Mammalian peroxiredoxin isoforms can reduce hydrogen peroxide generated in response to growth factors and tumor necrosis factor-alpha

Mitochondrial GPx1 decreases induced but not basal oxidative damage to mtDNA in T47D cells

The production of oxyradicals by mitochondria (mt) is a source of oxidative damage to mtDNA such as 8-oxo-dG lesions that may lead to mutations and mitochondrial dysfunction. The potential protection of mtDNA by glutathione peroxidase-1 (GPx1) was investigated in GPx1-proficient (GPx-2) and GPx1-deficient (Hygro-3) human breast T47D cell transfectants. GPx activity and GPx1-like antigen concentration in mitochondria were respectively at least 100-fold and 20- to 25-fold higher in GPx2 than Hygro-3 cells. In spite of this large difference in peroxide-scavenging capacity, the basal 8-oxo-dG frequency in mtDNA, assessed by carefully controlled postlabeling assay, was strikingly similar in both cell lines. In contrast, in response to menadione-mediated oxidative stress, induction of 8-oxo-dG and DNA strand breaks was much lower in the GPx1-proficient mitochondria (e.g., +14% 8-oxo-dG versus +54% in Hygro-3 after 1-h exposure to 25 microM menadione, P < 0.05). Our data indicate that the mitochondrial glutathione/GPx1 system protected mtDNA against damage induced by oxidative stress, but did not prevent basal oxidative damage to mtDNA, which, surprisingly, appeared independent of GPx1 status in the T47D model.

Mitochondria of Saccharomyces cerevisiae contain one-conserved cysteine type peroxiredoxin with thioredoxin peroxidase activity

Peroxiredoxins are ubiquitously expressed proteins that reduce hydroperoxides using disulfur-reducing compounds as electron donors. Peroxiredoxins (Prxs) have been classified in two groups dependent on the presence of either one (1-Cys Prx) or two (2-Cys Prx) conserved cysteine residues. Moreover, 2-Cys Prxs, also named thioredoxin peroxidases, have peroxide reductase activity with the use of thioredoxin as biological electron donor. However, the biological reducing agent for the 1-Cys Prx has not yet been identified. We report here the characterization of a 1-Cys Prx from yeast Saccharomyces cerevisiae that we have named Prx1p. Prx1p is located in mitochondria, and it is overexpressed when cells use the respiratory pathway, as well as in response to oxidative stress conditions. We show also that Prx1p has peroxide reductase activity in vitro using the yeast mitochondrial thioredoxin system as electron donor. In addition, a mutated form of Prx1p containing the absolutely conserved cysteine as the only cysteine residue also shows thioredoxin-dependent peroxide reductase activity. This is the first example of 1-Cys Prx that has thioredoxin peroxidase activity. Finally, exposure of null Prx1p mutant cells to oxidant conditions reveals an important role of the mitochondrial 1-Cys Prx in protection against oxidative stress.

Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages

Electrophiles and reactive oxygen species have been implicated in the pathogenesis of many diseases. Transcription factor Nrf2 was recently identified as a general regulator of one defense mechanism against such havoc. Nrf2 regulates the inducible expression of a group of detoxication enzymes, such as glutathione S-transferase and NAD(P)H:quinone oxidoreductase, via antioxidant response elements. Using peritoneal macrophages from Nrf2-deficient mice, we show here that Nrf2 also controls the expression of a group of electrophile- and oxidative stress-inducible proteins and activities, which includes heme oxygenase-1, A170, peroxiredoxin MSP23, and cystine membrane transport (system x(c)(-)) activity. The response to electrophilic and reactive oxygen species-producing agents was profoundly impaired in Nrf2-deficient cells. The lack of induction of system x(c)(-) activity resulted in the minimum level of intracellular glutathione, and Nrf2-deficient cells were more sensitive to toxic electrophiles. Several stress agents induced the DNA binding activity of Nrf2 in the nucleus without increasing its mRNA level. Thus Nrf2 regulates a wide-ranging metabolic response to oxidative stress.

Molecular cloning and characterization of the mouse peroxiredoxin V gene

We have cloned two cDNA isoforms as well as genomic sequences of the mouse Prx V gene and characterized their molecular genetic features. Two isoforms of the mouse Prx V cDNA were identified from liver and testis. The testis-originated long transcripts had extra 1164-bp 5'-UTR sequences compared to the liver-originated short transcripts. Primer extension and sequence analyses revealed that the two isoforms were presumably transcribed at the same gene locus. The gene was composed of six exons spanning 3.2 kb. The short transcript was abundantly expressed in the kidney, liver, and heart of the adult mouse tissues and in the extra-membrane of the 10.5 dpc embryos. The long transcript of 1985 bp was abundantly detected in testis with trace amounts in other tissues. Interestingly, in testis and fetus, only mRNA expression of the long form was identified. However, the protein expression was not found in testis, implying that the long form could not properly direct the protein expression. The long Prx V cDNA has eight uORFs in the extra 5'-UTR, which proceed the major ORF. The inability of protein expression for the long-form cDNA in testis suggests that the uORFs might inhibit translation of the major ORF and thereby confer the tissue-specific regulation of the mouse Prx V gene.

Peroxiredoxin I (macrophage 23 kDa stress protein) is highly and widely expressed in the rat nervous system

Expression pattern of peroxiredoxin I (Prx I), a novel stress-inducible 23 kDa protein, initially found in macrophages, was investigated in the normal rat nervous system by western blot and immunohistochemistry. The protein was widely expressed in the central and peripheral nervous system, particularly in oligodendroglia and Schwann cells. Weak staining of axons and neuropil suggests neuronal processes also may contain Prx I. Immunostaining of neuronal cell bodies were not evident. Since Prx I reduces hydrogen peroxide, the widespread glial expression of Prx I indicates that it may play an important protective role against oxidative damage in the nervous system.

Distinct mitochondrial and cytosolic enzymes mediate trypanothione-dependent peroxide metabolism in Trypanosoma cruzi

The American trypanosome Trypanosoma cruzi is exposed to toxic oxygen metabolites that are generated by drug metabolism and immune responses in addition to those produced by endogenous processes. However, much remains to be resolved about the parasite oxidative defense system, including the mechanism(s) of peroxide reduction. Here we show that reduction of peroxides in T. cruzi is catalyzed by two distinct trypanothione-dependent enzymes. These were localized to the cytosol and mitochondrion. Both are members of the peroxiredoxin family of antioxidant proteins and are characterized by the presence of two conserved domains containing redox active cysteines. The role of these proteins in protecting T. cruzi from peroxide-mediated damage was demonstrated following overexpression of enzyme activity. The parasite-specific features of T. cruzi cytoplasmic peroxiredoxin and T. cruzi mitochondrial peroxiredoxin may be exploitable in terms of drug development.

