Human cataractous lenses contain cross-links produced by crystallin-derived tryptophanyl and tyrosyl radicals

Abstract

Protein insolubilization, cross-linking and aggregation are considered critical to the development of lens opacity in cataract. However, the information about the presence of cross-links other than disulfides in cataractous lenses is limited. A potential role for cross-links produced from tryptophanyl radicals in cataract development is suggested by the abundance of the UV light-sensitive Trp residues in crystallin proteins. Here we developed a LC-MS/MS approach to examine the presence of Trp−Trp, Trp−Tyr and Tyr−Tyr cross-links and of peptides containing Trp−2H (−2.0156 Da) in the lens of three patients diagnosed with advanced nuclear cataract. In the proteins of two of the lenses, we characterized intermolecular cross-links between βB2-Tyr153−Tyr104-βA3 and βB2-Trp150−Tyr139-βS. An additional intermolecular cross-link (βB2-Tyr61−Trp200-βB3) was present in the lens of the oldest patient. In the proteins of all three lenses, we characterized two intramolecular Trp−Trp cross-links (Trp123−Trp126 in βB1 and Trp81−Trp84 in βB2) and six peptides containing Trp −2H residues, which indicate the presence of additional Trp−Trp cross-links. Relevantly, we showed that similar cross-links and peptides with modified Trp−2H residues are produced in a time-dependent manner in bovine β-crystallin irradiated with a solar simulator. Therefore, different crystallin proteins cross-linked by crystalline-derived tryptophanyl and tyrosyl radicals are present in advanced nuclear cataract lenses and similar protein modifications can be promoted by solar irradiation even in the absence of photosensitizers. Overall, the results indicate that a role for Trp−Tyr and Trp−Trp cross-links in the development of human cataract is possible and deserves further investigation.