Purified red blood cell Ca2+-pump ATPase: evidence for direct inhibition by presumed anti-calmodulin drugs in the absence of calmodulin

A variety of presumed anti-calmodulin (anti-CaM) drugs was tested for their potential inhibitory effects on the isolated, purified and reconstituted Ca2+-pump ATPase of human red blood cell membranes. Anti-CaM drugs inhibited the Ca2+-pump ATPase both in the absence and presence of added CaM. Qualitatively similar inhibition was observed in two different ATPase assay systems. In asolectin vesicles in the absence of added CaM trifluoperazine (TFP), N-(6-aminohexyl)-5-chloro-1-naphthalene- sulfonamide (W-7), vinblastine, dibucaine, imipramine, propranolol and dimethylpropranolol (UM-272) were all inhibitory. Potency of anti-CaM drugs was generally greater on the enzyme reconstituted in asolectin vesicles than on the enzyme reconstituted in phosphatidylcholine vesicles, either in the presence or absence of CaM. The results emphasize that anti-CaM drugs have actions other than to bind to CaM. Possible direct interaction of amphipathic cationic anti-CaM drugs with the Ca2+-pump ATPase and/or its lipid environment is suggested.