Dityrosine formation in the proteins of the eye lens

The presence of dityrosine crosslinks in the proteins of the lens is a subject of some debate. We have investigated the formation of dityrosine in the lens proteins, the crystallins, through reactions mediated by reactive oxygen species, as well as through direct photolysis of proteins in the UVB region. Multiple methods were used to identify dityrosine. These include amino acid analysis of protein hydrolysates, quenching of the fluorescence at 400-410 nm by borate/boric acid solutions specific for dityrosine, and inhibition of formation of dityrosine in the presence of dithiothreitol. We find that reaction of the crystallins, alpha (alpha), beta (beta) and gamma (gamma), with singlet oxygen (1O2), hydroxyl radicals (.OH) and hydrogen peroxide (H2O2), does not result in the formation of either intermolecular or intramolecular dityrosine. It appears that the formation of dityrosine involves a radical mediated electron transfer reaction. Consistent with this, direct photolysis in the UVB region produces intramolecular dityrosine in all three proteins, with the efficiency varying in the order gamma - > beta - > alpha. Since the levels of electron transfer agents (e.g., Fe ions) in the lens are usually negligible, and since the amount of UVB radiation reaching the gamma--crystallin--rich lens nucleus is normally quite low, the changes of dityrosine formation in the crystallins in vivo appear remote.