title: Metabolism of opioid peptides by cerebral microvascular aminopeptidase M.
abstract: Aminopeptidase M (EC 3.4.11.2), which can degrade low molecular weight opioid peptides, has been reported in both peripheral vasculature and in the CNS. Thus, we have studied the metabolism of opioid peptides by membrane-bound aminopeptidase M derived from cerebral microvessels of hog and rabbit. Both hog and rabbit microvessels were found to contain membrane-bound aminopeptidase M. At neutral pH, microvessels preferentially degraded low molecular weight opioid peptides by hydrolysis of the N-terminal Tyr1-Gly2 bond. Degradation was inhibited by amastatin (I50 = 0.2 microM) and bestatin (10 microM), but not by a number of other peptidase inhibitors including captopril and phosphoramidon. Rates of degradation were highest for the shorter peptides (Met5- and Leu5-enkephalin) whereas beta-endorphin was nearly completely resistant to N-terminal hydrolysis. Km values for the microvascular aminopeptidase also decreased significantly with increasing peptide length (Km = 91.3 +/- 4.9 and 28.9 +/- 3.5 microM for Met5-enkephalin and Met5-enkephalin-Arg6-Phe7, respectively). Peptides known to be present within or in close proximity to cerebral vessels (e.g., neurotensin and substance P) competitively inhibited enkephalin degradation (Ki = 20.4 +/- 2.5 and 7.9 +/- 1.6 microM, respectively). These data suggest that cerebral microvascular aminopeptidase M may play a role in vivo in modulating peptide-mediated local cerebral blood flow, and in preventing circulating enkephalins from crossing the blood-brain barrier.
mesh_terms: D000626:Aminopeptidases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D000900:Anti-Bacterial Agents; D001921:Brain / Q000098:blood supply*; D018826:CD13 Antigens; D002462:Cell Membrane / Q000201:enzymology; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D004723:Endorphins / Q000378:metabolism*; D007122:Immunoelectrophoresis; D007700:Kinetics; D007930:Leucine / Q000031:analogs & derivatives / Q000494:pharmacology; D008833:Microcirculation / Q000201:enzymology; D009842:Oligopeptides / Q000494:pharmacology; D010455:Peptides; D011817:Rabbits; D013379:Substrate Specificity; D013552:Swine
chemical_list: D000900:Anti-Bacterial Agents; D004723:Endorphins; D009842:Oligopeptides; D010455:Peptides; C016977:amastatin; D000626:Aminopeptidases; D018826:CD13 Antigens; D007930:Leucine; C012211:ubenimex