Redox-active complexes formed during the interaction between glutathione and mercury and/or copper ions

Prompted by the recently reported capacity of the physiologically occurring Cu(I)-[glutathione](2) complex (Cu(I)-[GSH)](2)) to reduce oxygen, the effect of various GSH-binding metals (Co(2+), Cd(2+), Zn(2+), Pb(2+), Al(3+), Hg(2+) and Ni(2+)) on the superoxide-generating capacity of such complex was investigated. Amongst all tested metals, only Hg(2+) was able to substantially affect the capacity of Cu(I)-[GSH](2) to generate superoxide. When Hg(2+) and Cu(I)-[GSH](2) were mixed equimolarly, the superoxide formation, assessed through the cytochrome c reduction and dihydroethidium oxidation, was increased by over 50%. Such effect was totally inhibitable by SOD. Based on the reportedly higher affinity of Hg(2+) for GSH and the observed ability of Hg(2+) to lower the concentration of Cu(I)-[GSH](2) (spectroscopically assessed), we suggest that Hg(2+) displaces Cu(I) from Cu(I)-[GSH](2), to release Cu(I) ions and form a Hg(II)-[GSH](2) complex. The latter species would account for the Hg(2+)-induced exacerbation of the superoxide production. In fact, the present study provides first time evidence that a preformed Hg(II)-[GSH](2) complex is able to concentration-dependently reduce oxygen. Such redox-activity was evidenced using cytochrome c and confirmed by EPR studies using DMPO (5,5-dimethyl-1-pyrroline N-oxide, a spin-trapping agent). Considering this novel ability of Hg(II)-[GSH](2) to generate superoxide, a further characterization of its redox-activity and its potential to affect superoxide-susceptible biological targets appears warranted.