TY - JOUR

T1 - Protein-bound kynurenine is a photosensitizer of oxidative damage

AU - Parker, Nicole R

AU - Jamie, Joanne F

AU - Davies, Michael Jonathan

AU - Truscott, Roger J W

PY - 2004/11/1

Y1 - 2004/11/1

N2 - 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.

AB - 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.

KW - Animals

KW - Cattle

KW - Chromatography, High Pressure Liquid

KW - Crystallins

KW - Hydrolysis

KW - Kinetics

KW - Kynurenine

KW - Lens, Crystalline

KW - Nitrogen

KW - Oxygen

KW - Peptide Fragments

KW - Photolysis

KW - Photosensitizing Agents

KW - Protein Binding

KW - Ultraviolet Rays

U2 - 10.1016/j.freeradbiomed.2004.07.015

DO - 10.1016/j.freeradbiomed.2004.07.015

M3 - Journal article

C2 - 15454288

VL - 37

SP - 1479

EP - 1489

JO - Free Radical Biology & Medicine

JF - Free Radical Biology & Medicine

SN - 0891-5849

IS - 9

ER -