Calmodulin, activated cyclic nucleotide phosphodiesterase, microtubules, and vinca alkaloids.

Calmodulin 1.8 and 10.6 microM, inhibited the polymerization of bovine brain microtubules by 30 and 50%, respectively. Two 55,000- to 68,000-dalton calmodulin-binding protein as well as calmodulin-dependent and -independent phosphodiesterases (PDE) were found associated with microtubule proteins. Among the antimicrotubule drugs, such as colchicine, podophyllotoxin, griseofulvin, and vinca alkaloids (vinblastine, desacetylvinblastine amide, and vincristine), the vinca alkaloids were selective inhibitors of calmodulin-activated PDE activity. This action of vinca alkaloids resides in the catharanthine moiety of vinblastine molecule. An alpha 2 inhibitor, yohimbine, affects the microtubules, and a series of alpha-adrenoceptor blocking agents were examined for their effects on calmodulin-dependent PDE. The relative order of the potency is phenoxybenzamine = dibenamine greater than phentolamine greater than yohimbine greater than prazosin greater than tolazoline, and the first four drugs in this series were selective inhibitors of calmodulin action. Inasmuch as phenoxybenzamine and dibenamine are alkylating agents, the effects of antineoplastic alkylating agents on the calmodulin action were also examined. Busulphan, melphalan, 1-(2-chloroethyl)-3-cyclohexyl-l-nitrosourea, and streptozotocin (up to 4 mM) were not selective inhibitors of calmodulin action. Maytansine, a vinca alkaloid-type antimicrotubule agents as well as an alkylating agent, selectively inhibited the calmodulin with a potency similar to vincristine. In addition, phenoxybenzamine affected Ca2+-dependent fluorescence induced by the interaction between calmodulin and hydrophobic fluorescent probes, whereas vinblastine was ineffective. However, the binding of vinblastine to calmodulin is calcium dependent. Studies such as these suggest the importance of physical and structural considerations in drugs binding to calmodulin as well as at least two different binding sites for drugs on calmodulin.