Preparation and characterization of a new cholecystokinin receptor probe that can be oxidatively radioiodinated

Oxidative iodination is the simplest, most efficient, and least expensive method for radiolabeling peptide hormones. It has not been applied to cholecystokinin (CCK), however, because this peptide lacks an unsubstituted tyrosine residue and oxidation abolishes its biologic activity. We report the synthesis, purification, and characterization of the first derivative of CCK that can be radioiodinated with an oxidative method while maintaining full biologic activity and receptor binding affinity. Boc-Tyr-[Thr²⁸, Nle³¹)CCK-25-33] was synthesized, iodinated using a solid-phase oxidant, and purified on reverse-phase high-pressure liquid chromatography to a specific activity of 2125 Ci/mmol. Both native and iodinated Boc-Tyr-[(Thr²⁸, Nle³¹)CCK-25-33] exhibited efficacy and potency for stimulation of in vitro pancreatic enzyme secretion that were identical to CCK-8. Binding of Boc-¹²⁵I-Tyr-[(Thr²⁸, Nle³¹)CCK-25-33] to rat pancreatic plasma membranes was rapid, reversible, temperature-dependent, saturable, and specific. The abilities of various molecular forms of CCK to compete for this binding paralleled their potencies for stimulation of pancreatic secretion (K_d: CCK-8, 0.8 nM; CCK-33, 3 nM; CCK-8DS, 1 μM; CCK-4, 50 μM), whereas structurally unrelated pancreatic ligands (1 μM) demonstrated no significant competition. These findings suggest that Boc-¹²⁵I-Tyr-[(Thr²⁸, Nle³¹)CCK-25-33] interacts with the same high-affinity pancreatic receptor as the CCK receptor probes previously reported, i.e., the Bolton-Hunter-labeled CCK-33 and CCK-8. This probe also possesses the major advantages of simplicity, efficiency, and cost of the labeling procedure, and the stability and relative oxidation-resistance of the product. Boc-¹²⁵I-Tyr-[(Thr²⁸, Nle³¹)CCK-25-33] will be useful for the characterization of binding interactions between CCK and its target tissues.