Photodegradation of Tryptophan Residues and Attenuation of Molecular Chaperone Activity in α-Crystallin Are Correlated

Abstract

The major eye-lens protein α-crystallin is known to possess a remarkable sequence homology to the low molecular weight heat-shock proteins and has been shown to protect several proteins against thermally induced aggregation. In this work we demonstrate that the rapid aggregation of rabbit muscle phosphoglycerate kinase during incubation at 52°C is completely inhibited in presence of 1/3 moles α-crystallin monomer per mole enzyme. Upon irradiation by UV light, tryptophan fluorescence intensity of α-crystallin declines, reflecting the destruction of these residues. A remarkable correlation is revealed between the reduction in α-crystallin fluorescence during UV-irradiation and the loss of its ability to protect phosphoglycerate kinase against aggregation. Since a loss of tryptophan fluorescence in intact eye lenses in vivo has been demonstrated to occur upon exposure to UV light, as well as during aging, it is proposed that the enhanced rate of lens opacification and cataract formation, as well as the increased levels of damaged lens proteins, which accumulate under these conditions, are the result of the gradual loss of the chaperone-protein efficacy of α-crystallin.