Solubilization and characterisation of the cholecystokininB binding site from pig cerebral cortex

Cholecystokinin (CCK) binding sites were solubilized from pig cerebral cortical membranes with digitonin (2%, w/v) in the presence of Na+ (120 mM) and Mg2+ (5 mM). Scatchard plot transformation of equilibrium binding data obtained with 125I-CCK-8S gave an apparent dissociation constant (Kd) of 0.6 nM, comparable to that obtained in membranes in the presence of these cations. Hill coefficients close to unity suggested the presence of a single population of receptor sites. Competitive inhibition studies with pentagastrin, gastrin(1-17)S and the CCKA receptor antagonist L-364,718 indicated that the solubilized receptor sites were of the B-type (CCKB), with the same pharmacological profile as that observed in membranes. Optimal specific binding of 125I-CCK-8S to membrane-bound and solubilized receptors was obtained in the presence of divalent cations. Both the membrane-bound and the solubilized receptor activity were attenuated by guanylyl-imidodiphosphate (Gpp(NH)p) indicating that the brain CCKB receptors are coupled to G proteins.