Interactions of Quinones with Thioredoxin Reductase (original) (raw)
2003, Journal of Biological Chemistry
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Interaction of quinones with Arabidopsis thaliana thioredoxin reductase
Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology, 1998
. In view of the ubiquitous role of the thioredoxinrthioredoxin reductase TRXrTR system in living cells, the interaction Ž . of Arabidopsis thaliana NADPH-thioredoxin reductase EC 1.6.4.5 with quinones, an important class of redox cycling Ž . and alkylating xenobiotics, was studied. The steady-state reactions of A. thaliana TR with thioredoxin TRX and reaction q Ž product NADP inhibition patterns were in agreement with a proposed model of E. coli enzyme B.W. Lennon, C.H. Ž . . Williams, Jr., Biochemistry, vol. 35 1996 , pp. 4704-4712 , that involved enzyme cycling between four-and two-electron reduced forms with FAD being reduced. Quinone reduction by TR proceeded via a mixed single-and two-electron transfer, Ž . the percentage of single-electron flux being equal to 12-16%. Bimolecular rate constants of quinone reduction k rk cat m Ž . 7 and reaction catalytic constants k increased upon an increase in quinone single-electron reduction potential, E . In cat 1 several cases, the k of quinone reduction exceeded k of TRX reduction, suggesting that quinones intercepted electron cat cat flux from TR to TRX. Incubation of reduced TR with alkylating quinones resulted in a rapid loss of TRX-reductase activity, while quinone reduction rate was unchanged. In TRX-reductase and quinone reductase reactions of TR, NADP q exhibited different inhibition patterns. These data point out that FAD and not the catalytic disulfide of TR is responsible for quinone reduction, and that quinones may oxidize FADH before it reduces catalytic disulfide. Most probably, quinones may 2 oxidize the two-electron reduced form of TR, and the enzyme may cycle between two-electron reduced and oxidized forms in this reaction. The relatively high rate of quinone reduction by A. thaliana thioredoxin reductase accompanied by their redox cycling, confers pro-oxidant properties to this antioxidant enzyme. These factors make plant TR an attractive target for redox active and alkylating pesticide action. q 1998 Elsevier Science B.V.
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