Tryptophan-derived ultraviolet filter compounds covalently bound to lens proteins are photosensitizers of oxidative damage

TY - JOUR

T1 - Tryptophan-derived ultraviolet filter compounds covalently bound to lens proteins are photosensitizers of oxidative damage

AU - Mizdrak, Jasminka

AU - Hains, Peter G

AU - Truscott, Roger J W

AU - Jamie, Joanne F

AU - Davies, Michael Jonathan

PY - 2008/3/15

Y1 - 2008/3/15

N2 - The human eye is chronically exposed to light of wavelengths >300 nm. In the young human lens, light of wavelength 300-400 nm is predominantly absorbed by the free Trp derivatives kynurenine (Kyn), 3-hydroxykynurenine (3OHKyn), and 3-hydroxykynurenine-O-beta-D-glucoside (3OHKynG). These ultraviolet (UV) filter compounds are poor photosensitizers. With age, the levels of the free UV filters in the lens decreases and those of protein-bound UV filters increases. The photochemical behavior of these protein-bound UV filters and their role in UV damage are poorly elucidated and are examined here. UVA illumination of protein-bound UV filters generated peroxides (principally H2O2) in a metabolite-, photolysis-time-, and wavelength-dependent manner. Unmodified proteins, free Trp metabolites, and Trp metabolites that do not bind to lens proteins gave low peroxide yields. Protein-bound 3OHKyn (principally at Cys residues) yielded more peroxide than comparable Kyn and 3OHKynG adducts. Studies using D2O and sodium azide implicated 1O2 as a key intermediate. Illumination of the protein-bound adducts also yielded protein-bound Tyr oxidation products (DOPA, di-tyrosine) and protein cross-links via alternative mechanisms. These data indicate that the covalent modification of lens proteins by Kyn derivatives yields photosensitizers that may enhance oxidation in older lenses and contribute to age-related nuclear cataract.

AB - The human eye is chronically exposed to light of wavelengths >300 nm. In the young human lens, light of wavelength 300-400 nm is predominantly absorbed by the free Trp derivatives kynurenine (Kyn), 3-hydroxykynurenine (3OHKyn), and 3-hydroxykynurenine-O-beta-D-glucoside (3OHKynG). These ultraviolet (UV) filter compounds are poor photosensitizers. With age, the levels of the free UV filters in the lens decreases and those of protein-bound UV filters increases. The photochemical behavior of these protein-bound UV filters and their role in UV damage are poorly elucidated and are examined here. UVA illumination of protein-bound UV filters generated peroxides (principally H2O2) in a metabolite-, photolysis-time-, and wavelength-dependent manner. Unmodified proteins, free Trp metabolites, and Trp metabolites that do not bind to lens proteins gave low peroxide yields. Protein-bound 3OHKyn (principally at Cys residues) yielded more peroxide than comparable Kyn and 3OHKynG adducts. Studies using D2O and sodium azide implicated 1O2 as a key intermediate. Illumination of the protein-bound adducts also yielded protein-bound Tyr oxidation products (DOPA, di-tyrosine) and protein cross-links via alternative mechanisms. These data indicate that the covalent modification of lens proteins by Kyn derivatives yields photosensitizers that may enhance oxidation in older lenses and contribute to age-related nuclear cataract.

KW - Aging

KW - Animals

KW - Cattle

KW - Chromatography, High Pressure Liquid

KW - Crystallins

KW - Electrophoresis, Polyacrylamide Gel

KW - Hydrogen Peroxide

KW - Lens, Crystalline

KW - Oxidative Stress

KW - Tryptophan

KW - Ultraviolet Rays

U2 - 10.1016/j.freeradbiomed.2007.12.003

DO - 10.1016/j.freeradbiomed.2007.12.003

M3 - Journal article

C2 - 18206985

SN - 0891-5849

VL - 44

SP - 1108

EP - 1119

JO - Free Radical Biology & Medicine

JF - Free Radical Biology & Medicine

IS - 6

ER -