melanin, water-insoluble polymer of various compounds derived from the amino acid tyrosine. It is one of two pigments found in human skin and hair and adds brown to skin color; the other pigment is carotene, which contributes yellow coloring. The synthesis of melanin reactions is catalyzed by the enzyme tyrosinase; an inherited lack of tyrosinase activity results in one of the forms of albinism. Tyrosinase is found in only one specialized type of cell, the melanocyte, and in this cell melanin is found in membrane-bound bodies called melanosomes. Melanosomes can be transferred from their site of synthesis in the melanocytes to other cell types. The various hues and degrees of pigmentation found in the skin of human beings are directly related to the number, size, and distribution of melanosomes within the melanocytes and other cells. Besides it role in pigmentation, melanin, which absorbs ultraviolet light, plays a protective role when skin is exposed to the damaging rays of the sun (see sunburn; skin cancer).

Any of several organic compounds, dark biological pigments that give coloration (shades of yellow to brown) to skin, hair, feathers, scales, eyes, and some internal tissues, notably the substantia nigra in the brain. In humans, melanins help protect the skin against the damaging effects of ultraviolet radiation, but melanoma may arise from cells that produce it. The amount in the skin depends on both genetic and environmental factors. Melanin is produced from the amino acid tyrosine; albinos lack the enzyme that catalyzes that reaction (see albinism).

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