Adaptive responses of the endothelium to stress

It is now established that endothelial cells acquire several functional properties in response to a diverse array of extracellular stimuli. This expression of an altered phenotype is referred to as endothelial cell activation, and it includes several activities that promote inflammation and coagulation. While it is recognized that endothelial cell activation has a principal role in host defense, recent studies also demonstrate that endothelial cells are capable of complex molecular responses that protect the endothelium against various forms of stress including heat shock, hypoxia, oxidative stress, shock, ischemia-reperfusion injury, toxins, wounds, and mechanical stress. In this review, we examine endothelial cell genotypic and phenotypic responses to stress. Also, we highlight important cellular stress responses that, although not yet demonstrated directly in endothelial cells, likely exist as part of the repertoire of stress responses in endothelium. A detailed understanding of the molecular mechanisms mediating the adaptive responses of endothelial cells to stress should facilitate the development of novel therapeutics to aid in the management of diverse surgical diseases and their complications.

Mitochondrial oxidative stress in mice lacking the glutathione peroxidase-1 gene

Oxidative stress resulting from mitochondrially derived reactive oxygen species (ROS) has been hypothesized to damage mitochondrial oxidative phosphorylation (OXPHOS) and to be a factor in aging and degenerative disease. If this hypothesis is correct, then genetically inactivating potential mitochondrial antioxidant enzymes such as glutathione peroxidase-1 (Gpx1; EC 1.11.1.9) should increase mitochondrial ROS production and decrease OXPHOS function. To determine the expression pattern of Gpx1, isoform-specific antibodies were generated and mutant mice were prepared in which the Gpx1 protein was substituted for by beta-galactosidase, driven by the Gpx1 promoter. These experiments revealed that Gpx1 is highly expressed in both the mitochondria and the cytosol of the liver and kidney, but poorly expressed in heart and muscle. To determine the physiological importance of Gpx1, mice lacking Gpx1 were generated by targeted mutagenesis in mouse ES cells. Homozygous mutant Gpx1(tm1Mgr) mice have 20% less body weight than normal animals and increased levels of lipid peroxides in the liver. Moreover, the liver mitochondria were found to release markedly increased hydrogen peroxide, a Gpx1 substrate, and have decreased mitochondrial respiratory control ratio and power output index. Hence, genetic inactivation of Gpx1 resulted in growth retardation, presumably due in part to reduced mitochondrial energy production as a product of increased oxidative stress.

Mouse peroxiredoxin V is a thioredoxin peroxidase that inhibits p53-induced apoptosis

We have identified human and mouse peroxiredoxin V (Prx-V) by virtue of the sequence homologies to yeast peroxisomal antioxidant enzyme PMP20. Prx-V represents the fifth of the six currently known subfamilies of mammalian peroxiredoxins. It is a novel organellar enzyme that has orthologs in bacteria. Biochemically, Prx-V is a thioredoxin peroxidase. One important aspect of p53 function in mammalian cells involves induction of apoptosis likely mediated by redox. We show that overexpression of Prx-V prevented the p53-dependent generation of reactive oxygen species. Likewise, Prx-V inhibited p53-induced apoptosis. Thus, Prx-V is critically involved in intracellular redox signaling.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Isolation, purification, and characterization of a rat liver mitochondrial protein disulfide isomerase

The isolation and purification to electrophoretical homogeneity and characterization of a protein disulfide isomerase from rat liver mitochondria is reported. The purified enzyme exhibits a single band on sodium dodecylsulfatepolyacrylamide gel electrophoresis with an apparent molecular weight of approximately 54 kDa. Comparatively, the microsomal form shows an apparent molecular weight of 57 kDa indicating that the two forms are slightly different. The antibody raised against the microsomal isoform does not recognize the mitochondrial enzyme. To characterize the enzyme, different classical methodologies utilized for protein disulfide isomerase estimation have been adopted. The isolated enzyme is active with all of them, indicating that it comprises all the features of a typical protein disulfide isomerase. At the mitochondrial level the enzyme appears mostly localized at the membrane level. Its potential involvement in mitochondrial membrane permeability control is also discussed.

Characterization of human and mouse peroxiredoxin IV: evidence for inhibition by Prx-IV of epidermal growth factor- and p53-induced reactive oxygen species

The aim of this study was to identify and characterize human and mouse Prx-IV. We identified mouse peroxiredoxin IV (Prx-IV) by virtue of sequence homology to its human ortholog previously called AOE372. Mouse Prx-IV conserves an amino-terminal presequence coding for signal peptide. The amino acid sequences of mature mouse and human Prx-IV share 97.5% identity. Phylogenetic analysis demonstrates that Prx-IV is more closely related to Prx-I/-II/-III than to Prx-V/-VI. Previously, we mapped the mouse Prx-IV gene to chromosome X by analyzing two sets of multiloci genetic crosses. Here we performed further comparative analysis of mouse and human Prx-IV genomic loci. Consistent with the mouse results, human Prx-IV gene localized to chromosome Xp22.135-136, in close proximity to SAT and DXS7178. A bacterial artificial chromosome (BAC) clone containing the complete human Prx-IV locus was identified. The size of 7 exons and the sequences of the splice junctions were confirmed by PCR analysis. We conclude that mouse Prx-IV is abundantly expressed in many tissues. However, we could not detect Prx-IV in the conditioned media of NIH-3T3 and Jurkat cells. Mouse Prx-IV was specifically found in the nucleus-excluded region of cultured mouse cells. Intracellularly, overexpression of mouse Prx-IV prevented the production of reactive oxygen species induced by epidermal growth factor or p53. Taken together, mouse Prx-IV is likely a cytoplasmic or organellar peroxiredoxin involved in intracellular redox signaling.

Polypeptides differentially expressed in imaginal discs define the peroxiredoxin family of genes in Drosophila

2D gel electrophoresis followed by microsequencing has been used to purify and identify a protein (catalogued in the database as SSP5111) from Drosophila wing imaginal discs of third instar larvae that showed significant differences in their level of expression when compared with other imaginal discs of the same age. The microsequence data showed identity with amino acids encoded by the human proliferation association gene, pag, which is a thiol-specific antioxidant. By virtue of this homology we have cloned and sequenced two cDNAs that appear to define the peroxiredoxin family of Drosophila. One of them, Jafrac1, encodes the SSP5111 protein searched, had 194 amino acids and mapped in the region 11E in the X chromosome. The other, Jafrac2, encodes a protein of 242 amino acids and mapped in the region 62F in the 3 L chromosome. Both new peroxidases contain two conserved cysteines and share homology with other peroxidases that extends over the entire sequence and ranges between 47% and 76%. An antiserum raised against the SSP5111 protein showed significant changes in the amount of protein in different stages of Drosophila development, being a major product in early embryos. In 2D gels the antibody not only recognizes the SSP5111 polypeptide but also a related one (catalogued in the database as SSP6107) that exhibits identical amino-acid sequence over at least 85% of its sequence. The data also suggest that the SSP5111 polypeptide could be a maternal-effect product.

