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⁺.