L-Dopachrome

S. japonicum and S. haematobium) (Pennock et al. 1998). cDNAs were obtained for MIF/L-dopachrome methyl ester tantomerase homolognes from the nematodes... 

Pennock JL, Behnke JM, Bickle QD, Devaney E, Grends RK, Isaac RE, Joshua GWR Selkirk ME, Zhang Y, Meyer DJ (1998) Rapid purification and characterization of L-dopachrome-methyl ester tautomerase (macrophage-migration-inhibitory factor) from Trichinella spiralis, Trichuris muris and Brugia pahangi. Biochem J 335 495 98... 

Further, recent studies 132) have revealed the presence in melanocytes of a melanosomal protein different from tyrosinase, which has the ability to catalyze the rearrangement of dopachrome to DHICA. This enzymic reaction is highly stereospecific for normal L-dopachrome, is unaffected by metal chelators and has an optimal pH of about 6.8. Different names have been proposed for this enzyme, i.e. dopachrome conversion factor 132, 256), dopachrome oxidoreductase 143), dopachrome isomerase 201), and dopachrome tautomerase 4). It is of interest that another enzyme named dopaquinoneimine conversion factor seems to exist which has the remarkable ability to catalyze the decaibox-ylative rearrangement of dopachrome to DHI rather than DHICA 193). 

Melanin biosynthesis in animals is a complex process starting with the L-tyrosine amino acid. In the first step, L-tyrosine is converted first into DOPA and then into dopaquinone, a process catalyzed by tyrosinase. In the biosynthesis of eumelanins, dopaquinone undergoes a cyclization to form dopachrome and subsequently a tau-tomerization into 5,6-dihydroxyindole-2-carboxylic acid (DHICA). DHICA is further oxidized to indole-5,6-quinone2-carboxylic acid, the precnrsor of DHICA eumelanins. Tyrosinase-related proteins TRP-2 and TRP-1, respectively, are responsible for the last two steps, and they are under the control of the tyrosinase promoter. 

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. 

From Lys hydrochloride pyrolyzed in vacuo at 600°C for 8-10 min, a tricyclic compound 134 is formed in low yield (80TL2679). Cystine reacts with dopachrome to give an unstable product, but the methyl ester of dopachrome gave a stable pyrrolo[2,3-/i][l,4]benzothiazine 135 (87T5357). 

Tyrosinase, a copper-containing oxidoreductase, catalyzes the orthohydroxy-lation of monophenols and the aerobic oxidation of catechols. The enzyme activity will be assayed by monitoring the oxidation of 3,4-dihydroxyphenyl-alanine (dopa) to the red-colored dopachrome. The kinetic parameters Ku and Vmax will be evaluated using Lineweaver-Burk or direct linear plots. Inhibition of tyrosinase by thiourea and cinnamate will also be studied. Two stereoisomers, L-dopa and D-dopa, will be tested and compared as substrates. 

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-... 

Kinetically slow steps in the formation of melanin from DOPA are the formation of dopaquinone from DOPA (step 1, kD), the reaction of dopachrome to dihydroxyindole (step 2), and the polymerization to form melanin (step 3, kM). Step 1 and step 2 proceed with about the same rate in the oxidative coupling polymerization catalyzed by tyrosinase. However, step 1 becomes remarkably slow when a macromolecule-metal complex is used as a catalyst. The copper complex in poly(l-vinylimidazole-co-vinylpyrrolidone) has been found [38] to act as an excellent catalyst and to exhibit the highest activity for melanin formation. The ratio of the rate constants ( m/ d) is approximately 3 (tyrosinase... 

Condensation of dopachrome methyl or ethyl esters with cysteine ethyl ester in pH 6.8 aqueous phosphate buffer led to the l,2-dihydro-3//,8//-pyrrolo[2,3-/ ][l,4]benzothiazines (87), as shown in... 

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. 

The most significant 2,3-dihydro-5,6-dihydroxyindole is L-cyclodopa 78, also known as leucodopachrome, which is an intermediate in the biosynthesis of eumelanin and is formed by the spontaneous cyclization of dopaquinone 79 (92MI2) (04MI1). In turn, cyclodopa is readily oxidized to dopachrome 80 but this oxidation can be reversed chemically using sodium dithionite. Wyler and Chiovini have described the preparation of both enantiomers of cyclodopa using transformations... 

Cabanes J, Garcia-Canovas F, Lozano JA, Garcia-Carmona P (1987) A Kinetic Study of the Melanization Pathway Between L-Tyrosine and Dopachrome. Biochim Biophys Acta 923 187... 

Palumbo A, d Ischia M, Misuraca G, De Martino L, Prota G (1994) A New Dopachrome Rearranging Enzyme from the Ejected Ink of the Cuttlefish Sepia officinalis. Biochem J 299 839... 

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