Transport of Ca2+ and ATP Hydrolysis by the Ca2+ Pump

To study kinetics of active Ca²⁺ transport through the plasma membrane in cells possessing intracellular organelles is a difficult task, because in the presence of these organelles that could transport Ca²⁺ and consume or produce ATP, it is almost impossible to keep constant known concentrations of Ca²⁺ and ATP in the cytosol through the experiment. Thus, most transport studies have been made in vesicular preparations derived from the plasma membrane rather than in intact cells. Four kinds of membrane preparations have shown to be suitable to study Ca²⁺ fluxes across the plasma membrane, that is: intact red blood cells, resealed ghosts from red blood cells, inside-out vesicles, reconstituted liposomes, and squid axons. The experimental values of the apparent affinity for Ca²⁺ and the maximum ATPase activity measured at saturating Ca²⁺ are highly dependent on calmodulin, acidic phospholipids, partial proteolysis, EGTAs and other factors.