Spectroscopic and rapid kinetic experiments were performed to detail the interaction of human glutathione S-transferases GSTA1-1, GSTM2-2, and GSTP1-1 with 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol (NBDHEX). This compound is a representative molecule of a new class of 7-nitro-2,1,3-benzoxadiazole (NBD) derivatives (non-GSH peptidomimetic compounds) that have been designed both to give strong GST inhibition and to accumulate in tumor cells avoiding the extrusion mechanisms mediated by the multidrug resistance protein pumps. We have recently shown that submicromolar amounts of NBDHEX trigger apoptosis in several human tumor cell lines through the dissociation of the JNK.GSTP1-1 complex (Turella, P., Cerella, C., Filomeni, G., Bullo, A., De Maria, F., Ghibelli, L., Ciriolo, M. R., Cianfriglia, M., Mattei, M., Federici, G., Ricci, G., and Caccuri, A. M. (2005) Cancer Res. 65, 3751-3761). Results reported in the present study indicated that NBDHEX behaves like a suicide inhibitor for GSTs. It bound to the H-site and was conjugated with GSH forming a sigma complex at the C-4 of the benzoxadiazole ring. This complex was tightly stabilized in the active site of GSTP1-1 and GSTM2-2, whereas in GSTA1-1 the release of the 6-mercapto-1-hexanol from the sigma complex was the favored event. Docking studies demonstrated the likely localization of the sigma complex in the GST active sites and provide a structural explanation for its strong stabilization.