title: Methionine residue acts as a prooxidant in the •OH-induced oxidation of enkephalins.
abstract: Enkephalins are bioactive pentapeptides (Tyr-Gly-Gly-Phe-Leu (Leu-enk) and Tyr-Gly-Gly-Phe-Met (Met-enk)) produced while an organism is under mental and/or physical stress. In the course of their biological action they are exposed to reactive oxygen and nitrogen species. We have reinvestigated the reactions of (•)OH radicals toward these peptides in order to elucidate the oxidation mechanisms and the final products. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Additional insight into details of the oxidation mechanism was gained by identification of main final products by means of UV-vis spectrophotometry, HPLC coupled with fluorescence spectroscopy, and mass spectrometry. The key processes are different in both peptides. In Leu-enk, the first step is an (•)OH radical addition to the aromatic rings of Tyr and Phe residues that leads to hydroxylated residues, dihydroxyphenylalanine (DOPA) from Tyr and tyrosine isomers from Phe, respectively. In Met-enk, these processes are less important, an additional target being the sulfur atom of the methionine residue. Depending on pH either an OH-adduct (hydroxysulfuranyl radical) or a sulfur radical cation undergo intramolecular electron transfer with Tyr residue resulting in a repair of Met and oxidation of Tyr to tyrosyl radicals and a final formation of dityrosine. At low pH, the OH-adducts to Tyr residue are precursors of tyrosyl radicals and dityrosine. Thus, the final products coming from oxidation of the Tyr residue depend strongly on the neighboring residues and the pH.
authors.affiliation: Laboratory of Physical Chemistry and CNRS  Bldg 350, Centre Universitaire, F-91405 Orsay, F-91405 Orsay, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Mozziconacci
authors.forename: Jacek
authors.initials: J
authors.lastname: Mirkowski
authors.forename: Filippo
authors.initials: F
authors.lastname: Rusconi
authors.forename: Gabriel
authors.initials: G
authors.lastname: Kciuk
authors.forename: Pawel B
authors.initials: PB
authors.lastname: Wisniowski
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Bobrowski
authors.forename: Chantal
authors.initials: C
authors.lastname: Houée-Levin
mesh_terms: D004745:Enkephalins / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D011789:Quantum Theory
chemical_list: D004745:Enkephalins; D016877:Oxidants; D017665:Hydroxyl Radical; D008715:Methionine