The Role of Bestatin-Sensitive Aminopeptidase, Angiotensin Converting Enzyme and Thiorphan-Sensitive "Enkephalinase" in the Potency of Enkephalins in the Guinea-Pig Ileum

The role of each enkephalin-hydrolyzing peptidase in the inhibitory potency of exogenously added enkephalins in the myenteric plexus-longitudinal muscle preparation of guinea-pig ileum was studied by using the relatively specific inhibitor of each enzyme. Results showed that three distinct enzymes, bestatin-sensitive aminopeptidase(s), angiotensin converting enzyme, and thiorphan-sensitive "enkephalinase", played a critical role in the inactivation of enkephalins. Additionally, these enzymes are likely to be located close to opioid receptors, since they produce a significant concentration difference of enkephalin between the surrounding organ bath and the vicinity of opioid receptors. In contrast to these three enzymes, both L-tyrosyl-L-tyrosine-sensitive dipeptidyl aminopeptidase and D-phenylalanine-sensitive carboxypeptidase are indicated not to be involved significantly in the degradation of exogenously added enkephalins in the guinea-pig ileum.