INTERACTION OF QUINONES WITH ARABIDOPSIS-THALIANA THIOREDOXIN REDUCTASE

In view of the ubiquitous role of the thioredoxin/thioredodin reductas e (TRX/TR) system in living cells, the interaction of Arabidopsis thal iana NADPH-thioredoxin reductase (EC 1.6.4.5) with quinones, an import ant class of redox cycling and allylating xenobiotics, was studied, Th e steady-state reactions of A. thaliana TR with thioredoxin (TRX) and reaction product NADP(+) inhibition patterns were in agreement with a proposed model of E. coli enzyme (B.W. Lennon, C.H. Williams, Jr., Bio chemistry, vol. 35 (1996), pp. 4704-4712), that involved enzyme cyclin g between four-and two-electron reduced forms with FAD being reduced. Quinone reduction by TR proceeded via a mixed single-and two-electron transfer, the percentage of single-electron flux being equal to 12-16% . Bimolecular rate constants of quinone reduction (k(cat)/k(m)) and re action catalytic constants (k(cat)) increased upon an increase in quin one single-electron reduction potential, E-1(7). In several cases, the k(cat) of quinone reduction exceeded k(cat) of TRX reduction, suggest ing that quinones intercepted electron flux from TR to TRX. Incubation of reduced TR with alkylating quinones resulted in a rapid loss of TR X-reductase activity, while quinone reduction rate was unchanged. In T RX-reductase and quinone reductase reactions of TR, NADP(+) exhibited different inhibition patterns. These data point out that FAD and mt th e catalytic disulfide of TR is responsible for quinone reduction, and that quinones may oxidize FADH(2) before it reduces catalytic disulfid e. Most probably, quinones may oxidize the two-electron reduced form o f TR, and the enzyme may cycle between two-electron reduced and oxidiz ed forms in this reaction, The relatively high rate of quinone reducti on by A. thaliana thioredoxin reductase accompanied bq-their redox cyc ling, confers pro-oxidant properties to this antioxidant-enzyme. These factors make plant TR an attractive target for redox active and alkyl ating pesticide action. (C) 1998 Elsevier Science B.V.