Polarographic studies of membrane particles containing Na−K ATPase

Summary

The properties of a suspension of membrane particles containing Na−K ATPase have been investigated with the aid of d−c and a−c polarography. In particular, we have studied the interaction of three cations, two very effective enzyme inhibitors and one activator, with the enzyme preparation. Ag⁺ and Cu⁺⁺, which inhibit the enzyme at very low concentrations, bind very strongly. No binding could be found with the activating ion, Tl⁺, however. Adsorption of a substance with an isoelectric point between pH 4 and pH 5.5 occurred at the electrode surface between −0.1 and −1.2 V at pH 7, and was associated with the random currents that appear during the measurements. The random currents arise when the membrane particles collide with the electrode and cause changes in the structure of the electrical double layer. (Added substances that adsorb more strongly at the mercury/water interface eliminate the random currents.) The adsorbed film impedes the flow of the free Ag⁺ and Cu⁺⁺ ions, and to a smaller extent, the flow of Tl⁺ ions. The differences between the binding of inhibiting and activating ions are correlated with their effects on the ATPase enzyme activity.