[Subcellular localization, inhibiting specificity and catalytic properties of several aminooxidases from the human...

Human placenta was shown to contain practically all known types of aminooxidase, i.e., Membrane-bound and soluble monoamine oxidases A that predominantly oxidize serotonin (Km approximately 0.05 and 0.2 mM) and tyramine (Km approximately 0.03 and 0.085 mM), partly oxidize phenylethylamine (Km approximately 0.013 and 0.1 mM) and slightly oxidize benzylamine; Monoamine oxidase B and its intermediate form, B', with equal sensitivity towards the inhibitors, Lilly 51641 and deprenyl. The main substrates for these enzymes are phenylethylamine (Km = 0.011 mM for the membrane-bound and 0.019 mM for the soluble enzymes); Membrane-bound and soluble benzylamine oxidases that are stable to MAO inhibitors but are highly labile towards semicarbazide and aminoguanidine and that predominantly oxidize benzylamine. The Km value for the soluble enzyme is 0.19 mM, its specific activity is 0.058 nmol aldehyde/min/mg protein, which markedly exceeds that for serum benzylamine oxidase (i.e., 0.014 nmol/min/mg) and thus excludes its serum origin; Diamine oxidase that oxidizes putrescine (Km = 0.025 mM), histamine and cadaverine and only slightly oxidizes benzylamine. One characteristic feature of the placenta is the presence of soluble MAO as well as MAO incorporated into the endoplasmic reticulum membrane (microsomes). In all probability, these enzymes are precursors of the mitochondrial enzyme. The concentration of MAO A in the mitochondria is approximately 1.3%, that in microsomes--approximately 1%, kcat = 270 and 320 min-1, respectively.