Advanced glycation endproducts induce photocrosslinking and oxidation of bovine lens proteins through type-I mechanism

Advanced glycation endproducts (AGEs) have been suggested as photosensitizers that are capable of mediating eye lens photo-damage during aging. In the present work, we investigate the photo-crosslinking and oxidation of bovine lens proteins sensitized by AGEs, with special regard to low oxygen conditions. A mechanistic study was conducted using different oxygen concentrations and specific additives with the aim either to scavenge or enhance Type-I or Type-II photoprocesses. Quantum yields for Trp decomposition were determined at 5%, 20% and 100% O₂, in the presence of ferricyanide and D₂O to elucidate the mechanism of action of AGEs. Type-I mechanism proved to be the most efficient pathway for AGE-sensitized Trp decomposition at low oxygen concentration. Photocrosslinking of lens proteins and crystallin fractions due to Type-I interaction was observed. The influence of the oxygen concentration and additives was also studied. The results show that both Type-I mechanism and oxygen-mediated reactions contribute to protein crosslinking. Carbonyl group formation due to protein photo-oxidation was detected with Oxyblot technique. The generation of high levels of hydrogen peroxide during the irradiations was detected and attributed mainly to Type-I reactions. The results support that AGEs act preferentially as Type-I sensitizers at the low oxygen concentration found in the lens and are capable of inducing protein crosslinking, oxidation and peroxide formation.