Pheomelanins

Biosynthetic studies provided evidence that pigment formation involves the oxidative polymerization of cysteinyldopas of type 55CD (5-S-cysteinyldopa) via 

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Kolb, A. M., Lentjes, E. G., Smit, N. P., Schothorst, A., Vermeer, B. J., and Pavel, S., Determination of pheomelanin by measurement of aminohydroxypheny-lalanine isomers with high-performance liquid chromatography, Anal. Bio-chem., 252, 293, 1997. 

Melanin from natural sources falls into two general classes. The first component is pheomelanin (I), which has a yellow-to-reddish brown colour, and is found in red feathers and red hair. The other component is eumelanin (which has two principal components, II and III). Eumelanin is a dark brown-black compound, and is found in skin, hair, eyes, and some internal membranes, and in the feathers of birds and scales of fish. Melanin is particularly conspicuous in the black dermal melanocytes (pigment cells) of dark-skinned peoples and in dark hair and is conspicuous in the freckles, and moles of people with lighter skins. 

Figure 9.7 UV-visible spectrum of the skin pigment melanin. The spectrum contains two traces (a) eumelanin and (b) pheomelanin. Both component compounds protect the skin by absorbing harmful UV light. All pigment concentrations were 1 mgdm 3...

Catechol oxidation catalyzed by peroxidases can be used not only for the synthesis of sulfur-substituted catechols but also for the preparation of synthetic compounds related to pheomelanins, which contain benzothiazine units. In fact, the quinone undergoes an extremely easy nucleophilic addition by thiols. For example, treating the neurotransmitter dopamine with cysteine, in the presence of HRP/H2O2, gives rise to 2-S- and 5-5-cysteinyl-catecholamine and a smaller amount of the 2-S,5-S,-di-cysteinyl-catecholamme conjugate [48, 49] (Fig. 6.3e). 

Enzymes present in melanosomes synthesize two types of melanin, eumelanin and pheomelanin. Figure 2 illustrates the proposed biosynthetic pathways of eumelanin and pheomelanin. The synthesis of eumelanin requires tyrosinase, an enzyme located in melanosomes. Tyrosinase catalyzes the conversion of tyrosine to dopa, which is further oxidized to dopaquinone. Through a series of enzymatic and nonenzymatic reactions, dopaquinone is converted to 5,6-indole quinone and then to eumelanin, a polymer. This polymer is always found attached to proteins in mammalian tissues, although the specific linkage site between proteins and polymers is unknown. Polymers affixed to protein constitute eumelanin, but the exact molecular structure of this complex has not been elucidated. Pheomelanin is also synthesized in melanosomes. The initial steps in pheomelanin synthesis parallel eumelanin synthesis, since tyrosinase and tyrosine are required to produce dopaquinone. Dopaquinone then combines with cysteine to form cysteinyldopa, which is oxidized and polymerized to pheomelanin. The exact molecular structure of pheomelanin also has not been determined. 

There are distinct chemical properties of eumelanin and pheomelanin which affect the physiochemical properties of hair. - Pheomelanin granules are smaller and less resistant to chemical degradation than eumelanin granules. Pheomelanin is soluble in dilute alkali in comparison to eumelanin, which is insoluble in almost all solvents. Pheomelanin and eumelanin also differ in sulfur content. Pheomelanin is high in sulfur content (9 to 12%) in contrast to eumelanin, which contains only 0 to 1% sulfur. ... 

Melanin Varies with hair color (blond, eumelanin (+) brown, eumelanin (++) red, pheomelanin (++) and eumelanin (+) Eumelanin (+++) Eumelanin (+++) ... 

Blond hair contains elliptical and oval melanosomes. The granules in melanosomes are small, few in number, and likely represent eumelanin, although pheomelanin also may be present in blond hair. T22,24 Ortonne and Prota suggested that blond hair results from a quantitative decrease in the synthesis of melanin in comparison to black and brown hair. The main features which distinguish brown hair from blond hair are the presence of more melanosomes and a higher concentration of melanin in brown hair. - Melanosomes present in the shaft of blond hair also appear to be more susceptible to degradation than melanosomes in black hair. Cesarini reported that melanosomes may not be present in the shaft of blond hair, possibly due to digestion of melanosomes by lysosomes. 

Although the structures of pheomelanin and eumelanin have not been resolved, melanin is one of several suspected binding sites in hair for metals, chemicals, and drugs of abuse. The quantity and type of melanin in hair should determine the extent to which drugs bind to hair. Melanin is present in several mammalian tissues including the brain, skin, hair, iris of the eye, vas deferens, and cochlea of the inner ear. - Since melanin is present in many human tissues, drug may bind to other bodily tissues as well as hair. 

Ito, S. and Fujita, K., Microanalysis of eumelanin and pheomelanin in hair and melanomas by chemical degradation and liquid chromatography. Anal. Biochem., 144, 527,1985. 

Birbeck, M. S. C. and Barnicot, N. A., Electron microscope studies on pigment formation in human hair follicles, in Pigment Cell Biology, Gordon, M., Ed., Academic Press, New York, 1959, 549. Thody A. J., Higgins, E. M., Wakamatsu, K., Ita, S., Burchill, S. A., and Marks, J. M., Pheomelanin as well as eumelanin is present in human epidermis, J. Invest. Dermatol., 97, 340,1991. 

