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Ferredoxin:NADP + oxidoreductase from Plasmodium falciparum ( Pf FNR) catalyzes the NADPH-dependent reduction of ferredoxin ( Pf Fd), which provides redox equivalents for the biosynthesis of isoprenoids and fatty acids in the apicoplast. Like other flavin-dependent electrontransferases, Pf FNR is a potential source of free radicals of quinones and other redox cycling compounds. We report here a kinetic study of the reduction of quinones, nitroaromatic compounds and aromatic N- oxides by Pf FNR . We show that all these groups of compounds are reduced in a single-electron pathway, their reactivity increasing with the increase in their single-electron reduction midpoint potential ( E 1 7 ). The reactivity of nitroaromatics is lower than that of quinones and aromatic N -oxides, which is in line with the differences in their electron self-exchange rate constants. Quinone reduction proceeds via a ping-pong mechanism. During the reoxidation of reduced FAD by quinones, the oxidation of FADH . to FAD is the possible rate-limiting step. The calculated electron transfer distances in the reaction of Pf FNR with various electron acceptors are similar to those of Anabaena FNR, thus demonstrating their similar "intrinsic" reactivity. Ferredoxin stimulated quinone- and nitro-reductase reactions of Pf FNR, evidently providing an additional reduction pathway via reduced Pf Fd. Based on the available data, Pf FNR and possibly Pf Fd may play a central role in the reductive activation of quinones, nitroaromatics and aromatic N- oxides in P. falciparum, contributing to their antiplasmodial action., Competing Interests: The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results.

With the aim to clarify the mechanism(s) of action of nitroaromatic compounds against the malaria parasite Plasmodium falciparum , we examined the single-electron reduction by P. falciparum ferredoxin:NADP + oxidoreductase ( Pf FNR) of a series of nitrofurans and nitrobenzenes ( n = 23), and their ability to inhibit P. falciparum glutathione reductase ( Pf GR). The reactivity of nitroaromatics in Pf FNR-catalyzed reactions increased with their single-electron reduction midpoint potential ( E 1 7 ). Nitroaromatic compounds acted as non- or uncompetitive inhibitors towards Pf GR with respect to NADPH and glutathione substrates. Using multiparameter regression analysis, we found that the in vitro activity of these compounds against P. falciparum strain FcB1 increased with their E 1 7 values, octanol/water distribution coefficients at pH 7.0 (log D ), and their activity as Pf GR inhibitors. Our data demonstrate that both factors, the ease of reductive activation and the inhibition of Pf GR, are important in the antiplasmodial in vitro activity of nitroaromatics. To the best of our knowledge, this is the first quantitative demonstration of this kind of relationship. No correlation between antiplasmodial activity and ability to inhibit human erythrocyte GR was detected in tested nitroaromatics. Our data suggest that the efficacy of prooxidant antiparasitic agents may be achieved through their combined action, namely inhibition of antioxidant NADPH:disulfide reductases, and the rapid reduction by single-electron transferring dehydrogenases-electrontransferases.

Derivatives of tirapazamine and other heteroaromatic N-oxides (ArN→O) exhibit tumoricidal, antibacterial, and antiprotozoal activities, which are typically attributed to bioreductive activation and free radical generation. In this work, we aimed to clarify the role of NAD(P)H:quinone oxidoreductase (NQO1) in ArN→O aerobic cytotoxicity. We synthesized 9 representatives of ArN→O with uncharacterized redox properties and examined their single-electron reduction by rat NADPH:cytochrome P-450 reductase (P-450R) and Plasmodium falciparum ferredoxin:NADP+ oxidoreductase (PfFNR), and by rat NQO1. NQO1 catalyzed both redox cycling and the formation of stable reduction products of ArN→O. The reactivity of ArN→O in NQO1-catalyzed reactions did not correlate with the geometric average of their activity towards P-450R- and PfFNR, which was taken for the parameter of their redox cycling efficacy. The cytotoxicity of compounds in murine hepatoma MH22a cells was decreased by antioxidants and the inhibitor of NQO1, dicoumarol. The multiparameter regression analysis of the data of this and a previous study (DOI: 10.3390/ijms20184602) shows that the cytotoxicity of ArN→O (n = 18) in MH22a and human colon carcinoma HCT-116 cells increases with the geometric average of their reactivity towards P-450R and PfFNR, and with their reactivity towards NQO1. These data demonstrate that NQO1 is a potentially important target of action of heteroaromatic N-oxides. [ABSTRACT FROM AUTHOR]

Ferredoxin:NADP+ oxidoreductase from Plasmodium falciparum (PfFNR) catalyzes the NADPH-dependent reduction of ferredoxin (PfFd), which provides redox equivalents for the biosynthesis of isoprenoids and fatty acids in the apicoplast. Like other flavin-dependent electrontransferases, PfFNR is a potential source of free radicals of quinones and other redox cycling compounds. We report here a kinetic study of the reduction of quinones, nitroaromatic compounds and aromatic N-oxides by PfFNR. We show that all these groups of compounds are reduced in a single-electron pathway, their reactivity increasing with the increase in their single-electron reduction midpoint potential (E17). The reactivity of nitroaromatics is lower than that of quinones and aromatic N-oxides, which is in line with the differences in their electron self-exchange rate constants. Quinone reduction proceeds via a ping-pong mechanism. During the reoxidation of reduced FAD by quinones, the oxidation of FADH. to FAD is the possible rate-limiting step. The calculated electron transfer distances in the reaction of PfFNR with various electron acceptors are similar to those of Anabaena FNR, thus demonstrating their similar "intrinsic" reactivity. Ferredoxin stimulated quinone- and nitro-reductase reactions of PfFNR, evidently providing an additional reduction pathway via reduced PfFd. Based on the available data, PfFNR and possibly PfFd may play a central role in the reductive activation of quinones, nitroaromatics and aromatic N-oxides in P. falciparum, contributing to their antiplasmodial action. [ABSTRACT FROM AUTHOR]