Characterization and immunological properties of selenium-containing glutathione peroxidase induced by selenite in Chlamydomonas reinhardtii

Effects of the Chloroplast Fructose-1,6-Bisphosphate Aldolase Gene on Growth and Low-Temperature Tolerance of Tomato

Tomato (Solanum lycopersicum) is one of the most important greenhouse vegetables, with a large cultivated area across the world. However, in northern China, tomato plants often suffer from low-temperature stress in solar greenhouse cultivation, which affects plant growth and development and results in economic losses. We previously found that a chloroplast aldolase gene in tomato, SlFBA4, plays an important role in the Calvin-Benson cycle (CBC), and its expression level and activity can be significantly altered when subjected to low-temperature stress. To further study the function of SlFBA4 in the photosynthesis and chilling tolerance of tomato, we obtained transgenic tomato plants by the over-expression and RNA interference (RNAi) of SlFBA4. The over-expression of SlFBA4 led to higher fructose-1,6-bisphosphate aldolase activity, net photosynthetic rate (Pn) and activity of other enzymes in the CBC than wild type. Opposite results were observed in the RNAi lines. Moreover, an increase in thousand-seed weight, plant height, stem diameter and germination rate in optimal and sub-optimal temperatures was observed in the over-expression lines, while opposite effects were observed in the RNAi lines. Furthermore, over-expression of SlFBA4 increased Pn and enzyme activity and decreased malonaldehyde (MDA) content under chilling conditions. On the other hand, Pn and MDA content were more severely influenced by chilling stress in the RNAi lines. These results indicate that SlFBA4 plays an important role in tomato growth and tolerance to chilling stress.

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

Structural elucidation of selenocysteine insertion machinery of microalgal selenoprotein T and its transcriptional analysis

Essential trace element selenium in association with selenoproteins, which is found in almost all organisms except higher plants and fungi, is involved in various biological functions. Advancement in the field of whole genome sequencing and data analyzing bioinformatic tools led to the accumulation of genome information of organisms. However, selenoproteins are unique and it needs specialized genomics tool for its identification as well as characterization. In this study, the presence of selenoprotein T (SelT) from Scenedesmus quadricauda was shown for the first time with experimental evidence and compared with SelT of marine microalgae Nannochloropsis oceanica. Along with SelT, all the associated machineries required to synthesize the selenoproteins were also identified. Also, the present study tried to explicate the evolutionary relatedness of SelT of these two organisms with other known bacteria and eukaryotes. Transcript level analysis in S. quadricauda under endoplasmic reticulum stress showed a 1.2 ± 0.28-fold increase in SelT expression. Thus, it provided the first experimental evidence on SelT expression from microalgae.

Biochemistry and Physiology of Reactive Oxygen Species in Euglena

Reactive oxygen species (ROS) such as superoxide and hydrogen peroxide are by-products of various metabolic processes in aerobic organisms including Euglena. Chloroplasts and mitochondria are the main sites of ROS generation by photosynthesis and respiration, respectively, through the active electron transport chain. An efficient antioxidant network is required to maintain intracellular ROS pools at optimal conditions for redox homeostasis. A comparison with the networks of plants and animals revealed that Euglena has acquired some aspects of ROS metabolic process. Euglena lacks catalase and a typical selenocysteine containing animal-type glutathione peroxidase for hydrogen peroxide scavenging, but contains enzymes involved in ascorbate-glutathione cycle solely in the cytosol. Ascorbate peroxidase in Euglena, which plays a central role in the ascorbate-glutathione cycle, forms a unique intra-molecular dimer structure that is related to the recognition of peroxides. We recently identified peroxiredoxin and NADPH-dependent thioredoxin reductase isoforms in cellular compartments including chloroplasts and mitochondria, indicating the physiological significance of the thioredoxin system in metabolism of ROS. Besides glutathione, Euglena contains the unusual thiol compound trypanothione, an unusual form of glutathione involving two molecules of glutathione joined by a spermidine linker, which has been identified in pathogenic protists such as Trypanosomatida and Schizopyrenida. Furthermore, in contrast to plants, photosynthesis by Euglena is not susceptible to hydrogen peroxide because of resistance of the Calvin cycle enzymes fructose-1,6-bisphosphatse, NADP⁺-glyceraldehyde-3-phosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase to hydrogen peroxide. Consequently, these characteristics of Euglena appear to exemplify a strategy for survival and adaptation to various environmental conditions during the evolutionary process of euglenoids.

