title: Angiotensin-converting enzyme 2 catalytic activity in human plasma is masked by an endogenous inhibitor.
abstract: Angiotensin-converting enzyme 2 (ACE2) is thought to act in an opposing manner to its homologue, angiotensin-converting enzyme (ACE), by inactivating the vasoconstrictor peptide angiotensin II and generating the vasodilatory fragment, angiotensin(1-7). Both ACE and ACE2 are membrane-bound ectoenzymes and may circulate in plasma as a consequence of a proteolytic shedding event. In this study, we show that ACE2 circulates in human plasma, but its activity is suppressed by the presence of an endogenous inhibitor. Partial purification of this inhibitor indicated that the inhibitor is small, hydrophilic and cationic, but not a divalent metal cation. These observations led us to develop a method for removal of the inhibitor, thus allowing detection of plasma ACE2 levels using a sensitive quenched fluorescent substrate-based assay. Using this technique, ACE2 activity measured in plasma from healthy volunteers (n = 18) ranged from 1.31 to 8.69 pmol substrate cleaved min-1 ml-1 (mean +/- s.e.m., 4.44 +/- 0.56 pmol min-1 ml-1). Future studies of patients with cardiovascular, renal and liver disease will determine whether plasma ACE2 is elevated in parallel with increased tissue levels observed in these conditions.
authors.affiliation: Department of Biochemistry & Molecular Biology, PO Box 13D, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Lew
authors.forename: Fiona J
authors.initials: FJ
authors.lastname: Warner
authors.forename: Iresha
authors.initials: I
authors.lastname: Hanchapola
authors.forename: Michael A
authors.initials: MA
authors.lastname: Yarski
authors.forename: Jay
authors.initials: J
authors.lastname: Ramchand
authors.forename: Jay
authors.initials: J
authors.lastname: Manohar
authors.forename: Louise M
authors.initials: LM
authors.lastname: Burrell
authors.forename: A Ian
authors.initials: AI
authors.lastname: Smith
mesh_terms: D000097:Acetonitriles / Q000737:chemistry / Q000302:isolation & purification; D000328:Adult; D000803:Angiotensin I / Q000378:metabolism; D000804:Angiotensin II / Q000378:metabolism; D000085962:Angiotensin-Converting Enzyme 2; D000806:Angiotensin-Converting Enzyme Inhibitors / Q000097:blood*; D017920:Antiporters / Q000378:metabolism; D015153:Blotting, Western; D002413:Cations, Divalent / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D005260:Female; D006801:Humans; D047468:Immunoprecipitation; D008297:Male; D008875:Middle Aged; D010446:Peptide Fragments / Q000378:metabolism; D007703:Peptidyl-Dipeptidase A / Q000097:blood*; D010949:Plasma / Q000201:enzymology; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000302:isolation & purification; D013347:Subcellular Fractions / Q000201:enzymology / Q000378:metabolism
chemical_list: D000097:Acetonitriles; D000806:Angiotensin-Converting Enzyme Inhibitors; D017920:Antiporters; D002413:Cations, Divalent; D002614:Chelating Agents; D010446:Peptide Fragments; D011994:Recombinant Proteins; D000804:Angiotensin II; D000803:Angiotensin I; D007703:Peptidyl-Dipeptidase A; C000705307:ACE2 protein, human; D000085962:Angiotensin-Converting Enzyme 2; C118790:angiotensin I (1-7); C032159:acetonitrile