Dopachrome oxidoreductase

Purification procedure via treatment with HCl Ultrastructural and cytochemical study Magnesium dependence and hair graying Study of dopachrome oxidoreductase and tyrosinase activity Correlation between melanin charge and intensity of ESR signal of irradiated hair... 

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). 

Leonard LJ, Townsend D, King RA (1988) Dopachrome Oxidoreductase and Metal Ions in Dopachrome Conversion in the Eumelanin Pathway. Biochemistry 27 6156... 

For many years the biosynthesis of melanin was thought to result from the spontaneous oxidation and polymerization of dopachrome produced by the tyrosinase-catalyzed hydroxylation of tyrosine to dopa and subsequent oxidation (5 ). In addition to tyrosinase, however, several enzymatic factors have been recently identified in mammalian tissues that appear to regulate melanogenesis at intermediate steps distal to those involving tyrosine and dopa. The factors include dopachrome conversion factor, dihydroxyindole blocking factor, dihydroxyindole conversion factor and dopachrome oxidoreductase (54-59). 

Dihydroxyindole blocking factor blocks the indolization of quinone imine derivatives. Dihydroxyindole conversion factor catalyzes the dehydrogenation of 5,6-dihydroxyindole to indole-5,6-qui-none. Dopachrome oxidoreductase converts dopachrome to 5,6-dihydroxyindole and also may block 5,6-dihydroxyindole oxidation and subsequent melanogenic reactions. Relatively little information is available about the physical, chemical and kinetic properties of these proteinaceous factors in mammals. Controversy about melanin-related regulatory factors has focused on whether activity is due to unique individual proteins or is only an expression of activities of a multicatalytic enzyme (61.62). For example, dihydroxyindole conversion activity in mice melanoma is apparently due to tyrosinase, not a unique factor (56). 

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. 

Some reports associate the rearrangement of dopachrome to DI in melanoma with an enzyme distal from tyrosinase (179-182) characterized as an oxidoreductase (183). Owing to the preliminary nature of the supporting experiments, however, the regulatory effect may be associated with the catalytic effect of metal ions rather than that of the new enzyme (184). 

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