Melanosomes are the dominant membrane-limited pigment granules in young RPE cells. They contain melanin, a complex oligomeric aggregate consisting of products of enzymatic oxidation of tyrosine and nonenzymatic transformations of melanin precursors deposited on a protein matrix.
31,32 When illuminated under aerobic conditions, melanin can generate superoxide anion and hydrogen peroxide.
33–35 Despite its photoreactivity, RPE melanin usually is considered photoprotective,
36,37 although how melanin protects from light damage is not entirely clear. Apart from absorbing light, melanin may act as a cellular antioxidant.
37,38 In a variety of model systems, melanin has been shown to scavenge reactive free radicals,
39,40 quench singlet oxygen
41,42 and excited states of photosensitizing molecules,
43–45 and to inhibit lipid peroxidation.
46–48 Melanin also can sequester redox-active metal ions, making them less likely to induce lipid peroxidation,
46 and less available to act as cofactors in Fenton-type reactions that yield extremely reactive hydroxyl radicals.
49,50 Despite the many properties that predict melanin would photoprotect RPE cells by acting as an antioxidant, showing this effect in a cellular system has presented an experimental challenge. Using conventional measures of antioxidant-mediated cytoprotection, we previously were unable to detect photoprotection of ARPE-19 cells by phagocytized porcine melanosomes when compared to phagocytized black beads, which were used to control for the effect of optical screening.
51 Subsequent experiments, using a more sensitive dynamic imaging protocol to quantify the timing of cell injury, revealed no protection, but rather detected a slight increase in phototoxicity conferred by melanosomes compared to beads,
52 an outcome consistent with observations that melanosomes generate superoxide and hydrogen peroxide on blue light irradiation.
35 In the same study, we observed that untreated melanosomes conferred protection when compared to melanosomes that were photobleached, a process that changes the properties of melanin rendering it more photoreactive.
52 This raised the possibility that, despite melanin's photoreactivity, melanosomes may help photoprotect RPE cells when irradiation occurs in the presence of other, more photoreactive substances or organelles, a context in which the light screening and the antioxidant properties of melanosomes could be functionally relevant. This is the typical state for RPE cells in which melanosomes coexist with the naturally-occurring and more highly photoreactive organelle, the lipofuscin granule.
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