Activation and deactivation kinetics of Ca transport in inside-out erythrocyte membrane vesicles

The kinetics of active Ca2+ transport in inside-out vesicles of human erythrocyte membranes has been studied. Hemolysate or purified calmodulin increased the apparent affinity of the Ca2+ transport system for Ca2+ and increased the apparent maximum velocity of Ca2+ transport. However, as Ca2+ concentration was increased above 90 microM in the presence of hemolysate or calmodulin, the extent of activation of Ca2+ fluxes decreased. This deactivation was less prominent if the Mg2+ concentration was increased. These results may be explained by postulating that calmodulin has a site which binds either Mg2+ or Ca2+ and if Mg2+ occupies this site calmodulin activates Ca2+ fluxes, while if Ca2+ occupies this site, calmodulin is unable to activate the transport system.