Abstract
Human lens proteins become progressively modified by tryptophan-derived UV filter compounds in an age-dependent manner. One of these compounds, kynurenine, undergoes deamination at physiological pH, and the product binds covalently to nucleophilic residues in proteins via a Michael addition. Here we demonstrate that after covalent attachment of kynurenine, lens proteins become susceptible to photo-oxidation by wavelengths of light that penetrate the cornea. H2O2 and protein-bound peroxides were found to accumulate in a time-dependent manner after exposure to UV light (lambda > 305-385 nm), with shorter-wavelength light giving more peroxides. Peroxide formation was accompanied by increases in the levels of the protein-bound tyrosine oxidation products dityrosine and 3,4-dihydroxyphenylalanine, species known to be elevated in human cataract lens proteins. Experiments using D2O, which enhances the lifetime of singlet oxygen, and azide, a potent scavenger of this species, are consistent with oxidation being mediated by singlet oxygen. These findings provide a mechanistic explanation for UV light-mediated protein oxidation in cataract lenses, and also rationalize the occurrence of age-related cataract in the nuclear region of the lens, as modification of lens proteins by UV filters occurs primarily in this region.
| Original language | English |
|---|---|
| Journal | Free Radical Biology & Medicine |
| Volume | 37 |
| Issue number | 9 |
| Pages (from-to) | 1479-89 |
| Number of pages | 11 |
| ISSN | 0891-5849 |
| DOIs | |
| Publication status | Published - 1 Nov 2004 |
| Externally published | Yes |
Keywords
- Animals
- Cattle
- Chromatography, High Pressure Liquid
- Crystallins
- Hydrolysis
- Kinetics
- Kynurenine
- Lens, Crystalline
- Nitrogen
- Oxygen
- Peptide Fragments
- Photolysis
- Photosensitizing Agents
- Protein Binding
- Ultraviolet Rays