Post-translational activation of non-selenium glutathione peroxidase of Chlamydomonas reinhardtii by specific incorporation of selenium

Glutathione peroxidase (GPX) plays a pivotal role in the protection of cells against oxidative damage. The green alga Chlamydomonas reinhardtii expresses both selenocysteine-containing GPX and the non-selenium GPX homolog (GPXH). We previously reported that supplementation of selenium to algal culture induces GPXH to exhibit GPX activity. Here we investigated the incorporation of selenium into GPXH and its causal relationship with the upregulation of the enzymatic activity. GPXH was purified from algal cells grown with selenium and proteolytically digested into four fragments. Selenium content analysis for these proteolytic fragments confirmed that GPXH-incorporated selenium is predominantly enriched in a fragment that carries the putative catalytic residue Cys-38. We next constructed three kinds of engineered GPXH proteins by substituting Ser for one of three Cys residues in native GPXH, Cys-38, -66, and -84, using a bacterial overexpression system, resulting in Cys38Ser, Cys66Ser, and Cys84Ser derivatives, respectively. Of these, the Cys66Ser and Cys84Ser derivatives exhibited the same level of selenium-dependent GPX activity as the normal recombinant GPXH, whereas the Cys38Ser mutant GPXH not only lost its activity completely but also demonstrated severely impaired incorporation of selenium. These findings strongly suggest that selenium is post-translationally assimilated into the Cys-38 of the GPXH protein, thereby enhancing its enzymatic activity.

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Non-Se algal GPX was characterized in terms of Se-associated structure–function.

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Se was found to be specifically bound to the catalytic Cys of the GPX.

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Se-binding targeted to the active site was required for GPX up-regulation.

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This is the first evidence for Se-mediated post-translational activation of plant GPX.

Evidence of translocation and physiological impacts of foliar applied CeO2 nanoparticles on cucumber (Cucumis sativus) plants

Currently, most of the nanotoxicity studies in plants involve exposure to the nanoparticles (NPs) through the roots. However, plants interact with atmospheric NPs through the leaves, and our knowledge on their response to this contact is limited. In this study, hydroponically grown cucumber (Cucumis sativus) plants were aerially treated either with nano ceria powder (nCeO2) at 0.98 and 2.94 g/m(3) or suspensions at 20, 40, 80, 160, and 320 mg/L. Fifteen days after treatment, plants were analyzed for Ce uptake by using ICP-OES and TEM. In addition, the activity of three stress enzymes was measured. The ICP-OES results showed Ce in all tissues of the CeO2 NP treated plants, suggesting uptake through the leaves and translocation to the other plant parts. The TEM results showed the presence of Ce in roots, which corroborates the ICP-OES results. The biochemical assays showed that catalase activity increased in roots and ascorbate peroxidase activity decreased in leaves. Our findings show that atmospheric NPs can be taken up and distributed within plant tissues, which could represent a threat for environmental and human health.

Arsenic and selenium interactive effect on alga Desmodesmus quadricauda

Substances known to be toxic in one-component solutions often exhibit unexpected effects when present in mixtures. Only a few efforts have been made to assess the effect of As-Se mixture in algae or plants in general. Due to the lack of information on this topic, the aim of this study was to examine the As-Se interactive effect in the alga species Desmodesmus quadricauda. The initial density of algal cells was 1.9×10(4), cultures were permanently illuminated (70μEm(-2)s(-1)) and As and Se adverse effect was expressed as EC (effective concentration) value. For all experiments three EC (EC(10), EC(20), EC(50)) values for both metalloids were used: for As 26.20, 29.05, 35.38mg L(-1) and for Se 1.93, 3.65, 12.24mg L(-1), respectively. During this study algal biomass growth, lipid peroxidation and protein-bound thiol content parameters were used to assess the As-Se interactions. The reciprocal effect of the elements on their uptake by the alga was also determined. The As-treated algae supplemented with Se exhibited impaired growth indicating a synergistic interaction between the two elements. In samples treated with As-Se mixture, the total algal As content showed marked increase depending on the Se concentration in the mixture. Se uptake was also positively affected by rising As concentrations in the mixture. Consequently, the As-Se-treated algae experienced greater damage to membranes, evidenced by marked elevation of the TBARS (thiobarbituric acid reactive substances) content. The TBARS content increased to a maximum level by 29.05mg L(-1) of As and 3.65mg L(-1) of Se, which was around 70 percent higher than that of the control. The thiol content was very close to that of the control treatment over the entire concentration range and for all As and Se combinations tested. Possible explanation for the synergism observed in D. quadricauda, is that the elevated uptake of As and Se upon their interaction and impaired antioxidant system, has added to the toxicity of the elements.

