ABSTRACT
Mammalian melanins are the end products of complex multistep transformations of L-tyrosine to polymorphous and multifunctional biopolymers, represented by eumelanin, pheomelanin, neuromelanin, and mixed melanin pigment. Melanin biosynthesis commences either from the hydroxylation of L-phenylalanine to L-tyrosine or directly from L-tyrosine, which is then hydroxylated to L-dihydroxyphenylalanine (L-DOPA). Tyrosinase is the key enzyme of melanogenesis, and L-DOPA is the precursor for both melanins and catecholamines. Oxidation of L-DOPA to dopaquinone is common to both eumelanogenic and pheomelanogenic pathways. Melanin may also be produced by other cells, such as cells of pigmented epithelium of the retina, epithelia of the iris, and the ciliary body of the eye; some neurons; and adipocytes. Mature melanosomes from basal melanocytes are engulfed by keratinocytes and via their lysosomal compartment become melanin dust in the upper nonviable layers of the epidermis. The main action of melanin in human skin appears to be attenuation of ultra violet penetration to skin and protection against DNA damage and carcinogenicity.
