The effect of cholera toxin on human red cell Ca-ATPase

The effect of cholera toxin on transport Ca-ATPase was studied in membrane fragments from human red cells. A consistently moderate inhibition was found when fragments were previously incubated with toxin in the presence of beta-NAD but not in its absence of after treatment with non-activated toxin. In calmodulin-free preparations, both Ca affinity and maximal rate of hydrolysis were affected whereas only affinity was altered in calmodulin-deficient membranes.