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Photooxidation of Lens Proteins with Xanthurenic Acid: A Putative Chromophore for Cataractogenesis ¶
Author(s) -
Roberts Joan E.,
Finley Eric L.,
Patat Severine A.,
Schey Kevin L.
Publication year - 2001
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(2001)0740740polpwx2.0.co2
Subject(s) - histidine , crystallin , chemistry , tryptophan , xanthurenic acid , singlet oxygen , chromophore , metabolite , polyacrylamide gel electrophoresis , biochemistry , photochemistry , sodium dodecyl sulfate , gel electrophoresis , amino acid , oxygen , organic chemistry , enzyme
The tryptophan metabolite, xanthurenic acid (Xan), is produced through a transamination reaction in high concentrations in human lenses with age and has been isolated from aged human cataractous lenses. It has appreciable absorption between 300 and 400 nm (λ max = 334 nm), the range absorbed by the human lens. Our recent studies have shown that unlike most tryptophan metabolites in the eye, Xan is photochemically active, producing both superoxide and singlet oxygen. To determine if Xan could act as a photosensitizer and photooxidize cytosolic lens proteins, α‐, β‐ and γ‐crystallins were irradiated (λ > 300 nm, 12 mW/cm 2 ) in the presence and absence of Xan. Upon irradiation and in the presence of Xan, lens proteins polymerized in the order α > β > γ as assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Further analysis of the photolyzed α‐crystallin by mass spectrometry indicated that histidine, tryptophan and methionine residues were oxidized at specific positions in a dose‐dependent (irradiation time) manner. In αA‐crystallin two forms of oxidized histidine 154 were observed, 2‐imidazolone and 2‐oxohistidine. Our results suggest that naturally occurring Xan is a chromophore capable of photosensitization and photooxidation of lens proteins. Furthermore, this compound could play a role in age‐related cataractogenesis.