Establishment of a new short, protease-resistant, affinity labeling reagent for the cholecystokinin receptor

Proteolytic degradation of radioligands is an important source of artifact in affinity labeling of receptor proteins. To complement our previous characterization of the pancreatic acinar cell cholecystokinin (CCK) receptor, we synthesized D-Tyr-Gly[(Nle^28,31)CCK-26-33]. The amino terminal D-enantiomer of tyrosine provided a site for oxidative iodination, a free amino group for cross-linking and rendered the peptide resistant to aminopeptidases. The deca-peptide was oxidatively iodinated and purified by reverse-phase HPLC to 2,000 Ci/mmol, to yield a probe which was equal in potency and efficacy to CCK-8, and which bound to rat pancreatic membranes in a rapid, reversible, temperature-dependent, specific, saturable and high affinity manner. This probe was resistant to aminopeptidase degradation, and maintained its ability to bind to receptor after incubation with pancreatic membranes or dispersed cells. Affinity labeling of pancreatic membranes with this analogue identified an M_r=85,000-95,000 molecule. This analogue offers several advantages over existing probes and should be useful for future studies of this and other CCK receptors.