The agonist SR 146131 and the antagonist SR 27897 occupy different sites on the human CCK1 receptor

Abstract 1-[2-(4-(2-Chlorophenyl)thiazol-2-yl) aminocarbonyl indoyl] acetic acid (SR 27897) is an effective CCK 1 receptor antagonist, while the structurally related molecule 2-[4-(4-chloro-2,5-dimethoxyphenyl)-5-(2-cyclohexyl-ethyl)-thiazol-2-ylcarbamoyl]-5,7-dimethyl-indol-1-yl-1-acetic acid (SR 146131) is a highly potent and specific agonist for the same receptor. To discover how the two molecules interact with the human cholecystokinin (CCK) CCK 1 receptor, we have carried out binding and activity studies with 33-point mutated receptors. Only six mutants showed altered [ 3 H]SR 27897 binding properties, Lys 115 , Lys 187 , Phe 198 , Trp 209 , Leu 214 and Asn 333 . In contrast, numerous mutations throughout the receptor either reduced SR 146131 agonist potency, Phe 97 , Gly 122 , Phe 198 , Trp 209 , Ile 229 , Asn 333 , Arg 336 and Leu 356 or increased it, Tyr 48 , Cys 94 , Asn 98 , Leu 217 and Ser 359 . Only mutations of Phe 198 , Trp 209 and Asn 333 affected both SR 27897 and SR 146131 binding or activity. The collated information was used to construct molecular models of SR 27897 and SR 146131 bound to the human CCK 1 receptor. The clear difference in the binding sites of SR 27897 and SR 146131 offers a molecular explanation for their contrasting pharmacological characteristics.