Induction of peroxiredoxins in transplanted livers and demonstration of their in vitro cytoprotection activity

Peroxiredoxin (Prx)-I and -II belong to a new class of antioxidants. Here, we report that they are induced by ischemia/reperfusion (I/R) in transplanted livers. Hypothesizing that Prxs are induced to protect liver from oxidative damage, we transduced these human genes into murine NIH-3T3 cells. The overexpressed Prxs made the cells more resistant to t-butylhydroperoxide-induced apoptosis. These results indicate that Prx-I and Prx-II are induced by the transplantation process and can protect cells against oxidant damage in tissue culture. Thus, proper genetic manipulations of Prxs may be useful in increasing the success rate of organ transplantation.

Thioredoxin inhibits tumor necrosis factor- or interleukin-1-induced NF-kappaB activation at a level upstream of NF-kappaB-inducing kinase

Gene induction by tumor necrosis factor-alpha (TNFalpha) or interleukin-1beta (IL-1beta) is mediated in part by activation of the transcription factor nuclear factor kappaB (NF-kappaB), and requires signal adaptor molecules such as TNF receptor-associated factor (TRAFs). The latter interact with the NF-kappaB-inducing kinase (NIK), which is believed to be part of the IkappaB kinase complex. Although the precise mechanism is to be elucidated, it is well-known that antioxidant treatments inhibit the inflammatory cytokine-induced NF-kappaB activation. Thioredoxin (TRX) is a 12-kDa endogenous protein that regulates various cellular functions by modulating the redox state of proteins, overexpression of this molecule inhibits NF-kappaB activation. To elucidate the roles of TRX in the signal transduction of the cytokines, we investigated the effects of TRX on NF-kappaB activation induced by cytokine treatment or by overexpression of the signaling molecules. Our data show that TRX treatment inhibits NF-kappaB-dependent transcription at the level of downstream of TRAFs and upstream of NIK: TRX inhibited TRAF2-, TRAF5-, and TRAF6-induced NF-kappaB activation but does not inhibit NIK-, IKKalpha-, and MEKK-induced activation. In addition, we show that TRX inhibits NF-kappaB activation in a manner different from that for SAPK (stress activated protein kinase) inhibition.

Proteomic analysis of simulated occupational jet fuel exposure in the lung

We analyzed protein expression in the cytosolic fraction prepared from whole lung tissue in male Swiss-Webster mice exposed 1 h/day for seven days to aerosolized JP-8 jet fuel at concentrations of 1000 and 2500 mg/m3, simulating military occupational exposure. Lung cytosol samples were solubilized and separated via large scale, high resolution two-dimensional electrophoresis (2-DE) and gel patterns scanned, digitized and processed for statistical analysis. Significant quantitative and qualitative changes in tissue cytosol proteins resulted from jet fuel exposure. Several of the altered proteins were identified by peptide mass fingerprinting, confirmed by sequence tag analysis, and related to impaired protein synthetic machinery, toxic/metabolic stress and detoxification systems, ultrastructural damage, and functional responses to CO2 handling, acid-base homeostasis and fluid secretion. These results demonstrate a significant but comparatively moderate JP-8 effect on protein expression and corroborate previous morphological and biochemical evidence. Further molecular marker development and mechanistic inferences from these observations await proteomic analysis of whole tissue homogenates and other cell compartment, i.e., mitochondria, microsomes, and nuclei of lung and other targets.

Management of oxidative stress in the CNS: the many roles of glutathione

An outline is given of mechanisms that generate oxidative stress and inflammation. Considered are the metabolic mechanisms that give rise to peroxides, the source of strong oxidants; the production of dicarbonyls that interact with macromolecules to form advanced glycation endproducts; and the role that activation of the transcription factor NF(Kappa)B has in the expression of pro-inflammatory genes. Management of oxidative stress is considered by outlining the central role of reduced glutathione (GSH) in peroxide scavenging, dicarbonyl scavenging and activation of NF(Kappa)B. Cellular GSH levels are dictated by the balance between consumption, oxidation of GSH, reduction of oxidized-glutathione, and synthesis. The rate-limiting enzyme in GSH synthesis is L-gamma-glutamyl-L-cysteine synthase, a phase II enzyme. Phase II enzyme inducers are found in many fruits and vegetables. It is suggested that dietary phase II enzyme inducers be investigated for their potential for preventing or retarding the development of degenerative diseases that have an underlying oxidative stress and inflammatory component.

Rapid degradation of PrxI and PrxII induced by silica in Rat2 cells

Peroxidases of the peroxiredoxin (Prx) family catalyze the reduction of H(2)O(2) and lipid peroxides. The effects of H(2)O(2), 12-O-tetradecanoylphorbol 13-acetate (TPA), and silica on the abundance of two cytosolic isoforms of Prx (PrxI and PrxII) were examined in Rat2 cells. TPA induces the production of reactive oxygen species (ROS) in various mammalian cell types, and silica induces the production of ROS in Rat2 cells. Whereas H(2)O(2) and TPA did not affect the concentration of PrxI or Prx II, silica triggered a rapid degradation of both Prx enzymes. Silica also induced degradation of the NF-kappaB inhibitor IkappaB-alpha. N-Acetylcysteine and diphenyleneiodonium, both of which inhibit the accumulation of intracellular ROS, each blocked silica-induced degradation of IkappaB-alpha but had no effect on that of the Prx enzymes, suggesting that ROS do not contribute to Prx proteolysis. The silica-induced degradation of Prx enzymes was also insensitive to the proteasome inhibitors MG132 and lactacystin, whereas IkappaB-alpha proteolysis was completely blocked by these inhibitors. Experiments with the Ca(2+) ionophore A23187 indicated that a Ca(2+)-dependent protease such as calpain might contribute substantially to silica-induced degradation of PrxII, but only moderately to that of PrxI. These results indicate that silica increases cellular oxidative stress not only by inducing ROS production, but also by triggering the degradation of Prx enzymes that are responsible for elimination of cellular ROS. Such aggravated oxidative stress might be important in the initial pathogenesis of silica-associated pulmonary diseases.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.

