Melanin phaeomelanins

Melanins are complex polymeric structures, which are usually mixtures of macromolecules. Melanins are classified as eumelanins, phaeomelanins and allomelanins. ... 

Melanins are prodnced in mammals in two types of cells of different developmental origin (1) the melanocytes of the skin, hair, choroids and iris and (2) the retinal pigment epithelium (RPE). Specialized organelles of the melanocytes, the melano-somes, synthesize and store eumelanins and phaeomelanins. 

Dopachrome also undergoes a nonenzymatic reaction to form dihidroxyindole (DHI), the precursor of DHI-eumelanins. For the formation of phaeomelanins, dopaquinone is first transformed in cysteinil-DOPA and then in cysteinyl-dopaquinone which suffers a nonenzymatic polymerization. The polymerization of monomers and the association of melanins with proteins is not yet completely elucidated and may involve other intermediates. ... 

Figure 25-6 Postulated pathways for synthesis of the black pigment melanin and pigments (phaeomelanins) of reddish hair and feathers. Dopachrome reacts in two ways, with and without decarboxylation. The pathway without decarboxylation is indicated by green arrows. To the extent that this pathway is followed the green carboxylate groups will remain in the polymer. The black eumelanin is formed by reactions at the left and center while the reddish phaeomelanin is derived from polymers with cysteine incorporated by reactions at the right.

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Melanins are classified according to their chemical structure into the insoluble black eumelanins (poly-5,6-indole quinones) and the alkali-soluble red phaeomelanins (polydihydrobenzothiazines). Nicolaus (5) includes another group, the homoaromatic phenolic allomelanins (per-... 

The content and relative ratios of heteroelements have been used as criteria in the differentiation of melanin families. Even in the same family, however, the content of heteroatom(s) depends on the origin (756). Thus, the nitrogen content for eumelanins ranges from 5.18% for one synthetic dopa-melanin to 12.13% for melanoma melanin (Ref. 7, p. 97) (9.42% N calculated for 5,6-indolequinone homopolymer). For phaeomelanins the sulfur and nitrogen content varies between 10 and 12% and between 7 and 9%, respectively (Ref. 7, p. 116) (10.26% S and 8.97% N calculated for the cysteinyldopa homopolymer). 

The process of photoinduced interaction of melanin with molecular oxygen is accompanied by photobleaching (301,304,331). Using ESR measurements the rates of photooxidation of eumelanin and phaeomelanin have been found to be comparable. In other reports, however (337), phaeomelanin has been found to be more resistant toward photobleaching than eumelanin. A similar pattern of reactivity has been noticed in photo- and chemical oxidation of these two pigments in their native milieu (hair) (338). 

Characteristics of the ESR spectra of phaeomelanins are quite different and deserve some comment. The usual spectrum is composed of a triplet with g = 2.0052, a value typical of immobilized radicals with hyperfine splitting due to nitrogen (1 = 1). Accurate measurements were accomplished on synthetic cysteinyldopa melanins (382) at various pH in DiO... 

Two basic types of melanins exist in mammals reddish brown phaeomelanins and brownish black eumelanins. Both are produced by enzymatically catalyzed and chemical reactions in specific subcellular melanogenic compartments, known as melanosomes. 

The 1,4-benzothiazine ring is the basic unit for the phaeomelanins and trichochrome melanin pigments found in mammals, particularly in red hair... 

The process of photoinduced interaction of melanin with molecular oxygen is accompanied by photobleaching (232), and phaeomelanin has been found to be more photolabile and susceptible to photodegradation on prolonged photolysis (55). 

Sama T, Menon I A, Sealy RC (1984) Photoinduced Oxygen Consumption in Melanin Systems - II. Action Spectra and Quantum Yields for Phaeomelanins. Photochem Photobiol 39 805... 

Hair contains two pigments — melanin and phaeomelanin. Melanin has a dark-brown color, and phaeomelanin has a reddish-brown color. The natural color of the hair is determined by the relative amounts of these pigments. 

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