The role of oxidative stress on the effect of 1,4,7,10,13,16-hexathiacyclooctadecane on copper and zinc toxicity in HepG2 cells

Experiments have shown that 1,4,7,10,13,16-hexathiacy-clooctadecane (U) increased the Cu2+ toxicity on HepG2 cells, whereas the combination Zn2+/L3 was less toxic relative to the metal control. In all cases, glutathione (GSH) levels were decreased and vitamins C and E supplementation partially counteracted the increased toxicity in the Cu2+/L3-treated cells. The previously observed effects of this hexathiamacrocyclic ligand (U) on the Cu2+ and Zn2+ toxicity were further investigated by first depleting the intracellular GSH levels by means of L-buthionine S,R-sulphoximine. Combined treatment with Cu2+/L3 resulted in complete cell death, whereas for Zn2+/L3 no severe effects were observed. Direct measurement of reactive oxygen species (ROS) revealed that Cu2+ induced a high degree of oxidative stress on the cells. This was not the case for Zn2+. The results proved a previously proposed mechanism in which GSH is used to conjugate the metal-ligand complex, but as a result of this, GSH is no longer available for inactivation of ROS. Also, both the intracellular copper and zinc content were determined for each experiment by means of inductively coupled plasma-atomic emission spectroscopy. According to these data, zinc is depleted in Cu2+/L3-treated cells, which could have consequences on superoxide dismulase and as a result of this on the amount of oxidative stress.