Melanin from tyrosine, formation

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

In alkaptonuria, homogentisic acid (28) is excreted in excessive amounts indicating that in this metabolic disease the opening of the aromatic ring and complete oxidation to CO2 and H2O is blocked. In albinism, there is a deficiency of the enzyme systems responsible for the formation of melanin from tyrosine (24) (Figure 3.7). 

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

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-... 

Metabolism 3,4-D. is formed from tyrosine by means of tyrosine 3-monooxygenase (EC 1.14.16.2.) and tet-rahydrobiopterin. DOPA is a biosynthetic precursor of dopamine, noradrenaline, and adrenaline. It is used for therapy of Parkinson s disease (dopamine deficiency). Oxidation and subsequent polymerization lead to the formation of melanins. Extradiol cleavage of the benzene ring gives rise to betalamic acid, " stizolobic acid, stizolobinic acid, muscaflavin, etc. 3,4-D. is toxic for the development of certain insects and for the reproduction of the duckweed Lemna minor. ... 

The unique property of this enzyme is that the product of the first monoxygenation step, o-diphenol, serves as the electron donor for the reduction of the cupric ions with formation of the corresponding o-quinone 161, 274). The products formed by catecholase activity from tyrosine are extremely reactive and undergo intermolecular reactions to form indole derivatives which subsequently polymerize to melanin. 

According to Pawelek et al. 200), the biosynthesis of melanin in Cloudman melanoma cells is a complex process and is regulated by three factors (a) a dopamine conversion factor which converts dopamine to 5,6-dihydroxyindole (13), (b) a 5,6-dihydroxyindole conversion factor which catalyzes the conversion of 5,6-dihydroxyindole to melanin and is active when cells are exposed to melanotropin (MSH), and (c) a 5,6-dihydroxyindole blocking factor which restricts melanogenesis at the 5,6-dihydroxyindole stage. They have also shown that at least three steps in the Raper-Mason scheme of melanin formation from tyrosine are catalysed by tyrosinase (Fig. 6). 

It appears from the previous discussion that melanogenesis in vivo or in vitro is regulated by various factors. Hence, to study the dynamics of melanin formation (monooxygenase reaction, i.e. constructive metabolism) and breakdown (dioxygenase reaction, i.e. catabolism of melanin precursors), a non-enzymatic melanin synthesis from tyrosine and tryptophan, respectively, was devised by Roy et al. (227), using a prototype of a monooxygenase reaction, i.e. the Udenfriend reaction (Fe+ /EDTA/ ascorbic acid) (272). 

Further, owing to deamination, p-hydroxyphenylpyruvic acid (15), a well-known inhibitor of tyrosinase, was produced from tyrosine. This product underwent further transformation to yield biphenyl 16 while indolylpyruvic acid (22) and subsequently indole (19) were formed from tryptophan. Hence, in biological systems, the loss of the starting material, i.e. tyrosine, for melanin synthesis due to the formation of 15, 16 and 18 may be counteracted by tryptophan, as an alternative substrate in the pathway of melanogenesis through 22 and 19. However, such a replacement of substrate for melanin by tryptophan is not possible in... 

On irradiation with ultraviolet light, tyrosine is readily converted to DOPA, which is then oxidised further, probably to dopachrome and then to melanin [29, 85-88]. Synephrine (41) behaves similarly, being oxidised first to adrenaline and then probably to adrenochrome which rearranges to adrenolutin [89]. These oxidations probably involved the initial formation of the semi-quinone followed by oxidation to the open-chain quinone [90]. Ultraviolet irradiation was also found to increase the rate of oxidation of tyrosine by tyrosinase in rat skin. OrrAo-quinones were produced in the reaction and it was concluded that the acceleration was due to the formation of low levels of these compounds from tyrosine [91]. 

Tyrosiaase occurs in lower animals, especially meal-worms, in many plants, notably clover, potato, and higher fungi. It catalyses the oxidation of the amino acid tyrosine with the production of a red pigment which turns black and becomes insoluble, in which condition it is called melanin. Tyrosinase is believed to function in the formation of natural pigments from tyrosine, and also in the metabolism of the amino acid in lower organisms. 

Albinism, due to lack of tyrosinase and dopa oxidase, which form the natural melanin pigments of skin, hair and retina from tyrosine. The condition is unassociated with any abnormal urinary metabolite. Of these inborn errors, albinism is the most common, cystinuria is moderately rare, and the others are very uncommon. The most dangerous are porphyrinuria, with its hypersensitisation to light, and cystinuria, which tends to the formation of renal calculi. 

Very few radicals exist in tissues at rest . Two important exceptions are melanin radicals and tyrosine radicals. The former exist in low concentrations in samples of melanin, which is a high polymer made up of units which include potential semiquinone units. It is reasonable to expect some orf/io-semiquinone formation from such structures. The radicals can be thought of as occluded, being sterically protected by surrounding polymer. So far as we know, they have no special chemical significance. 

The first two steps in the synthesis of melanin are catalyzed by tyrosinase, a copper-containing oxidase, which converts tyrosine to dopaquinone. All subsequent reactions presumably occur through nonenzymatic auto-oxidation, in the presence of zinc, with formation of the black to brown pigment eumelanin. The yellow to reddish brown, high-molecular-weight polymer known as pheomelanin and the low-molecular-weight trichromes result from addition of cysteine to dopaquinone and further modification of the products. Pheome-lanins and trichromes are primarily present in hair and feathers. 

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). 

Using crystalline amino acids, they report growth failure for this organism upon the omission from the medium of any one of the nine essential amino acids, except phenylalanine. Tyrosine or phenylalanine, but not both, were necessary, especially for melanine formation. The ability to reduce tyrosine to phenylalanine is rare among animals, and this report needs confirmation before acceptance. 

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]. 

Natural melanins usually occur in the form of melanoproteins and thioether linkages, such as those mentioned above, may be important in the overall molecular structure. However, in this regard several attempts to demonstrate the formation of addition products between the oxidation products of either 5,6-dihydroxyindole or DOPA with certain proteins and peptides including ovalbumin [218, 224] or bovine serum albumin [224] have been unsuccessful. More recently, however, it has been shown that when tyrosine was oxidised in the presence of bovine lens protein, brown or black melanoproteins were formed [225]. On hydrolysis these pigments gave rise to a compound with similar properties to those of a (110)-type compound, which could have been formed from the oxidation of DOPA in the presence of cysteine. The thiol groups of the protein may react with some of the intermediates produced by the oxidation of tyrosine [225]. Reactions such as this may be involved in the formation of cataracts in the eye [225]. 

---