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B. Rozanowski, M. Rozanowska; Degradation Products of Xanthophyll Include Blue–Light Absorbing Photosensitizers . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3032.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Carotenoids are potent antioxidants that absorb blue light, quench excited triplet states of photosensitizers and singlet oxygen, and scavenge free radicals. However, as a result of their antioxidant action, carotenoids undergo degradation. In case of ß–carotene, its degradation results in the formation of a potent photosensitizer all–trans retinal. The aim of our study was to determine whether degradation of carotenoids accumulating in the retina – xanthophylls containing two hydroxyl groups, leads to the formation of products of photosensitizing properties, and if so, to evaluate their ability to photogenerate singlet oxygen. Methods: In order to model physiologically relevant conditions xanthophyll (Sigma) was incorporated into egg yolk phosphatidylcholine liposomes and degraded by the exposure to blue light (406–490 nm; 2.6 mW/cm2) and/or iron ions in the presence or absence of hydrogen peroxide. Xanthophyll depletion and formation of degradation products was monitored by spectrophotometry after liposomes extraction with organic solvents. Nanosecond laser flash photolysis combined with kinetic absorption spectroscopy was used to monitor transient products formed in the mixture of xanthophyll and its degradation products solubilized in acetone. Photosensitized formation and decay of singlet oxygen was monitored by time–resolved detection of characteristic singlet oxygen phosphorescence at 1270 nm after 5 ns laser pulse of light of selected wavelength from the range 415 – 490 nm. Results:Irradiation of xanthophylls with blue light for up to 8 h led to a slow gradual decrease in xanthophyll concentration. Exposure of xanthophyll in liposomes to iron ions led to four times faster degradation of xanthophyll and after 8 h exposure only 50% of xanthophyll was remaining. The rate of xanthophyll degradation was the greatest during concomitant exposure to blue light and iron ions. During degradation several different products were formed absorbing at shorter wavelengths than xanthophyll. Photoexcitation of the degradation products in the presence of native xanthophyll resulted in the photosensitized generation of a xanthophyll triplet state, while in the presence of oxygen – in photosensitized generation of singlet oxygen. The yields of singlet oxygen generation were increasing with the progress of xanthophyll degradation. Conclusions:Xanthophylls are highly susceptible to (photo)oxidative degradation and as a result potent photosensitizers are generated which may mediate blue light–induced oxidative damage to the retina.
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