An antioxidative role of ocular screening pigments

The action of ocular screening pigments of vertebrates (melanins) as well as those of invertebrates (ommochromes) on lipid peroxidation has been studied. Lipid peroxidation has been induced by one of the following systems: Fe2+ + ascorbic acid; Fe2+ + NADPH + liver microsomes; xanthine + xanthine oxidase; u.v. illumination; intense visible light, high concentration of O2. Measurements of the lipid peroxidation rate, as estimated from the accumulation of malonic dialdehyde, showed a sharp decrease of the lipid peroxidation rate in the presence of either melanosomes or ommochromes. Synthetic DOPA melanin was also found to exert a strong inhibiting effect on lipid peroxidation. A comparative study of lipid peroxidation in retinal pigment epithelium (RPE) of pigmented and albino rabbits demonstrated that the latter tissue is more sensitive to the effect of the above mentioned prooxidant systems. Apparently this finding is related to the presence of melanin-containing granules in the pigmented tissue rather than to differences in efficiency of other endogenous antioxidant systems. The activities of superoxide dismutase and glutathione peroxidase are practically equal in the RPE of pigmented and albino rabbits whereas the alpha-tocopherol content is higher in albinos. Possible mechanisms of inhibition of lipid peroxidation by melanosomes and ommochromes are discussed. It is proposed that their antioxidant function is one of the most important physiological features of melanins (vertebrate eye) and ommochromes (invertebrate eye).