Direct Measurement of the Ultraviolet Absorption Coefficient of Single Retinal Melanosomes

A novel approach to photoemission electron microscopy is used to enable the first direct measurement of the absorption coefficient from intact melanosomes isolated from bovine retinal pigment epithelial cells. The difference in absorption between newborn and adult melanosomes is in good agreement with that predicted from the relative amounts of the monomeric precursors present in the constituent melanin as determined by chemical degradation analyses. The results demonstrate that for melanosomes containing eumelanins, there is a direct relation between the absorption coefficient and the relative 5,6-dihydroxyindole: 5,6-dihydroxyindole-2-carboxylic acid (DHI:DHICA) content, with an increased UV absorption coefficient associated with increasing DHICA content.