Electrochemical behaviors of sulfhydryl compounds in the presence of elemental mercury

In the expression of bioaccumulated elemental mercury (Hg-0) toxicity, the first Hg-0 oxidation step is crucial. Therefore, to clarify the mechanism underlying the interactions of sulfhydryl (SH) compounds and Hg-0 in the present study, we analyzed the oxidation of reduced glutathione (GSH) and L-cysteine (Cys) in the presence of Hg-0 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 Hg-0, but the oxidation of Cys after a 24-h incubation was suppressed. Conversely, GSH oxidation increased with incubation time in the absence of Hg-0. In the presence of Hg-0, the oxidation of GSH was suppressed. The different reactivities of Cys and GSH with Hg-0 may arise from differences in their oxidation/reduction potentials and pH. The important SH compound interactions with Hg-0 oxidation were as follows: (i) oxidation of Hg-0 to form either mercurous ion (Hg+) or mercuric ion (Hg-2%) which both form stable complexes in aqueous solution as Hg-I (RS) or Hg-II (RS)(2); (ii) catalyzed oxidation of SH compounds in the presence of Hg-0, 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 Hg-0 dissolves in aqueous solution in the presence of SH compounds, and contribute to our understanding of SH compounds in non-enzymatic Hg-0 oxidation in vivo. (C) 2007 Elsevier Ltd. All rights reserved.