UVA irradiation of the lens – a damaging factor or a healing tool?

Abstract

Abstract: : Purpose: The yellowing of the lens due to post–translational modifications of proteins is a typical event during aging and in some types of cataract. Previous publications from this laboratory demonstrated that anaerobic irradiation of aged and cataractous lenses with UVA light causes significant bleaching of their coloration and that ascorbic acid (Asc) is an important participant in this process. The purpose of the present study was to investigate the photochemical changes of the two major UVA–absorbing post–translational modifications known in the lens – OP–lysine (1) and argpyrimidine (2). Methods: The bleaching of OP–lysine was studied using aged human lense pairs. One of the lenses from each pair was assigned as a dark control and the other one was irradiated with UVA light in presence of artificial aqueous humor. Solutions of OP–lysine (1), argpyrimidine (2) and OP–phenethylamine (3) in 100 mM phosphate buffer were irradiated with UVA light. The bleaching of the starting compounds was followed by UV spectroscopy. The products obtained after irradiation of OP–phenethylamine (3) was analyzed by HPLC, NMR–spectroscopy and mass spectrometry. Results: The irradiation of aged human lenses with UVA resulted in a decrease of OP–lysine content compared to the corresponding dark controls. This was in agreement with the decrease in purfied OP–lysine solutions due to UVA irradiation in the presence of Asc. In contrast argpyrimidine was relatively stable under those conditions. Surprisingly the major product of OP–phenethylamine (3) after irradiation in presence of Asc was phenethylamine (4). Conclusions: UVA can destroy some post–translational modifications like OP–lysine while others like argpyrimidine are more stable. The restoration of a free amino group in phenethylamine (4) due to irradiation of OP–phenethylamine (3) in presence of Asc was an unexpected and remarkable result. If such transformation could be performed in a living lens it would result in elimination of some of the advanced glycation endproducts with restoration of the lysine residues in lens. This opens an opportunity for healing of aged or cataractous lenses if appropriate technical means are available.  

Keywords: radiation damage: light/UV • aging: visual performance • protein modifications–post translational