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There are various causes for the darkening of skin color, UV rays considered the primary source. When skin is exposed to UV rays, melanin is synthesized in melanocytes and released to darken the skin. Melanin pigmentation in human skin is a major defense mechanism against UV light from the sun, but abnormal pigmentation such as freckles or chloasma can be a serious aesthetic problem. Tyrosinase and other proteins, such as tyrosinase-related protein 1 (Tyrp-1) and tyrosinase-related protein 2 (Tyrp-2, Dopachrome tautomerase), are responsible for the biosynthesis of melanin. Among these enzymes, tyrosinase plays the most important role in melanin synthesis. Tyrosinase is a copper-containing enzyme and is expressed exclusively in melanocytes, the melanin-producing cells in the epidermis. Tyrosinase itself can produce melanin pigments in the absence of other melanogenic enzymes. In fact, fi broblast cells transfected with tyrosinase cDNA produce melanin in their lysosome-like structures. It is not fully understood how the expression of tyrosinase is regulated. Studies are being conducted to discover melanogenic inhibitory compounds to prevent or to cure these hyperpigmentary disorders with tyrosinase as the major target.
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