TY - JOUR

T1 - Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

AU - Sormacheva, Ekaterina D.

AU - Sherin, Peter S.

AU - Tsentalovich, Yuri P.

N1 - Publisher Copyright: © 2017 Elsevier Inc.

PY - 2017/12/1

Y1 - 2017/12/1

N2 - 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 (315–400 nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human 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 and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA• − radicals with the rate constant of (2.0 ± 0.2) × 109 M−1 s−1, leading to the restoration of KNA and the formation of superoxide radical O2 • −. The latter reacts with NTrp• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

AB - 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 (315–400 nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human 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 and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA• − radicals with the rate constant of (2.0 ± 0.2) × 109 M−1 s−1, leading to the restoration of KNA and the formation of superoxide radical O2 • −. The latter reacts with NTrp• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

KW - Ditryptophan crosslinks

KW - Kynurenic acid

KW - Oxidation

KW - Radicals

KW - Tryptophan

KW - UV-A light

KW - XANTHURENIC ACID

KW - CROSS-LINKING

KW - N-FORMYLKYNURENINE

KW - MASS-SPECTROMETRY

KW - ALPHA-CRYSTALLIN

KW - SINGLET MOLECULAR-OXYGEN

KW - AMINO-ACIDS

KW - HUMAN EYE

KW - IN-VIVO

KW - CATARACTOUS HUMAN LENSES

KW - Electron Transport

KW - Photosensitizing Agents/chemistry

KW - Chemical Fractionation/methods

KW - Oxygen/chemistry

KW - Ultraviolet Rays

KW - Photolysis

KW - Cattle

KW - Lens, Crystalline/chemistry

KW - Singlet Oxygen/chemistry

KW - Dimerization

KW - Oxidation-Reduction

KW - Tryptophan/analogs & derivatives

KW - alpha-Crystallins/chemistry

KW - Animals

KW - Kynurenic Acid/chemistry

KW - Superoxides/chemistry

UR - http://www.scopus.com/inward/record.url?scp=85032176896&partnerID=8YFLogxK

U2 - 10.1016/j.freeradbiomed.2017.10.007

DO - 10.1016/j.freeradbiomed.2017.10.007

M3 - Article

C2 - 29024806

AN - SCOPUS:85032176896

VL - 113

SP - 372

EP - 384

JO - Free Radical Biology and Medicine

JF - Free Radical Biology and Medicine

SN - 0891-5849

ER -