CA2-INDUCED INHIBITION OF SODIUM-PUMP - NONCOMPETITIVE INHIBITION IN RESPECT OF MAGNESIUM AND SODIUM-CATIONS()

Calcium inhibits the activity of the (Na+ /K+)-ATPase from dog kidney in a dose-dependent manner. Other 2A group cations of the periodic tab le such as Sr2+ and Ba2+ were able to inhibit the ATPase activity but to a lesser degree. Any considerable competition between Ca2+ (Ba2+, S r2+) ions and magnesium or sodium ions could not be detected using enz yme kinetic analysis. Thus, the above three inhibitory acting ions dep ress the ATPase activity of sodium pump by interaction with loci dista nt from the sodium and potassium binding sites. This suggests that the (Na+ /K+)-ATPase molecule contains an inhibitory acting binding site for calcium. This putative binding site could recognize magnesium ions as well as calcium, strontium and barium ions. The specificity of the binding site may describe herein be secured by a structure complement ary to the coordination structure of Ca2+, Ba2+ and Sr2+ ions characte rized by coordination number 8. Mg2+ ions can form coordination struct ure with a maximum coordination number 6, and do not interact specific ally with this binding site.