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B. Li, P. S. Bernstein; The Singlet Oxygen Scavenging Mechanism of Human Macular Pigment. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2140.
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The protective function of macular pigment (MP) is thought to be performed by filtering out high-energy short-wavelength light to reduce free radical and singlet oxygen formation or by directly scavenging reactive oxygen species, particularly singlet oxygen. It is easy to understand that macular pigment can work as an optical filter since its absorption peak is around 460 nm. Up to now, however, there still is no direct evidence of MP quenching singlet oxygen. In the present work, we investigated whether or not MP can play a protective role by directly quenching singlet oxygen.
Production of singlet oxygen in macula and foveal retinal pigment epithelium (RPE) was measured by electron paramagnetic resonance (EPR) under light treatment. The yield of singlet oxygen between macula and macula residual (the portion of macula remaining after removing the MP by solvent extraction) were compared. In addition, the singlet oxygen quenching abilities of the MP were investigated by absorption spectroscopy.
Singlet oxygen was detected in foveal RPE but not in macula, macula residual, or vitreous. No singlet oxygen signal was present when macula was added to the foveal RPE sample, consistent with a mechanism whereby high levels of carotenoids in macular tissue quench singlet oxygen generated in RPE. The singlet oxygen quenching abilities of MP were compared by absorption spectroscopy, and the results were meso-zeaxanthin > zeaxanthin >lutein.
The high amounts of macular pigment in the foveal inner retina are most likely functioning as light filters since they are not in close proximity to the RPE where the bulk of singlet oxygen is generated. Lower levels of carotenoids are found endogenously in the RPE where they may partially quench locally generated reactive oxygen species.
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