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B. J. Ortwerth, J. Bhattacharyya; The Presence of Ultraviolet a Light-Responsive Sensitizers in the Low Molecular Weight Fractions of Various Lenses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2424.
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To determine whether there are low molecular weight (LMW) compounds in the lenses of various species that are capable of absorbing UVA light and acting as sensitizers to cause the oxidation of ascorbic acid.
Lenses from different species were decapsulated, homogenized and separated into WS and WI fractions. Each fraction was filtered through a 5kD filter and both the LMW and the protein components were analyzed for the ability to oxidize ascorbic acid. Aliquots were irradiated with UVA light in the presence of 0.1 mM ascorbic acid and the extent of ascorbic acid oxidation was measured by the decrease in absorbance at 265nm over 30 min. Measurements were made on at least two preparations in duplicate or triplicate. Protein levels were measured by the BCA method and the results expressed as nmoles of ascorbate oxidized/mg of lens protein in the whole lens homogenate.
Bovine lens showed little activity in any of the four fractions, however, 31 year old human lens exhibited considerable activity in the LMW fraction. In 68 year old human lens the LMW activity decreased markedly, but corresponded to a 7-fold increase in the activity of the WI protein fraction. A similar distribution of activity was seen in Type I cataracts obtained from India. Little or no ascorbic acid hydrolyzing- activity was seen in the protein fractions from guinea pig, rat and rabbit lenses, but high activity was present in the LMW fraction from these lenses. This activity correlated with the published content of NADPH in these lenses, and in vitro studies with pyridine nucleotides showed high activity with both NADH and NADPH, suggesting these could be significant UVA sensitizers in these lenses.
UVB light causes the development of cataracts in rodents and has been implicated in human cataract. UVA light is 30-fold higher in sunlight that reaches the earth and is roughly 1000-fold greater than UVB light that reaches the lens, where it is all absorbed . The protein modifications present in aged human lenses and in cataracts can be mimicked by glycating calf lens proteins with ascorbic acid oxidation products for 4 weeks. The role of UVA light, therefore, may be to initiate the oxidation of ascorbic acid leading to advanced glycation endproduct formation on lens proteins. The data here show high levels of this activity in the WI protein fraction from aged human lens and cataracts, consistent with this idea. UVA sensitizers in rodent and rabbit lenses are present in the LMW fraction only, and may correlate with the levels of NADPH in these lenses.
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