Characterization of mouse peroxiredoxin I genomic DNA and its expression

Peroxiredoxins (Prxs) are a newly defined family of anti-oxidant proteins that have been implicated, via their anti-oxidant activity, in a number of cellular functions, including cell proliferation and differentiation, protection of other proteins from oxidative damage, and intracellular signaling. We isolated genomic DNA sequences of the Prx I genes from the mouse, and characterized their molecular genetic features. Prx I was found to form a small gene family with two and three members; one functional and two pseudogenes. The Prx I-1 gene has splice donor/acceptor site sequences and five or six exons, whereas the Prx I-2 clone has several structural features characteristic of a typical retroposon found to have ORF sequences. We analyzed the expression of pseudogenes, which were not expressed on the transcription levels in the investigated organs. The functional copy of the Prx I-1 gene was expressed abundantly in liver and kidney of the adult, as well as in early developing embryos. This report, together with amino acid/nucleotide sequence similarity between human and mice, provides a basis for speculating on an even earlier event in the evolution of the Prx I gene family, i.e. the Prx I gene was well conserved in human and mice via its anti-oxidant activity.

Cloning and characterization of AOEB166, a novel mammalian antioxidant enzyme of the peroxiredoxin family

Using two-dimensional electrophoresis, we have recently identified in human bronchoalveolar lavage fluid a novel protein, termed B166, with a molecular mass of 17 kDa. Here, we report the cloning of human and rat cDNAs encoding B166, which has been renamed AOEB166 for antioxidant enzyme B166. Indeed, the deduced amino acid sequence reveals that AOEB166 represents a new mammalian subfamily of AhpC/TSA peroxiredoxin antioxidant enzymes. Human AOEB166 shares 63% similarity with Escherichia coli AhpC22 alkyl hydroperoxide reductase and 66% similarity with a recently identified Saccharomyces cerevisiae alkyl hydroperoxide reductase/thioredoxin peroxidase. Moreover, recombinant AOEB166 expressed in E. coli exhibits a peroxidase activity, and an antioxidant activity comparable with that of catalase was demonstrated with the glutamine synthetase protection assay against dithiothreitol/Fe3+/O(2) oxidation. The analysis of AOEB166 mRNA distribution in 30 different human tissues and in 10 cell lines shows that the gene is widely expressed in the body. Of interest, the analysis of N- and C-terminal domains of both human and rat AOEB166 reveals amino acid sequences presenting features of mitochondrial and peroxisomal targeting sequences. Furthermore, human AOEB166 expressed as a fusion protein with GFP in HepG2 cell line is sorted to these organelles. Finally, acute inflammation induced in rat lung by lipopolysaccharide is associated with an increase of AOEB166 mRNA levels in lung, suggesting a protective role for AOEB166 in oxidative and inflammatory processes.

Peroxiredoxin I expression in human thyroid tumors

Peroxiredoxin I (Prx I) is newly discovered oxidative stress inducible protein, having a thioredoxin peroxidase activity. The Prx I expression level in 107 samples out of 60 thyroid lesions, including normal thyroid, tumors and thyroiditis including Graves' disease were examined using immunoblotting. Prx I expression levels in follicular neoplasm (P = 0.00005) and thyroiditis group (P = 0.0037) were significantly higher than that of the control group, while papillary carcinoma group did not show statistical significance. Immunohistochemistry indicated that Prx I was in epithelial cells of thyroid follicles. These results suggest that Prx I is expected to be a candidate for novel tumor markers to discriminate tissue types of tumors.

Oxidative stress-inducible proteins in macrophages

Macrophages produce reactive oxygen species such as O2-, H2O2 and *OH that contribute to the pathogenesis of diseases such as inflammation and atherosclerosis. The cells have multiple defense systems against those reactive oxygen species, and we describe here such an oxidative stress-inducible defense system. Upon exposure to reactive oxygen species and electrophilic agents, murine peritoneal macrophages induce stress proteins to protect themselves. Using differential screening, we cloned two novel proteins designated MSP23 and A170 that are induced in the cells by low levels of reactive oxygen species, electrophilic agents and other oxidative stress agents. MSP23 is murine peroxiredoxin I having a thioredoxin peroxidase activity and A170 is known as an ubiquitin- and PKC xi-binding protein. In addition to these two proteins, heme oxygenase-1 (HO-1) and cystine transport activity are also induced in the cells under oxidative stress conditions. Using nrf2-deficient macrophages, we found that transcription factor Nrf2, which is known to interact with antioxidant responsive elements (AREs) in the regulatory sequences of the genes, plays an important role in the oxidative stress-inducible response in the cells.

Distinct roles of thioredoxin in the cytoplasm and in the nucleus. A two-step mechanism of redox regulation of transcription factor NF-kappaB

Oxidative stresses such as UV irradiation to mammalian cells triggers a variety of oxistress responses including activation of transcription factors. Recently, activation of nuclear factor-kappaB (NF-kappaB) has been shown to be under oxidoreduction (redox) regulation controlled by thioredoxin (TRX), which is one of major endogenous redox-regulating molecules with thiol reducing activity. In order to elucidate where in the cellular compartment TRX participates in NF-kappaB regulation, we investigated the intracellular localization of TRX. UVB irradiation induced translocation of TRX from the cytoplasm into the nucleus. In our in vitro diamide-induced cross-linking study, we showed that TRX can associate directly with NF-kappaB p50. Overexpression of wild-type TRX suppressed induction of luciferase activity under NF-kappaB-binding sites in response to UV irradiation compared with the mock transfectant. In contrast, overexpression of nuclear-targeted TRX enhanced the luciferase activity. Thus, TRX seems to play dual and opposing roles in the regulation of NF-kappaB. In the cytoplasm, it interferes with the signals to IkappaB kinases and blocks the degradation of IkappaB. In the nucleus, however, TRX enhances NF-kappaB transcriptional activities by enhancing its ability to bind DNA. This two-step TRX-dependent regulation of the NF-kappaB complex may be a novel activation mechanism of redox-sensitive transcription factors.

