Arabidopsis thaliana NADPH:thioredoxin reductase (TR, EC 1.6.4.5) catalyzed redox cycling of aromatic nitrocompounds, including the explosives 2,4,6-trinitrotoluene and tetryl, and the herbicide 3,5-dinitro-o-cresol. The yield of nitro anion radicals was equal to 70-90%. Redox cycling of tetryl was accompanied by formation of N-methylpicramide. Bimolecular rate constants of nitroaromatic reduction (kcat/Km) and reaction catalytic constants (kcat) increased upon an increase in oxidant single-electron reduction potential (E(1)7). Using compounds with an unknown E(1)7 value, the reactivity of TR increased parallelly to the increase in reactivity of ferredoxin:NADP+ reductase of Anabaena PCC 7119 (EC 1.18.1.2). This indicated that the main factor determining reactivity of nitroaromatics towards TR was their energetics of single-electron reduction. Incubation of reduced TR in the presence of tetryl or 2,4-dinitrochlorobenzene resulted in a loss of thioredoxin reductase activity, most probably due to modification of reduced catalytic disulfide, whereas nitroreductase reaction rates were unchanged. This means that on the analogy of quinone reduction by TR (D. Bironaite, Z. Anusevicius, J.-P. Jacquot, N. Cenas, Biochim. Biophys. Acta 1383 (1998) 82-92), FAD and not catalytic disulfide of TR was responsible for the reduction of nitroaromatics. Tetryl, 2,4,6-trinitrotoluene and thioredoxin increased the FAD fluorescence intensity of TR. This finding suggests that nitroaromatics may bind close to the thioredoxin-binding site at the catalytic disulfide domain of TR, and induce a conformational change of enzymes (S.B. Mulrooney, C.H. Williams Jr., Protein Sci. 6 (1997) 2188-2195). Our data indicate that certain nitroaromatic herbicides, explosives and other classes of xenobiotics may interfere with the reduction of thioredoxin by plant TR, and confer prooxidant properties to this antioxidant enzyme.