title: CD13 (GP150; aminopeptidase-N): predominant functional activity in blood is localized to plasma and is not cell-surface associated.
abstract: This study is the first to report the presence of CD13/glycoprotein 150 (GP150)/aminopeptidase-N activity in cell-free plasma. We have determined that aminopeptidase-N in plasma provides, quantitatively, aminopeptidase-N's predominant functional activity within flowing blood. Thus, while aminopeptidase-N activity observed in whole blood can be partly, but significantly, blocked by the CD13 monoclonal antibody (MAB) WM15, the magnitude of such inhibition is low (< 25%) and similar to that observed using washed cell fractions selectively enriched for neutrophils (30.6% inhibition) or monocytes (21.8% inhibition). Plasma, free of cell components, possesses substantial aminopeptidase-N activity that is largely inhibitable (> 70%) by WM15. Blood collected into heparin or citrate yields similar data, while blood collected into EDTA gives rise to reduced CD13/aminopeptidase-N activity, consistent with inhibition of the known heavy-metal ion association necessary for proper functioning of this molecule. Although monocyte- and granulocyte-enriched cell fractions possess aminopeptidase-N activity significantly inhibitable by CD13 antibodies, lymphocyte-enriched cell fractions also possess aminopeptidase-N-like activity; however, in the latter case, this activity is not inhibitable by CD13 antibodies. Immunoaffinity isolation of plasma aminopeptidase-N has also been carried out; further characterization using functional studies and sodium dodecyl sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis indicates that CD13 MABs can completely clear plasma of aminopeptidase-N activity and that the purified protein has similar electrophoretic characteristics to cell-derived material. These data, therefore, provide evidence for the presence within blood both of a soluble (that is, non-cell-associated) form of CD13/GP150/aminopeptidase-N localizable to plasma and of cell-associated, aminopeptidase-N-like proteins other than CD13/GP150. These findings have significant implications for our understanding of the many functions of this molecule in blood.
authors.affiliation: Department of Haematology, Westmead Hospital, New South Wales, Australia.
authors.forename: E J
authors.initials: EJ
authors.lastname: Favaloro
authors.forename: T
authors.initials: T
authors.lastname: Browning
authors.forename: D
authors.initials: D
authors.lastname: Facey
mesh_terms: D000626:Aminopeptidases / Q000037:antagonists & inhibitors / Q000097:blood*; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D015703:Antigens, CD / Q000097:blood* / Q000276:immunology; D015214:Antigens, Differentiation, Myelomonocytic / Q000097:blood* / Q000276:immunology; D018826:CD13 Antigens; D002474:Cell-Free System; D002951:Citrates; D019343:Citric Acid; D004492:Edetic Acid; D004591:Electrophoresis, Polyacrylamide Gel; D006098:Granulocytes / Q000201:enzymology; D006493:Heparin; D006801:Humans; D008214:Lymphocytes / Q000201:enzymology; D009000:Monocytes / Q000201:enzymology
chemical_list: D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; D002951:Citrates; D019343:Citric Acid; D006493:Heparin; D004492:Edetic Acid; D000626:Aminopeptidases; D018826:CD13 Antigens