Characterization of three isoforms of mammalian peroxiredoxin that reduce peroxides in the presence of thioredoxin

A peroxidase from yeast that reduces H2O2 with the use of electrons provided by thioredoxin (Trx) together with homologs from a wide variety of species constitute the peroxiredoxin (Prx) family of proteins. Twelve mammalian Prx members have been previously identified in association with various cellular functions apparently unrelated to peroxidase activity. These mammalian proteins have now been divided into three distinct types, Prx I, II, and III, on the basis of their deduced amino acid sequences and immunological reactivity. With the use of recombinant proteins, Prx I, II, and III have now been shown to possess peroxidase activity and to rely on Trx as a source of reducing equivalents. None of the three proteins exhibited peroxidase activity in the presence of glutaredoxin. All three enzymes showed similar kinetic properties: the Vmax was 6-13 micromol/min per mg at 37 degrees C, the Km for Trx was 3-6 microM, and the Km for H2O2 was < 20 microM. Immunoblot analysis of various rat tissues and cultured cells indicated that most cell types contain the three Prx isoforms, the sum of which amounts to approximately 1-10 microg per milligram of soluble protein. Prx I and II are cytosolic proteins, whereas Prx IlI is localized in mitochondria. These results suggest that, together with glutathione peroxidase and catalase, Prx enzymes likely play an important role in eliminating peroxides generated during metabolism as well as during stimulation of cell surface receptors.

Oligogalacturonic acid and chitosan reduce stomatal aperture by inducing the evolution of reactive oxygen species from guard cells of tomato and Commelina communis

Stomatal opening provides access to inner leaf tissues for many plant pathogens, so narrowing stomatal apertures may be advantageous for plant defense. We investigated how guard cells respond to elicitors that can be generated from cell walls of plants or pathogens during pathogen infection. The effect of oligogalacturonic acid (OGA), a degradation product of the plant cell wall, and chitosan (beta-1,4-linked glucosamine), a component of the fungal cell wall, on stomatal movements were examined in leaf epidermis of tomato (Lycopersicon esculentum L.) and Commelina communis L. These elicitors reduced the size of the stomatal aperture. OGA not only inhibited light-induced stomatal opening, but also accelerated stomatal closing in both species; chitosan inhibited light-induced stomatal opening in tomato epidermis. The effects of OGA and chitosan were suppressed when EGTA, catalase, or ascorbic acid was present in the medium, suggesting that Ca(2+) and H(2)O(2) mediate the elicitor-induced decrease of stomatal apertures. We show that the H(2)O(2) that is involved in this process is produced by guard cells in response to elicitors. Our results suggest that guard cells infected by pathogens may close their stomata via a pathway involving H(2)O(2) production, thus interfering with the continuous invasion of pathogens through the stomatal pores.

Redox regulation of cell signaling by selenocysteine in mammalian thioredoxin reductases

The intracellular generation of reactive oxygen species, together with the thioredoxin and glutathione systems, is thought to participate in redox signaling in mammalian cells. The activity of thioredoxin is dependent on the redox status of thioredoxin reductase (TR), the activity of which in turn is dependent on a selenocysteine residue. Two mammalian TR isozymes (TR2 and TR3), in addition to that previously characterized (TR1), have now been identified in humans and mice. All three TR isozymes contain a selenocysteine residue that is located in the penultimate position at the carboxyl terminus and which is encoded by a UGA codon. The generation of reactive oxygen species in a human carcinoma cell line was shown to result in both the oxidation of the selenocysteine in TR1 and a subsequent increase in the expression of this enzyme. These observations identify the carboxyl-terminal selenocysteine of TR1 as a cellular redox sensor and support an essential role for mammalian TR isozymes in redox-regulated cell signaling.

Mitochondrial thioredoxin reductase in bovine adrenal cortex its purification, properties, nucleotide/amino acid sequences, and identification of selenocysteine

Mitochondrial thioredoxin reductase was purified from bovine adrenal cortex. The enzyme is a first protein component in the mitochondrial thioredoxin-dependent peroxide reductase system. The purified reductase exhibited an apparent molecular mass of 56 kDa on SDS/PAGE, whereas the native protein was about 100 kDa, suggesting a homodimeric structure. It catalysed NADPH-dependent reduction of 5, 5'dithiobis(2-nitrobenzoic acid) and thioredoxins from various origins but not glutathione, oxidized dithiothreitol, DL-alpha-lipoic acid, or insulin. Amino acid and nucleotide sequence analyses revealed that it had a presequence composed of 21 amino acids which had features characteristic of a mitochondrial targeting signal. The amino acid sequence of the mature protein was similar to that of bovine cytosolic thioredoxin reductase (57%) and of human glutathione reductase (34%) and less similar to that of Escherichia coli (19%) or yeast (17%) enzymes. Human and bovine cytosolic thioredoxin reductase were recently identified to contain selenocysteine (Sec) as one of their amino acid constituents. We also identified Sec in the C-terminal region of mitochondrial (mt)-thioredoxin reductase by means of MS and amino acid sequence analyses of the C-terminal fragment. The four-amino acid motif, Gly-Cys-Sec-Gly, which is conserved among all Sec-containing thioredoxin reductases, probably functions as the third redox centre of the enzyme, as the mitochondrial reductase was inhibited by 1-chloro-2,4-dinitrobenzene, which was reported to modify Sec and Cys covalently. It is known that mammalian thioredoxin reductase is different from bacterial or yeast enzyme in, for example, their subunit molecular masses and domain structures. These two different types of enzymes with similar activity are suggested to have evolved convergently. Our data clearly show that mitochondria, which might have originated from symbiotic prokaryotes, contain thioredoxin reductase similar to the cytosolic enzyme and different from the bacterial one.

In vivo characterization of a thioredoxin h target protein defines a new peroxiredoxin family

Disruption of the two thioredoxin genes in yeast dramatically affects cell viability and growth. Expression of Arabidopsis thioredoxin AtTRX3 in the Saccharomyces thioredoxin Delta strain EMY63 restores a wild-type cell cycle, the ability to grow on methionine sulfoxide, and H2O2 tolerance. In order to isolate thioredoxin targets related to these phenotypes, we prepared a C35S (Escherichia coli numbering) thioredoxin mutant to stabilize the intermediate disulfide bridged complex and we added a polyhistidine N-terminal extension in order to purify the complex rapidly. Expression of this mutant thioredoxin in the wild-type yeast induces a reduced tolerance to H2O2, but only limited change in the cell cycle and no change in methionine sulfoxide utilization. Expression in the Delta thioredoxin strain EMY63 allowed us to isolate a complex of the thioredoxin with YLR109, an abundant yeast protein related to PMP20, a peroxisomal protein of Candida. No function has so far been attributed to this protein or to the other numerous homologues described in plants, animals, fungi, and prokaryotes. On the basis of the complementation and of low similarity with peroxiredoxins, we produced YLR109 and one of its Arabidopsis homologues in E. coli to test their peroxiredoxins activity. We demonstrate that both recombinant proteins present a thioredoxin-dependent peroxidase activity in vitro. The possible functions of this new peroxiredoxin family are discussed.

Vitamin C: poison, prophylactic or panacea?

