Pituitary adenylate cyclase activating polypeptide is a potent calmodulin inhibitor

We report here that pituitary adenylate cyclase activating polypeptide (PACAP38), a new 38-residue neuropeptide of the secretin/glucagon family, is a potent inhibitor of calmodulin in vitro in the activation of bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase. The concentration of PACAP38 for half-maximal inhibition of the phosphodiesterase is 15 nM, one of the lowest for known calmodulin inhibitors. In the presence of Ca2+, PACAP38 binds strongly to calmodulin in a 1:1 ratio with a dissociation constant of about 28 nM. The binding is not dissociated by 4 M urea. In the absence of Ca2+ the binding is at random and can be dissociated by 4 M urea. Studies with PACAP38 derivatives show that the carboxyl half of the PACAP38 molecule is essential for the inhibition of calmodulin.