Inhibition of the human thioredoxin system - A molecular mechanism of mercury toxicity

Mercury toxicity mediated by different forms of mercury is a major health problem; however, the molecular mechanisms underlying toxicity remain elusive. We analyzed the effects of mercuric chloride (HgCl2) and monomethylmercury (MeHg) on the proteins of the mammalian thioredoxin system, thioredoxin reductase (TrxR) and thioredoxin (Trx), and of the glutaredoxin system, glutathione reductase (GR) and glutaredoxin (Grx). HgCl2 and MeHg inhibited recombinant rat TrxR with IC50 values of 7.2 and 19.7 nM, respectively. Fully reduced human Trx1 bound mercury and lost all five free thiols and activity after incubation with HgCl2 or MeHg, but only HgCl2 generated dimers. Mass spectra analysis demonstrated binding of 2.5 mol of Hg2+ and 5 mol of MeHg+/mol of Trx1 with the very strong Hg2+ complexes involving active site and structural disulfides. Inhibition of both TrxR and Trx activity was observed in HeLa and HEK 293 cells treated with HgCl2 or MeHg. GR was inhibited by HgCl2 and MeHg in vitro, but no decrease in GR activity was detected in cell extracts treated with mercurials. Human Grx1 showed similar reactivity as Trx1 with both mercurial compounds, with the loss of all free thiols and Grx dimerization in the presence of HgCl2, but no inhibition of Grx activity was observed in lysates of HeLa cells exposed to mercury. Overall, mercury inhibition was selective toward the thioredoxin system. In particular, the remarkable potency of the mercury compounds to bind to the selenol-thiol in the active site of TrxR should be a major molecular mechanism of mercury toxicity.