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B.J. Ortwerth, R. Cheng; UVA Light-Induced Changes in Human Lens Yellow Proteins . Invest. Ophthalmol. Vis. Sci. 2003;44(13):299.
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Purpose: To determine the effect of UVA irradiation in the presence of oxygen on the absorption spectrum of aged human lens proteins, and to see whether the newly-formed HPLC peaks resemble the peaks isolated from brunescent cataracts. Methods: The water-insoluble (WI) protein fraction from aged human lenses was solubilized by sonication and irradiated with UVA light (338 nm cutoff filter, 200 mW/cm2) for a 2 h period in the presence of air, and the change in the absorption spectrum was measured over time. Similar experiments were carried out with the yellow sensitizer-enriched fraction isolated from a proteolytic digest of human lens water-insoluble (WI) proteins. During irradiation the changes in the elution profile from a Prodigy HPLC column were monitored at 330 nm. Results: UVA light in the absence of oxygen is known to cause a photobleaching of the yellow chromophores in aged human WI proteins, however, there was a 2-3 fold increase in yellow color (A330) of WI proteins irradiated in air-equilibrated solutions. Difference spectra showed a continual increase in absorbance over 2 h with absorbance maxima at 250 and 305 nm. The WI fraction from brunescent cataracts and in vitro ascorbylated calf lens proteins also exhibited the same difference spectrum, but the spectrum of calf lens proteins was unchanged by UVA. No absorbance increase was seen with a partially-purified human lens sensitizer fraction, unless Trp was added prior to the UVA irradiation. Several new HPLC peaks were observed in the presence of Trp, but not with the sensitizer fraction alone. The addition of 10 mM sodium azide, a singlet oxygen quencher, markedly diminished the formation of several of the Trp oxidation peaks. HPLC analysis of digests of the WI proteins from human lens over 60 y of age showed the presence of significant amounts of kynurenine, anthranylic acid and a compound of mass 406. Only a small amount of kynurenine, and none of the other products was formed by the UVA irradiation of the sensitizers with Trp. The profile of digested brunescent cataract proteins, however, contained several peaks that were similar to the peaks produced in the UVA irradiation of the sensitizers with Trp, suggesting the possible generation of singlet oxygen within these lenses. Conclusions: The aerobic irradiation of aged human lens proteins by UVA light caused a marked increase in browning due to the oxidation of Trp residues. Singlet oxygen, while not responsible for the formation of the Trp degradation products in digests of normal aged human lens proteins, may be a significant Trp oxidant in brunescent cataracts. The path of Trp degradation may depend upon different oxygen levels in normal and cataractous lenses.
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