MAMMALIAN TYROSINASE: MELANIN FORMATION BY ULTRAVIOLET IRRADIATION

Raper 1 and his associates, in a comprehensive study of the tyrosine-tyrosinase reaction, showed that the enzyme tyrosinase obtained from plant sources catalyzes the conversion of tyrosine to dopa (3,4-dihydroxyphenyl-l-alanine). Dopa is then oxidized, in the presence of the same enzyme, to a red indole derivative, which changes spontaneously to melanin. Melanin formation in mammals has remained somewhat of an enigma, despite the demonstration by Bloch 2 of an intracellular oxidase which catalyzes the oxidation of dopa to melanin but is without apparent effect on tyrosine. The presence of dopa in mammalian tissue or fluids has never been demonstrated, and it has not been known how tyrosine is converted into dopa. Recently, however, the enzymatic formation of melanin from tyrosine has been conclusively demonstrated in several types of mammalian tissue, including beef ciliary bodies 3 and human, 4 horse 5 and mouse 6 melanomas. The enzyme has been shown to