Electrochemical behaviors of sulfhydryl compounds in the presence of elemental mercury

In the expression of bioaccumulated elemental mercury (Hg0) toxicity, the first Hg0 oxidation step is crucial. Therefore, to clarify the mechanism underlying the interactions of sulfhydryl (SH) compounds and Hg0 in the present study, we analyzed the oxidation of reduced glutathione (GSH) and l-cysteine (Cys) in the presence of Hg0 in aqueous solution by cyclic voltammetry (CV). Production of Hg2+ in the reaction mixture was found to increase along with a decrease in free SH residues. CV showed that the oxidation of Cys increased after a 4-h incubation in the presence of Hg0, but the oxidation of Cys after a 24-h incubation was suppressed. Conversely, GSH oxidation increased with incubation time in the absence of Hg0. In the presence of Hg0, the oxidation of GSH was suppressed. The different reactivities of Cys and GSH with Hg0 may arise from differences in their oxidation/reduction potentials and pH. The important SH compound interactions with Hg0 oxidation were as follows: (i) oxidation of Hg0 to form either mercurous ion (Hg+) or mercuric ion (Hg2+) which both form stable complexes in aqueous solution as HgI (RS) or HgII(RS)2; (ii) catalyzed oxidation of SH compounds in the presence of Hg0; and (iii) suppression of the oxidation of SH compounds due to the reduced concentration of free SH compounds through the binding of SH compounds with Hg+ or Hg2+. The present results demonstrate the chemical reaction processes by which Hg0 dissolves in aqueous solution in the presence of SH compounds, and contribute to our understanding of SH compounds in non-enzymatic Hg0 oxidation in vivo.