V. Miskiniene et al., NITROREDUCTASE REACTIONS OF ARABIDOPSIS-THALIANA THIOREDOXIN REDUCTASE, Biochimica et biophysica acta. Bioenergetics, 1366(3), 1998, pp. 275-283
Arabidopsis thaliana NADPH:thioredoxin reductase (TR, EC 1.6.4.5) cata
lyzed redox cycling of aromatic nitrocompounds, including the explosiv
es 2,4,6-trinitrotoluene and tetryl, and the herbicide 3,5-dinitro-o-c
resol. The yield of nitro anion radicals was equal to 70-90%, Redox cy
cling of tetryl was accompanied by formation of N-methylpicramide. Bim
olecular rate constants of nitroaromatic reduction (k(cat)/K-m) and re
action catalytic constants (k(cat)) increased upon an increase in oxid
ant single-electron reduction potential (E-7(1)). Using the compounds
with an unknown E-7(1) value, the reactivity of TR increased parallell
y to the increase in reactivity of ferredoxin:NADP(+) reductase of Ana
baena PCC 7119 (EC 1.18.1.2). This indicated that the main factor dete
rmining reactivity of nitroaromatics towards TR was their energetics o
f single-electron reduction. Incubation of reduced TR in the presence
of tetryl or 2,4-dinitrochlorobenzene resulted in a loss of thioredoxi
n reductase activity, most probably due to modification of reduced cat
alytic disulfide, whereas nitroreductase reaction rates were unchanged
. This means that on the analogy of quinone reduction by TR (D. Birona
ite, Z. Anusevicius, J.-P. Jacquot, N. Cenas, Biochim. Biophys. Acta 1
383 (1998) 82-92), FAD and not catalytic disulfide of TR was responsib
le for the reduction of nitroaromatics. Tetryl, 2,4,6-trinitrotoluene
and thioredoxin increased the FAD fluorescence intensity of TR. This f
inding suggests that nitroaromatics may bind close to the thioredoxin-
binding site at the catalytic disulfide domain of TR, and induce a con
formational change of enzymes (S.B. Mulrooney, C.H. Williams Jr., Prot
ein Sci. 6 (1997) 2188-2195). Our data indicate that certain nitroarom
atic herbicides, explosives and other classes of xenobiotics may inter
fere with the reduction of thioredoxin by plant TR, and confer prooxid
ant properties to this antioxidant enzyme. (C) 1998 Elsevier Science B
.V. All rights reserved.