Action spectrum of oxidative reactions mediated by light-activated melanin

The melanin of the retinal pigment epithelial (RPE) cells is generally thought to have a photoprotective role in the eye, yet it is excited by light to a free radical which can react with cellular components. Soluble proteins extracted from the retina are photo-oxidized by the output of a Xenon arc lamp containing UVA and visible wavelengths. The oxidative damage in this model consists of carbonyl adducts to the peptides, and is proportional to the amount of UVA present. Melanosomes isolated from bovine RPE cells and added to the retinal protein extract partly protect the proteins from photo-oxidation resulting from this broadband exposure. However, if the proteins are instead exposed to the 488 and 514.5 nm outputs of an Argon continuous wave laser, the amount of protein oxidation is markedly increased when melanosomes are present. This observation suggests that the melanin free radical is optimally excited by wavelengths in the blue-green region of the visible spectrum, and in fact the action spectrum for the photo-oxidation of NADPH by laser-excited melanin peaks between 450 and 500 nm. The present data do not distinguish between two alternative hypotheses, i.e. that the apparent action spectrum peak is due to (1) a chromophore different from the one determining the overall optical absorption of melanin, or (2) the lower efficiency of UVA photons in activating melanosomes because of their strong absorption at the solution surface. Nevertheless these data implicate melanin in the so-called 'blue light' retinal hazard.