme in comparison with heart muscle Na+,K+-ATPase, in agreement with experimental results. The voltage dependence is enhanced at high extracellular Na+ concentrations. This effect can be explained by a voltage-dependent depopulation of extracellular K+ ion binding sites on the E2P state and an increase in the proportion of enzyme in the E1P(Na+)(3) state in the steady state. This causes a decrease in the effective rate constant for occlusion of K+ by the E2P state and hence a drop in turnover. Around a membrane potential of zero, negligible voltage dependence is observed because the voltage-independent E2(K+)(2) -> E1 + 2K(+) transition is the major rate-determining step.