Products of cholecystokinin (CCK)-octapeptide proteolysis interact with central CCK receptors

Peptidases present in central nervous system (CNS) synaptic membranes, hydrolyze the neuroactive peptide cholecystokinin-octapeptide (CCK-8; Asp-Tyr-SO₃H-Met-Gly-Trp-Met-Asp-Phe-NH₂). In order to determine the pathway of degradation, synthetic CCK-8 was incubated at 37°C with purified synaptic membranes; at various intervals reaction samples were removed from the reaction mixture and analysed by high-performance liquid chromatography to identify and quantify the peptide fragments. The results indicate an initial endopeptidase cleavage at the Met-Gly bond producing CCK-5 (Gly-Trp-Met-Asp-Phe-NH₂). The carboxyl-terminal pentapeptide is further proteolysed to CCK-4 (Trp-Met-Asp-Phe-NH₂) by a puromycin-sensitive aminopeptidase and to CCK-3 (Met-Asp-Phe-NH₂) and Gly-Trp by an endopeptidase action, CCK-3 and CCK-2 appear to be relatively stable end-products. Moreover, these proteolytic fragments are shown to bind to the CCK receptor in brain with varying potencies.