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D Balasubramanian, G Thiagarajan, E Shirao, K Ando, Y Shirao, A Inoue; What Is The Role Of Xanthurenic Acid 8-O-beta-Glucoside- A Pigment That Accumulates In The Brunescent Human Lens? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):846.
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Purpose:The human lens receives far more UVA light (320-400 nm) than UVB or UVC (<320 nm). Light-induced damage to the lens is thus more likely to occur through endogenous UVA chromophores that can act as sensitizers. The lens is known to accumulate several metabolites of the kynurenine, hydroxyquinoline and b-carboline families. Their photodynamic properties are thus of cataractogeic interest. We have studied those of kynurenine (Ky), 3-hydroxykynurenine (3HK) and its glucoside (3HKG), xanthurenic acid (XA) and its 8-O-b-glucoside (XAOG). We have recently detected XAOG in human lenses and have identified it to be responsible for the brunescent color of nondiabetic lenses (Exp. Eye Res., in press). Methods:Each of these compounds was irradiated at its absorption maximum in the UVA and the generation of reactive oxygen species (ROS) monitored. Singlet oxygen was detected by the RNO, and superoxide by the ferricytochrome assay methods. The fluorescence lifetimes and quantum yields were also measured. Their photodynamic ability to cause covalent damage to target proteins was followed by SDS-PAGE. Results:Ky, 3HK and 3HKG are found to be photodynamically inert; they do not generate any ROS nor effect any covalent damage to proteins upon irradiation. Their fluorescence lifetimes appear too fast to do so. XA is found to be an efficient sensitizer (emission lifetime 6 ns) that produces both singlet oxygen and superoxide. It also photodynamically crosslinks and degrades test proteins. XAOG too is seen to be a sensitizer (major emission 12 ns, high quantum yield), and produces singlet oxygen, but the ferricytochrome assay did not detect superoxide generation. It also effects covalent chemical damage to test proteins, upon irradiation in the UVA. Conclusion:While 3HKG, which is known to decrease with age in the human lens, is inert, XAOG, a compound that is formed as a function of time in situ from 3HKG, is seen to be a potential photodynamic pro-oxidant that can play a role in cataractogenesis. However, it appears less harmful that its unglycosylated parent, XA.
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