Mutational analysis of the putative devazepide binding site of the CCK(A) receptor

Recently a molecular model was proposed for the binding site of the antagonist 3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl) -1H-indole-2-carboxamide (devazepide) on the cholecystokinin-A (CCK(A)) receptor (Van der Bent et al., 1994. Drug Design Discov. 12, 129-148). Fifteen amino acids were identified, including hydrophilic ones such as Ser139, Asn349 and Ser379, that might interact with the carboxamide moiety in devazepide. To provide mutational evidence for this model, wild-type and mutant receptors (S139A, N349A and S379A) were transiently expressed and compared with respect to the ability of devazepide to inhibit binding of radiolabelled cholecystokinin-(26-33)-peptide amide (CCK-8) and CCK-8-evoked Ca2+ mobilization. The data presented suggest the involvement of the three residues in antagonist binding, although to a different extent. However, it does not seem likely that hydrogen bonds are the driving force in view of the relatively minor changes in receptor affinity and activity.