Characterization of melanin in human iridal and choroidal melanocytes from eyes with various colored irides

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Abstract
Summary: Variance in iris color is related to the incidence of several important ocular diseases, including uveal melanoma and age‐related macular degeneration. The purposes of this study were to determine the quantity and the types of melanin in cultured human uveal melanocytes in relation to the iris color. Sixty‐one cell cultures of pure uveal melanocytes were isolated from donor eyes with various iris colors. The amount of eumelanin (EM) and pheomelanin (PM) of these cells was measured by chemical degradation and microanalytical high‐performance liquid chromatography (HPLC) methods. The total amount of melanin was measured by both microanalytical methods and spectrophotometry. Total melanin content, measured by HPLC and spectrophotometry, correlated well with r = 0.872 (P < 0.0001). The quantity and type of melanin in iridal and choroidal melanocytes showed no significant difference (P > 0.05). When cells became senescent, the levels of EM, PM and total melanin were significantly increased. In both growing and senescent melanocytes, the quantity and type of melanin were closely correlated to the iris color. In cells from eyes with dark‐colored irides (dark brown and brown), the amount of EM, the ratio of EM/PM and total melanin were significantly greater than that from eyes with light‐colored irides (hazel, green, yellow‐brown and blue) (P < 0.0001). The quantity of PM in uveal melanocytes from eyes with light‐colored irides was slightly greater than that from dark‐colored irides, although not statistically significant (P > 0.05). The present study shows that iris color is determined by both the quantity and the type of melanin in uveal melanocytes. These results suggest a possibility that uveal melanin in eyes with dark‐colored irides is eumelanic at the surface and acts as an antioxidant while that in eyes with light‐colored irides exposes pheomelanic core and behaves as a pro‐oxidant.