Catechol melanin formation from DOPA

In his classical studies on melanin formation from DOPA (3), Raper proposed the following scheme for the formation of the red pigment now known to be the aminochrome dopachrome (4). The first stage involved the oxidation of the catechol nucleus of-3 to give the quinone dopa-quinone (16). The second stage was the non-oxidative intramolecular cyclization of 16 to leuco-dopachrome (17), which was in turn oxidized to dopachrome (4).72,73 Since... 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

The polyphenoloxidase, tyrosinase, is a widely distributed enzyme found in plants and animals. It is the enzyme responsible for the formation of melanin pigments from phenylethylamine derivatives such as tyrosine and the catecholamines. According to Mason, it is a mixed function oxidase with two independent catalytic activities the hydroxylation of tyrosine to DOPA and the subsequent oxidation of DOPA, or another catechol derivative, to an open-chain quinone (cf. 12), which is then converted non-enzymati-cally to melanin [48-51]. 

Contrary to plants, higher animals can not synthesize compounds with benzenoid rings from aliphatic precursors, the very few exceptions include estrone and related phenolic steroids (Singleton, 1981). Plants are the source of nearly all the phenols found in animals. Even the phenols that are essential for animals (such as the catechol amines and phenolic indole amines involved in nerve action and associated effects), the vitamin E tocopherols, the vitamin K napthoquinones or menadiones, the ubiquinone benzoquinones, thyroxine, the tyrosine of proteins, and the tyrosine-DOPA derivatives involved in melanin pigment formation, are all drawn either directly or indirectly from plants or are modified from an essential plant precursor, usually phenylalanine (Singleton and Kratzer, 1969). 

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