Melanin structure

As the melanin structure grows, it becomes more colored giving various shades of brown color to our skin. This brown coloration acts to help protect deeper skin elements from being damaged by the UV radiation. The absence of the enzyme tyrosinase that converts tyrosine to melanin can lead to albinism. 

The dendrites of a melanocyte contact about 36 keratinocytes and are able to transfer melanosomes to these adjacent cells. The numbers and sizes of the melanosomes as well as melanin structure determine differences in skin color.131 Similar cells in amphibians, the melanophores, also contain light receptors p Their melanosomes are not transferred to other cells but may be either clustered near the center of the cell or dispersed. The location can be changed quickly by transport of the melanosomes along a network of microtubules allowing the animals to change in response to changes in light color.0)... 

The second approach to solubilization involves treatment of natural melanosomes and synthetic melanins with a dilute solution of hydrogen peroxide at pH 9-10 (303). The solubilized melanin precipitates under acidic conditions and is readily redissolved in basic media. There is only a slight increase in the carboxyl content, suggesting only limited degradation of the pigment. The fact that melanins can be solubilized in both polar and nonpolar media is a clear manifestation of the ability of the melanin structure to accommodate highly diverse demands on its solvation characteristics. 

The understanding of melanin structure has been attempted via analytical and biosynthetic approaches. The analytical one originally explored by Nicolaus (3,7,310) has led to the development of a number of useful methods for characterizing natural and synthetic melanins in terms of elemental composition, functional groups, and structural features of the pigment backbone. These methods helped in the elucidation of the partial polymeric structure of the eumelanin sepiomelanin (3) seen below. 

Kinetic experiments and pulse irradiation studies on the mechanism of DHI polymerization (2, 136) revealed that the dominating species formed by DHI oxidation was the quinone methide and that coupling proceeds via oxygen-centered semiquinone radicals, which can also account for the complexity of the later stages of melanogenesis and the heterogeneity of melanin structure. 

Swan G (1973) Current Knowledge of Melanin Structure. In McGovern VJ, Russell P (eds.) Pigment Cell, Vol. 1, p. 151. Karger, Basel... 

Figure 8.14 The formation of melanin from DOPA, showing a fragment of the melanin structure with two of the resonance forms that contribute to the conjugated system of double bonds that gives it its strong light absorption...

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

Allomelanins — These are structurally different compounds containing little or no nitrogen. They are considered polymers of phenolic compounds like catechol. Fnngi prodnce melanin pigments, predominantly dihydroxyphenylalanine (DOPA)-melanin and dihydroxynaphthalene (DHN)-melanin. ... 

Because of their very complex chemical structures and heterogeneity, melanins are difficult to extract, separate, and characterize from tissues. Eumelanins are insoluble in water and organic solvents. They can be extracted from tissues with strong chemicals that are capable of removing lipids, proteins, and other tissue components but also lead to the formation of degradation products. Enzymatic procedures were developed for the isolation of eumelanins from mammalian hair and irises. The first step is sequential digestion with protease, proteinase K, and papaine in the presence... 

Pezzella, A. et al., An integrated approach to the structure of sepia melanin evidence for a high proportion of degraded 5,6-dihydroxyindole-2-carboxylic acid units in the pigment backbone, Tetrahedron, 53, 8281, 1997. 

Double, K.L. et al., Structural characteristics of human substantia nigra neuromelanin and synthetic dopamine melanins, J. Neurochem., 75, 2583, 2000. 

A cross section of excised human skin, histologically stained, is shown in Figure 6.11. It shows a layer structure of the tissue and the increased homogeneity in the bloodless stratum corneum layer, where the cell nuclei are absent, and where the potentially confounding melanin concentrations are... 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

In addition to their well known role in protein structure, amino acids also act as precursors to a number of other important biological molecules. For example, the synthesis of haem (see also Section 5.3.1), which occurs in, among other tissues, the liver begins with glycine and succinyl-CoA. The amino acid tyrosine which maybe produced in the liver from metabolism of phenylalanine is the precursor of thyroid hormones, melanin, adrenaline (epinephrine), noradrenaline (norepinephrine) and dopamine. The biosynthesis of some of these signalling molecules is described in Section 4.4. 

---