Oxidation of free, peptide and protein tryptophan residues mediated by AAPH-derived free radicals: role of alkoxyl and peroxyl radicals

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E. Fuentes-Lemus, E. Dorta, E. Escobar, A. Aspee, E. Pino, M.L. Abasq, H. Speisky, E. Silva, E. Lissi, Michael Jonathan Davies, C. Lopez-Alarcon

The oxidation of tryptophan (Trp) residues, mediated by peroxyl radicals (ROOc), follows a complex
mechanism involving free radical intermediates, and short chain reactions. The reactivity of Trp towards
ROOc should be strongly affected by its inclusion in peptides and proteins. To examine the latter, we
investigated (by fluorescence) the kinetic of the consumption of free, peptide- and protein-Trp residues
towards AAPH (2,20
-azobis(2-amidinopropane)dihydrochloride)-derived free radicals. Interestingly, the
initial consumption rates (Ri
) were only slightly influenced by the inclusion of Trp in small peptides and
proteins (human serum albumin and human superoxide dismutase). Depending on the Trp
concentration, the Ri versus Trp concentration ([Trp]) plots showed three regions. At low Trp
concentrations (1–10 mM), a linear dependence was observed between Ri and [Trp]; at intermediate Trp
concentrations (10–50 mM), the values of Ri were nearly constant; and at high Trp concentrations (50 mM
to 1 mM), a slower increase of Ri than expected for chain reactions. Similar behavior was detected for all
three systems (free Trp, and Trp in peptides and proteins). For the first time we are showing that alkoxyl
radicals, formed from self-reaction of ROOc, are responsible of the Trp oxidation at low concentrations,
while at high Trp concentrations, a mixture of peroxyl and alkoxyl radicals are involved in the oxidation
of Trp residues.
Original languageEnglish
JournalRSC Advances
Volume6
Issue number63
Pages (from-to)57948-57955
Number of pages8
ISSN2046-2069
DOIs
Publication statusPublished - 2016

ID: 164414087