Kinetic, spectroscopic and in silico characterization of the first step of the reaction between glutathione and selenite

Abstract

Reduction of dietary selenite (SeO3H−, SeO3H2) is an important process in vivo, which predominantly involves glutathione (GSH). Although the reaction between selenite and thiols has been studied extensively, its mechanism and the identification of products remain controversial. Herein, we present kinetic, spectroscopic and in silico data on the first step of the reaction between GSH and SeO32− in aqueous solutions of varying acidity. We found that the reaction reversibly produces glutathione-S-selenite (GS-SeO2−) absorbing at 259 nm in the UV spectrum. Assignment of the absorption maximum at 259 nm to GS-SeO2− was performed using TDDFT and mass spectrometry. GS-SeO2− undergoes protonation in acidic medium to form the corresponding conjugated acid, GS-SeO2H (pKa = 1.9 at 25 °C), which exhibits reduced absorption intensity at 259 nm. According to the kinetic data, the mechanism of GS-SeO2−(H+) formation includes two pathways: (i) nucleophilic substitution of HO-group in biselenite by the thiolate group of GSH, and (ii) nucleophilic substitution of HO-group in selenous acid by the thiol group of GSH. The complex GS-SeO2−(H+) is unstable in aqueous medium and undergoes hydrolysis to initial reactants, which is accelerated by an increase in alkalinity.