Purpose: Human ultraviolet light (UV) filters, such as kynurenine (Kyn), readily deaminate to reactive unsaturated ketones that covalently modify proteins in older human lenses. The aim of this study was to examine in vitro rates of formation and decomposition of the three major Kyn-amino acid adducts and possible consequences for the lens.
Methods: The t-Boc-protected Kyn-His, Kyn-Lys, and Kyn-Cys adducts and Kyn-Cys were synthesized from the corresponding amino acids and Kyn. Calf lens proteins were modified with Kyn by incubation at pH 7. Stability and competition studies of the adducts were conducted under physiological conditions. Kyn-amino acids and their decomposition products were quantified using HPLC.
Results: At physiological pH, Kyn-Cys adducts formed more rapidly than either Lys or His adducts, but they also decomposed readily. By contrast, His adducts were stable. Cysteine (Cys) residues in beta-crystallins were major sites of modification. The Kyn moiety, initially bound to Cys residues, was found to transfer to other amino acids. Glutathione promoted the breakdown of Kyn-Cys.
Conclusions: These data may help explain why proteins in young lenses are not modified by UV filters in situ. The initial phase of the modification of proteins in the human lens by UV filters may be a dynamic process. In lenses, Cys residues of crystallins modify preferentially, but these adducts also decompose to release deaminated Kyn. This can then potentially react with other amino acids. Glutathione, which is present in high concentrations in the lenses of young people, may play a vital role in keeping proteins free from modification by intercepting reactive deaminated kynurenines formed by the spontaneous breakdown of free UV filters, promoting the decomposition of Kyn-Cys residues, and sequestering the unsaturated ketones once they are released from modified proteins.