The Tyrosinase-tyrosine Reaction: Production from Tyrosine of 5: 6-Dihydroxyindole and 5: 6-Dihydroxyindole-2-carboxylic Acid-the Precursors of Melanin.

IN the last communication of this series [Raper, 1926] it was shown that if a solution of the red pigment which is produced from tyrosine by tyrosinase is allowed to decolorise under suitable conditions, then 3: 4-dihydroxyphenylalanine may be isolated from the solution. It was pointed out also that in the solution obtained by this method, the immediate precursor of melanin is present and that it possesses a structure in which the nitrogen is no longer in the amino form. In the present paper an account is given of the identification of this precursor of melanin and the light it throws on the chemical changes involved in its production from tyrosine. It has not been possible to isolate the precursor itself because it is too easily oxidised, but its methyl derivative has been isolated in crystalline form and satisfactorily identified. When the red solution obtained from tyrosine by the action of tyrosinase, as described pteviously [Raper, 1926], is allowed to decolorise in vacuo or in presence of sulphurous acid and then, after concentration in an atmosphere of C02, is methylated by means of methyl sulphate in an atmosphere of hydrogen, from the reaction product two crystalline methylated products can be obtained. One of them is a feeble base and the other an acid. The former is obtained principally when the red substance has been allowed to decolorise in vacuo and the latter when decolorisation takes place in the presence of sulphurous acid. Both products give colour reactions characteristic of indole derivatives. It was postulated in the paper already mentioned that the 3: 4-quinone of phenylalanine (I) is formed prior to the production of the red substance in the tyrosinase-tyrosine reaction; and in attempting to determine the structure of the indole derivatives to which the red substance gives rise, it had to be considered whether in the formation of the indole ring the N of the amino group of the phenylalanine quinone attached itself in the 2 or 6 position in the benzene nucleus. Union in the 6 position would produce