The conversion of Met-enkephalin-Arg6, Phe7 to Met-enkephalin in rabbit ear artery

We studied the opioid agonist activities of met-enkephalin-Arg6, Phe7 heptapeptide (ME-A,P) and its D-Ala2-analogue in several isolated organs including rabbit ear artery (ART), guinea-pig ileum (GPI) mouse-, rat and rabbit vas deferens preparation. Both heptapeptides followed the in vitro activity pattern of met-enkephalin (ME). There was a significant parallelism in the sensitivity of ART and mouse vas deferens to opioid peptides. In ART, ME-A,P must be converted by a captopril sensitive enzyme to exert opioid agonist action. Incubation of heptapeptide with membranes prepared from rabbit ear artery revealed the captopril-sensitive enzymic conversion of ME-AP to ME as detected by high voltage paper electrophoresis.