Melanins, production Membranes

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

Hydroquinone is closely related to phenol and can reduce melanin production, ft appears that it can also degrade melanosomes. It has a tyrosinase-inhibiting activity and can change the membrane structure of the intracellular organelles of melanocytes. Hydroquinone acts mostly on the first stages of melanin synthesis. Its action is therefore gradual, like any tyrosinase inhibitor. 

The waste products are partially deposited on the Bruch s membrane (Young, 1987) in the form of drusen. The accumulation of lipofuscin in RPE cells appears detrimental to its function (Flood et al., 1984) and causes photoreceptor death (Dorey et ak, 1989). With age, the number of RPE cells decreases in the central retina, and they become pleomorphic (Dorey et ak, 1989). Other changes are also frequent. They include atrophy, depigmentation, hyperplasia, hypertrophy, and cell migration. The melanin concentration in the RPE cells decreases with age, especially in Caucasians, but also in blacks (Feeney-Burns et al., 1984). The melanin granules are slowly (over decades) digested by lysosomes (Bums and Feeney-Burns 1980). 

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