Ascorbate is an essential enzyme cofactor but is often also regarded as an important antioxidant in vivo, protecting against cancer by scavenging DNA-damaging reactive oxygen species. Recent studies suggest that ascorbate sometimes increases DNA damage in humans. Although there is no evidence that any of these effects are deleterious to humans, we might need to change our thinking about the mechanisms of the antioxidant action of ascorbate in vivo.

Effects of kainate-mediated excitotoxicity on the expression of rat counterparts of A170 and MSP23 stress proteins in the brain

Stress proteins play important roles in the protective mechanisms under critical conditions for cell survival. We report here the expression of A170 and MSP23, oxidative stress-inducible proteins, under kainate-mediated excitotoxicity in the rat brain. A170 mRNA was significantly induced in the brain 5-8 h after i.p. kainate administration. MSP23 mRNA was observed at quite a low level in the rat brain, and the induction of MSP23 mRNA was not observed during the period 24 h after kainate administration. Immunoblot analysis demonstrated that the maximal expression level of A170 protein occurred 8 h after treatment in each part of the brain. MSP23 protein was constitutively expressed in the brain and the level of this protein was significantly decreased during the period 24 h after kainate administration. In situ hybridization and immunohistochemical studies showed that A170 was expressed predominantly in neurons, especially in pyramidal neurons of the cerebrum and cerebellar Purkinje cells, while MSP23 was expressed in oligodendrocytes. The induction of A170 was observed in the regions which are affected by excitotoxicity and this induction was observed in the earlier phase than cell death. Also, the region which shows high vulnerability to excitotoxicity such as pyramidal cell layer in the hippocampus, showed lower A170 expression than that which shows resistance to excitotoxicity, such as the dentate gyrus in the hippocampus. These results suggest that A170 may play a protective role in the brain under kainate-mediated excitotoxicity.

Crystallographic characterization of a stress-induced multifunctional protein, rat HBP-23

HBP-23 is a stress-induced multifunctional rat protein that belongs to a novel family of antioxidant proteins, referred to as peroxiredoxins, and exhibits heme-binding and inhibition of c-Abl protein tyrosine kinase. Recombinant HBP-23 was crystallized by a hanging-drop vapor-diffusion method. The crystals belong to space group P41212 or P43212 with unit-cell dimensions of a = b = 73.47 A, c = 210.37 A and contain two protein molecules in the asymmetric unit. A data set at 2.7-A resolution was collected with a cryo-crystallographic technique. Crystals of selenomethionyl HBP-23 were also obtained under the same conditions.

Cloning and expression of a new isotype of the peroxiredoxin gene of Chinese cabbage and its comparison to 2Cys-peroxiredoxin isolated from the same plant

A cDNA encoding a newly identified isotype of peroxiredoxin (Prx) was isolated from a Chinese cabbage flower bud cDNA library and designated CPrxII. Database searches using the predicted CPrxII amino acid sequence revealed no substantial homology to other proteins with the exception of the yeast type II Prx with which CPrxII shares 27.8% sequence identity. Recombinant CPrxII expressed in Escherichia coli was able to protect glutamine synthetase from inactivation in a metal-catalyzed oxidation system and to reduce H2O2 with electrons provided by thioredoxin. This specific antioxidant activity of CPrxII was about 6-fold higher than that of 2Cys-Prx of the same plant. In contrast to 2Cys-Prx, which is predominantly expressed in leaf tissue of cabbage seedlings, CPrxII is highly expressed in root tissue as revealed by Northern and Western blot analyses. The CPrxII gene exists as a small multigene family in the cabbage genome.

Role of heme in intracellular trafficking of thyroperoxidase and involvement of H2O2 generated at the apical surface of thyroid cells in autocatalytic covalent heme binding

Thyroperoxidase (TPO) is a glycosylated hemoprotein that plays a key role in thyroid hormone synthesis. We previously showed that in CHO cells expressing human TPO (hTPO) only 2% of synthesized hTPO reaches the cell surface. Herein, we investigated the role of heme moiety insertion in the exit of hTPO from the endoplasmic reticulum. Peroxidase activity at the cell surface and cell surface expression of hTPO were decreased by approximately 30 and approximately 80%, respectively, with succinyl acetone, an inhibitor of heme biosynthesis, and were increased by 20% with holotransferrin and aminolevulinic acid, precursors of heme biosynthesis. Results were similar with holotransferrin plus aminolevulinic acid or hemin, but hemin increased cell surface activity more efficiently (+120%) relative to the control. It had been suggested (DePillis, G., Ozaki, S., Kuo, J. M., Maltby, D. A., and Ortiz de Montellano, P. R. (1997) J. Biol. Chem. 272, 8857-8960) that covalent attachment of heme to mammalian peroxidases could be an H2O2-dependent autocatalytic processing. In our study, heme associated intracellularly with hTPO, and we hypothesized that there was insufficient exposure to H2O2 in Chinese hamster ovary cells before hTPO reached the cell surface. After a 10-min incubation, 10 microM H2O2 led to a 65% increase in cell surface activity. In contrast, in thyroid cells, H2O2 was synthesized at the apical cell surface and allowed covalent attachment of heme. Two-day incubation of primocultures of thyroid cells with catalase led to a 30% decrease in TPO activity at the cell surface. In conclusion, we provide compelling evidence for an essential role of 1) heme incorporation in the intracellular trafficking of hTPO and of 2) H2O2 generated at the apical pole of thyroid cells in the autocatalytic covalent heme binding to the TPO molecule.

Identification and functional characterization of a novel mitochondrial thioredoxin system in Saccharomyces cerevisiae

The so-called thioredoxin system, thioredoxin (Trx), thioredoxin reductase (Trr), and NADPH, acts as a disulfide reductase system and can protect cells against oxidative stress. In Saccharomyces cerevisiae, two thioredoxins (Trx1 and Trx2) and one thioredoxin reductase (Trr1) have been characterized, all of them located in the cytoplasm. We have identified and characterized a novel thioredoxin system in S. cerevisiae. The TRX3 gene codes for a 14-kDa protein containing the characteristic thioredoxin active site (WCGPC). The TRR2 gene codes for a protein of 37 kDa with the active-site motif (CAVC) present in prokaryotic thioredoxin reductases and binding sites for NADPH and FAD. We cloned and expressed both proteins in Escherichia coli, and the recombinant Trx3 and Trr2 proteins were active in the insulin reduction assay. Trx3 and Trr2 proteins have N-terminal domain extensions with characteristics of signals for import into mitochondria. By immunoblotting analysis of Saccharomyces subcellular fractions, we provide evidence that these proteins are located in mitochondria. We have also constructed S. cerevisiae strains null in Trx3 and Trr2 proteins and tested them for sensitivity to hydrogen peroxide. The Deltatrr2 mutant was more sensitive to H2O2, whereas the Deltatrx3 mutant was as sensitive as the wild type. These results suggest an important role of the mitochondrial thioredoxin reductase in protection against oxidative stress in S. cerevisiae.

