Preparation and characterization of a probe for the cholecystokinin octapeptide receptor, N alpha (125I-desaminotyrosyl)CCK-8, and its interactions with pancreatic acini.

A biologically active, high specific activity (2000 Ci/mmol), radiolabeled derivative of cholecystokinin octapeptide (CCK-8) was synthesized, purified, characterized, and used to investigate interactions between CCK-8 and its receptor on rat pancreatic acini. N/sup ..cap alpha../(/sup 125/I-desaminotyrosyl)CCK-8 was prepared by acylation of CCK-8 with /sup 125/I-labeled N-hydroxysuccinimidyl 3-(4-hydroxyphenyl)propionate in dimethylformamide and triethylamine. It was purified by cation exchange chromatography (sulfopropyl-Sephadex), followed by either gel filtration on Sephadex G-25 or high performance liquid chromatography on a reversed-phase C-18 column. Its nonradioactive analogue, N/sup ..cap alpha../(/sup 125/I-desaminotyrosyl)CCK-8 to pancreatic acini was rapid, reversible, temperature-dependent, saturable, and specific. This binding was inhibited by each molecular species of cholecystokinin, but not by structurally unrelated ligands. Good correlation existed between potencies of the species of cholecystokinin for inhibiting binding of this receptor probe and for stimulating pancreatic secretion. N/sup ..cap alpha../(/sup 125/I-desaminotyrosyl) CCK-8 binding was inhibited by its nonradioactive analogue, N/sup ..cap alpha../(/sup 127/I-desaminotyrosyl)CCK-8, in a manner identical with that by iodinated desaminotyrosyl moiety to the NH/sub 2/-terminal end of CCK-8 does not compromise the ability of this peptide to either bind to its pancreatic receptor or stimulate a biologic response.