Melanin utilization

Oxidative coupling polymerization provides great utility for the synthesis of high-performance polymers. Oxidative polymerization is also observed in vivo as important biosynthetic processes that, when catalyzed by metalloenzymes, proceed smoothly under an air atmosphere at room temperature. For example, lignin, which composes 30% of wood tissue, is produced by the oxidative polymerization of coniferyl alcohol catalyzed by laccase, an enzyme containing a copper complex as a reactive center. Tyrosine is an a-amino acid and is oxidatively polymerized by tyrosinase (Cu enzyme) to melanin, the black pigment in animals. These reactions proceed efficiently at room temperature in the presence of 02 by means of catalysis by metalloenzymes. Oxidative polymerization is observed in vivo as an important biosynthetic process that proceeds efficiently by oxidases. 

Utilizing knowledge concerning the structural relationship of known melanin biosynthesis inhibitors (MBI) to 1,3,8-trihydroxynaphthalene (1,3,8-THN), the substrate of a target enzyme in this pathway, Omata et al. (58) have recently designed phthalazine derivatives that are good MBI and rice blast control agents. 

Catecholamines are thermodynamically and photochemically unstable compounds. This property has been utilized in the photochemical synthesis of melanin (74, 277). Thus, a dilute solution of adrenaline, iso-prenaline and noradrenaline saturated with oxygen on irradiation (254 nm) gives the corresponding aminochromes in 65, 56, and 35% yield, respectively. Longer irradiation produces melanins (74). Studies of the action spectrum confirmed the excited state of the catecholamine as the primary factor in the transformation processes. N-suhstituted catecholamines have been found to react more rapidly than the corresponding N-unsubstituted ones (74). 

The melanin of sepia is found in the ink sac of cuttlefish. Aristotle in his book Historia Animalia mentioned that instead of intestines molluscs have an organ known as the mytin where a black substance is found which is especially abundant in cutdefish they put forth this black substance when frightened, particularly the cuttlefish . The ink has considerable staining power, and is alkaline and odorless. When fresh it disperses readily in water and is partly soluble in alkali, but insoluble in acid. This biological function of melanin appears to be defensive but the mechanism is uncertain (270). However, like other pigments, melanin can be utilized for camouflage purposes in insects and fishes. 

Biopterin (9.20), an ivory-coloured pteridine, biosynthesized by animals and widely distributed in Nature, is L-erythro-2-amino-4-oxo-6-(i,2-dihydroxypropyl-)pteridine. Once again, GTP is the precursor. A reduced form, 5,6,7,8-tetrahydrobiopterin is the co-factor required for the set of enzymes that use molecular oxygen to oxidize phenylalanine to tyrosine (Kaufman, 1964), tryptophan to 5-hydroxytryptophan, and possibly in in the formation of melanin. This co-factor utilizes oxygen directly from the air. 

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