Calcium ion dependent covalent modification of calmodulin with norchlorpromazine isothiocyanate

Calmodulin forms a covalent, one to one, complex with 3H-labeled norchlorpromazine isothiocyanate. Complex formation was monitored by high-performance liquid chromatography using a CN reverse-phase column which resolves calmodulin, the calmodulin-norchlorpromazine adduct, and norchlorpromazine isothiocyanate. Formation of the adduct requires Ca2+ and is not observed with norchlorpromazine. The one to one calmodulin-norchlorpromazine complex does not activate phosphodiesterase but can interact with the enzyme and competitively inhibit its stimulation by calmodulin. High concentrations of trifluoperazine inhibit whereas low concentrations stimulate complex formation. This apparent potentiation of the interaction of calmodulin with norchlorpromazine by another phenothiazine suggests that calmodulin contains at least two phenothiazine binding sites and that the binding of phenothiazine to calmodulin is cooperative.