DIFFERENTIAL CATABOLIC FATE OF NEUROMEDIN-N AND NEUROTENSIN IN THE CANINE INTESTINAL-MUCOSA

We have established the peptidase content of a P2 fraction (enriched i n synaptosomes) and plasma membranes prepared from canine intestinal m ucosa. Fourteen exo- and endopeptidases were assayed with fluorimetric or chromogenic substrates and identified by means of specific peptida se inhibitors. Post-proline dipeptidyl aminopeptidase IV, aminopeptida se M, and carboxypeptidase A were the most abundant exopeptidases, whi le aminopeptidases A and B, dipeptidyl aminopeptidase, pyroglutamyl pe ptide hydrolase I, and carboxypeptidase B displayed little, if any, ac tivity. Endopeptidase 24.11 was the only endopeptidase that was detect ed in high amount. By contrast, proline endopeptidase exhibited a low activity, while angiotensin-converting enzyme, endopeptidase 24.15, en dopeptidase 24.16, and cathepsin B and D-like activities were not dete cted. The catabolic rates of the two related neuropeptides, neurotensi n (NT) and neuromedin N (NN), established that NN was inactivated 16 t o 24 times faster than NT by plasma membrane and P2 fractions, respect ively. Furthermore, the two peptides underwent qualitatively distinct mechanisms of degradation. A phosphoramidon-sensitive formation of NT( 1-10) was detected as the major NT catabolite, indicating that NT was susceptible to an endoproteolytic cleavage elicited by endopeptidase 2 4.11. By contrast, NN was inactivated by the action of an exopeptidase at its N-terminus, leading to the formation of [des-Lys1]NN. The occu rrence of this NN metabolite was prevented by bestatin and actinonin, but not by the aminopeptidase B inhibitor, arphamenine B, indicating t hat the release of the N-terminal residue of NN was likely due to amin opeptidase M. The drastically different catabolic rates of NT and NN c ould support the possibility that inactivating mechanisms directly inf luence the fate of the two peptides in the intestine and, therefore, m odulate their putative paracrine/endocrine function in the periphery.