An endogenous inhibitor of erythrocyte membrane (Ca2+ + Mg2+)-ATPase involved in calcium transport.

1. An inhibitor of erythrocyte membrane (Ca2+ + Mg+)-ATPase was isolated from membrane-free pig haemolysate through treatment with diethylaminoethyl-Sephacel and carboxymethyl-Sephadex at pH 6.8. The resulting inhibitor preparation was devoid of (Ca2+ + Mg2+)-ATPase activator protein. 2. The inhibitor was without effect on erythrocyte membrane acetylcholinesterase, ouabain-sensitive ATPase and spectrin (Ca2+ + Mg2+)-ATPase. 3. The inhibitor was active in suppressing activator sensitive, ATP-dependent calcium transport across erythrocyte membrane.