Human RPE melanosomes protect from photosensitized and iron-mediated oxidation but become pro-oxidant in the presence of iron upon photodegradation.

To determine the effects of human retinal pigment epithelial (RPE) cell pigment granules on photosensitized and iron ion-mediated oxidation and the effect of the photodegradation of melanosomes on their antioxidant properties. RPE cells were isolated from human and bovine eyes; pigmented and nonpigmented bovine retinal pigment epithelia were isolated separately. Melanosomes, melanolipofuscin, and lipofuscin granules were isolated from human RPE donors older than 60. Melanosomes were photodegraded by exposure to blue light. Oxidation of RPE cells or of linoleate was induced by iron/ascorbate in the presence and absence of pigment granules. The photosensitized oxidation of histidine was induced by blue light irradiation of cationic porphyrin. The progress of oxidation was monitored by electron spin resonance oximetry. Iron/ascorbate induced rapid oxidation in suspensions of nonpigmented bovine RPE cells. The rates of oxidation were diminished approximately four times in suspensions of pigmented bovine RPE cells. Adding bovine melanosomes or synthetic melanin to nonpigmented bovine RPE cells resulted in a concentration-dependent decrease in the rate of oxidation to levels similar to those of pigmented bovine retinal pigment epithelium. Human melanosomes exerted a concentration-dependent inhibitory effect on photosensitized and iron-mediated oxidation. Photodegradation of human melanosomes led to loss of the inhibitory effect on iron-mediated oxidation, whereas their ability to inhibit photosensitized oxidation was enhanced. Human melanosomes act as effective antioxidants by preventing iron ion-induced oxidation. Photodegradation of melanosomes results in the loss of these antioxidant properties while it preserves their ability to deactivate cationic photosensitizers.