Effects of caerulein-related peptides on cholecystokinin receptor bindings in brain and pancreas

A number of caerulein (CLN)-related peptides were synthesized and compared in terms of their affinities for cholecystokinin (CCK) receptors. We have found that these peptides can be classified into three types according to their relative affinities for the brain and pancreatic receptors. The first group (type A) of peptides includes CLN and analogs retaining the Tyr(SO3H)4 residue and the COOH-terminal amide group. Type A peptides were as potent as CLN in inhibiting [125I]BH-CCK-8 binding and showed almost the same affinities for pancreatic and brain receptors. When the Tyr(SO3H)4 residue was either deleted or desulfated (type B), the affinities of the peptides decreased remarkably for the pancreatic receptors but much less for the brain receptors. The type C peptides were deamidated, oxidized, or shortened in the COOH-terminal region and exhibited greatly decreased affinities for both brain and pancreatic receptors but a much greater decrease for the brain receptor. These results indicate that, although the Tyr(SO3H)4 residue and the COOH-terminal structure are both essential for CLN to bind to the CCK receptors, the former is of critical importance for the binding to the pancreas and the latter is rather important for the binding to the brain.