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Abstract
Melanin is an important phenolic skin pigment found throughout the animal kingdom. Tyrosine and its hydroxylated product dopa provide the starting material for melanin biosynthesis in all animals. Through a set of well-established reactions, they are converted to 5,6-dihydroxyindole (DHI) and DHI-2-carboxylic acid (DHICA). Oxidative polymerization of these two indoles produces the brown to black eumelanin pigment. The steps associated with these transformations are complicated by the extreme instability of the starting materials and the transient and highly reactive nature of the intermediates. We have used mass spectral studies to explore the nonenzymatic mechanism of oxidative transformation of DHI in water. Our results indicate the facile production of not only dimeric and trimeric products but also higher oligomeric forms of DHI upon exposure to air in solution, even under nonenzymatic conditions. Such instantaneous polymerization of DHI avoids toxicity to self-matter and ensures the much-needed deposition of melanin at (a) the wound site and (b) the infection site in arthropods. The rapid deposition of DHI melanin is advantageous for arthropods given their open circulatory system; the process limits blood loss during wounding and prevents the spread of parasites by encapsulating them in melanin, limiting the damage.
Highlights
The brown to black pigment widely distributed in the skin, hair, and fur of animals is due to the phenolic pigment eumelanin [1,2,3,4,5,6,7,8,9]
Elaborate studies conducted on the biosynthesis of eumelanin pigment have led to the delineation of its pathway, which is depicted in Figure 1 [1,2,3,4,5,6,7,8,9]
An enzyme catalyzing the conversion of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) was identified and named dopachrome tautomerase [18,19,20,21,22]
Summary
The brown to black pigment widely distributed in the skin, hair, and fur of animals is due to the phenolic pigment eumelanin [1,2,3,4,5,6,7,8,9] It is ubiquitously present in insects as an exoskeletal pigment and, more importantly, associated with two physiologically important biochemical processes, viz. Dopachrome was initially believed to undergo a nonenzymatic transformation to 5,6-dihydroxyindole (DHI), studies carried out in mammals and insects revealed the presence of two distinctly different enzymes associated with dopachrome conversion.
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