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BJ Ortwerth, V Chemoganskiy, PR Olesen; The Oxidation Of Ascorbic Acid Under Anaerobic Conditions By The UVA Irradiation Of Human Lens Yellow Proteins . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3574.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the extent to which UVA light can cause the anaerobic degradation of ascorbic acid (Asc) by aged human lens proteins, and whether the products formed are capable of glycating lens proteins. Methods: A sonication-solubilized WI fraction from aged human lenses was irradiated anaerobically with UVA light (338 nm cutoff filter, 200 mW/cm2) in the presence of ascorbic acid. Ascorbic acid levels were followed both by the loss of A265 and by HPLC analysis. Glycation was measured by the UVA-induced incorporation of [U-14C]Asc into lens proteins by the filter paper disc method. Results: UVA irradiation of anaerobic solutions containing 1.0 mg/ml of WI proteins from aged human lens and 0.05 mM Asc caused the loss of 20 nmoles of Asc in 20 minutes, and almost complete loss after one hour. The WS fraction exhibited less than half of this acitivity. A WS fraction from young human lenses had only slight activity, and no loss of Asc was seen in any of the dark controls. The action spectrum for Asc destruction showed maximum activity in the UVA region of the spectrum and correlated with the action spectrum for chromophore bleaching. Irradiation of whole human lenses for 2 hours produced a 37% decrease in Asc content, and 3-4 oxidation products were detected. When the WI proteins from aged human lenses were irradiated with [U-14C]ascorbate for 2 hours followed by 3 days of dark incubation, a marked increase in the incorporation of Asc into protein was observed, when compared to the unirradiated control. The irradiation of whole lenses also produced ascorbate labeling of protein, presumably due to glycation by the oxidation products of ascorbic acid. Conclusions: Limiting oxygen levels in the lens need not preclude the glycation of lens proteins by Asc, because UVA light is able to cause both the degradation of Asc and the glycation of lens proteins even in the absence of oxygen. The observed Asc oxidation likely represents the transfer of electrons from Asc to the triplet state of the yellow sensitizers in aged human lenses resulting in chromophore bleaching.
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