Search

Three new regulatory factors in the melanogenesis pathway were recently described: dopachrome conversion factor accelerates the conversion of dopachrome to 5,6-dihydroxyindole; indole conversion factor accelerates the conversion of 5, 6-dihydroxyindole into melanin; and indole blocking factor retards the conversion of 5, 6-dihydroxyindole into melanin. Exposure of wild-type Cloudman melanoma cells in culture to melanotropin (MSH) removes blocking factor activity and increases indole conversion factor activity. The chemical nature of factors has not yet been determined. In this report we demonstrate that highly purified isozymes of tyrosinase from C57B1/6N murine hair bulbs and B16 murine melanoma are closely associated with conversion and blocking factor activities. The soluble isozymes. T1, T2, and T2 contain blocking factor activity, while isozyme T4, the major tyrosinase species found in melanosomes, contains activity that accelerates melanin formation from dopachrome. The results suggest that melanogenesis is regulated by the association of these different factors with the specific tyrosinase isozymes.

The oxidation of dopachrome to melanin occurs spontaneously in vitro but has been shown to be under enzymatic control in vivo. Dopachrome conversion factor, purified from Cloudman S91 melanoma cells, converts dopachrome, which is orange, to a colorless compound. We have purified this colorless compound and identified it as 5,6-dihydroxyindole-2-carboxylic acid. This compound is generated enzymatically from a solution containing dopa and dopachrome and, under these conditions, is not readily oxidized to melanin in solution at pH 6.8. The synthesis in vitro of 5,6-dihydroxyindole-2-carboxylic acid suggests that this compound may be an intermediate in the synthesis of melanin.