Mechanisms that lead to damage of the human lens during aging

Includes bibliographical references (pages [276]-284)The transparency of the lens is important for proper vision. Because there is little protein turnover in the lens, modification to the protein accumulates over many years and may lead to cataract formation. Approximately 95% of Americans over 65 years old have some type of cataract formation. Cataracts are also the cause of 42% of blindness worldwide. During aging, the lens protein undergoes yellowing, cleavage, an increase in fluorescence, and a decrease in solubility. Concomitantly, the concentrations of several antioxidants and the tryptophan metabolites, kynurenine, 3-hydroxykynurenine (3-HK) and 3-hydroxykynurenine glucoside (3-HKG), decrease. Several groups have proposed that 3-HK or 3-HKG, which protect the retina by absorbing UV light, may become covalently attached to the protein. The yellowed protein may cause further damage in the lens through photosensitization reactions. The mechanisms of lens protein modification are not known, but likely reactions involve light, reactive oxygen species, protein glycation, and decreases in enzyme activity. The mechanisms involved in aging of the lens are complex; therefore, simple models have been used in this work. α-Crystallin, the most abundant lens protein, and 3-HK have been irradiated with UV light under conditions similar to those in the lens to study the mechanisms of photochemical damage. Changes in absorption, fluorescence and of the transient excited states of photochemically modified α-crystallin are similar to those found in cataractous lens protein. An absorbed dose of light has been correlated with an apparent age of the model and protein yellowing allowing lenticular aging to be quantified for the first time. The 3-HK mediated photolysis of α-crystallin has been found to lead to cleavage of the protein backbone and amino acid side chains which may contribute to the formation of degraded forms of the crystallins and the high molecular weight aggregates observed in vivo . Photochemical damage of glutathione reductase, an enzyme responsible for protection against oxidative damage, may also occur via excited states of its cofactors. Cleavage of 3-HKG by a β-glucosidase may contribute to damage of the lens nucleus. Once the mechanisms of damage in the lens are better understood, cataract formation may be slowed or prevented.Ph.D. (Doctor of Philosophy