Mechanisms of oxygen activation during plant stress

Green plants, within certain limitations, can adapt to a wide variety of unfavourable conditions such as drought, temperature changes, light variations, infectious attacks, air pollution and soil contamination. Depending on the strength of the individual impact(s), fluent or abrupt changes in visible or measurable stress symptoms indicate the deviation from normal metabolic conditions. Most of the visible or measurable symptoms are connected with altered oxygen metabolism principally concerning the transition from mostly heterolytic (two-electron transition) to increased homolytic (one-electron transition) processes. Homolytic reactions within metabolic sequences create, however, free radicals and have to be counteracted by the increase in radical-scavenging processes or compounds, thus warranting reaction sequences under metabolic control. At later states of stress episodes, the above control is gradually lost and more or less chaotic radical processes take over. Finally, cellular decompartmentalisations induce lytic and necrotic processes which are visible as the collapse of darkening cells or tissues. Every episode during this process is governed by a more or less denned balance between pro- and antioxidative capacities, including photosynthetic (strongly under metabolic and oxygen-detoxifying control) and photodynamic (only controlled by scavenger- and/or quencher-availability) reactions. This (theoretical) sequence of events in most cases can only be characterised punctually by strongly defined (analytical) indicator reactions (ESR) and is certainly species- and organ-specific.

Prenatal and perinatal acrylamide disrupts the development of cerebellum in rat: Biochemical and morphological studies

Acrylamide is known to cause neurotoxicity in the experimental animals and humans. The literature on its neurotoxic effect in the adult animals is huge, but the effect of acrylamide on the embryonic and postnatal development is relatively less understood. The present study examined its effects on the development of external features and cerebellum in albino rats. Acrylamide was orally administered to non-anesthetized pregnant females by gastric intubation 10 mg/kg/day. The animals were divided into three groups as follows. (1) Group A, newborn from control animals; (2) Group B; newborns from mothers treated with acrylamide from day 7 (D7) of gestation till birth (prenatal intoxicated group); (3) Group C; newborns from mothers treated with acrylamide from D7 of gestation till D28 after birth (perinatally intoxicated group). Acrylamide administered either prenatally or perinatally has been shown to induce significant retardation in the newborns’ body weights development, increase of thiobarbituric acid-reactive substances (TBARS) and oxidative stress (significant reductions in glutathione reduced [GSH], total thiols, superoxide dismutase [SOD] and peroxidase activities) in the developing cerebellum. Acrylamide treatment delayed the proliferation in the granular layer and delayed both cell migration and differentiation. Purkinje cell loss was also seen in acrylamide-treated animals. Ultrastructural studies of Purkinje cells in the perinatal group showed microvacuolations and cell loss. The results of this study show that prenatal and perinatal acrylamide or its metabolites disrupts the biochemical machinery, cause oxidative stress and induce structural changes in the developing rat cerebellum.

Effect of prenatal and perinatal acrylamide on the biochemical and morphological changes in liver of developing albino rat

Acrylamide has been employed as an experimental probe to investigate biochemical and morphological changes in developing rat liver following toxin administration in pregnant rats. Non-anesthetized pregnant rats were given acrylamide by gastric intubation at a dose of 10 mg/kg/day. The pups were divided into three groups: Group A, mothers were treated with saline (control group); Group B, mothers were treated with acrylamide from day D7 of gestation till birth (prenatal intoxication); Group C, mothers were treated with acrylamide from D7 of gestation to D28 after birth (perinatal intoxication). Acrylamide-induced biochemical changes (in liver and serum) and morphological changes (in liver) were studied in control and acrylamide-treated developing pups. Prenatally and perinatally administered acrylamide significantly increased lipid peroxidation and reduced glutathione and total thiol levels in liver. Significant inhibition of peroxidase and superoxide dismutase activities was observed in liver tissue. Total lipids including cholesterol and triglycerides were significantly increased in the serum. Acrylamide treatment increased serum alanine aminotransferase and aspartate aminotransferase activities and inhibited alkaline phosphatase activity. Sodium and potassium concentrations were increased, but calcium, phosphorus and iron levels were significantly reduced in the serum. Acrylamide produced significant electrophoretic changes in serum proteins. The most noticeable change was splitting of beta-globulin into beta1- and beta2-globulins. Light microscopy showed acrylamide-induced fatty deposits, congested central vein, vacuolization and chromatolysis in hepatocytes. Ultrastructural studies revealed vacuolated cytoplasm, lipid droplets of variable size and mitochondria with damaged cristae and vacuolization. The nuclei in acrylamide-treated groups showed marked decrease in the staining of nuclear DNA.

