Dopachrome conversion factor

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). 

Pawelek JM (1990) Dopachrome Conversion Factor Functions as an Isomerase. Biochim Biophys Res Commun 166 1328... 

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). 

Dopachrome conversion factor catalyzes the decolorization of dopachrome. The mechanism of this conversion apparently involves an isomeric rearrangement of a hydrogen atom from one position of the dopachrome molecule to another, an intramolecular oxidoreduction which results in a tautomeric shift forming 5,6-dihydroxyindole-2-... 

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). 

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