Alessandra Napolitano graduated in chemistry in 1984 at the University of Naples Federico II under the guidance of Prof. G. Prota. In 2001 she was made associate professor of organic chemistry. Her main research interests lie in the field of heterocyclic compounds, with special reference to hydroxyindoles and benzothiazines, oxidative chemistry of phenolic natural products, food chemistry, lipid peroxidation, and analytical chemistry. Currently, she is involved in several research projects dealing with the chemistry of natural pigments, including pheomelanins, and the chemical basis of diseases. 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

The agouti protein is secreted by specialized cells and is bound by adjacent melanocytes. These specialized cells and the melanocytes reside near each other in the skin. Melanocytes are cells that create skin pigment, Le., eumelanin, w hich is black, and pheomelanin, which is yellow. The agouti protein binds to a receptor on the melanocyte, called the melanocortin receptor, and instructs the melanocytes to switch from making the black to the yellow pigment. 

The natural colour of animal fibre is closely related to the character of environment in which the animal lives [19]. Wool lots completely free of dark fibres do not exist [20]. In animal (and human) hair two kinds of pigments occur, namely eumelanin (responsible for black, dark brown and grey colours and commonly referred to as melanin) and pheomelanin (present in yellow, reddish-brown and red hair). Both are thought to be formed by different mechanisms and chemically differed [21]. Eumelanin is formed by enzymatic (tyrosinase) oxidation of tyrosine and polymerisation of several oxidation product [22]. Pheomelanin occurs in form of discrete grannules. Melanin grannules can occur in the cortex or in the cuticle. 

The first two steps in the synthesis of melanin are catalyzed by tyrosinase, a copper-containing oxidase, which converts tyrosine to dopaquinone. All subsequent reactions presumably occur through nonenzymatic auto-oxidation, in the presence of zinc, with formation of the black to brown pigment eumelanin. The yellow to reddish brown, high-molecular-weight polymer known as pheomelanin and the low-molecular-weight trichromes result from addition of cysteine to dopaquinone and further modification of the products. Pheome-lanins and trichromes are primarily present in hair and feathers. 

Whittaker, J. R. Biosynthesis of a thiouracil pheomelanin in embryonic pigment cells exposed to thiouracil. J. Biol. Chem. 1971, 246, 6217-6226. 

ESR spectra of melanins are fairly characteristic. Eumelanins (derived mostly from dopa), which are the most common, show a single line with a g value close to 2.004 and a linewidth of approximately 4 G (Fig. 10a) [68]. Pheomelanins (from cysteinyldopa) have a quite different spectrum (Fig. 10b) which shows evidence of hyperfine interaction with nitrogen [164]. Many natural melanins are evidently derived from mixtures of dopa and cysteinyldopa and show intermediate spectra (Fig. 10c). 

Figure 10. ESR spectra ( — 196°C) from different forms of melanin, a. Melanin from oxidation of dopa (a eumelanin) b, melanin from oxidation of cysteinyldopa (a pheomelanin) c, melanin from cooxidation of dopa and cysteinyldopa. From [164], with permission.

From a chemical point of view, and in a very simplified way, in the animal kingdom the so-called eumelanins (or melanins tout-court), black or dark brown, and completely insoluble in water, differ from pheomelanins, that contain sulfur, varying from yellow to brown-red, and alkali-soluble. Both the kinds of melanin can be present at the same time within the same organism. For a deeper insight in melanin occurrence and properties, see [238],... 

It has been hypothesised [239] that eumelanins derive biosynthetically from tyrosine, whereas pheomelanins of hair involve the participation of other monomers besides tyrosine, for example HK which can polymerise to a yellow pigment (an ommochrome-like substance ), owing to interferences with the main biosynthetic pathway of eumelanin synthesis. 

The presence of phenoloxidase in all phyla of living organisms demonstrates that their origin is very early in the history of life. It is involved in the biosynthesis of melanins and other polyphenolic compounds. Depending on the species, phenoloxidases are involved in the primary immune response, wound healing, sclerotization, and coloring processes. In mammals, L-tyrosine is the initial substrate in the pathway leading to the final products of black-brown eumelanins, red-yellow pheomelanins, or a mixture of pheo-and eumelanins.Ti ... 

The principal pigments of human hair are the brown-black melanins (eume-lanins) and the less prevalent red pigments (pheomelanins). These latter pigments at one time were called trichosiderins. For this discussion, the brown-black pigments of hair will be referred to as melanins, and the yellow and red pigments will be referred to as pheomelanins. 

Pheomelanins are the yellow-red pigments and are lighter in color than the brown-black pigments of human hair. Both pheomelanins and eumelanins occur as granules in melanocytes. Prota and co-workers [14,62] have proposed that red hair pigments are formed by a modihcation of the eume-lanin pathway described earlier. The pheomelanin pathway, however, involves the interaction of cysteine with dopaquinone. Figure 4-28 summarizes Prota s [62] description of a common metabolic pathway for formation of all melanins and shows how pheomelanin formation relates to eumelanin biosynthesis. 

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