Excess tyrosine stimulates eumelanin and pheomelanin synthesis in cultured slaty melanocytes from neonatal mouse epidermis

The mouse slaty (Dct(slt)) mutation is known to reduce the activity of dopachrome tautomerase (DCT). The reduced DCT activity inhibits melanosome maturation and reduces the melanin content in the skin, hair and eyes. It is not known whether eumelanin and pheomelanin synthesis in slaty melanocytes is modulated by melanogenic factors. In this study, to address this point, epidermal melanocytes derived from 0.5-, 3.5- and 7.5-day-old wild-type mice (Dct(+)/Dct(+) at the slaty locus) and from congenic mice mutant (Dct(slt) /Dct(slt) at that locus) were cultured in serum-free primary culture with or without additional L-tyrosine (Tyr). The content of melanin was measured by high-performance liquid chromatography in the cultured melanocytes as well as culture supernatants in serum-free primary culture. L-Tyr was found to increase the content of pheomelanin in addition to eumelanin in cultured slaty melanocytes and cuture supernatants at all ages tested. The eumelanin and pheomelanin contents in culture supernatants were greater than in cultured melanocytes. The eumelanin and pheomelanin contents in culture supernatants from 7.5-day-old slaty melanocytes in the presence of L-Tyr were greater than those from wild-type melanocytes. These results suggest that the inhibition of eumelanin synthesis by the slaty mutation can be partly restored by the addition of excess L-Tyr. Eumelanin and pheomelanin may accumulate with difficulty in slaty melanocytes and be easily released from them during skin development. L-Tyr may stimulate this release.