With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract.
Photochemical reactions involving kynurenines, viz., molecules present in the eye lens, can result in modifications of the lens proteins and cause a development of a cataract. The rate constants of the reactions of photoexcited kynurenine with several amino acids and antioxi dants contained in the lens were measured. The most efficient quenchers of triplet kynurenine are amino acids tryptophan and tyrosine, as well as antioxidant ascorbate. In all cases, the quenching reaction proceeds by the electron transfer mechanism, except for the reaction with oxygen where transfer of the triplet energy to the oxygen molecule occurs.Cataract is a progressive clouding of the natural eye lens preventing light transmission to the retina. Most fre quently, the cataract develops in persons more than 60 years old under unfavorable environmental factors and because of poor genes or metabolic dysfunctions in the eye lens. One of the main risk factors for cataract devel opment is the ultraviolet irradiation followed by the chemi cal transformations in the eye tissues. 1 The development of cataract is related to the oxidative processes in the eye lens involving tryptophan metabolites, viz., kynurenines. Kynurenines (1) are structurally similar to natural amino acids and characterized by the absorption band at 300-400 nm and short fluorescence times. 2-9 Therefore, in the eye lens kynurenines act as light filters protecting the retina from UV radiation and improving visual acuity by decreasing the chromatic aberrations. 10 However, the concentration of UV filters in the eye lens decreases with ag ing. 6 The lenses of the human eye turn yellow, 11 and the UV spectra of the proteins of aged eye lenses are similar to the ab sorption spectra of the UV filter compounds. 8 Cataract develop
The photophysics and photochemistry of kynurenine (KN) covalently bound to the amino acids lysine, cysteine, and histidine, the antioxidant glutathione, and the protein lysozyme have been studied by optical spectroscopy with femto- and nanosecond time resolution. The fluorescence quantum yield of the adducts of KN to amino acids is approximately 2 times higher than that of the free KN in solution; KN attached to protein exhibits a 7-fold increase in the fluorescence quantum yield. The S(1) state dynamics of KN-modified lysozyme reveals a multiphasic decay with a broad dispersion of time constants from 1 ps to 2 ns. An increase of the triplet yield of KN bound to lysozyme is also observed; the triplet state undergoes fast intramolecular decay. The obtained results reveal an increase of the photochemical activity of KN after its covalent attachment to amino acids and proteins, which may contribute to the development of oxidative stress in the human lenses-the main causative factor for the cataract onset.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.