The superoxide reductase from the early diverging eukaryote Giardia intestinalis

Fabrizio Testa, Daniela Mastronicola, Diane E. Cabelli, Eugenio Bordi, Leopoldo P. Pucillo, Paolo Sarti, Lígia M. Saraiva, Alessandro Giuffrè, Miguel Teixeira

Research output: Contribution to journalArticlepeer-review


Unlike superoxide dismutases (SODs), superoxide reductases (SORs) eliminate superoxide anion (O 2 •-) not through its dismutation, but via reduction to hydrogen peroxide (H 2O 2) in the presence of an electron donor. The microaerobic protist Giardia intestinalis, responsible for a common intestinal disease in humans, though lacking SOD and other canonical reactive oxygen species-detoxifying systems, is among the very few eukaryotes encoding a SOR yet identified. In this study, the recombinant SOR from Giardia (SOR Gi) was purified and characterized by pulse radiolysis and stopped-flow spectrophotometry. The protein, isolated in the reduced state, after oxidation by superoxide or hexachloroiridate(IV), yields a resting species (T final) with Fe 3+ ligated to glutamate or hydroxide depending on pH (apparent pK a = 8.7). Although showing negligible SOD activity, reduced SOR Gi reacts with O 2 •- with a pH-independent second-order rate constant k 1 = 1.0 × 10 9 M - 1 s - 1 and yields the ferric-(hydro)peroxo intermediate T 1; this in turn rapidly decays to the T final state with pH-dependent rates, without populating other detectable intermediates. Immunoblotting assays show that SOR Gi is expressed in the disease-causing trophozoite of Giardia. We propose that the superoxide-scavenging activity of SOR in Giardia may promote the survival of this air-sensitive parasite in the fairly aerobic proximal human small intestine during infection.

Original languageEnglish
Pages (from-to)1567-1574
Number of pages8
JournalFree Radical Biology and Medicine
Issue number8
Publication statusPublished - Oct 15 2011


  • Anaerobic protozoa
  • Free radicals
  • Nonheme iron protein
  • Oxidative stress
  • Superoxide detoxification
  • Time-resolved spectroscopy

ASJC Scopus subject areas

  • Biochemistry
  • Physiology (medical)


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