Nitric oxide rapidly scavenges tyrosine and tryptophan radicals

By utilizing a pulse-radiolytic technique, we demonstrate for the first time that the rate constant for the reaction of nitric oxide (^.NO) with biologically relevant tyrosine and tryptophan radicals (Tyr^. and Trp^. respectively) in amino acids, peptides and proteins is of the order of (1-2) x 10⁹ M^-1·s^-1. We also show that ^.NO effectively interferes with electron-transfer processes between tryptophan and tyrosine residues in proteins subjected to pulse radiolysis. The near diffusion-controlled rates of these reactions, coupled with the increasingly recognized role of protein radicals in enzyme catalysis and oxidative damage, suggest that Tyr^. and Trp^. are likely and important targets for ^.NO generated in vivo.