Gramicidin S inhibition of the Ca2+-ATPase of human red blood cells

The cationic amphiphilic polypeptide gramicidin S inhibits the Ca2+-ATPase of human red-cell membranes by lowering the maximum velocity of the high-affinity component and the apparent affinity of the low-affinity component of the velocity-versus-ATP concentration curve of the enzyme. Gramicidin S does not alter the apparent affinity of the Ca2+-ATPase for Ca2+. Calmodulin is not essential for the inhibition, but increases the sensitivity of the enzyme to the inhibitor. The effects of gramicidin S on the Ca2+-ATPase can be reversed with phosphatidylcholine vesicles but not with buffer solutions, suggesting that gramicidin S acts from the lipid phase of the membrane.