Use of a Nitrotryptophan-Containing Peptide for Photoaffinity Labeling the Pancreatic Cholecystokinin Receptor

We report the preparation and characterization of a new type of intrinsic photoaffinity labeling probe, on the basis of the incorporation of a photolabile nitrotryptophan into a biologically relevant domain of a peptide. The model system used was the pancreatic cholecystokinin (CCK) receptor, previously affinity labeled with a variety of probes. Those studies have suggested that an M_r= 85 000–95 000 protein is more likely to be labeled as the site of covalent attachment approaches the receptor-binding domain of this hormone. Indeed, CCK has a Trp in the center of its receptor-binding region, and replacement of that residue with 6-nitrotryptophan resulted in a photolabile probe which affinity labeled the same M_r = 85 000–95 000 pancreatic membrane protein. This probe, ¹²⁵I-D-Tyr-Gly-[(Nle²⁸'³¹,6-NO₂-Trp³⁰)CCK-26-33], was synthesized by solid-phase and solution techniques and characterized by mass spectrometry. Following oxidative iodination, it was purified on HPLC to 2000 Ci/mmol. Binding to pancreatic membranes was rapid, temperature dependent, reversible, saturable, and specific and was with high affinity (K_d = 3 nM). While its binding affinity was only 3-fold lower than that of native CCK-8, this probe was 70-fold less potent than native hormone in stimulating amylase secretion (EC₅₀ = 1 nM) and equally efficacious to native hormone. Despite the slight decrease in affinity, this probe demonstrated a high relative efficiency of covalent labeling of the M_r= 85 000–95 000 protein. This confirms that the M_r = 85 000–95 000 protein represents the hormone-binding subunit of the CCK receptor and demonstrates the utility of this type of photoaffinity labeling probe. This should provide a generally applicable substitution for photoaffinity labeling of molecules in which a Trp resides in a domain relevant for the characterization of a bimolecular interaction.