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 h aving 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 r ecombinant rat secretin receptors but had a greater affinity than porc ine secretin on recombinant rat VIP1 and PACAP I receptors. Synthetic [L(6)] porcine secretin had a reduced potency on secretin and VIP1 rec eptors 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 introductio n 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- t o 10-fold higher affinities than VIP and PACAP for VIP1 and PACAP I re ceptors, and 3-fold lower affinities for the secretin receptors. The t hree [R]peptides also had a reduced potency on the chimeric receptor c onsisting of the N-terminal part of the secretin receptor grafted on t he VIP1 receptor, and an enhanced potency on the chimeric receptor con sisting of the N-terminal part of VIP1 receptor grafted on the secreti n receptor, indicating that position 16 of each ligand interacted with the N-terminal extracellular domain of the receptors.