The reaction of oxygen with radicals from oxidation of tryptophan and indole-3-acetic acid

The oxidation of tryptophan and indole-3-acetic acid (IAA) by the dibromine radical anion or peroxidase from horseradish in aqueous solution was investigated and compared, especially with respect to the involvement of oxygen and superoxide. Using EPR with spin-trapping, the tryptophanyl radical, generated by either method was found to react with oxygen, although this reaction is too slow to be observed by pulse radiolysis (k < 5 x 10(6) dm(3) mol(-1) s(-1)). No superoxide results from this reaction, thus excluding an electron-transfer mechanism and suggesting the formation of a tryptophan peroxyl radical, possibly in a reversible process. These observations imply that in proteins when the tryptophanyl radical exists as a stable species it must either have its reactivity modified by the protein environment or be inaccessible to oxygen. The related molecule IAA is oxidized by either peroxidase or Br2(.-) to a radical cation that decarboxylates to yield a skatolyl radical. The latter reacts with oxygen to give a peroxyl radical that does not release superoxide. However, O-2(.-) is formed during the peroxidase-catalyzed oxidation of indoleacetic acid. This supports the hypothesis that the peroxidase can act in an oxidase cycle involving ferrous enzyme and compound III, with superoxide as a product. (C) 1997 Elsevier Science B.V.