The identification of uric acid as the major reducing agent on the corneal surface

Aim: To identify the compound accountable for the high reducing power lining the surface of the corneal epithelium. Materials and methods: Chinoy’s histochemical method was used to demonstrate the occurrence of a high reducing agent on the corneal surface. Ascorbic and uric acid concentration in the tear and cornea were determined by highpressure liquid chromatography using a μ-Bondapak-NH 2 column eluted by 10 mM ammonium phosphate. Results: A solid black stain was observed on the surface of a cornea after incubating a rabbit eye in a fixative with silver nitrate. Uric acid concentration in the epithelium was much higher than in the stroma and endothelium. A supersaturated uric acid concentration was observed in human tears. The concentration of uric acid was much higher than that of ascorbic acid. Mixing ascorbic acid with silver nitrate produced fine silver granules. Uric acid and silver nitrate formed coarse precipitate. Conclusion: Uric acid is the major reducing agent on the surface of the cornea. Uric acid on the corneal surface may have a role in preventing oxidative damage by air.