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

Hitomi Tanaka,Shosuke Ito,Kazumasa Wakamatsu,Kazunari Kondo,Makoto Ojika,Tomoko Nishimaki-Mogami

doi:10.3390/ijms22179145

Abstract

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.

Highlights

Equol (7-hydroxy-3-(4 -hydroxyphenyl)-chroman, EQ 1) has recently attracted increased interest as a health-beneficial compound for estrogen-dependent diseases [1,2]
UV/visible spectral changes were followed for 60 min, which showed the rapid production of a quinoid chromophore with absorption at around 400–450 nm (Figure 2a)
EQ 1 was consumed within 15 min, giving new compounds with retention times in high-performance liquid chromatography (HPLC) of 7.0, 9.9, and 12.0 min (EQ 1 appeared at 19.4 min)

Summary

Introduction

Equol (7-hydroxy-3-(4 -hydroxyphenyl)-chroman, EQ 1) has recently attracted increased interest as a health-beneficial compound for estrogen-dependent diseases [1,2]. In 1982, EQ 1 was first detected in human urine and blood as a non-steroidal estrogenic compound [4] and was found in high concentrations in the urine of about 40% of adults who consumed soy foods [5]. EQ 1 is one of the main metabolites of daidzein and is an isoflavane phenolic compound with a non-planar structure that is optically active with an asymmetric carbon atom at the C3 position giving rise to (R)-(+)and (S)-(−)-EQ enantiomers (Figure 1). EQ 1 has been shown to have antioxidant activity [10,11] and binds to estrogen receptors to show a stronger estrogenic activity than that of any other isoflavone or isoflavone-derived metabolite [8,12]

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