Glutathione Transferase Omega 1 Catalyzes the Reduction of S-(Phenacyl)glutathiones to Acetophenones

S-(Phenacyl)glutathione reductase (SPG-R) plays a significant role in the biotransformation of reactive alpha-haloketones to nontoxic acetophenones. Comparison of the apparent subunit size, amino acid composition, and catalysis of the reduction of S-(phenacyl)glutathiones indicated that a previously described rat SPG-R (Kitada, M., McLenithan, J. C., and Anders, M. W. (1985) J. Biol. Chem. 260, 11749-11754) is homologous to the omega-class glutathione transferase GSTO1-1. The available data show that the SPG-R reaction is catalyzed by GSTO1-1 and not by other GSTs, including the closely related GSTO2-2 isoenzyme. In the proposed reaction mechanism, the active-site cysteine residue of GSTO1-1 reacts with the S-(phenacyl)glutathione substrate to give an acetophenone and a mixed disulfide with the active-site cysteine; a second thiol substrate (e.g., glutathione or 2-mercaptoethanol) reacts with the active-site disulfide to regenerate the catalytically active enzyme and to form a mixed disulfide. A new spectrophotometric assay was developed that allows the rapid determination of SPG-R activity and specific measurement of GSTO1-1 in the presence of other GSTs. This is the first specific reaction attributed to GSTO1-1, and these results demonstrate the catalytic diversity of GSTO1-1, which, in addition to SPG-R activity, catalyzes the reduction of dehydroascorbate and monomethylarsonate(V) and also possesses thioltransferase and GST activity.