Differential responses to copper-induced oxidative stress in the marine macroalgae Lessonia nigrescens and Scytosiphon lomentaria (Phaeophyceae)

In order to help explain the absence of the brown kelp Lessonia nigrescens from a coastal environment chronically enriched with copper, we characterized the biochemical responses induced by copper stress in this kelp and compared them with those displayed by the copper tolerant brown alga Scytosiphon lomentaria. These algae were cultivated with increasing concentrations of copper (20, 40 and 100microgL(-1)) for 96h and the temporal production of hydrogen peroxide, superoxide anions and lipoperoxides as well as the activities of antioxidant enzymes catalase (CAT), glutathione peroxidase (GP), ascorbate peroxidase (AP), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) and the activity of the defense enzyme lipoxygenase (LOX) were determined. In L. nigrescens and S. lomentaria, a single peak of hydrogen peroxide was detected, with similar maxima after 3h of copper exposure, although in L. nigrescens buffering took longer. Superoxide anions, on the other hand, were only detected in L. nigrescens. The production of lipoperoxides in L. nigrescens increased steadily at higher copper levels, in a pattern clearly different to their rapid stabilization in S. lomentaria. We suggest that the accumulation of lipoperoxides might be related to LOX, whose activity also increases with exposure time. Furthermore, activities of the antioxidant enzymes CAT, GP, AP and DHAR were lower in L. nigrescens than in S. lomentaria, and GP and DHAR were completely inhibited at higher copper concentrations. Since these enzymes also detoxify fatty acid hydroperoxides, their inhibition, together with the activation of LOX, may explain the persistent and copper-dependent levels of lipoperoxides in L. nigrescens. Based on terrestrial plant models demonstrating toxic effects of lipoperoxides, and on our results on organellar ultrastructural changes, we suggest that copper toxicity induced an uncontrolled lipoperoxide accumulation which may lead to cell damage and dysfunction in L. nigrescens, explaining at least partially, the absence of this kelp in a copper-enriched coastal environment.

Effects of selenium on arsenic uptake in arsenic hyperaccumulator Pteris vittata L

Selenium (Se) is a non-metallic element, which has the capability to increase the antioxidative capacity and stress tolerance of plants to heavy metals. Plants vary considerably in their physiological response to Se. The reported research investigated the effects of Se on arsenic (As) uptake by As hyperaccumulator Pteris vittata L. and determined possible mechanisms of interaction. Pteris vittata plants were exposed hydroponically to 0, 150 or 300 microM of Na(2)HAsO(4) in the presence of 0, 5 or 10 microM of Na(2)SeO(4) for 5 or 10d. Application of 5 microM Se enhanced As concentration by P. vittata fronds by 7-45%. At 5 microM, Se acted as an antioxidant, inhibiting lipid peroxidation (reduced by 26-42% in the fronds) via increased levels of thiols and glutathione (increased by 24% in the fronds). The results suggest that Se is either an antioxidant or it activates plant protective mechanisms, thereby alleviating oxidative stress and improving arsenic uptake in P. vittata.

Selenium toxicosis assessment (in vivo and in vitro) and the protective role of vitamin B12 in male quail (Coturnix Coturnix)

The present study was undertaken to elucidate the toxicity induced by sodium selenite in male quail through in vivo and in vitro studies and the role played by vitamin B12 in alleviating selenium toxicity. Administration of selenite orally for 1 month induced hepatic oxidative damage. Selenite decreased body weight gain and increased relative liver weight. Selenite reduced hemoglobin and iron concentrations and elevated total bilirubin concentration. Serum transaminases and alkaline phosphatase activities were increased in selenium-intoxicated quails. Total protein concentration was decreased associated with the appearance of prealbumin fraction, an increased γ-globulin and a decreased α- and β-globulins. The highest level of selenium was found in liver followed by kidney, testis, faeces and blood. Supplementation of vitamin B12 orally for 1 month simultaneously with selenite caused less marked biological alteration in the investigated parameters. In vitro study using isolated quail hepatocytes incubated with sodium selenite showed a dose-dependent response for toxicity markers. These results suggest that selenosis can be reduced by vitamin B12 supplementation.

