- * * *Any of the dark brown to black polymers of indole-5,6-quinone and/or 5,6-dihydroxyindole 2-carboxylic acid that normally occur in the skin, hair, pigmented coat of the retina, and inconstantly in the medulla and zona reticularis of the adrenal gland. M. may be formed in vitro or biologically by oxidation of l-tyrosine or l-tryptophan, the usual mechanism being the enzymatic oxidation of l-tyrosine to 3,4-dihydroxy-l-phenylalanine (dopa) and dopaquinone by monophenol monooxygenase, and the further oxidation (probably spontaneous) of this intermediate to m.. Cf.:eumelanin, pheomelanin. SYN: melanotic pigment. [G. melas (melan-), black]
* * *mel·a·nin 'mel-ə-nən n any of various black, dark brown, reddish brown, or yellow pigments of animal or plant structures (as skin, hair, the choroid, or a raw potato when exposed to air) esp any of numerous animal pigments that are essentially polymeric derivatives of indole formed by enzymatic modification of tyrosine
* * *n.a dark-brown to black pigment occurring in the hair, the skin, and in the iris and choroid layer of the eyes. Melanin is contained within special cells (chromatophores or melanophores) and is produced by the metabolism of the amino acid tyrosine. Production of melanin by melanocytes in the epidermis of the skin is increased by the action of sunlight (producing tanning), which protects the underlying skin layers from the sun's radiation.
* * *mel·a·nin (melґə-nin) [Gr. melas black] any of several closely related dark, amorphous pigments of the skin, hair, and various tumors, of the choroid coat of the eye and of the brain (neuromelanin, q.v.). Melanins are produced by polymerization of oxidation products of tyrosine and dihydroxyphenyl compounds, and contain carbon, hydrogen, nitrogen, oxygen, and often sulfur. See also eumelanin and pheomelanin.
Medical dictionary. 2011.