The inhibition of the calcium dependent ATPase from human red cells by erythrosin B.

Erythrosin B, a tetraiodinated derivative of fluorescein, completely inhibits the Ca2+-ATPase activity of human red cell membranes. The effect is exerted by reversible combination to a single class of site(s) of high apparent affinity (Ki = 70 nM). Erythrosin B decreases the maximum velocity and leaves unaltered the apparent affinity of the catalytic site for ATP and, conversely, it decreases the apparent affinity and leaves unaltered the maximum effect of ATP on its regulatory site in the Ca2+-ATPase. These results are consistent with the idea that erythrosin B displaces ATP from the regulatory site without replacing it in its effects.