Structure and biological activity of natural melanin pigments

Abstract

Melanin pigments are heterogeneous biopolymers and are widely distributed in nature. They are produced by animals, plants, and microorganisms, such as pathogenic fungi and bacteria. In humans, melanins are present in the skin, hair, eyes, and in other locations of the body, including the inner ear, and the substantia nigra and the locus coeruleus of the brain (Marsden 1983; Tolleson 2005). Traditionally, the pigments can be classified into brown to black eumelanins and allomelanins, and yellow or reddish-brown, sulfur-containing pheomelanins (Nicolaus 1968). Allomelanins occur in the plant kingdom, e.g., in certain fungi, and in the seeds of some flowering plants, and are formed by the oxidation of nitrogen-free diphenols, such as catechol, 1,8-dihydroxynaphtalene, and γ-glutaminyl-3,4-dihydroxybenzene (Swan 1974; Wheeler and Bell 1988). Eumelanins are polymers consisting mainly of indole-type units that arise from L-tyrosine or L-DOPA (L-3,4-dihydroxyphenylalanine) oxidation, whereas pheomelanins are derived from the oxidative polymerization of cysteinyl conjugates of DOPA via benzothiazine intermediates (Prota 1992).