ROLE OF ENDOPEPTIDASE-3.4.24.16 IN THE CATABOLISM OF NEUROTENSIN, IN-VIVO, IN THE VASCULARLY PERFUSED DOG ILEUM

1 The degradation of tritiated and unlabelled neurotensin (NT) following close intra-arterial infusion of the peptides in ileal segments of anaesthetized dogs was examined. 2 Intact NT and its catabolites recovered in the venous effluents were purified by chromatography on Sep-Pak columns followed by reverse-phase h.p.l.c. and identified by their retention times or by radioimmunoassay. 3 The half-life of neurotensin was estimated to be between 2 and 6 min. Four labelled catabolites, corresponding to free tyrosine, neurotensin (1-8), neurotensin (1-10) and neurotensin (1-11), were detected. 4 Neurotensin (1-11) was mainly generated by a phosphoramidon-sensitive cleavage, probably elicited by endopeptidase 24-11. 5 Two endopeptidase 3.4.24.16 inhibitors, phosphodiepryl 03 and the dipeptide Pro-Ile, dose-dependently potentiated the recovery of intact neurotensin. Furthermore, both agents inhibited the formation of neurotensin (1-10), the product that results from the hydrolysis of neurotensin by purified endopeptidase 3.4.24.16. In contrast, the endopeptidase 3.4.24.15 inhibitor Cpp-AAY-pAB neither protected neurotensin from degradation nor modifed the production of neurotensin (1-10). 6 Our study is the first evidence to indicate that endopeptidase 3.4.24.16 contributes to the catabolism of neurotensin, in vivo, in the dog intestine.