Eumelanin
Eumelanin is a high molecular weight polymer whose exact molecular structure has not yet been determined, in part because eumelanin is insoluble in most solvents and resists most chemical changes and degradation.
Observations based on Raper's work with plant tyrosinases and modified by Mason led to the conclusion that melanin is a polymer composed of indole5,6-quinone units. However, the use of labeled antecedents showed that this was an oversimplification.
The ratio, molecular chain length, and molecular weight of the various subunits of eumelanin are still unknown. Blois et al. thinks that melanin is a highly disordered three-dimensional polymer joined by covalent bonds. Melanin has a free radical character associated with the semi-quinonoid form of 5,6-dihydroxyindole, which is highly stabilized by resonance through the conjugated polymer. Can act as a mild free radical quencher and also have some weak acid cation exchange abilities. Skin Melanin has specific absorption peaks at 3 and 6 J.L. only in the infrared region; however, it shows a broad absorption band between 200 and 2400 nm, which is not quite specific.
Phaeomelanin
Phaeomelanins are yellow and red sulfur-containing pigments found in mammalian hair. Unlike eumelanin, pheomelanin is soluble in dilute alkali. Pheomelanins, like eumelanin, are derived from tyrosine via dopaquinone. However, it is the interaction of cysteine with dopaquinone at this level that results in the synthesis of pheomelanin. By adding 1,6 cysteine to dopaquinone, 13-(5-S-cysteinyl-3,4-dehydroxyphenyl)alanine or 2-S-cysteinyldopa is formed. This is further oxidized to form pheomelanin. A minor product of the addition of 1.6 may be 2-S-cysteinyldopa.
Sulfhydryl compounds are involved in in vivo synthesis; indeed, under some experimental conditions, sulfhydryl compounds can induce pigment cells to produce yellow pigment. The interesting genetic control influencing the common pathway of eumelanin and pheomelanin synthesis is mirrored in the agouti mouse, which has only a few subapical pheomelanoms, the rest of which are eumelanogenic. Follicular melanocytes initially produce brown-black eumelanin, temporarily switch to yellow pheomelanin production, and soon revert to the original eumelanin synthesis.
Guinea pig studies have shown that melanosomes in red or yellow follicles are spherical, while those of black follicles are ellipsoidal. Various differences in biochemical behavior were observed between eumelanin and pheomelanin.
It has been shown that more sulfhydryl compounds, including glutathione and cysteine, are more incorporated into pheomelanogenic melanocytes and pheomelanin than eumelanic melanocytes and eumelanin. In the presence of sufficient reduced glutathione in vitro, only melanocytes that synthesize eumelanin in vivo produce pheomelanin. It has been suggested that the agouti band in the agouti mouse is derived from cyclically altered competition patterns by keratinocytes and melanocytes for substrate common to hair growth and melanogenesis.
Therefore, it was suggested that cyclical changes in the type of melanin synthesized might involve histochemical changes, not genetic intervention. Knisely et al. corroborating these findings with yellow (AY/a) samples in similar conditions and cultures of (A/A) agouti skin may be related to such histochemical factors not present in these systems.
Although cysteine is an established substrate for pheomelanogenesis, cysteinyldopa and pheomelanin can be produced by eumelanogenic melanocytes. It has also become clear that there is a third melanocyte pigment called trichochrome, which has dopaquinone as a critical intermediate, such as eumelanin and pheomelanin. The factors that favor one final path over another remain unclear.
Neuromelanin
The trigeminal and dorsal root ganglia, substantia nigra, locus caeruleus, and pigmented nuclei of the basal ganglia contain cytoplasmic organelles containing a brown pigment called "neuromelanin." There seem to be significant differences between eumelanin and neuromelanin.
Since patients with oculocutaneous albinism have a normal amount of neuromelanin, it seems unlikely that neuromelanin is formed by the action of tyrosinase. There are other important differences between melanin and neuromelanin.
Pigment particles of the substantia nigra appear to have a higher electron density than melanosomes, a size of 0.5 to 2.5 f.L, a single limiting membrane, and the longitudinal or diagonal striations typical of eumelanin melanosomes. The presence of labeled tyrosine in the area of pigmented granules in neurons convinced the researchers that tyrosine must be present.
The enzyme that catalyzes the hydroxylation of tyrosine to dopa is probably tyrosine hydroxylase and not the copper-requiring oxidase tyrosinase. Since tyrosine hydroxylase only catabolizes CNS tyrosine, disorders of eumelanin synthesis and CNS abnormalities should not be expected, and neuromelanin disorders (such as Parkinson's disease) should not be expected to have obligatory abnormalities of cutaneous eumelanogenesis.
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