Exhibits glutathione-dependent thiol transferase and dehydroascorbate reductase activities. Has S-(phenacyl)glutathione reductase activity. Has also glutathione S-transferase activity. Participates in the biotransformation of inorganic arsenic and reduces monomethylarsonic acid (MMA) and dimethylarsonic acid. glutathione + RX = a halide anion + an S-substituted glutathione + H(+) 2 glutathione + L-dehydroascorbate = glutathione disulfide + L-ascorbate 2 glutathione + H(+) + methylarsonate = glutathione disulfide + H2O + methylarsonous acid Monomethylarsonic acid reductase activity is competitively inhibited by 1-chloro 2,4-dinitrobenzene (CDNB) and by deoxycholate. Optimum pH is 8. Homodimer. Ubiquitous. Highest expression in liver, pancreas, skeletal muscle, spleen, thymus, colon, blood leukocyte and heart. Lowest expression in brain, placenta and lung. Belongs to the GST superfamily. Omega family. Extended N-terminus.