Alterations of the glutathione-redox balance induced by metals in CHO-K1 cells

The effects of cadmium (Cd2+), mercury (Hg2+), lead (Pb2+), copper (Cu2+) and nickel (Ni2+) on the glutathione (GSH)-redox cycle were assessed in CHO-K1 by the neutral red uptake inhibition (NR) assay (NR6.25, NR12.5 and NR25). Mercury proved to be the most and lead the least toxic of the metals tested. The effects on GSH content and intracellular specific activities of enzymes involved in the GSH-redox balance were measured after a 24-h exposure. Total GSH content increased significantly in cultures exposed to the lowest metal concentration assayed (NR6.25), but fell to below control values when exposed to concentrations equivalent to NR25. Oxidised glutathione content dropped significantly at NR6.25, while somewhat higher values were obtained for cultures exposed to higher doses. Glutathione peroxidase (Gpx) activities were 1.2-, 1.5-, 1.6-, 2.0- and 2.5-fold higher than untreated controls for cadmium, copper, mercury, nickel and lead, respectively, at concentrations equivalent to NR6.25. Gpx activity declined at metal concentrations equivalent to NR12.5 and NR25. Glutathione reductase activity remained almost unchanged except at low doses of mercury, nickel and lead. Glutathione-S-transferase activity decreased at rising metal concentrations. The results suggest that a homeostatic defence mechanism was activated when cells were exposed to doses equivalent to NR6.25 while the ability of the cells to respond weakened as the dose increased. A close relationship was also observed between metal cytotoxicity, total GSH content and the dissociation energy of the sulphur-metal bonds. These facts confirm the involvement of antioxidant defence mechanisms in the toxic action of these ions. (C) 2002 Elsevier Science Inc. All rights reserved.