The peroxiredoxin and glutathione peroxidase families in Chlamydomonas reinhardtii

Thiol/selenol peroxidases are ubiquitous nonheme peroxidases. They are divided into two major subfamilies: peroxiredoxins (PRXs) and glutathione peroxidases (GPXs). PRXs are present in diverse subcellular compartments and divided into four types: 2-cys PRX, 1-cys PRX, PRX-Q, and type II PRX (PRXII). In mammals, most GPXs are selenoenzymes containing a highly reactive selenocysteine in their active site while yeast and land plants are devoid of selenoproteins but contain nonselenium GPXs. The presence of a chloroplastic 2-cys PRX, a nonselenium GPX, and two selenium-dependent GPXs has been reported in the unicellular green alga Chlamydomonas reinhardtii. The availability of the Chlamydomonas genome sequence offers the opportunity to complete our knowledge on thiol/selenol peroxidases in this organism. In this article, Chlamydomonas PRX and GPX families are presented and compared to their counterparts in Arabidopsis, human, yeast, and Synechocystis sp. A summary of the current knowledge on each family of peroxidases, especially in photosynthetic organisms, phylogenetic analyses, and investigations of the putative subcellular localization of each protein and its relative expression level, on the basis of EST data, are presented. We show that Chlamydomonas PRX and GPX families share some similarities with other photosynthetic organisms but also with human cells. The data are discussed in view of recent results suggesting that these enzymes are important scavengers of reactive oxygen species (ROS) and reactive nitrogen species (RNS) but also play a role in ROS signaling.

The Calvin cycle in cyanobacteria is regulated by CP12 via the NAD(H)/NADP(H) ratio under light/dark conditions

In Synechococcus PCC7942 cells grown in the dark, the concentrations of NAD(H) and NADP(H) were 128+/-2.5 and 483+/-4.0 microm, respectively, while those in the cells under light conditions were 100+/-5.0 and 649+/-7.0 microm, respectively. Analysis of gel filtration indicated that the change of the ratio of NADP(H) to NAD(H) in cyanobacterial cells under light/dark conditions controls the reversible dissociation of the PRK/CP12/GAPDH complex (approximately 520 kDa) consisting of phosphoribulokinase (PRK), CP12, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). S. 7942 CP12 lacked the two Cys residues essential for formation of the N-terminal peptide loop in the CP12 of higher plants, but the N-terminal region of S. 7942 CP12 had the ability to be associated with PRK. The growth of mutant cells in which the CP12 gene was disrupted by a kanamycin resistance cartridge gene was almost the same as that of wild-type cells under continuous light conditions. However, under the light/dark cycle (12 h/12 h), the growth of CP12-disrupted mutant cells was significantly inhibited compared with that of wild-type cells. The mutant cells showed a decreased rate of O2 consumption and an increased level of ribulose 1,5-bisphosphate compared with wild-type cells in the dark. These data suggest that under light and dark conditions, the oligomerization of CP12 with PRK and GAPDH regulates the activities of both enzymes and thus the carbon flow from the Calvin cycle to the oxidative pentose phosphate cycle.

Molecular characterization of glutathione peroxidase-like protein in halotolerant Chlamydomonas sp. W80

A cDNA clone encoding a glutathione peroxidase (GPX)-like protein was isolated from the cDNA library from halotolerant Chlamydomonas W80 (C. W80) by a simple screening method based on the bacterial expression system. The cDNA clone contained an open reading frame encoding a mature protein of 163 amino acids with a calculated molecular mass of 18 267 Da. No potential signal peptide was found. The deduced amino acid sequence of the cDNA showed 40-63% and 37-46% homology to those of GPX-like proteins from higher plants and mammalian GPXs, respectively. The C. W80 GPX-like protein contained a normal cysteine residue instead of a selenocysteine at the catalytic site. However, it contained amino acid residues (glutamine and tryptophan) that are involved in three protein loops and are important for the catalytic activity in the mammalian GPX. Interestingly, the native and recombinant GPX-like proteins showed activities towards unsaturated fatty acid hydroperoxides, but not towards either H2O2 or phospholipid hydroperoxide. Transformed E. coli cells expressing the C. W80 GPX-like protein showed enhanced tolerance to 5% NaCl or 0.2 mM paraquat treatments. Accession number: The nucleotide sequence data reported have been submitted to the DDBJ, EMBL, and GenBank nucleotide sequence databases with the following accession number AB009083.

