Ascorbate enhances photogeneration of hydrogen peroxide mediated by the iris melanin.

The iris in the human eye is exposed to UV and visible light transmitted by the cornea. This pigmented tissue is bathed with the aqueous humor (AqH), which contains high concentration of ascorbate. It has been postulated that the presence of ascorbate in AqH can contribute to increased photoproduction of H2O2 mediated by the iris melanin. In this study, we monitored generation of H2O2 induced by UV-VIS irradiation of bovine irides, bovine and human iris homogenates and iris melanin. Our data show that exogenous ascorbate significantly amplified the rate of H2O2 photoformation in all melanotic samples. Deactivation of endogenous catalase with NaN3 in bovine irides increased the level of the accumulated H2O2 in the bathing solution following sample irradiation. Photoformation of H2O2 in samples with exogenous ascorbate was accompanied by its photo-oxidation. Both photoprocesses exhibited significant wavelength dependence. EPR spectroscopy measurements showed that ascorbyl radical is an intermediate product of the ascorbate photo-oxidation. The photoproduction of H2O2 and photo-oxidation of ascorbate appear to be stoichiometric processes. No significant differences in the photoreactivity of iridial melanin from donors of different age and iris color was found. We postulate that also in vivo ascorbate increases the rate of iris melanin-mediated photoformation of H2O2 and its steady state concentration in AqH.