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  • Title: Metabolism of enkephalins in head membranes of the leech Theromyzon tessulatum by peptidases: isolation of an enkephalin-degrading aminopeptidase.
    Author: Laurent V, Salzet M.
    Journal: Regul Pept; 1996 Sep 09; 65(2):123-31. PubMed ID: 8884979.
    Abstract:
    Metabolism of leucine and methionine enkephalins by enzyme preparations from head parts of the leech Theromyzon tessulatum was investigated. Leech homogenate degraded enkephalins by cleavage of the Tyr1-Gly2 and Gly3-Phe4 bonds. The Tyr1-Gly2-Gly3 was detected as a major metabolite when amastatin (aminopeptidase inhibitor) was present to prevent Tyr1-Gly2 breakdown. Around 50% of enkephalin-degrading activity was isolated in a 20000 x g membrane fraction and was shown to be almost entirely due to an aminopeptidase activity. This enzyme, a homodimer of approx. 70 kDa, has been purified to homogeneity by a combined approach including gel permeation and anion exchange chromatographies followed by reversed-phase HPLC. This enkephalin-degrading aminopeptidase is a typical integral membrane 'zincin' metalloprotein with an apparent k(m) of 30 microM, a specific activity of 12 nmol GGFM min-1 mg protein-1 and a catalytic efficiency (kcat/k(m)) of 46 x 10(6) mol-1 min-1. This enzyme is specifically inhibited by amastatin (IC50 = 0.5 microM), but not by bestatin and actinonin. In leech membranes, the other degrading activities performed at the same time were due to a neuropeptide-endopeptidase (NEP)-like enzyme attack, inhibited by phosphoramidon (IC50 = 0.1 microM) and in the case of the Met-enkephalin by a combined action of an angiotensin-converting-like enzyme, inhibited by captopril (IC50 = 0.2 microM) and the NEP-like enzyme. These two enzymes were previously isolated from head membranes of T. tessulatum and possess towards Met-enkephalin a catalytic efficiency (kcat/k(m)) of, respectively, 12 x 10(6) mol-1 min-1 and 78 x 10(6) mol-1 min-1. These findings constitute the first report in leeches on the nature and the sites of attack of the membrane peptidases involved in the metabolism of enkephalins and also the first biochemical evidence for a novel member of the aminopeptidase family.
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