Trends in selenium biochemistry

The biochemistry of selenium-containing natural products, including selenoproteins, is reviewed up to May 2002. Particular emphasis is placed on the assimilation of selenium from inorganic and organic selenium sources for selenoprotein synthesis, the catalytic role of selenium in enzymes, and medical implications of an unbalanced selenium supply. The review contains 393 references on key discoveries and recent progress.

A selenoprotein in the plant kingdom. Mass spectrometry confirms that an opal codon (UGA) encodes selenocysteine in Chlamydomonas reinhardtii gluththione peroxidase

Selenoproteins that contain the rare amino acid selenocysteine in their primary structure have been identified in diverse organisms such as viruses, bacteria, archea, and mammals, but so far not in yeast or plants. Among the most thoroughly investigated families of selenoenzymes are the animal glutathione peroxidases (GPXs). In the last few years, genes encoding GPX-like homologues from Chlamydomonas and higher plants have been isolated, but, unlike the animal ones, all of them have cysteine (rather than selenocysteine) residues in their catalytic site. In all organisms investigated that contain selenoproteins, selenocysteine is encoded by a UGA opal codon, which is usually a stop codon. We report here that, in Chlamydomonas reinhardtii, the cDNA-cloned sequence of a GPX homologue contains an internal TGA codon in frame to the ATG. Specific mRNA expression, protein production, and enzyme activity are selenium-dependent. Sequence analysis of the peptides produced by proteolytic digestion, performed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), confirmed the presence of a selenocysteine residue at the predicted site and suggest its location in the mitochondria. Thus, our data present the first direct proof that a UGA opal codon is decoded in the plant kingdom to incorporate selenocysteine.

Selenoproteins and selenocysteine insertion system in the model plant cell system, Chlamydomonas reinhardtii

Known eukaryotic selenocysteine (Sec)-containing proteins are animal proteins, whereas selenoproteins have not been found in yeast and plants. Surprisingly, we detected selenoproteins in a member of the plant kingdom, Chlamydomonas reinhardtii, and directly identified two of them as phospholipid hydroperoxide glutathione peroxidase and selenoprotein W homologs. Moreover, a selenocysteyl-tRNA was isolated that recognized specifically the Sec codon UGA. Subsequent gene cloning and bioinformatics analyses identified eight additional selenoproteins, including methionine-S-sulfoxide reductase, a selenoprotein specific to Chlamydomonas: Chlamydomonas selenoprotein genes contained selenocysteine insertion sequence (SECIS) elements that were similar, but not identical, to those of animals. These SECIS elements could direct selenoprotein synthesis in mammalian cells, indicating a common origin of plant and animal Sec insertion systems. We found that selenium is required for optimal growth of Chlamydomonas: Finally, evolutionary analyses suggested that selenoproteins present in Chlamydomonas and animals evolved early, and were independently lost in land plants, yeast and some animals.

Acquisition of a new type of fructose-1,6-bisphosphatase with resistance to hydrogen peroxide in cyanobacteria: molecular characterization of the enzyme from Synechocystis PCC 6803

