Mechanism of Cu(II) Detoxification by Ergothioneine: Reduction and Formation of Stable Cu(I) Complexes

Ergothioneine (ESH) is one of the most abundant antioxidants in the human body, but its biological functions remain unclear. In this work, we elucidated the detailed mechanism of the deactivation of divalent copper ions Cu(II) by ESH using optical spectroscopy, NMR, and LC-MS. We found that the reduction of Cu(II) ions to Cu(I) by ESH occurs in a bimolecular reaction of Cu^II(ES^-)₂ complexes, leading to the formation of the disulfide ESSE. Cu(I) ions remain bound in stable complexes with ESH, preventing their reactions with molecular oxygen. Over longer timescales, ESSE decomposes, regenerating reduced ESH and forming the final reaction product, hercynine (EH). The conversion of one ESH molecule to EH causes the reduction of four Cu(II) ions. We determined the rate constants of the bimolecular reaction between Cu^II(ES^-)₂ complexes to be k₃ = (8 ± 4) × 10⁴ M^-1s^-1 and of ESSE hydrolysis to be k₄ = (1.7 ± 0.4) × 10^-5 s^-1 and also estimated the formation constant K₁₂ ≈ 1.2 × 10¹⁷ M^-2 for the Cu^II(ES^-)₂ complex. The high efficiency of Cu(II) reduction and the chelation of the resulting Cu(I) ions in a stable complex supports the hypothesis that protecting cells from oxidation by metal ions is likely one of the main functions of ESH.