Structure-activity relationship of pituitary adenylate cyclase activating polypeptide

AIM: To investigate the structure-activity relationship of pituitary adenylate cyclase activating polypeptide (PACAP) in guinea pig gallbladder using a synthetic PACAP/vasoactive intestinal peptide (VIP) hybrid.METHODS: We synthesized PACAP-VIP hybrid peptides using the Fmoc strategy and a simultaneous multiple solid-phase peptide synthesizer. The peptides were tested in isolated guinea pig gallbladders using an improved horizontal type organ bath.RESULTS: VIP induced relaxation of gallbladder smooth muscle strips, while PACAP27 contracted them. Amino acids at positions 4, 5, 9, and 24-26 were replaced without significant loss of activity. [Leu¹³]-PACAP27, a substitution in the α-helix domain, also had no significant loss in activity (P < 0.05). It was more potent than [Gly⁸]- and [DAsp⁸]-PACAP27 and could substitute peptides at position 21. Des-[His¹] and [Ala⁶]-PACAP27 had no activity at 10^-7 mol/L. [Gly⁸]-, [DAsp⁸]-, [Phe²¹]- and [Pro²¹]-PACAP27 at 10^-7 mol/L had about 25% of the activity of PACAP27 at 10^-7 mol/L (P < 0.05).CONCLUSION: The N-terminal disordered region is more important than other regions for determining the physiological activity of PACAP in the guinea pig gallbladder.