In primates UV light is filtered by the lens and cornea so that only visible light (lambda more than 400nm) is transmitted to the retina. The main absorbers of radiation in the young primate lens are low molecular weight metabolites of tryptophan including kynurenine, 3- hydroxykynurenine and 3-hydroxykynurenine-O-glucoside, which act as filters. In the retina, the two principal chromophores are the retinoids and lipofuscin granules of the retina pigment epithelium (RPE) that accumulate during aging.
The aim of this project is to understand the photochemistry of all these components so that appropriate strategies can be developed to ameliorate or prevent light-induced damage. UV irradiation of kynurenine (KN) reduces oxygen to superoxide, and photo- oxidizes cysteine, NADH and ascorbate. In addition, KN undergoes photodegradation to products which are more photo-active than KN itself. These findings indicate that the kynurenines are capable of photo- initiating free radical reactions in vivo and that oxygen is an important factor determining the yields of free radicals induced by lenticular chromophores. UV irradiation of retinoids (retinal, retinol and retinylpalmitate) results in hydrogen atom (or electron) abstraction from suitable substrates. In contrast to previous reports which used chemical methods, direct detection of singlet oxygen (via luminescence at 1270nm) indicate that the quantum yield for retinal and retinol are quite low, 0.05 and 0.03 respectively. These findings suggest that retinal components can be damaged via hydrogen atom (or electron) extraction forming two reactive species, the oxidized substrate radical(s) and the reduced retinoid radical(s). Active oxygen species do not appear to be important. The photochemistry of other lens components 3-OH-kynurenine and 3-OH-kynurenine-O-glycoside will be studied. The photoproducts derived from the kynurenines will be identified and their photochemistry studied. The components of RPE lipofuscin will be separated, identified and their photochemistry evaluated. The photochemistry of anti-malarial drugs that bind to eye melamin and lens protein will also be elucidated.
