Dihydroxyphenylalanine quinone

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Tyrosine, itself a degradation product of phenylalanine (Sec. 15.1), is initially converted to 3.4-dihydroxyphenylalanine (dopa), and the corresponding do pa quinone, by the copper-containing enzyme tyrosinase. Tyrosinase is found in melanocytes and is a mixed-function oxidase. It catalyzes the following reaction ... 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

Prior to the hydrolysis of the ester mentioned above, Hatano, Nozawa, Ikeda and Yamamoto (29) studied the catalytic action of poly(S-lysine) (PEL) - copper(II) complex for the oxidation of 3,4-dihydroxyphenylalanine (DOPA) to the corresponding quinone. 

The most attractive detailed hypotheses (Fig. 11) suggest the formation of the erythrinane skeleton by oxidation of LXXXVIII, a symmetrical intermediate derived from two molecules of tyrosine or dihydroxyphenylalanine. The two additional bonds necessary might be formed in either order. In one hypothesis (1, 57) oxidation of one aromatic ring to the o-quinone (LXXXIX) is followed by nucleophilic addition of the amino group and further oxidation to XC (or the related o-quinone). This sequence is exactly analogous to the in vitro oxidation of dihydroxyphenylalanine itself to the quinone dopachrome (3S). Nucleophilic or radical addition of the second phenolic ring to the quinoid system would complete the spiro skeleton of XCI. 

In normal melaninogenesis, the amino acid tyrosine is hydroxylated to form 3,4-dihydroxyphenylalanine ( dopa ), which is then oxidized to dopa-quinone. The latter moiety is polymerized to form melanin, thereafter combining with melanoprotein to form a stable complex within premelanosomes and melanosomes. Tyrosinase plays a central role in this process, by catalyzing the first step in the stated sequence. As such, it is a specific marker for melanocytic differentiation. This premise has been affirmed by studies showing that tyrosinase gene transcripts are strictly confined to melanin-producing cells. 

Recent studies have identified shell precursors from S. mansoni and Fasciola hepatica with molecular masses of 14-48 kDa (5,6) further suggesting a role for sclerotinization in eggshell formation. A 31 kDa precursor from F. hepatica is particularly unique in that it is rich in dihydroxyphenylalanine (DOPA), a potential precursor of quinone crosslinking (11). A 35 kDa eggshell precursor, identified by pulse labeling S. mansoni with [ CJtyrosine, is converted into an approximately 100 kDa protein in shells of newly laid eggs most likely due to a quinone-mediated erosslinking mechanism (12). 

A apart. Dioxygen binding results in the two-electron reduced peroxide in a side-on bound dicopper(II) structure (Fig. 1). Crystallographic information is (also) established for the reduced, as well as the oxidized met forms (3-5). Based on their spectroscopic similarities, a similar active-site structure is proposed for the monooxygenase tyrosinase (Tyr) X-ray structures are available for catechol oxidase (CO) depicting a similar active-site structure to He and Tyr (6). However, unlike the reversible binding reactivity of hemocyanin, tyrosinase catalyzes the aromatic hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (l-DOPA), and can further perform catechol oxidase activity which oxidizes the catechol to the quinone (Fig. 2) (7). 

Ascorbic acid in this system is acting as an immediate hydrogen donator to the quinone and, in conjunction with the oxidase, confers the properties of a hydrogen carrier (reversible oxidation and reduction) on substances such as 3,4-dihydroxyphenylalanine. Tyrosine under such conditions serves as a reservoir for the supply of the dihydric phenolic derivative. 

Matheis and Whitaker (59) reviewed the chemistry of the modification of proteins by polyphenol oxidase and peroxidase. Monophenols such as tyrosine are first hydroxylated to dihydroxyphenylalanine (DOPA). The DOPA is then oxidized to an ortho-quinone. Ortho-quinones can undergo at least two types of reactions with thiols. Two molecules of a thiol can participate in an oxidation-reduction reaction with a quinone to form the corresponding hydroquinone and a disulfide, or the thiol anions may add to the conjugated system of a quinone to form a substituted hydroquinone. An excess of quinone in the reaction mixture usually oxidizes the monosubstituted quinone, which may then participate in another nucleophilic addition, and so on (Figure 13). The cysteine adduct(s) cannot further react with amino groups to form brown products. 

Similar chemical reactions are involved in the reaction of other catechols such as the catecholamines, dopamine, and L-dihydroxyphenylalanine (L-DOPA) with cysteine or GSH [149-152] and can lead to the generation of mitochondrial toxins with relevance to Parkinson s disease [153-155], As well as possible cytotoxic effects of lowering cellular thiol levels or binding to cysteine residues at die active site of specific enzymes, flavonoids could act beneficially by acting to limit the formation of the potentially cytotoxic catecholamine-thiol adducts, in a manner similar to that observed for dihydro-lipoic acid [151], Because of the structural similarity of the flavonoid B-ring and their ability to donate electrons efficiently to form quinones, it is conceivable that specific flavonoids may be of use to prevent such neurotoxic compounds as 5-S-cysteinyl dopamine from forming in vivo. 

Raper suggested that the red product obtained on oxidation of 3,4-dihydroxyphenylalanine (DOPA, 3) was a cyclic 2,3-dihydroindole derivative, namely indoline-5,6-quinone-2-carboxylic acid (4)[14]. It was originally believed that (4) was identical with hallachrome, a red pigment that can be isolated from the marine worm Halla parthenopaea Costa [15]. 

Melanins are widely distributed in biopigments in bacteria, fungi, plants, and animals. Melanins are heterogeneous polyphenol-like biopolymers with a complex structure and color varying from yellow to black [210]. They are biosynthesized by a combination of enzymatic and chemical reactions which are initiated by the oxidation of tyrosine. Tyrosinase catalyzes the hydroxylation (monophenolase activity) of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and the oxidation (diphenolase or catecholase activity) of DOPA to the corresponding -quinone [211] as shown in eq. (28.)... 

Phenolase Complex. Phenolase (= tyrosinase, or phenol oxidase) converts tyrosine to dopa (= dihydroxyphenylalanine) and oxidizes the dihydroxy derivative further to the quinone stage. Through a series of subsequent reactions, some of which occur spontaneously and wathout enzymic catalysis, the black or brownish black melanin is finally formed (for a schematic representation of the reactions see Chapt. VIII-11). The phenolase is an oxidase with mixed functions, where the product of oxygenation, the hydroquinone derivative, simultaneously acts as... 

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