Oxidative denitrification of N-omega-hydroxy-L-arginine by the superoxide radical anion

The superoxide radical anion (O-2(-.)) produced during the catalytic activity of nitric oxide synthase (NOS) and cytochrome P-450 has been implicated in the oxidative denitrification of hydroxy-guanidines (> C = NOH). The reactivity of the radiolytically generated O-2(-.) radical with N-omega-hydroxy-L-arginine (NHA) is pH dependent and appears to parallel the prototropic equilibrium of the hydroxyguanidino group (> C = NOH reversible arrow > C = NO- +H+; pK = 8). The N-omega-hydroxyguanidino group is more reactive towards O-2(-.) when deprotonated but exhibits negligible reactivity when protonated. Based on a model, the rate constant for the reaction of the O-2(-.) with NHA was estimated as k (O-2(-.) + > C = NO-) approximate to 200-500 M(-1). s(-1), which is probably too low to compete with O-2(-.) reactions with NO. or superoxide dismutase, which occur many orders of magnitude faster. The oxidative elimination of NO from NHA by O-2(-.), 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 O-2(-.) radicals will prove a major free-radical pathway to NO. and L-citrulline.