The Toxicity of Melanin Precursors

The quinone intermediates resulting from tyrosinase-mediated oxidation of tyrosine were evaluated as sulfhydryl reagent inhibitors of purified calf thymus DNA polymerase alpha in order to determine which of these might be cytotoxic. Dopachrome and an oxidation product of 2,4,5-trihydroxyphenylalanine were relatively ineffective as inhibitors of DNA polymerase alpha. On the other hand, a dopaquinone analogue, 4-(2-N-acetylaminoethyl)-1,2-benzoquinone, synthesized from N-acetyl dopamine, was demonstrated to have marked affinity for this sulfhydryl enzyme. This property was shared by 1,2-benzoquinone. These studies point to dopaquinone as a significant toxic metabolite in melanin biosynthesis.