Owing to their various physiological activities, thiol compounds, such as l-cysteine with UV-protection properties and captopril that inhibits the catalytic activity of angiotensin converting enzyme (ACE), are currently used as supplements and pharmaceuticals. Glutathione (GSH) plays an important role in intracellular protective effects and is currently used for the treatment of cataract and detoxification from metal poisoning. In contrast to GSH, the GSH precursor γ-glutamyl cysteine (γ-EC) has been reported to exhibit neuroprotective effects, thus making it an attractive key biological protective molecule. However, its characteristics are largely unknown. Here, we evaluated the ACE inhibitory function of γ-EC and its mechanism by comparing it with that of GSH in vitro. ACE inhibitory analysis showed that the IC 50 of GSH and γ-EC against ACE were 8.3 μM and 3.9 × 10 2 μM, respectively. These data suggested that γ-EC exerted ACE inhibitory activity, but it was weaker than that of GSH. Docking simulation showed that the ACE inhibitory activity of both compounds was due to the interaction of their carboxyl groups of Glu with Zn 2+ in the active center of ACE. Moreover, GSH could fit more compactly in the pocket of ACE, forming more hydrogen bonds with the enzyme than γ-EC. By analyzing its kinetics and in vivo efficacy, we hope that γ-EC could be used as a promising compound for lowering blood pressure in applications with moderate activity, such as functional foods.Key words thiol, γ-glutamyl cysteine, angiotensin converting enzyme Fig. 1. The Structure of GSH and Its Precursor γ-EC Structural formulas of (a) GSH and (b) γ-EC. GSH is a tripeptide consisting of Glu, Cys, and Gly. γ-EC is a dipeptide consisting of Glu and Cys. Both have unusual γ-peptide bonds between Glu and Cys.