Photoaffinity labeling of central cholecystokinin receptors with high efficiency

A new photoreactive tritiated cholecystokinin (CCK) analogue was synthesized which contains the p-benzoylbenzoyl moiety linked to an ornithine residue at the N-terminus of the sulfated CCK octapeptide (CCK-8s). p-Benzoylbenzoyl-Orn(propionyl)-CCK-8s bound specifically and with high affinity to CCK binding sites in membranes both from pig cerebral cortex and from rat pancreatic membranes. The apparent dissociation constants KD were 1.2 nM and 0.5 nM, respectively. [3H]-p-Benzoylbenzoyl-Orn(propionyl)-CCK-8s was incubated with CCKB receptor preparations enriched by lectin chromatography and subsequently photoactivated. A polypeptide migrating with an apparent molecular weight M(r) of 56,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis was specifically labeled. The labeling was suppressed by the CCKB receptor agonist pentagastrin. The efficiency of incorporation of radioactivity was high, reaching up to 70% of specifically bound radioactivity. After treatment with trifluoromethanesulfonic acid, the molecular weight of the labeled protein decreased to 45,000, indicating that the receptor is glycosylated. Photoaffinity labeling of CCKA receptors from rat pancreas resulted in the specific labeling of a protein band with M(r) of 80,000-100,000. Our synthetic approach should be useful for the design of photoreactive analogues of a variety of peptides. The high efficiency photolabeling of the CCKB receptor will be valuable for further characterization and purification of this receptor.