Identifying sites of attachment of UV filters to proteins in older human lenses

Recent results indicate that covalent modification of proteins by tryptophan-derived UV filters may explain the age-dependent coloration of human lenses, and play a role in age-related cataract. The sites of attachment of the UV filters to the lens crystallins, however, have not been determined. This study utilized a database of predicted masses of UV filter-modified tryptic peptides to target sites of UV filter attachment. Proteins were isolated from old normal lenses and digested with trypsin at pH 6, in order to preserve the integrity of the sites of modification. Peptides were separated by high-performance liquid chromatography and characterized by mass spectrometry. Major colored and fluorescent peaks in the digest were found to correspond to cysteine-containing peptides in which the sulfur atom of the sidechain was linked to the major UV filter compound, 3-hydroxykynurenine glucoside. Three of the peptides originated from γS-crystallin and one from βB1-crystallin. These results show that a predicted mass database can be used to facilitate the identification of sites of UV filter modification in human lens crystallins. Furthermore, this work represents the first evidence that UV filters bind to specific residues on lens proteins in vivo, and suggests that sulfhydryl groups may be important sites for the attachment of UV filters.