UVA Light-excited Kynurenines Oxidize Ascorbate and Modify Lens Proteins through the Formation of Advanced Glycation End Products: IMPLICATIONS FOR HUMAN LENS AGING AND CATARACT FORMATION

Mikhail Linetsky,Cibin T Raghavan,Kaid Johar,Xingjun Fan,Vincent M Monnier,Abhay R Vasavada,Ram H Nagaraj

doi:10.1074/jbc.m114.554410

Mikhail Linetsky, Cibin T Raghavan + Show 5 more

Open Access

https://doi.org/10.1074/jbc.m114.554410

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Abstract

Advanced glycation end products (AGEs) contribute to lens protein pigmentation and cross-linking during aging and cataract formation. In vitro experiments have shown that ascorbate (ASC) oxidation products can form AGEs in proteins. However, the mechanisms of ASC oxidation and AGE formation in the human lens are poorly understood. Kynurenines are tryptophan oxidation products produced from the indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway and are present in the human lens. This study investigated the ability of UVA light-excited kynurenines to photooxidize ASC and to form AGEs in lens proteins. UVA light-excited kynurenines in both free and protein-bound forms rapidly oxidized ASC, and such oxidation occurred even in the absence of oxygen. High levels of GSH inhibited but did not completely block ASC oxidation. Upon UVA irradiation, pigmented proteins from human cataractous lenses also oxidized ASC. When exposed to UVA light (320-400 nm, 100 milliwatts/cm(2), 45 min to 2 h), young human lenses (20-36 years), which contain high levels of free kynurenines, lost a significant portion of their ASC content and accumulated AGEs. A similar formation of AGEs was observed in UVA-irradiated lenses from human IDO/human sodium-dependent vitamin C transporter-2 mice, which contain high levels of kynurenines and ASC. Our data suggest that kynurenine-mediated ASC oxidation followed by AGE formation may be an important mechanism for lens aging and the development of senile cataracts in humans.

Highlights

Human lens proteins accumulate pigmented, protein-cross-linked Advanced glycation end products (AGEs) adducts during cataract formation, and the mechanisms of their formation are poorly understood
The purpose of this study was to investigate the hypothesis that kynurenine-mediated ASC oxidation leads to AGE formation in the lens proteins
This hypothesis was based on the fact that human lenses are chronically exposed to UVA light from the sun and ambient UVA light, and UVA sensitizing kynurenines are present in significant quantities in free and protein-bound forms in the lens

Summary

Introduction

Human lens proteins accumulate pigmented, protein-cross-linked AGE adducts during cataract formation, and the mechanisms of their formation are poorly understood. Significance: This study provides a mechanism for UVA light-mediated damage to lens proteins during cataract formation. Advanced glycation end products (AGEs) contribute to lens protein pigmentation and cross-linking during aging and cataract formation. The mechanisms of ASC oxidation and AGE formation in the human lens are poorly understood. When exposed to UVA light (320 – 400 nm, 100 milliwatts/cm min to 2 h), young human lenses (20 –36 years), which contain high levels of free kynurenines, lost a significant portion of their ASC content and accumulated AGEs. A similar formation of AGEs was observed in UVA-irradiated lenses from human IDO/human sodium-dependent vitamin C transporter-2 mice, which contain high levels of kynurenines and ASC.

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