The Dual Nature of Sesamol: Tyrosinase-Dependent Bioactivation from an Inhibitor to a Cytotoxic Pro-Oxidant.

Sesamol (3,4-methylenedioxyphenol) is a unique natural phenolic compound derived from sesame seeds that is characterized by its concurrent phenolic and methylenedioxy moieties. The purpose of this study was to investigate the tyrosinase-mediated metabolic activation of sesamol to determine whether a cytotoxic quinone intermediate is produced, mirroring the mechanism established for p-substituted phenols such as resveratrol. Although sesamol is recognized as a potent tyrosinase inhibitor, we demonstrate that in the presence of catalytic amounts of L-DOPA, it serves as an efficient substrate for tyrosinase-mediated bioactivation. The resulting metabolite, sesamol-5,6-quinone, was identified via reduction with NaBH4 or ascorbic acid to its corresponding catechol, followed by isolation through acetylation to stabilize the transient species. The reactive nature of sesamol-5,6-quinone was confirmed through the formation of covalent adducts with N-acetylcysteine. In metabolic experiments using human tyrosinase-expressing 293T cells, we identified the reactive metabolite, sesamol-5,6-quinone, through the formation of intracellular thiol adducts released into the medium. While sesamol is systemically protective, it induced a marked tyrosinase-dependent cytotoxicity in B16BL6 melanoma cells, which was significantly suppressed by tyrosinase knockdown. Crucially, sesamol oligomers exhibited potent pro-oxidant activity, generating hydrogen peroxide and depleting reduced glutathione through its oxidation to GSSG. These findings suggest that tyrosinase-mediated conversion transforms sesamol into a highly reactive pro-oxidant, which induces substantial cytotoxicity in melanin-producing cells via protein arylation and profound redox imbalance.