The Oxidation of Equol by Tyrosinase Produces a Unique Di-ortho-Quinone: Possible Implications for Melanocyte Toxicity

Equol (7-hydroxy-3-(4′-hydroxyphenyl)-chroman, EQ), one of the major intestinally derived metabolites of daidzein, the principal isoflavane found in soybeans and most soy foods, has recently attracted increased interest as a health-beneficial compound for estrogen-dependent diseases. However, based on its structure with two p-substituted phenols, this study aimed to examine whether EQ is a substrate for tyrosinase and whether it produces o-quinone metabolites that are highly cytotoxic to melanocyte. First, the tyrosinase-catalyzed oxidation of EQ was performed, which yielded three EQ-quinones. They were identified after being reduced to their corresponding catechols with NaBH4 or L-ascorbic acid. The binding of the EQ-quinones to N-acetyl-L-cysteine (NAC), glutathione (GSH), and bovine serum albumin via their cysteine residues was then examined. NAC and GSH afforded two mono-adducts and one di-adduct, which were identified by NMR and MS analysis. It was also found that EQ was oxidized to EQ-di-quinone in cells expressing human tyrosinase. Finally, it was confirmed that the EQ-oligomer, the EQ oxidation product, exerted potent pro-oxidant activity by oxidizing GSH to the oxidized GSSG and concomitantly producing H2O2. These results suggest that EQ-quinones could be cytotoxic to melanocytes due to their binding to cellular proteins.