We have previously described that Synechococcus PCC 7942 cells contain two fructose-1,6-bisphosphatase isozymes, designated F-I and F-II the former belongs to a new type of fructose-1,6-bisphosphatase, while the latter is a typical enzyme similar to the cytosolic and chloroplastic forms from eukaryotic cells [Tamoi et al., Arch. Biochem. Biophys., 334, 1996, 27-36]. The genes of F-I and F-II were found in three species of cyanobacteria, Synechocystis PCC 6803, Anabaena 7120, and Plectonema boryanum according to the results of Southern hybridization with a probe from the S. 7942 F-I and F-II genes. In Western blotting, antibody raised against the S. 7942 F-I cross-reacted with a protein band corresponding to the F-I in each crude extract from cyanobacterial cells, whereas the antibody against F-II failed to cross-react with any protein band corresponding to the F-II. In cyanobacterial cells, only one form of F-I has been resolved by ion-exchange chromatography at same concentration of NaCl as shown in the F-I of S. 7942. The F-I from Synechocystis 6803 has been purified to electrophoretic homogeneity. The enzyme hydrolyzed both fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate. The apparent K(m) values of the enzyme for fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate were 57 +/- 2.4 and 180 +/- 6.3 microM, respectively. The enzyme activity was inhibited by AMP with a Ki value of 0.57 +/- 0.03 mM for fructose 1,6-bisphosphate and 0.35 +/- 0.02 mM for sedoheptulose 1,7-bisphosphate. The enzyme showed a molecular mass of 168 kDa which was composed of four identical subunits. The activities of FBPase and SBPase from the F-I were resistant to hydrogen peroxide up to 1 mM. The nucleotide sequence of the S. 6803 F-I gene showed an open reading frame of 1164 bp that encoded a protein of 388 amino acid residues (approx. molecular mass of 41.6 kDa). The deduced amino acid sequences had homologous sequences with the S. 7942 F-I.

Purification and characterization of ascorbate peroxidase in Chlorella vulgaris

Chlorella vulgaris contained only one isoform of ascorbate peroxidase (AsAP) as the hydrogen peroxide (H2O2)-scavenging system except for catalase at a specific activity of 3.3 +/- 0.2 units/mg protein. The activity of glutathione peroxidase was not detected in the extracts from cells grown in the absence and presence of sodium selenite. We detected the activity of monodehydroascorbate reductase involved in the regeneration of ascorbate, but we failed to detect the dehydroascorbate reductase activity. AsAP has been purified to electrophoretic homogeneity from Chlorella cells. The enzyme was a monomer with a molecular mass of 32 kDa using gel filtration and SDS-polyacrylamide gel electrophoresis. The enzyme showed higher specificity with ascorbate than with pyrogallol. The K(m) values of the enzyme for ascorbate and H2O2 were 111 +/- 8.9 and 20 +/- 2.5 microM, respectively. When the enzyme was diluted with the ascorbate-deleted medium, the half inactivation time was approximately 15 min. The absorption spectra of the purified enzyme and the inhibition by cyanide and azide showed that it is a hemoprotein. The enzyme was markedly inhibited by 0.2 mM p-chloromercuribenzoate. The enzyme cross-reacted by immunoblotting with the monoclonal antibody raised against Euglena cytosolic AsAP. The amino acid sequences in the N-terminal region of Chlorella AsAP showed no significant similarity to any other AsAPs from higher plants and algae.

Metabolic activation of meta-phenylenediamine by the alga Chlamydomonas reinhardtii

Promutagens/procarcinogens arylamines are widely distributed in the environment. While it is accepted that these compounds can be metabolized to ultimate mutagens in mammals and higher plants, in aquatic plants they have not yet been explored. Intact wild-type and repair-deficient strains of Chlamydomonas reinhardtii and Saccharomyces cerevisiae D7 strain were assayed for their ability to activate meta-phenylenediamine (m-PDA) to an ultimate mutagen. The different responses of the algal wild-type strain and repair-deficient strains to the toxic and mutagenic effects of m-PDA were observed. Recombination repair played an important role in repair of damage induced to C. reinhardtii DNA by this arylamine. The examined isomer of phenylenediamine induced mutations in both algal and yeast cells. m-PDA was activated in the algal cell/microbe coincubation assay in which algal cells were used as an activating system and bacteria Salmonella typhimurium and yeast Saccharomyces cerevisiae as the genetic indicator organisms. This new assay is, in addition to the animal microsome metabolizing system and the plant cell/microbe coincubation assay, suitable for the detection of environmental promutagens and their conversion to mutagens mainly in aquatic environments.

Methods for purification of glutathione peroxidase and related enzymes

The different preparative techniques and related analytical methods used for purification of glutathione peroxidase, glutathione transferase and glutathione reductase, described in papers published in the last ten years, have been reviewed in this article. Among the different purification techniques, chromatography has played a relevant role, being reported in all the papers reviewed, whereas other preparative techniques such as electrophoresis and isoelectric focusing were less employed and have been reported in only ca. 3% of cases. Frequently, several different chromatographic modes and several rechromatography steps have been employed. The use of at least three different chromatographic modes has been reported in 53% of total reviewed papers, whereas 41% of them employed two differents modes and in only 6% a single preparative chromatographic step was used. To evaluate losses and improve recovery, analytical methods for quantitation of protein and assay of enzymatic activity must be used in each purification step. Among these analytical techniques, gel electrophoresis, under denaturing conditions, has been widely used to assess purity of enzyme preparation. A discussion of the different activity assay methods used for these three enzymes is also presented in this article.

