Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid.

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid. Free Radic Biol Med. 2017 Oct 09;: Authors: Sormacheva ED, Sherin PS, Tsentalovich YP Abstract Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins, leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315-400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA•(-) and NTrp• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA•(-) to NTrp• with the formation of oxidized NTrp species a...
Source: Free Radical Biology and Medicine - Category: Biology Authors: Tags: Free Radic Biol Med Source Type: research