Serial Review: Peroxiredoxins Serial Review Editor: Henry J. Forman
Peroxiredoxin 6, a 1-Cys peroxiredoxin, functions in antioxidant defense and lung phospholipid metabolism

https://doi.org/10.1016/j.freeradbiomed.2005.02.011Get rights and content

Abstract

Peroxiredoxin 6 (Prdx6), a bifunctional 25-kDa protein with both GSH peroxidase and phospholipase A2 activities, is the only mammalian 1-Cys member of the peroxiredoxin superfamily and is expressed in all major organs, with a particularly high level in lung. Prdx6 uses GSH as an electron donor to reduce H2O2 and other hydroperoxides including phospholipid hydroperoxides at ∼5 μmol/mg protein/min with K1 ∼ 3 × 106 M−1 s−1. Oxidation of the Cys47 to a sulfenic acid during catalysis requires πGST-catalyzed glutathionylation and reduction with GSH to complete the enzymatic cycle. Prdx6 stably overexpressed in cells protected against oxidative stress, whereas antisense treatment resulted in oxidant stress and apoptosis. Adenoviral-mediated overexpression of Prdx6 in mouse lungs protected against the toxicity of hyperoxia, whereas Prdx6-null mice were more sensitive to the effects of hyperoxia or paraquat. We postulate that Prdx6 functions in antioxidant defense mainly by facilitating repair of damaged cell membranes via reduction of peroxidized phospholipids. The PLA2 activity of Prdx6 is Ca2+ independent and maximal at acidic pH. Inhibition of PLA2 activity results in alterations of lung surfactant phospholipid synthesis and turnover. Thus, Prdx6, a unique mammalian peroxiredoxin, is an important antioxidant enzyme and has a major role in lung phospholipid metabolism.

Section snippets

Physiological electron donor for peroxidase activity

Prdx6, like other peroxiredoxins, functions as a peroxidase. However, as stated above, this enzyme, unlike other members of the Prdx family, does not utilize thioredoxin as the physiological reductant [9], [26] and the physiological electron donor for peroxidatic activity of Prdx6 has been controversial. The non-selenium GSH peroxidase isolated from the bovine ciliary body demonstrated peroxide reduction in the presence of GSH [5], [27]. Native protein isolated from the rat olfactory epithelium

Enzymatic activities of Prdx6

Prdx6 is a bifunctional protein with both GSH peroxidase and PLA2 activities and has been called a “moonlighting” protein [37]. The protein is composed of 224 amino acids with a molecular mass of 25.1 kDa. The structure of the protein shows the thioredoxin fold with a loop–helix region at the active site characteristic of the peroxiredoxin family [38]. GSH peroxidase activity has been measured using the GSH/GSH reductase assay coupled to NADPH reduction [26]. Fig. 2 shows that t-butyl

Structure–function relationships of Prdx6

Human recombinant Prdx6 has been crystallized as a homodimer formed by hydrophobic head-to-tail interaction of monomers [32]. To accomplish crystallization, the conserved catalytic Cys47 was oxidized to cysteine sulfenic acid by the addition of H2O2 and the nonconserved surface Cys91 was mutated to Ser [32]. The major structural feature of Prdx6 is a thioredoxin “fold,” which comprises ∼80 amino acids. The thioredoxin fold contains four central β strands and two α helices, which are essential

Species and tissue localization of Prdx6

The cDNA for Prdx6 has been identified for human (GenBank Accession No. D14662), rat (AF014009), mouse (AF004670), cow (AF090194), and pig (AJ243849) [7], [8], [10], [24], [25], [26], [30]. Comparisons of the cDNAs and deduced protein compositions of Prdx6 from these species have shown greater than 95% nucleotide and amino acid similarity [8], [44]. This high degree of conservation suggests an important role for Prdx6 in the metabolism of mammalian cells. Homologous 1-Cys peroxiredoxin proteins

Antioxidant function of Prdx6

An increasing number of studies have provided evidence that Prdx6 can function in vivo as an antioxidant enzyme. Initial experiments were carried out with isolated cells and more recently with intact animals. Overexpression of Prdx6 as a fusion construct with green fluorescent protein in H441 cells, a lung epithelial cell line that does not express Prdx6, protected against peroxide (H2O2, t-butyl hydroperoxide) or radical dotOH-mediated oxidative stress; overexpressing cells showed less lipid peroxidation

PLA2 function of Prdx6

The PLA2 activity of Prdx6 has not been studied as widely as the peroxidase activity and essentially all information related to the functional significance of this activity has been obtained from studies of lung surfactant phospholipids. Lung surfactant is a physiologically important secretion of the lung alveolar epithelium that functions to stabilize lung alveoli during respiration. DPPC is the major phospholipid component of surfactant and along with other lipids and specific proteins is

Summary

Prdx6 has both GSH peroxidase and PLA2 activities that play important physiological roles in antioxidant defense and lung surfactant metabolism. Although Prdx6 is a peroxiredoxin based on homology of structure, properties of the enzyme clearly differentiate Prdx6 from other mammalian members of this superfamily (Table 2). Prdx6 is a 1-Cys enzyme that has a different mechanism for enzymatic catalysis compared to other peroxiredoxins, which are 2-Cys enzymes; the latter form a disulfide either

Acknowledgments

This work was supported by HL19737 and HL65543.

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