Effect of introduction of an arginine(16) in VIP, PACAP and secretin on ligand affinity for the receptors

Rabbit secretin, which differs from all other mammalian secretins in having a Leu residue in position 6 (instead of Phe) and a basic residue (Arg) in position 16, had a lower affinity than porcine secretin on recombinant rat secretin receptors but had a greater affinity than porcine secretin on recombinant rat VIP1 and PACAP I receptors. Synthetic [L(6)] porcine secretin had a reduced potency on secretin and VIP1 receptors whereas [R(16)] porcine secretin had a similar binding profile as rabbit secretin. Thus, an arginine residue in position 16 reduced 3-fold the affinity of secretin for secretin receptors but increased 30-fold its affinity for the VIP1 and PACAP I receptors. The introduction of an arginine residue in position 16, instead of glutamine, in VIP and PACAP had a similar effect: [R(16)] VIP and [R(16)] PACAP had 3- to 10-fold higher affinities than VIP and PACAP for VIP1 and PACAP I receptors, and 3-fold lower affinities for the secretin receptors. The three [R]peptides also had a reduced potency on the chimeric receptor consisting of the N-terminal part of the secretin receptor grafted on the VIP1 receptor, and an enhanced potency on the chimeric receptor consisting of the N-terminal part of VIP1 receptor grafted on the secretin receptor, indicating that position 16 of each ligand interacted with the N-terminal extracellular domain of the receptors.