The pigmentation of human iris influences the uptake and storing of zinc

Age-related macular degeneration (AMD) is more prevalent among the elderly Caucasians than in Africans. A significant association between light iris colour, fundus pigmentation and incidence of AMD is reported, suggesting a possible correlation with melanin pigment. Zinc is known to bind to melanin in pigmented tissues and to enhance antioxidant capacity by function as a cofactor or gene expression factor of antioxidant enzymes in the eye. In this in vitro study, we investigated the uptake and storage of zinc in human irides. Irides of blue and brown human eyes were used. The number of melanocytes was measured. Tissues without any treatment served as controls. The irides were incubated with 100 microM zinc chloride in culture medium for 24 h. Specimens of the tissues were stored for the uptake examination. The remained pieces were further incubated for 3 and 7 d to investigate the storage of zinc. The concentration of zinc was measured by inductively coupled plasma mass spectrometry (ICP-MS). Melanocytes count was significantly higher in the brown tissues (P < 0.0001). Zinc concentration of blue coloured irides after 24 h zinc treatment was close to the controls. We did not observe any significant storing. In contrast, the concentration of zinc in brown irides was significantly increased after 24 h (P < or = 0.01) and remained at a high level for 7 d. The uptake of zinc is likely dependent on the amount of pigmentation in human iris. Therefore, we assume that in patients suffering from AMD the degree of pigmentation of the irides and eventually fundi should be under consideration when the patients are treated with zinc supplementation.