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Regulatory factors for polymerization of melanin monomers within coated vesicles and premelanosomes in melanoma cells.

Coated vesicles (CV) and premelanosomes (PMS) differ in the amount of melanin monomers produced from 2-carboxy-2,3-dihydroindole-5,6-quinone (dopachrome) in vitro. CV convert dopachrome preferentially to 5,6-dihydroxy-indole-2-carboxylic acid (DHI2C) and PMS mainly to 5,6-dihydroxyindole (DHI). We examined the effect of CV and PMS on in vitro spontaneous polymerization of DHI and DHI2C. The polymerization of DHI is accelerated by addition of extracts from both CV and PMS, while polymerization of DHI2C is suppressed. Factors associated with these two regulatory activities have been separated on concanavalin A-Sepharose. The ConA-adhesive glycoproteins isolated from PMS enhance polymerization of not only DHI but also DHI2C. The latter effect is stimulated by dopa. Both accelerating activities can be inhibited by phenylthiourea (PTU). In contrast to glycoproteins, the non-glycoprotein subfraction suppresses spontaneous polymerization of both melanin monomers. Our data suggest that the absence of in vivo melanin polymer formation within CV can be explained by formation by these organelles of relatively stable DHI2C rather than easily polymerizing DHI.