The glutathione S-transferases play a pivotal role in the detoxification of toxic and carcinogenic electrophiles. We have previously reported the isolation of two actively transcribed murine pi-class glutathione S-transferase genes. In this study the two proteins encoded by these genes, Gst p-1 and Gst p-2, were expressed in Escherichia coli and found to exhibit profoundly different catalytic activities, the activity of Gst p-2 toward a panel of electrophilic substrates being 1-3 orders of magnitude lower than that of Gst p-1. In order to establish the basis for the difference between these highly homologous proteins, mutants were generated where specific amino acids had been exchanged. Kinetic analysis of the wildtype and mutant enzymes revealed that the amino acid differences occurring at positions 10 (Val/Ser), 11 (Arg/Pro), and 104 (Val/Gly) are responsible for the reduced enzymatic activity of Gst p-2. This analysis together with computer graphics modeling for Gst p-2 indicated that these changes affected both substrate and glutathione binding to the enzyme.