Inhibition of the Fenton reaction by nitrogen monoxide

The toxicity of iron is believed to originate from the Fenton reaction which produces the hydroxyl radical and/or oxoiron2+. The effect of nitrogen monoxide on the kinetics of the reaction of iron(II) bound to citrate, ethylenediamine-N,N'-diacetate (edda), ethylenediamine-N,N,N',N'-tetraacetate (edta), (N-hydroxyethyl)amine-N,N',N'-triacetate (hedta), and nitrilotriacetate (nta) with hydrogen peroxide was studied by stopped-flow spectrophotometry. Nitrogen monoxide inhibits the Fenton reaction to a large extent. For instance, hydrogen peroxide oxidizes iron(II) citrate with a rate constant of 5.8x10(3) M(-1) s(-1), but in the presence of nitrogen monoxide, the rate constant is 2.9x10(2) M(-1) s(-1) . Similar to hydrogen peroxide, the reaction of tert-butyl hydroperoxide with iron(II) complexes is also efficiently inhibited by nitrogen monoxide. Generally, nitrogen monoxide binds rapidly to a coordination site of iron(II) occupied by water. The rate of oxidation is influenced by the rate of dissociation of the nitrogen monoxide from iron(II).