Abstract: | 1. 1. The Mg2+- plus Ca2+-dependent ATPase (Ca2+-ATPase) in human red cell membranes is susceptible to inhibition by low concentrations of vanadate. 2. 2. Several natural activators of Ca2+-ATPase (Mg2+, K+, Na+ and calmodulin) modify inhibition by increasing the apparent affinity of the enzyme for vanadate. 3. 3. Among the ligands tested, K+, in combination with Mg2+, had the most pronounced effect on inhibition by vanadate. 4. 4. Under conditions optimal for inhibition of Ca2+-ATPase, the K for vanadate was 1.5 μM and inhibition was nearly complete at saturating vanadate concentrations. 5. 5. There are similarities between the kinetics of inhibition of red cell Ca2+-ATPase and (Na+ + K+)-ATPase prepared from a variety of sources; however, (Na+ + K+)-ATPase is approx. 3 times more sensitive to inhibition by vanadate. Keywords: Ca2+-ATPase; Red cell membrane; Vanadate; Calmodulin |