Molecular cloning and characterization of a mitochondrial selenocysteine-containing thioredoxin reductase from rat liver

A thioredoxin reductase (TrxR), named here TrxR2, that did not react with antibodies to the previously identified TrxR (now named TrxR1) was purified from rat liver. Like TrxR1, TrxR2 was a dimeric enzyme containing selenocysteine (Secys) as the COOH-terminal penultimate residue. A cDNA encoding TrxR2 was cloned from rat liver; the open reading frame predicts a polypeptide of 526 amino acids with a COOH-terminal Gly-Cys-Secys-Gly motif provided that an in-frame TGA codon encodes Secys. The 3'-untranslated region of the cDNA contains a canonical Secys insertion sequence element. The deduced amino acid sequence of TrxR2 shows 54% identity to that of TrxR1 and contained 36 additional residues upstream of the experimentally determined NH2-terminal sequence. The sequence of this 36-residue region is typical of that of a mitochondrial leader peptide. Immunoblot analysis confirmed that TrxR2 is localized almost exclusively in mitochondria, whereas TrxR1 is a cytosolic protein. Unlike TrxR1, which was expressed at a level of 0.6 to 1.6 microgram/milligram of total soluble protein in all rat tissues examined, TrxR2 was relatively abundant (0.3 to 0.6 microgram/mg) only in liver, kidney, adrenal gland, and heart. The specific localization of TrxR2 in mitochondria, together with the previous identification of mitochondria-specific thioredoxin and thioredoxin-dependent peroxidase, suggest that these three proteins provide a primary line of defense against H2O2 produced by the mitochondrial respiratory chain.

Cloning of the peroxiredoxin gene family in rats and characterization of the fourth member

Peroxiredoxin (PRx) exhibits thioredoxin-dependent peroxidase activity and constitutes a family of proteins. Four members of genes from rat tissues were isolated by PCR using degenerated primers based on the sequences which encode a pair of highly conserved Cys-containing domains, and were then cloned to full-length cDNAs. These included two genes which have previously been isolated in rats, PRx I and PRx II, and two rat homologues of PRx III and PRx IV. We showed, for the first time, the simultaneous expression of all four genes in various rat tissues by Northern blotting. Since a discrepancy exists regarding cellular distribution, we further characterized PRx IV by expressing it in COS-1 cells. This clearly demonstrates that PRx IV is a secretory form and functions within the extracellular space.

Antioxidant function of the mitochondrial protein SP-22 in the cardiovascular system

The mitochondrial protein SP-22 has recently been reported to be a member of the thioredoxin-dependent peroxide reductase family, suggesting that it may be one of the antioxidant systems in mitochondria, which are the major site of reactive oxygen intermediate generation. The aim of this study was to examine whether SP-22 is involved in mitochondrial antioxidant mechanisms and whether its expression is induced by oxidative stresses, particularly those in mitochondria. The expression of SP-22 protein was enhanced by about 1.5-4.6-fold when bovine aortic endothelial cells (BAEC) were exposed to various oxidative stresses, including mitochondrial respiratory inhibitors which increased the superoxide generation in BAEC mitochondria. The expression of SP-22 mRNA increased 2.0-3.5-fold with a peak at 3-6 h after exposure to Fe2+/dithiothreitol or a respiratory inhibitor, antimycin A. BAEC with an increased level of SP-22 protein caused by pretreatment with mild oxidative stress became tolerant to subsequent intense oxidative stress. On the other hand, BAEC that had been depleted of SP-22 with an antisense oligodeoxynucleotide against SP-22 mRNA became more labile to oxidative stress than control BAEC. The induction of SP-22 protein by oxidative stress in vivo was demonstrated in an experimental model of myocardial infarction in rat heart. These findings indicate that SP-22 functions as an antioxidant in mitochondria of the cardiovascular system.

ATP-sensitive association of mortalin with the IL-1 receptor type I

Interleukin-1 (IL-1) is a major proinflammatory cytokine mediating local and systemic responses of the immune system. Two types of IL-1 receptors are known, but only the IL-1 receptor type I initiates biological responses. Here we show that two proteins with nucleic acid binding potential and mortalin, a member of the HSP70-family, are associated with the IL-1 receptor type I irrespective of IL-1 binding. The association of mortalin with the IL-1 receptor type I is specifically reversed by ATP concentrations in the physiological range. Other nucleotides are not or much less effective. The in vitro dissociation of mortalin effects neither the receptor association nor the activity of IRAK, which initiates the IL-1-dependent phosphorylation cascade. The roles of the receptor-associated proteins are therefore discussed in the context of receptor internalisation.

Characterization of the murine gene encoding Aop2 (antioxidant protein 2) and identification of two highly related genes

The mouse Aop2 (antioxidant protein 2) cDNA recently cloned from liver and kidney is a member of the thiol-specific antioxidant gene family. We have isolated the mouse gene encoding Aop2 and have shown that it comprises five exons and four introns. Analysis of the sequence upstream of the translation start site revealed several potential Sp1-binding sites and two putative transcription initiation sites. Primer extension studies were used to determine the 5' end of the Aop2 transcript. This upstream region also contains consensus recognition sequences for the transcription factors USF, SREBP, and ADR1, all of which have been shown to regulate genes involved in lipid metabolism, and multiple consensus binding sites for HSF, whose activity is modulated by oxidative stress. Since Aop2 has recently been proposed as a candidate gene for atherosclerosis susceptibility differences in mice, the presence of these binding sites may have biological significance. We also isolated two highly related intronless genes and determined their chromosomal locations. Further characterization of this highly conserved gene family and its regulation will help to elucidate their biological functions.

Evidence that peroxiredoxins are novel members of the thioredoxin fold superfamily

Peroxiredoxins catalyze reduction of hydrogen peroxide or alkyl peroxide, to water or the corresponding alcohol. Detailed analysis of their sequences indicates that these enzymes possess a thioredoxin (Trx)-like fold and consequently are homologues of both thioredoxin and glutathione peroxidase (GPx). Sequence- and structure-based multiple sequence alignments indicate that the peroxiredoxin active site cysteine and GPx active site selenocysteine are structurally equivalent. Homologous peroxiredoxin and GPx enzymes are predicted to catalyze equivalent reactions via similar reaction intermediates.

