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Singlet Oxygen–mediated Protein Oxidation: Evidence for the Formation of Reactive Side Chain Peroxides on Tyrosine Residues ¶
Author(s) -
Wright Adam,
Bubb William A.,
Hawkins Clare L.,
Davies Michael J.
Publication year - 2002
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2002)0760035sompoe2.0.co2
Subject(s) - singlet oxygen , side chain , tyrosine , chemistry , photochemistry , chain (unit) , oxygen , reactive intermediate , singlet state , organic chemistry , biochemistry , catalysis , physics , polymer , astronomy , nuclear physics , excited state
Singlet oxygen ( 1 O 2 ) is generated by a number of enzymes as well as by UV or visible light in the presence of a sensitizer and has been proposed as a damaging agent in a number of pathologies including cataract, sunburn, and skin cancers. Proteins, and Cys, Met, Trp, Tyr and His side chains in particular, are major targets for 1 O 2 as a result of their abundance and high rate constants for reaction. In this study it is shown that long‐lived peroxides are formed on free Tyr, Tyr residues in peptides and proteins, and model compounds on exposure to 1 O 2 generated by both photochemical and chemical methods. The yield of these species is significantly enhanced in D 2 O and decreased by azide. Nuclear magnetic resonance and mass spectroscopic analysis of reaction mixtures, or materials separated by high‐performance liquid chromatography, are consistent with the initial formation of an (undetected) endoperoxide that undergoes rapid ring‐opening to give a hydroperoxide situated at the C1 ring‐position ( i.e. para to the phenolic group). In the presence of a free α‐amino group ( e.g. with free Tyr), rapid ring‐closure occurs to give an indolic hydroperoxide that decays into the corresponding alcohol, 3a‐hydroxy‐6‐oxo‐2,3,3a,6,7,7a‐hexahydro‐1 H ‐indole‐2‐carboxylic acid. Hydroperoxides that lack a free α‐amino group ( e.g. those formed on 3‐(4‐hydroxyphenyl)propionic acid, N ‐Ac‐Tyr and Tyr‐containing peptides) are longer‐lived, with half‐lives of hours to days. These species undergo slow decay at low temperatures to give the corresponding alcohol. Their rate of decay is enhanced at 37°C, or on exposure to UV light or metal ions, and gives rise to reactive radicals, via cleavage of the peroxide bond. These radicals have been characterized by electron paramagnetic resonance spin trapping. These studies demonstrate that long‐lived Tyr‐derived peroxides are formed on proteins exposed to 1 O 2 and that these may promote damage to other targets via further radical generation.