15N-CIDNP investigations during tryptophan, N-acetyl-L-tryptophan, and melatonin nitration with reactive nitrogen species.

Tryptophan and melatonin are nitrated by peroxynitrite; tryptophan residues in proteins are susceptible to attack by reactive nitrogen species. Nitrated tryptophan might therefore be used as a biomarker for the involvement of reactive species derived from nitrogen oxide in a variety of pathophysiological conditions. The radical character of the tryptophan (Trp) and N-acetyl-L-tryptophan (N-AcTrp) nitration with peroxynitrite is shown using (15)N-CIDNP. During the decay of peroxynitrite-(15)N in the presence of Trp at pH 5 in the probe of a (15)N-NMR spectrometer, the (15)N-NMR signals of various nitrated tryptophans ((15)NO(2)-Trp) show emission (E). The effects are built up in radical pairs [Trp( radical), 15NO2 ](F) formed by diffusive encounters of radicals 15NO2 and Trp( radical) generated during decay of peroxynitrite-(15)N in the presence of Trp. Similar (15)N-CIDNP effects are observed during reaction of Trp and/or N-AcTrp using the nitrating systems H(15)NO(3), H(15)NO(4) and H(2)O(2)/15NO2 /HRP, which are also built up in radical pairs [Trp, 15NO2 ](F). During nitration of melatonin (Mel) with peroxynitrite-(15)N and H(15)NO(4), the (15)N-NMR signal of 4-nitromelatonin (4-(15)NO(2)-Mel) shows emission arising from radical pairs [Mel, 15NO2 ](F) which are formed in an analogous manner.