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NAD(P)H:menadione oxidoreductase of the amitochondriate eukaryote Giardia lamblia: a simpler homologue of the vertebrate enzyme

Laboratory of Biochemical Parasitology, The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA¹
Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA²

Author for correspondence: Lidya B. Sánchez. Tel: +1 212 327 8144. Fax: +1 212 327 7974. e-mail: sanchel{at}rockvax.rockefeller.edu

The amitochondriate eukaryote Giardia lamblia contains an NAD(P)H:menadione oxidoreductase (EC 1.6.99.2) (glQR) that catalyses the two-electron transfer oxidation of NAD(P)H with a quinone as acceptor. The gene encoding this protein in G. lamblia was expressed in Escherichia coli. The purified recombinant protein had an NAD(P)H oxidoreductase activity, with NADPH being a more efficient electron donor than NADH. Menadione, naphthoquinone and several artificial electron acceptors served as substrate for the enzyme. glQR shows high amino acid similarity to its homologues in vertebrates and also to a series of hypothetical proteins from bacteria. Although glQR is considerably smaller than the mammalian enzymes, three-dimensional modelling shows similar arrangement of the secondary structural elements. Most amino acid residues of the mammalian enzymes that participate in substrate binding or catalysis are conserved. Conservation of these features and the similarity in substrate specificity and in susceptibility to inhibitors establish glQR as an authentic member of this protein family.

Keywords: menadione oxidoreductase, Giardia lamblia, DT-diaphorase

Abbreviations: QR, NAD(P)H:menadione oxidoreductase; QR1, QR type 1; rQR1, QR1 from rat; hQR1, QR1 from human; hQR2, QR type 2 from human; glQR1, G. lamblia QR1; DCIP, 2,6-dichorophenolindophenol; , apparent Michaelis constant; MT, methyl red; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; SOD, superoxide dismutase

The GenBank accession number for the glQR1-encoding sequence reported in this paper is AF321405.

^a Present address: Biology Department, Georgetown University, Washington, DC 20057, USA.

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