The Role of Intracellular Glutathione in Inorganic Mercury-Induced Toxicity in Neuroblastoma Cells

It is well known that antioxidants containing sulfhydryl (−SH) groups are protective against the toxic effects of mercury. The current study was designed to elucidate the mechanism(s) of the cytoprotective effects of glutathione (GSH) and N-acetylcysteine (NAC) against the toxicity of inorganic mercury (HgCl₂) in neuroblastoma cells (N-2A). The obtained results demonstrated the protective effects of these compounds in a dose dependant manner up to 95 and 74% cell viability, respectively as compared to the control of HgCl₂ of 10%. The administration of buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, increased the toxicity of HgCl₂ in a dose dependent manner. Moreover, BSO treatment attenuated the levels of the cellular free −SH concentrations at low concentrations (1–100 μM) of HgCl₂. The data also show that cellular thiol concentrations were augmented in the presence of GSH and NAC and these compounds were cytoprotective against HgCl₂ and this is due to up regulating of GSH synthesis. A reduction in intracellular levels of GSH was observed with treatment of HgCl₂. In addition, the ratio of GSH/GSSG increased from 16:1 to 50:1 from 1 to 10 μM concentration of HgCl_2. The ratio of GSH/GSSG then decreased from 4:1 to 0.5:1 with the increase of concentration of HgCl₂ between 100 μM and 1 mM due to the collapse of the N-2A cells. It was of interest to note that the synthesis of GSH was stimulated in cells exposed to low concentration of HgCl₂ when extra GSH is available. These data support the idea that the loss of GSH plays a contributing role to the toxic effects of HgCl₂ and that inorganic mercury adversely affects viability, through altering intracellular −SH concentrations. The data further indicate that the availability of GSH to the cells may not be sufficient to provide protection against mercury toxicity and the de novo synthesis of intracellular GSH is required to prevent the damaging effects of mercury.