UV absorption by uric acid in diurnal bird aqueous humor

PURPOSE. To analyze the components responsible for the UV absorbance in diu rnal bird aqueous humor. METHODS. The absorbance studies were carried out using a Hitachi spectropho tometer (U 2000). Uric acid was determined by high-performance liquid chrom atography (LC-10 system; Shimadzu, Kyoto, Japan). Chicken and turkey eyes w ere examined. RESULTS. The UV absorbance in chicken aqueous was largely accounted for by the presence of protein, tryptophan, tyrosine, ascorbic acid, and uric acid . Ascorbic acid was low (23 mu mol/l). Uric acid was, on the other hand, re markably high (151 mu mol/l) compared with that in mammals (cattle, 16 mu m ol/l). Principally the same results were obtained in chicken and turkey. CONCLUSIONS. Uric acid is a significant UV-absorbing substance in the aqueo us humor of diurnal birds with its peak absorbance at 292 nm. The hypothesi s that the aqueous humor acts as a UV filter seems to be valid also for the avian eye. However, in these eyes uric acid fulfills the role that ascorbi c acid does in mammals.