Affinity labeling the bovine gallbladder cholecystokinin receptor using a battery of probes

Although the gallbladder was the first recognized target of the peptide hormone cholecystokinin (CCK) and is a physiologically important target, only one preliminary report of the biochemical characterization of this receptor exists. Recently, a series of molecular probes for the affinity labeling of different domains of the pancreatic CCK receptor have been developed. In this work we report the application of several of those probes toward the biochemical characterization of the bovine gallbladder muscularis receptor. These include "long" (125I-Bolton-Hunter-CCK-33) and "short" (125I-D-Tyr-Gly-[Nle28,31)CCK-(26-33)]) probes chemically cross-linkable through their amino-terminal amino groups and monofunctional probes with their photolabile moieties at their amino terminus (2-diazo-3,3,3-trifluoropropionyl-125I-D-Tyr-Gly-[(Nle28,31) CCK-(26-33)]) and carboxyl terminus (125I-D-Tyr-Gly-[(Nle28,31,pNO2-Phe33)CCK-(26-33)]), that span the receptor-binding region. Each of these bound specifically and saturably to a preparation enriched in plasma membranes from bovine gallbladder muscularis (mean inhibitor constants: 5.2, 1.1, 0.8, and 1.8 nM, respectively). A major relative molecular weight (Mr) 70,000-85,000 band was specifically and reproducibly labeled with the appropriate apparent affinity by each of the probes, whereas labeling of minor bands of Mr 40,000-50,000, Mr 92,000, Mr 120,000, and Mr 200,000 was dependent on cross-linker type or concentration. These observations support the identification of the Mr 70,000-85,000 protein as the bovine gallbladder CCK-binding subunit and, since this is a different size from the pancreatic CCK-binding subunit, provide biochemical evidence for molecular heterogeneity of peripheral CCK receptors.