Occurrekce in Melanoma Akd Binding Constant*

3,4-Dihydroxy-L-phenylalanine (dopa) has been shown to account for the cofactor activity for tyrosinase contained in extracts of hamster melanoma. Dopa has been identified by several different comparisons with an authentic sample and by conversion to two derivatives. The amount of dopa in the tumor (4 to 8 pg per g) has been estimated by cofactor assay and fluorometry. A Km value for dopa as a cofactor was estimated to be about 2 x low6 M, much lower than its K, as a sole substrate (5 x 10-“&x).