MAMMALIAN TYROSINASE AND DOPA OXIDASE

Most of the experimental work concerning the mechanism of melanin production in mammals indicates that the pigment is formed from the amino acid, dihydroxyphenylalanine (dopa), by the action of a specific oxidative enzyme. The evidence for this reaction has in large part been presented by Bloch (l), whose experiments were .confirmed by Laidlaw and others (2). Bloch treated frozen sections of skin with a 1 per cent aqueous solution of dihydroxyphenylalanine buffered at pH 7.4, and over a period of 24 hours noted the appearance of a dark pigment in the cytoplasm of the melanoblasts. The reaction was inhibited by cyanide ion and by heat and failed to occur with a number of other substrates, including tyrosine. Bloch concluded that a specific enzyme, which he named dopa oxidase, is responsible for the formation of melanin. Although several tyrosine-oxidizing enzymes have been extracted from certain plants and insects, there is no conclusive evidence for the existence of a mammalian tyrosinase. A number of attempts to demonstrate this enzyme in extracts of mammalian skin by means of a color reaction have been made, but the results are contradictory and inconclusive (3-9). Color reactions with dihydroxyphenylalanine have usually been obtained with such extracts, but the amount of dopa oxidase extractable is apparently very small. It seems evident, therefore, that normal pigmented tissues do not provide a rich enough source fey any detailed study of this enzyme system. The melanoma, a tumor composed chiefly of melanin-producing cells, might be expected t,o provide a richer source of the enzyme. Several experiments (10, 11) have indicated that extracts of mclanomata possess activity against, catechol derivatives, but the presence of tyrosinase has not been proved. The melanoma which we have utilized arose spontaneously in bhe skin of the ear of a chocolate-brown mouse and was reported by Harding and Passey in 1930 (12). It was found to be easily transplantable to mice of all colors, including albinos, and it has been carried for a number of years in the Rockefeller Institute strain of albino mice. Although slow growing, the neoplasm attains a relatively enormous size, often as much as one-third that of its host, in about 3 months. Grossly, it is a soft, encapsulated, jet-black nodule. Microscopically, it is made up of two cellular elements, one a cuboidal cell containing a relatively