[Characteristics and functions of melanin in retinal pigment epithelium]

Melanin granules in the retinal pigment epithelium (RPE) have many important functions which are not yet completely understood. Melanin in the RPE protects the cell from damage caused by oxidative stress. This pigment acts as a free radical sink and diminishes cytotoxic lipid peroxidation. Thus, melanin protects against light toxicity and against cytotoxic effects caused by ocular inflammation. Many enzymes, e.g. superoxide dismutase or carboanhydrase, are only activated in the presence of zinc. Melanin can store zinc and release it when required. The absence of melanin in patients with oculocutaneous albinism is accompanied by photophobia, poor visual acuity and nystagmus. Furthermore melanin is said to protect against damaging lipofuscin accumulation in the RPE. Melanosomes are involved in the lysosomal degradation pathways and possibly take part in the degradation of rod outer segments (ROS) in the RPE.