Synthesis of tritium labelled CCK4 (Trp-Met-Asp-[3H]Phe-NH2)

CCK4 (CCK30-33) is the shortest fragment of cholecystokinin which retains a high affinity (nanomolar range) for brain CCK receptors. In order to determine whether CCK4 interacts with binding sites distinct from those of CCK8, [3H]CCK4 was synthesized. This peptide was prepared in liquid phase using L-3(3',4',5'-tribromophenyl)alanine as the phenylalanine precursor. The reductive tritiation of H-Trp-Met-Asp-Phe(Br)3-NH2 was performed using PdO as the catalyst to yield [3H]CCK4 with a specific activity of 35 Ci/mmol (1295 GBq/mmol). (author)