The therapeutic effect of ebselen has been linked to its peroxidase activity. In the present study, the peroxidase activity of ebselen toward H2O2 with the endogenous thiols GSH and dihydrolipoate [L(SH)2] as cofactors was determined. When GSH was used, peroxide removal was described by a ter uni ping pong mechanism with Dalziel coefficients for GSH and H2O2 of 0.165 +/- 0.011 and 0.081 +/- 0.005 mM min, respectively. When L(SH)2 was used, peroxidase activity was independent of the concentration of L(SH)2 in the concentration range studied (5 microM to 2 mM) and peroxide removal was only dependent on the concentration of H2O2 and ebselen, with the second-order rate constant being 12.3 +/- 0.8 mM-1 min-1. To elucidate the difference between GSH and L(SH)2, the molecular mechanism of the peroxidase activity of ebselen was investigated, using UV spectrophotometry, high pressure liquid chromatography, 77Se NMR, and mass spectrometry. GSH was found to react quickly with ebselen to give a selenenyl sulfide, an adduct of GSH to ebselen. Subsequently, the GSH-selenenyl sulfide is converted into the diselenide of ebselen. Finally the diselenide reacts with a peroxide and ebselen is regenerated. The formation by GSH of the diselenide from the GSH-selenenyl sulfide of ebselen is slow and linearly dependent on the concentration of free thiol; however, no net consumption of GSH was observed. Furthermore, it is likely that a selenol is an intermediate in diselenide formation. After reaction between ebselen and L(SH)2 the diselenide of ebselen was immediately detected. The fast formation of the diselenide with L(SH)2 versus the slow formation of the diselenide with GSH accounts for our observation that L(SH)2 is a better cofactor than GSH in the peroxidase activity of ebselen. Our results suggest that the interaction between ebselen and L(SH)2 might be of major importance in the mechanism by which ebselen exerts its therapeutic effect.