Binding of equimolar mercury (Hg) and selenium (Se) to a specific plasma protein in the detoxification of Hg was studied in vitro by the HPLC/inductively coupled argon plasma-mass spectrometry (ICP-MS) method with use of an enriched stable isotope. Hg and 82Se became co-eluted with endogenous 78Se on a size exclusion column by incubation of 0-200 microM HgCl2 and 82Se-enriched selenite with rat serum in the presence of glutathione at 37 degrees C for 10 min. The endogenous 78Se peak was the most abundant plasma Se-containing protein, and it showed the affinity to heparin, indicating it to be selenoprotein P (Sel P). The 82Se/endogenous Se ratio of (Hg-Se)-Sel P complex changed with doses of HgCl2 and 82Se-enriched selenite and amounted to more than 100, suggesting that more than 1,000 units of (Hg-Se) bind to Sel P based on the fact that there are 10 selenocysteinyl residues per Sel P. These results indicate that equimolar Hg and Se bind to Sel P to form the {(Hg-Se)n}m-Sel P complex, where n is the number of Hg-Se complexes and m the number of binding sites in Sel P.