5,6-Dihydroxyindole oxidation by mammalian, mushroom and amphibian tyrosinase preparations.

The oxidation of 5,6-dihydroxyindole by tyrosinases from mushroom, Harding-Passey melanoma, bovine eye and Bufo bufo embryo has been investigated. The apparent Km values for this substrate were measured and found to be of the same order of magnitude as those for L-tyrosine and L-3,4-dihydroxyphenylalanine, as reported in the literature (5 x 10(-4) M). The 5,6-dihydroxyindole oxidases of mushroom and T4 melanoma isozyme are sensitive to phenylthiourea, while, on the other hand, those from crude preparations of bovine and B. bufo tyrosinases are not sensitive to the inhibitor in an evident manner. The action of some indole derivatives on the 5,6-dihydroxyindole oxidase of mushroom has also been investigated.