Cclxiv. the Accumulation of 1-3:4- Dihydroxyphenylalanine in the Tyrosinase-tyrosine Reaction

ALTHOUGH it is easily demonstrated that when tyrosinase acts upon tyrosine 1-3:4-dihydroxyphenylalanine (dopa) is produced, it has hitherto not been clear why this is possible. Tyrosinase acts far more rapidly on dopa than on tyrosine, and dopa would therefore not be expected to accumulate in the reaction. However, from a mealworm tyrosinase-tyrosine reaction, which has proceeded 2-5 hr., dopa can be isolated in yields varying from 10 to 20% of the actual tyrosine oxidized. Although, as shown in the preceding paper, the presence of peroxidase and other catechol oxidases explains satisfactorily why dopa accumulates to a smaller extent in the case of the plant tyrosinase preparation, it does not explain its accumulation in the animal tyrosinase-tyrosine reaction, in spite of the rate at which tyrosinase oxidizes dopa. Indeed, it seems to be generally true that tyrosinases from all sources oxidize o-dihydric phenols at a much faster rate than monohydric phenols. Pugh [1930] and Graubard & Nelson [1935] have investigated the actions of tyrosinase preparations subjected to different methods of purification on catechol