Pathway of inactivation of cholecystokinin octapeptide (CCK-8) by synaptosomal fractions

Cholecystokinin octapeptide (CCK-8) was degraded by peptidases present in intact synaptosomes isolated from rat cortex and hypothalamus. Most of the degrading activity was present in the cytoplasmic fraction although a small amount (7%) was membrane-bound. Products of the degradation were isolated by HPLC and characterized by amino acid analysis. The Met³-Gly⁴ bond was the main primary site of cleavage giving rise to Asp-Tyr(SO₃H)-Met and Gly-Trp-Met-Asp-Phe-NH₂. These products appeared to be further degraded by sequential removal of amino-terminal residues. The Asp¹-Tyr² and Asp⁷-Phe⁸ bonds were also sites of cleavage. p-Chloromercuribenzoate was the most effective inhibitor (90% inhibition) of CCK-8 degradation by synaptosomal peptidases at the concentrations tested.This peptidase activity in synaptosomes may be important in the regulation of levels of the neuropeptide CCK-8 at the synapse. Identification of the sites of cleavage of CCK-8 on incubation with synaptosomes will assist in the isolation and characterization of the enzymes involved.