Oxidative denitrification of N^ω-hydroxy-l-arginine by the superoxide radical anion

The superoxide radical anion (O₂^-•) produced during the catalytic activity of nitric oxide synthase (NOS) and cytochrome P-450 has been implicated in the oxidative denitrification of hydroxyguanidines (> C = NOH). The reactivity of the radiolytically generated O₂^-• radical with N^ω-hydroxy-l-arginine (NHA) is pH dependent and appears to parallel the prototropic equilibrium of the hydroxyguanidino group (> C = NOH ⇌ > C = NO^- + H⁺; pK = 8). The N^ω-hydroxyguanidino group is more reactive towards O₂^-• when deprotonated but exhibits negligible reactivity when protonated. Based on a model, the rate constant for the reaction of the O₂^-• with NHA was estimated as k (O₂^-•+ > C = NO^-) ≈ 200–500 M^-1·s^-1, which is probably too low to compete with O₂^-• reactions with NO^• or superoxide dismutase, which occur many orders of magnitude faster. The oxidative elimination of NO from NHA by O₂^-• was not accompanied by the formation of l-citrulline. Since only 21% of NHA will exist in the deprotonated > C = NO^- form at physiological pH, it is unlikely that oxidative denitrification of NHA by cytochrome P-450 or NOS-derived O₂^-• radicals will prove a major free-radical pathway to NO^• and l-citrulline.