Human erythrocyte membranes exhibit a cooperative calmodulin-dependent Ca2+-ATPase of high calcium sensitivity

Abstract

A simple method is reported for preparing human erythrocyte membranes in 2.5 mM HEPES [2-[4-(2-hydroxyethyl)-l-piperazinyl]-ethane sulfonic acid]/lmM EGTA [ethylene glycol bis [2-aminoethyl ether]-N,N''-tetraacetic acid] at pH 7.0 in which the activation of the Ca2+-ATPase by Ca2+ and intracellular concentrations of calmodulin is highly cooperative and is complete by .apprx. 1 .mu.M Ca2+. Unlike other available erythrocyte membrane preparations, the pattern of activation by Ca2+ and calmodulin is not complicated by partial resealing of the ghosts during and after isolation. This cooperative activation of the Ca2+ pump may explain how healthy erythrocytes maintain their normal cytosol Ca2+ concentration at a threshold value at or below .apprx. 0.1 .mu.M. Several other calmodulin-dependent enzymes display similar cooperative activation kinetics.