THE EFFECTS OF NEAR‐UV RADIATION ON HUMAN LENS β‐CRYSTALLINS: PROTEIN STRUCTURAL CHANGES and THE PRODUCTION OF O2_ and H2O2

Abstract

.beta.-Crystallins (.beta.1-, .beta.2- and .beta.3-crystallin) comprise nearly half the protein of the human lens. The effect of near-UV radiation, which is one of the possible risk factors in cataract formation, on the .beta.-crystallins is investigated in this study. Protein intersubunit crosslinking, change in charge of the protein subunits to more acidic species and changes in protein tertiary structure (conformation) by 300 nm irradiation are reported. The fluorescence yield of protein tryptophan residues decreases by 300 nm irradiation. There is an increase in nontryptophan fluorescence (.lambda.cx 340 nm,.lambda.cm 400-600 nm), and in protein absorption at 340 nm, due to the formation of tryptophn photooxidation products. Both tryptophan and its oxidation products can be photoexcited by 300 nm irradiation and the latter are known to be good photosensitizers. The results provide evidence for the generation of H2O2 in the irradiated human .beta.-crystallin solutions by the Type I photosensitizing action of the chromophores absorbing at 300 nm. The H2O2 is generated via the intermediate production of O2- anion; the latter spontaneously dismutates H2O2, presumalby via O2--protein interactions. The amount of H2O2 generated per absorbed photon is compared for various solutions of .beta.1-, .beta.2- and .beta.3-crystallins from human lenses of different age.