A preparation of closed membrane vesicles derived from the longitudinal and circular smooth muscle of pig small intestine was enriched eight-fold in the activity of 5′-nucleotidase and six-fold in the activity of peptidyl dipeptidase A relative to the tissue homogenate. The membrane vesicles specifically bound [3H]bradykinin and the concentration of bradykinin required to inhibit 50% binding was 0.76 ± 0.05 nM. This concentration was not significantly different from the corresponding concentration of lysyl-bradykinin (0.45 ± 0.13 nM) but was less (P < 0.05) than the concentration of methionyl-lysyl-bradykinin (1.25 ± 0.10 nM). The concentration of des-Argy bradykinin (7.5 μM) required for 50% inhibition was >103 times less than bradykinin indicating the presence of a B2-type receptor. The membrane vesicles also degraded bradykinin and the principal metabolite was identified as bradykinin1-7. Des-Arg1 bradykinin, des-Arg9 bradykinin and bradykinin6-9 were also formed in low yield. Cleavage of the Pro7-Phe8 bond was inhibited by phosphoramidon but not by enalapril or captopril indicating that proteolytic inactivation of bradykinin in the muscle layer of the intestine is mediated through endopeptidase 24.11 ("enkephalinase") but not through peptidyl dipeptidase A ("angiotensin-converting enzyme").
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