Detoxification of mercury by selenium by binding of equimolar Hg-Se complex to a specific plasma protein

Toxicity of mercury (Hg) can be reduced by coadministration with selenium (Se), and this has been explained by the formation of a complex between a specific plasma protein and the two elements, which are bound to the protein at an equimolar ratio. The purpose of the present study was to characterize the specific binding protein in order to clarify the detoxification mechanism. The coadministration of 82Se-enriched selenite and mercuric chloride into a rat produced a 82Se- and Hg-binding peak on a gel filtration column as measured by high-performance liquid chromatography with detection by inductively coupled argon plasma-mass spectrometry (ICP-MS). The specific binding protein was also detected in vitro by incubating 82Se-enriched selenite and mercuric chloride in serum in the presence of glutathione. The molar ratio of Se/Hg = 1 was maintained in binding not only to the specific protein but also to other proteins under any condition. In in vitro experiments, it was shown that although the two elements could bind to many plasma proteins, the affinity to the specific protein was extremely high and it showed a binding capacity of 500 nmol Hg or Se/the specific protein in 1 ml of serum. These results suggest that the two elements form an equimolar complex at first and then bind specifically to the protein. Further, the binding of the two elements to the protein was inhibited by the addition of polylysine to the reaction mixture, suggesting that the two elements interact with the protein through basic amino acids in the molecule and also that the protein may be one of the heparin-binding proteins since the heparin-binding sites mainly consist of basic amino acids.