Structure-activity relationships of CCK26-33-related analogues modified in position 33

In the sequence of the C-terminal octapeptide of cholecystokinin, the phenylalanine amide residue in position 33 is of primary importance for the biological activity. Indeed, removal of Phe33-NH2 is a modification known to induce antagonist properties. The influence of the chemical nature of the Phe33-NH2 side chain on the biological activity of CCK8 was investigated through replacement of this residue by several amino acids with different lipophilic properties in the sequence of Boc(Nle28, Nle31)CCK27-33, an equipotent analogue of CCK8. The binding properties of these new CCK-related analogues: Boc(Nle28,Nle31,X33)CCK27-33 were measured on both mouse brain and guinea pig pancreatic membranes, and their peripheral activities on amylase secretion and contractions of guinea pig ileum. Among the various peptides modified in position 33, Boc(Nle28,Nle31,Naa33-NH2)CCK27-33(Naa = naphthylalanine) and Boc(Nle28,Nle31,Cha33-NH2)CCK27-33(Cha = cyclohexylalanine) displayed high affinities for central and peripheral CCK-receptors and proved to be full agonists of CCK8 in the peripheral tests while Boc(Nle28,Nle31,Ala33-NH2)CCK27-33 was completely inactive. This suggests that, at the level of the Phe33-NH2 subsite, the critical factor for optimal interaction with CCK-receptors is not the aromatic nature of the side chain but its size and hydrophobicity.