title: The superoxide reductase from the early diverging eukaryote Giardia intestinalis.
abstract: Unlike superoxide dismutases (SODs), superoxide reductases (SORs) eliminate superoxide anion (O(2)(•-)) not through its dismutation, but via reduction to hydrogen peroxide (H(2)O(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.
authors.affiliation: Department of Biochemical Sciences, CNR Institute of Molecular Biology and Pathology, Sapienza Università di Roma, I-00185 Rome, Italy.
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Testa
authors.forename: Daniela
authors.initials: D
authors.lastname: Mastronicola
authors.forename: Diane E
authors.initials: DE
authors.lastname: Cabelli
authors.forename: Eugenio
authors.initials: E
authors.lastname: Bordi
authors.forename: Leopoldo P
authors.initials: LP
authors.lastname: Pucillo
authors.forename: Paolo
authors.initials: P
authors.lastname: Sarti
authors.forename: Lígia M
authors.initials: LM
authors.lastname: Saraiva
authors.forename: Alessandro
authors.initials: A
authors.lastname: Giuffrè
authors.forename: Miguel
authors.initials: M
authors.lastname: Teixeira
mesh_terms: D002478:Cells, Cultured; D003001:Cloning, Molecular; D056890:Eukaryota; D016829:Giardia lamblia / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D005873:Giardiasis / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007421:Intestine, Small / Q000378:metabolism / Q000469:parasitology; D007495:Iridium / Q000378:metabolism; D007501:Iron / Q000737:chemistry / Q000378:metabolism; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D011675:Pulse Radiolysis; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism*; D013053:Spectrophotometry; D013482:Superoxide Dismutase / Q000378:metabolism
chemical_list: D015800:Protozoan Proteins; D011994:Recombinant Proteins; C010978:hexachloroiridic acid; D007495:Iridium; D006861:Hydrogen Peroxide; D007501:Iron; D010088:Oxidoreductases; C121849:superoxide reductase; D013482:Superoxide Dismutase