A selenium-containing phospholipid-hydroperoxide glutathione peroxidase in Schistosoma mansoni

The 100000Xg supernatant parasite platyhelminth Schistosoma mansoni exhibits a glutathione peroxidase activity with the substrate phosphatidylcholine hydroperoxide. Purification yielded a protein of 20 kDa molecular mass both on gel filtration column chromatography and SDS/PAGE, thus suggesting that S. mansoni expresses a protein similar to the mammalian selenoenzynic phospholipid-hydroperoxide glutathione peroxidase. Kinetic analysis and substrate specificity corroborated this assumption, the second-order rate constants for the oxidation of the ground-state enzyme (k+1) being higher with phosphatidylcholine hydroperoxide than with other peroxide substrates, such as cumene liydroperoxide or H2O2, and quantitatively similar to those of mammalian phospholipid-hydroperoxide glutathione peroxidase. Partial sequencing of the protein and selenium measurement by neutron activation analysis established that the purified peroxidase corresponded to the product of the S. mansoni gene previously reported and supposed to encode a selenium-containing glutathione peroxidase [Roche, C., Williams, D. L., Khalife, J., LePresle, T., Capron, A. & Pierce, R. J. (1994) Cloning and characterization of gene encoding Schistosoma mansoni glutathione peroxidase, Gene 138, 149 - 152]. S. mansoni thus contains a scienoperoxidase sharing molecular mass, catalytic efficiency and substrate specificity with phospholipid-hydroperoxide glutathione peroxidase, dismantling the concept that those enzymes are unique to vertebrate organisms.

Molecular characterization of Euglena ascorbate peroxidase using monoclonal antibody

Ascorbate peroxidase (EC 1.11.1.11) has been purified to electrophoretic homogeneity from Euglena gracilis Z. The enzyme showed a molecular mass of 58 kDa on SDS-PAGE and gel filtration, indicating that Euglena ascorbate peroxidase exists as a monomeric form. The substrate specificity for an electron donor and the stability of the purified enzyme were similar to those of cytosolic isozymes from higher plants. One of the characteristic properties was that Euglena ascorbate peroxidase reduces organic hydroperoxides as well as hydrogen peroxide. The N-terminal amino-acid sequence showed no significant similarity to any other ascorbate peroxidase from higher plants. However, the sequence of the peptides from the purified enzyme exhibited a high degree of homology to sequences of cytosolic and chloroplastic ascorbate peroxidases. Monoclonal antibodies against the purified Euglena ascorbate peroxidase were prepared. Two monoclonal antibodies (EAP1 and EAP2) showed high homology to cytosolic ascorbate peroxidases of higher plants, as judged by Western blot analysis. The EAP1 was also specific for chloroplastic ascorbate peroxidase from spinach. These findings indicate that Euglena ascorbate peroxidase exists in highly homologous regions with the ascorbate peroxidases of higher plants.

Effect of L-glutamate on 2-oxoglutarate decarboxylase in Euglena gracilis

The effect of tricarboxylic acid-cycle intermediates and related compounds on 2-oxoglutarate decarboxylase activity was investigated. The addition of L-glutamate to Euglena cells grown on glucose/(NH4)2SO4 medium resulted in an increase in 2-oxoglutarate decarboxylase activity, which was abolished by the simultaneous addition of cycloheximide. Immunochemical titration, immunoblot analysis and labelling in vivo with antibody raised against 2-oxoglutarate decarboxylase showed that the increase in 2-oxoglutarate decarboxylase activity was due to synthesis of new protein and not to activation of pre-existing protein. The experimental results reported here demonstrate that L-glutamate is assimilated by the pathway, via 2-oxoglutarate, that consists of L-glutamate-oxaloacetate aminotransferase, 2-oxoglutarate decarboxylase and succinate semialdehyde dehydrogenase, rather than by the gamma-aminobutyrate shunt, consisting of L-glutamate decarboxylase and gamma-aminobutyrate aminotransferase.