Electron paramagnetic resonance and spin trapping investigations of the photoreactivity of human lens proteins

Oxidation of cysteine, glutathione and ascorbate by photoexcited proteins f rom normal and cataractous lenses was investigated using electron paramagne tic resonance in combination with spin trapping. We report that illuminatio n of these proteins in pH 7 buffer with light >300 nm in the presence of th iols (RSH) and a spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), afforde d DMPO/.S-cysteine and DMPO/.SG adducts, suggesting the formation of the co rresponding thiyl radicals. In a nonbuffered aqueous solution, illumination of the proteins and glutathione also produced superoxide detected as a DMP O/.O2H adduct. Irradiation of these proteins in the presence of ascorbate g enerated ascorbate radical, We conclude that chromophores present in the na tural normal and cataractous lenses are capable of initiating photooxidativ e processes involving endogenous thiols and ascorbic acid. This observation may be pertinent to UV-induced development of cataract.