A comparative study of the production of l-3:4-dihydroxyphenylalanine from tyrosine by tyrosinase from various sources

Abstract

L-3:4-dihydroxyphenylalanine (dopa) was estimated in the presence of tyrosine by precipitating the former with Pb acetate at an alkaline pH not greater than 9. Pb was removed from the precipitate by H2S in weak acetic acid soln. and the dopa estimated either by the molybdate method of Rae or by Vulpian''s method. After removal of Pb from the filtrate, tyrosine was estimated by bromination. Tyrosinase from mealworms or Calliphora larvae produced more dopa than tyrosinase from bran or potatoes. The diminution of dopa production with the vegetable enzymes was due to the presence of peroxidase or polyphenolases. This effect of peroxidase provided evidence that H2O2 was produced in the oxidation of tyrosine by tyrosinase.