Phaeomelanins

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

Phaeomelanins — This gronp consists of sulfur-containing polymers composed of benzothiazines, derived from cysteinyl-DOPA. 

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-... 

Melanocytes Ultrastructural composition of eumelanins and phaeomelanins in animals... 

Method for quantitative determination of eu- and phaeomelanins Effect of UV radiation Co irradiation studies Effect of terf-butylcatechol... 

Scheme 1. Overall view of metabolic pathways leading to the formation of eumelanins and phaeomelanins (775).

A synthetic phaeomelanin can be easily obtained by the tyrosinase-catalyzed oxidation of L-tyrosine or L-dopa in the presence of excess L-cysteine at pH 6.8 followed by chromatography of the acid-soluble fraction on a Sephadex column. This procedure leads to the isolation of four major reddish brown pigments that are similar to natural... 

Enzymatic copolymerization of dopa and 5-5-cysteinyldopa led to products with both eu- and phaeomelanin properties. Thus, reaction of a... 

Methods based on quantitative markers combined with TLC and HPLC (using an electrochemical detector) have been developed both for eu- and phaeomelanins. Thus, for eumelanins the marker is pyrrole-2,3,5-... 

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). 

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