Nitric oxide is an irreversible human diamine oxidase inhibitor

Diamine oxidase (DAO) histamine-degradation rates are compromised in plasma of mastocytosis patients during severe mast cell activation events. Mast cell-liberated histamine induces the release of nitric oxide (NO) close to DAO extracellular storage sites. We hypothesized that NO inhibits DAO activity. Recombinant human DAO activity was measured after incubation with NO-releasing NONOates (R¹R²N-(NO^-)-N = O). Topaquinone reactivity was quantified by absorption measurements and by mass spectrometry. Several murine models of NO-production were assessed for DAO activity inhibition <i>in vivo</i>. Nitric oxide released from NONOates dose dependently and irreversibly inhibited DAO activity. The NO scavengers Trolox (Vitamin E derivative) and 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (C-PTIO), the reversible DAO inhibitors diminazene and ciproxifan, the substrates histamine (EC₅₀ = 32 µM) and putrescine (EC₅₀ = 39 µM), heparin whole blood and plasma protected DAO from inhibition. Nitric oxide reduced the reactivity of topaquinone to phenylhydrazine by 90%. None of the NO producing <i>in vivo</i> models showed DAO inhibition in plasma or tissue. Nitric oxide is a potent irreversible DAO inhibitor <i>in vitro</i> representing the first discovered natural inhibitor for this enzyme. Endogenous mouse DAO inhibition <i>in vivo</i> could not be demonstrated. The true nature of human DAO activity inhibition during severe mastocytosis events remains unknown.