Reactions of Antimalarial Peroxides with Each of Leucomethylene Blue and Dihydroflavins: Flavin Reductase and the Cofactor Model Exemplified

Richard K. Haynes, Kwan Wing Cheu, Maggie Mei Ki Tang, Min Jiao Chen, Zu Feng Guo, Zhi Hong Guo, Paolo Coghi, Diego Monti

Research output: Contribution to journalArticlepeer-review

40 Scopus citations

Abstract

Flavin adenine dinucleotide (FAD) is reduced by NADPH-E.coli flavin reductase (Fre) to FADH2 in aqueous buffer at pH7.4 under argon. Under the same conditions, FADH2 in turn cleanly reduces the antimalarial drug methylene blue (MB) to leucomethylene blue. The latter is rapidly re-oxidized by artemisinins, thus supporting the proposal that MB exerts its antimalarial activity, and synergizes the antimalarial action of artemisinins, by interfering with redox cycling involving NADPH reduction of flavin cofactors in parasite flavin disulfide reductases. Direct treatment of the FADH2 generated from NADPH-Fre-FAD by artemisinins and antimalaria-active tetraoxane and trioxolane structural analogues under physiological conditions at pH7.4 results in rapid reduction of the artemisinins, and efficient conversion of the peroxide structural analogues into ketone products. Comparison of the relative rates of FADH2 oxidation indicate optimal activity for the trioxolane. Therefore, the rate of intraparastic redox perturbation will be greatest for the trioxolane, and this may be significant in relation to its enhanced invitro antimalarial activities. 1HNMR spectroscopic studies using the BNAH-riboflavin (RF) model system indicate that the tetraoxane is capable of using both peroxide units in oxidizing the RFH2 generated insitu. Use of the NADPH-Fre-FAD catalytic system in the presence of artemisinin or tetraoxane confirms that the latter, in contrast to artemisinin, consumes two reducing equivalents of NADPH. None of the processes described herein requires the presence of ferrous iron. Ferric iron, given its propensity to oxidize reduced flavin cofactors, may play a role in enhancing oxidative stress within the malaria parasite, without requiring interaction with artemisinins or peroxide analogues. The NADPH-Fre-FAD system serves as a convenient mimic of flavin disulfide reductases that maintain redox homeostasis in the malaria parasite. Antimalarial peroxides and flavin reductase: NADPH-E.coli flavin reductase (Fre) reduces FAD to FADH2, which in turn rapidly reduces artemisinins and antimalarial peroxides to deoxy or ketone products under physiological conditions. Thus, antimalarial activity is due to perturbation of intraparasitic redox homeostasis by oxidation of FADH2 in critical flavoenzymes with consequent sequestration of NADPH. The tetraoxane uses both peroxide units in consuming two equivalents of NADPH in the NADPH-Fre-FAD system.

Original languageEnglish (US)
Pages (from-to)279-291
Number of pages13
JournalChemMedChem
Volume6
Issue number2
DOIs
StatePublished - Feb 7 2011
Externally publishedYes

Keywords

  • Artemisinins
  • Flavoenzymes
  • Malaria
  • Oxidative stress
  • Peroxides

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Medicine
  • Pharmacology
  • Drug Discovery
  • Pharmacology, Toxicology and Pharmaceutics(all)
  • Organic Chemistry

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