γ-Glutamylcysteine, a Glutathione Precursor, Exhibits Higher Thiol Reactivity for Complex Formation with Iron (III) Ions Compared to Glutathione

Abstract

Glutathione (GSH), the most abundant intracellular thiol compound, protects various cells from metal toxicities by forming complexes with metal ions through the thiol group. γ-Glutamylcysteine (γ-EC), a glutathione precursor, is anticipated to be a functional thiol compound. However, unlike GSH, the characteristics of γ-EC in metal complex formation are largely unclear. In this study, we analyzed the ability of γ-EC to form complexes with various metal ions. 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) assays demonstrated that the reaction ratios between DTNB and γ-EC and GSH were slightly reduced by adding light metal ions, such as K⁺, Mg²⁺, and Al³⁺. These results indicated that γ-EC and GSH exhibit low thiol reactivity and weak complex formation with these ions. In contrast, the reaction ratio was reduced in a concentration-dependent manner by the addition of heavy metal ions, such as Ag⁺, Cu²⁺, Zn²⁺, and Fe³⁺. Specifically, the reaction ratio in the γ-EC-treated group was significantly reduced by the addition of Fe³⁺ compared to that in the GSH-treated group. These data indicate that, while γ-EC as well as GSH form the complexes with Ag⁺, Cu²⁺, and Zn²⁺, γ-EC has a stronger interaction with Fe³⁺ than GSH. In the proposed complex model based on the hard and soft acids and bases (HSAB) principle, GSH theoretically forms unstable nine-membered rings with Fe³⁺, whereas γ-EC can form more stable six-membered rings, resulting in a strong interaction between γ-EC and Fe³⁺.