DECREASE IN VITAMIN-C CONCENTRATION IN HUMAN LENSES DURING CATARACT PROGRESSION

Cataract formation is believed to result from an oxidative insult whic h decreases the antioxidant defense of the lens, particularly the vita min C concentration. Upon oxidation, vitamin C contributes with glucos e to protein glycation, It also favours tryptophan oxidation, resultin g in fluorescent peptide cross-links and protein insolubilisation. The relationship between cataract and lenticular vitamin C was analysed i n 48 cataractous lens nuclei classified into four severity grades, con sidering the sum of the colour and opacity. Ascorbic and dehydroascorb ic acids were quantified by HPLC-fluorescence. The Amadori product was measured by means of furosine, advanced glycation endproducts by thei r fluorescence and tryptophan concentration by HPLC UV. The lens vitam in C concentration significantly decreased with cataract severity, but mostly in severe brown cataracts (around 88 mu mol/100 g lens in mild cataracts, and 50 mu mol/100 g in dark brown lenses). The dehydroasco rbic acid concentration was always low and stable (1.9 +/- 0.9 mu mol/ /100 g), as was the furosine concentration (0.4 +/- 0.1 mu mol/g). The fluorescence of insobuble advanced glycated end products was signific antly higher in severe cataracts than in milder ones. The peptide tryp tophan content was stable but the tryptophan to tyrosine ratio decreas ed and was highly correlated to the ascorbic acid concentration. Vitam in C content appears to be a good indicator of cataract severity, sugg esting that oxidation could take parr in cataract progression.