Characterization of a class alpha glutathione-S-transferase with glutathione peroxidase activity in human liver microsomes

A 25.5 kDa class alpha glutathione S-transferase (GST) designated as microsomal Ya-GST or M-GSTA has been purified to electrophoretic homogeneity from human liver microsomes. Limited proteolysis, gel filtration chromatography followed by EDTA, and alkaline Na2CO3 treatments of microsomes indicate that the M-GSTA is intrinsic to the microsomes. Western immunoblot analysis revealed that human liver M-GSTA and the previously reported 17-kDa microsomal GST (FEBS Lett. 315 (1993) 77) did not have immunological cross reactivity. The enzyme showed conjugation activity towards substrates like 1-chloro-2,4-nitrobenzene (CDNB) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, and 4-hydroxy-2-nonenal (4-HNE), a genotoxic α,β-unsaturated aldehyde product of lipid peroxidation. In addition, the M-GSTA exhibited significant glutathione peroxidase activity towards physiologically relevant fatty acid hydroperoxides as well as phosphatidylcholine hydroperoxide, but not with H2O2. C-terminal amino acid sequence analysis revealed a high homology with the human liver cytosolic GST-A1 and A3 isozymes. Western immunoblot analyses of the microsomes prepared from human hepatoblastoma (HepG2) showed that the expression of this M-GSTA was induced upon treatment with such prooxidants as H2O2, suggesting that it may play an important role in the protection of cellular membranes from peroxidative damage.