Sensitivity and reversibility of Ca-dependent inhibition of the (Na⁺ + K⁺)-ATPase of human red blood cells

The sensitivity of the (Na⁺ + K⁺)-ATPase to inhibition by Ca was increased 30-fold by a partially purified extract of human red cell hemolysate. The hemolysate fraction reduced the concentration of free Ca required for 50% inhibition from 30 μM to approx. 1 μM. Ca-dependent inhibition of the (Na⁺ + K⁺)-ATPase in the presence and absence of the hemolysate fraction was completely reversible. The hemolysate fraction also stimulated the Ca²⁺-ATPase and increased its affinity for Ca. In the presence of the hemolysate fraction, the concentration of free Ca that inhibited the (Na⁺ + K⁺)-ATPase by 50% was similar to that which half-maximally stimulated the Ca²⁺-ATPase. Boiling the fraction destroyed its effect on the (Na⁺ + K⁺)-ATPase, but did not impair its stimulation of the Ca²⁺-ATPase.