Characterization of the Mg(2+)-ATPase activity of the human erythrocyte membrane

The Mg(2+)-dependent ATPase activity of human erythrocyte ghosts undergoes biphasic activation in the presence of ATP (K0.5 approximately 20 microM and 400 microM). Inhibition by vanadate is also biphasic with the most sensitive component (IC50 approximately 100 microM) responsible for the remaining activity. La3+ inhibits approximately 70% of activity (IC50 approximately 25 microM) while Cd2+ can fully inhibit activity (IC50 approximately 60 microM). Partially purified activity was obtained by extracting ghosts with Triton X-100, followed by chromatography over Sepharose CL-2B. Triton distorts the curves for inhibition by vanadate and the activation by ATP. The partially purified activity is inhibited by Ca2+ (IC50 approximately 0.4 mM). These results, together with data from other studies, suggest that the total Mg(2+)-ATPase activity in the membrane represents at least two separate enzymes. Only the activity which is highly sensitive to vanadate appears to be involved in erythrocyte shape change.