Three cytosolic glutathione S-transferase (GST, EC 2.5.1.18) isozymes were purified from livers of male CD-1 mice to homogeneity using affinity chromatography and gradient elution. Isozyme molecular masses and purities were determined using electrospray ionization mass spectrometry (ESI/MS), HPLC, and electrophoretic methods. Isozymes were assigned to alpha, pi, and mu classes based on kinetic and electrophoretic properties. Molecular masses determined by mass spectrometry were 25 271.4 +/- 2, 23 478.8 +/- 2, and 25 838.4 +/- 2 Da for alpha, pi, and mu isozymes, respectively. Molecular masses for pi and mu class isozymes agreed with molecular masses derived from nucleic acid sequences, but the molecular mass of the alpha class isozyme was 42 Da greater than calculated for a reported sequence. Analysis of tryptic digests of GST-alpha using tandem mass spectrometry determined that the molecular weight discrepancy could be attributed to N-terminal acetylation. Effects of covalent attachment of 4-hydroxy-2-nonenal (HNE) on catalytic activities of each GST isozyme were investigated by incubating GSTs with HNE. The extent of HNE-protein adduct formation was determined using ESI/MS. Catalytic activity of HNE-modified GSTs toward CDNB was measured. ESI mass spectra of modified enzymes show that the extent of HNE modification to the isozymes (pi > alpha > mu) coincides with the loss of activity seen for each protein. ESI mass spectra also indicated that glutathione-protein adducts form in isozymes during incubations with HNE even though purified proteins had been dialyzed against water before incubations. Glutathionylation of isozymes increased as the concentration of HNE in the incubations was raised. Dialysis of GSTs against 2 mM S-hexylglutathione (to displace GSH) before incubations eliminated S-glutathionylation. Reactivities of these GSTs toward HNE differed and is related to the number of cysteine residues (alpha < mu < pi) within the amino acid sequence.