Non-amidated forms of VIP (glycine-extended VIP and VIP-free acid) have full bioactivity on smooth muscle.

The aim of the present study was to elucidate the importance of the C-terminal amide group for the biological activity of vasoactive intestinal peptide (VIP). Two synthetic peptides lacking the amide group: VIP having a carboxyl group at the C-terminus and the intermediate biosynthetic precursor, glycine-extended VIP were compared with VIP itself regarding the ability to inhibit spontaneous activity in smooth muscle strips from rat stomach and human Fallopian tube. Both the glycine-extended VIP and VIP having a carboxyl group at the C-terminus caused a significant and dose-dependent inhibition of smooth muscle activity and displayed dose-response curves similar to VIP. The potencies of the VIP variants did not differ significantly from that of VIP. Thus, alpha-carboxyamidation of VIP is not a prerequisite for biological activity.