Inactivation of glutathione S transferase zeta by dichloroacetic acid

The environmental contaminant dichloroacetate (DCA) is considered hazardous by the US environment protection agency (EPA) but is also an important drug in the clinical management of lactic acidosis. DCA has been shown to be carcinogenic in rats and causes peripheral neuropathy in humans. It is metabolized by glutathione S transferase zeta (GSTz) and inhibits its own metabolism by inactivating GSTz. This aspect of DCA metabolism is also important because GSTz also metabolizes endogenous substrates, namely maleylacetoacetate (MAA) and maleylacetone (MA), which are intermediates in tyrosine catabolism.; The objectives of this research were to study the time course of inactivation and recovery of GSTz following exposure to DCA at both clinical and environmentally relevant doses and to identify possibly adducts of DCA with GSTz. Research presented here also examined the in-vivo role of MA in inactivating GSTz. Experiments conducted showed that DCA completely inactivated GSTz after one week of exposure and that enzyme recovery was not as rapid as inactivation. It took more than two weeks for the enzyme activity to return to control levels and enzyme expression remained below control levels even after eight weeks of withdrawing DCA treatment. These studies showed that enzyme recovery was slow and that the protein needed to be re-synthesized for activity to be restored. Another important finding was that environmental levels of DCA similar to those present in drinking water inactivated GSTz. Exposure of rats to DCA in drinking water at levels of 2.5 and 250 mug/kg/day significantly inhibited GSTz activity and decreased GSTz expression. Longer duration of treatment had a more prominent effect suggesting a cumulative effect. Adduct studies with recombinant hGSTz 1c-1c showed the presence of adducts of GSTz with glutathione (GSH), and glyoxalate which is the primary metabolite of DCA. Another important finding of this study was that the reactive endogenous substrate for GSTz, maleylacetone, did not inhibit GSTz activity or expression in-vivo. Previous studies with MA have shown that it inhibits GSTz in-vitro but this is the first study, to our knowledge, which showed that it was not an inhibitor of GSTz in-vivo.