Cell surface monoamine oxidases: enzymes in search of a function

Ectoenzymes with a catalytically active domain outside the cell surface hav e the potential to regulate multiple biological processes. A distinct class of copper-containing semicarbazide-sensitive monoamine oxidases, expressed on the cell surface and in soluble forms, oxidatively deaminate primary am ines. Via transient covalent enzyme-substrate intermediates, this reaction results in production of aldehydes, hydrogen peroxide and ammonium, which a re all biologically active substances. The physiological functions of these enzymes have remained unknown, although they have been suggested to be inv olved in the metabolism of biogenic amines. Recently, new roles have been p roposed for these enzymes in regulation of glucose uptake and, even more su rprisingly, in leukocyte-endothelial cell interactions. The emerging functi ons of ectoenzymes in signalling and cell-cell adhesion suggest a novel mod e of molecular control of these complex processes.