Cloning, expression and reconstitution of the trypanothione-dependent peroxidase system of Crithidia fasciculata

As a consequence of aerobic metabolism, trypanosomatids are exposed to reactive oxygen intermediates such as superoxide, hydrogen peroxide and the hydroxyl radical. Metabolism of hydrogen peroxide in Crithidia fasciculata is accomplished by three distinct proteins, tryparedoxin, tryparedoxin peroxidase and trypanothione reductase, working in concert with the substrates NADPH and trypanothione. Here, we report the cloning and characterisation of the tryparedoxin (TryX) and tryparedoxin peroxidase (TryP) genes from C. fasciculata. Both genes are multicopy and organized in distinct tandem arrays in the genome. TryX encodes a 16 kDa protein, which belongs to the thioredoxin superfamily, sharing the WCPPC motif, whereas TryP encodes a 21 kDa protein belonging to a new class of peroxidases called 2-Cys peroxidoxins. Both TryX and TryP were expressed in Escherichia coli and the purified recombinant proteins shown to utilise hydrogen peroxide in the presence of NADPH, trypanothione and trypanothione reductase, similar to the native proteins. TryX is rapidly reduced by trypanothione, but weakly by glutathionylspermidine, glutathione or ovothiol A. TryP shows a broad substrate specificity and can reduced hydrogen peroxide, t-butyl hydroperoxide and cumene hydroperoxide with equal efficiency.

Resistance of filarial nematode parasites to oxidative stress

All filariae examined to date express a comprehensive repertoire of both cytoplasmic and secreted anti-oxidant enzymes, although significant differences exist between species and life-cycle stages. Adult Brugia malayi, Dirofilaria immitis and Onchocerca volvulus secrete CuZn superoxide dismutases, and the former two species also secrete a selenocysteine-independent glutathione peroxidase. This enzyme has been localised to the cuticular matrix of B. malayi, and the preferential reduction of fatty acid- and phospholipid hydroperoxides suggests that it may protect cuticular membranes from oxidative damage rather than directly metabolise hydrogen peroxide. Adult O. volvulus may compensate for an apparent deficiency in expression of this enzyme via a secreted variant of glutathione S-transferase. Recent studies have identified a highly expressed family of enzymes collectively termed peroxiredoxins, which most probably play an essential role in reduction of hydroperoxides. Data from cDNA cloning exercises indicate that all filarial species examined thus far express at least two peroxiredoxin variants which have been localised to diverse tissues. In-vitro studies have shown that B. malayi are particularly resistant to oxidative stress, and that the parasites do not rely solely on enzymatic mechanisms of defence. Cuticular lipids are relatively resistant to lipid peroxidation due to the low unsaturation indices of the constituent fatty acyl residues, but complete protection is afforded by the presence of alpha-tocopherol, presumably assimilated from host extracellular fluids. Brugia malayi are also relatively resistant to nitric oxide-mediated toxicity, and this may be due in part to incomplete dependence on aerobic metabolism. Little is known of potential mechanisms for detoxification of nitric oxide derivatives and adaptive responses to oxidative stress in general, and these represent goals for future research.

Thioredoxin peroxidase (natural killer enhancing factor) regulation of activator protein-1 function in endothelial cells

Thioredoxin peroxidase-1 (TxP-1), originally cloned as natural killer enhancing factor-B, belongs to a highly conserved antioxidant family. Tumor necrosis factor-alpha (TNF) activates the expression of activator protein-1 (AP-1) responsive genes. We show here that over-expression of TxP-1 blocks TNF-induced AP-1 activation in endothelial ECV304 cells, which was demonstrated by three independent experimental protocols: electromobility shift assay with AP-1 oligonucleotide probe; reporter gene expression with AP-1 binding site, and interleukin-8 production, which is dependent on AP-1. These results are consistent with the role of TxP-1 as an antioxidant and the previous reports that TNF-induced reactive oxygen species were responsible for AP-1 activation.

Crystal structure of a novel human peroxidase enzyme at 2.0 A resolution

Hydrogen peroxide (H2O2) has been implicated recently as an intracellular messenger that affects cellular processes including protein phosphorylation, transcription and apoptosis. A set of novel peroxidases, named peroxiredoxins (Prx), regulate the intracellular concentration of H2O2 by reducing it in the presence of an appropriate electron donor. The crystal structure of a human Prx enzyme, hORF6, reveals that the protein contains two discrete domains and forms a dimer. The N-terminal domain has a thioredoxin fold and the C-terminal domain is used for dimerization. The active site cysteine (Cys 47), which exists as cysteine-sulfenic acid in the crystal, is located at the bottom of a relatively narrow pocket. The positively charged environment surrounding Cys 47 accounts for the peroxidase activity of the enzyme, which contains no redox cofactors.

Characterization of a mammalian peroxiredoxin that contains one conserved cysteine

A new type of peroxidase enzyme, named thioredoxin peroxidase (TPx), that reduces H2O2 with the use of electrons from thioredoxin and contains two essential cysteines was recently identified. TPx homologs, termed peroxiredoxin (Prx), have also been identified and include several proteins, designated 1-Cys Prx, that contain only one conserved cysteine. Recombinant human 1-Cys Prx expressed in and purified from Escherichia coli has now been shown to reduce H2O2 with electrons provided by dithiothreitol. Furthermore, human 1-Cys Prx transiently expressed in NIH 3T3 cells was able to remove intracellular H2O2 generated in response either to the addition of exogenous H2O2 or to treatment with platelet-derived growth factor. The conserved Cys47-SH group was shown to be the site of oxidation by H2O2. Thus, mutation of Cys47 to serine abolished peroxidase activity. Moreover, the oxidized intermediate appears to be Cys-SOH. In contrast to TPx, in which one of the two conserved cysteines is oxidized to Cys-SOH and then immediately reacts with the second conserved cysteine of the second subunit of the enzyme homodimer to form an intermolecular disulfide, the Cys-SOH of 1-Cys Prx does not form a disulfide. Neither thioredoxin, which reduces the disulfide of TPx, nor glutathione, which reduces the Cys-SeOH of oxidized glutathione peroxidase, was able to reduce the Cys-SOH of 1-Cys Prx and consequently could not support peroxidase activity. Human 1-Cys Prx was previously shown to exhibit a low level of phospholipase A2 activity at an acidic pH; the enzyme was thus proposed to be lysosomal, and Ser32 was proposed to be critical for lipase function. However, the mutation of Ser32 or Cys47 has now been shown to have no effect on the lipase activity of 1-Cys Prx, which was also shown to be a cytosolic protein. Thus, the primary cellular function of 1-Cys Prx appears to be to reduce peroxides with the use of electrons provided by an as yet unidentified source; the enzyme therefore represents a new type of peroxidase.