The physiological role of the alpha class glutathione S-transferases against oxidative stress and lipid peroxidation

The role of glutathione S-transferases (GSTs) in detoxification of electrophilic xenobiotics including carcinogens through the conjugation of these electrophiles to glutathione (GSH) is well established. However, the physiological significance of Se-independent glutathione peroxidase (GPx) activity of GSTs, primarily associated with the Alpha class GST isoenzymes hGSTA1-1 and hGSTA2-2, is not clear and systematic studies in this area are lacking.; In this dissertation, the GPx activity of hGST A1-1 and hGSTA2-2 is characterized. Our results demonstrate that these isoenzymes can effectively catalyze GSH-dependent reduction of various physiological substrates including phospholipid hydroperoxides (PL-OOH) generated during lipid peroxidation (LPO) and the Km values for these substrates are within the range of concentrations during oxidative stress. More importantly, we demonstrate, for the first time, that GSH-dependent reduction of PL-OOH by these isoenzymes occurs in biological membranes from different origins in situ and these cytosolic enzymes have some interaction with membranes. The physiological role of the Alpha class GSTs against oxidative stress-induced LPO in tissues and cells is also studied. The immunoprecipitation studies using antibodies against the Alpha class GSTs show that these enzymes contribute to a major portion of GPx activity of mammalian liver, an organ that is continually subjected to a relatively high level of oxidative stress. Present studies demonstrate that overexpression of hGSTA1-1 or hGSTA2-2 attenuates LPO in human erythroleukemia cells (K562) and human lens epithelial cells (HLE B-3) under normal conditions as well as during oxidative stress. Furthermore, overexpression of hGSTA1-1 or hGSTA2-2 in K562 cells and HLE B-3 cells attenuates the cytotoxic effects of H₂O₂ and naphthalene and protects against apoptosis induced by these oxidants by inhibiting stress activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and caspase 3 activation.; Taken together, the studies presented in this dissertation provide strong evidence for an important physiological role of the Alpha class GST isoenzymes, hGSTA1-1 and hGSTA2-2, in the protection mechanisms against oxidative stress, in particular, lipid peroxidation (LPO). Our results suggest that the Alpha class GSTs should be conceived as one of the important “antioxidant enzymes”, and that these enzymes are involved in the protective mechanisms against cataractogenesis. In addition, the Alpha class GSTs also play a role in the apoptotic signaling mechanisms through regulation of intracellular concentrations of LPO products.