| Research Abstract |
Since 1978, 5,6-dihydroxyindole (DHI) is thought to be a most potent cytotoxic intermediate in melanin biosynthetic pathway. Tyrosinase, an essential enzyme for melanogenesis catalyzes L-tyrosine to dopaquinone, which is immediately cyclized to be dopachrome. Dopachrome (DC) is converted to DHI-2-carboxylic acid (DHICA) by DC tautomerase (Dct), or is auto-oxidized to be DHI without Dct. Since DC is a quite unstable intermediate, a conventional long time exposure to the cells is not an appropriate condition to estimate the cytotoxicity of this compound. In this study, we examined the cytotoxicities of L-DOPA, DC, DHI and DHICA under a short time exposure (30-min) using colony formation assays on murine melanoma cells. As the results, we found that if cytotoxicity of L-DOPA was referred to as one, DC was 156, DHI was 4.7 and DHICA was 1.3. In addition, 0.1 mM L-DOPA solution didn't show any cytotoxicity to the cells in our condition, if it was mixed with tyrosinase, the solution became significantly cytotoxic to the cells. These facts suggested that DC supposed to be a most potent cytotoxic intermediate in melanogenic pathway. In this context, Dct might act as an important survival enzyme against DC cytotoxicity in melanogenic cells, and inhibitors of this enzyme (Dct) could have a therapeutic effect against malignant melanoma. We are going to examine whether reported inhibitors, such as tryptophan and indole-3-propionic acid enhance the cytotoxicities derived from melanogenesis.
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