Tyrosinase is able to oxidize a great number of phenols and catechols to form ortho-quinones. Ortho-quinones are highly reactive compounds that exert cytotoxicity through binding with thiol enzymes and the production of reactive oxygen species. Certain phenolic (and catecholic) compounds are known to induce contact/occupational leukoderma through activation to ortho-quinones. We report a convenient screening method to follow the oxidation of those leukoderma-inducing phenols by mushroom tyrosinase. Oxidation of phenolic compounds by mushroom tyrosinase was followed periodically by UV-vis spectrophotometry. The production of ortho-quinones were confirmed by their absorptions around 400-420 nm. HPLC analysis after reduction with NaBH4 detected the corresponding catechols. Leukoderma-inducing phenols, rhododendrol, raspberry ketone, 4-methoxyphenol, 4-benzyloxyphenol, 4-tert-butylphenol, and 4-tert-butylcatechol, were readily oxidized by mushroom tyrosinase to form ortho-quinones. On the other hand, phenolic skin whitening tyrosinase inhibitors, ellagic acid, 4-n-butylresorcinol, potassium 4-methoxysalicylate, and 2,2'-dihydroxy-5,5'-di-n-propylbiphenyl, were not oxidized by mushroom tyrosinase, while arbutin was only slowly oxidized. This study has provided a convenient screening method to differentiate phenolic skin whitening tyrosinase inhibitors from leukoderma-inducing phenols. A common chemical feature of the latter group of compounds is that they are readily oxidized by tyrosinase to form reactive ortho-quinone species. The present results point out the necessity that tyrosinase inhibitors should also be examined as substrates if they are phenolic compounds.
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