Characterization of ascorbate peroxidases from unicellular red alga Galdieria partita

Ascorbate-Glutathione Cycle Genes Families in Euphorbiaceae: Characterization and Evolutionary Analysis

Ascorbate peroxidase (APX), Monodehydroascorbate Reductase (MDAR), Dehydroascorbate Reductase (DHAR) and Glutathione Reductase (GR) enzymes participate in the ascorbate-glutathione cycle, which exerts a central role in the antioxidant metabolism in plants. Despite the importance of this antioxidant system in different signal transduction networks related to development and response to environmental stresses, the pathway has not yet been comprehensively characterized in many crop plants. Among different eudicotyledons, the Euphorbiaceae family is particularly diverse with some species highly tolerant to drought. Here the APX, MDAR, DHAR, and GR genes in Ricinus communis, Jatropha curcas, Manihot esculenta, and Hevea brasiliensis were identified and characterized. The comprehensive phylogenetic and genomic analyses allowed the classification of the genes into different classes, equivalent to cytosolic, peroxisomal, chloroplastic, and mitochondrial enzymes, and revealed the duplication events that contribute to the expansion of these families within plant genomes. Due to the high drought stress tolerance of Ricinus communis, the expression patterns of ascorbate-glutathione cycle genes in response to drought were also analyzed in leaves and roots, indicating a differential expression during the stress. Altogether, these data contributed to the characterization of the expression pattern and evolutionary analysis of these genes, filling the gap in the proposed functions of core components of the antioxidant mechanism during stress response in an economically relevant group of plants.

Alleviation of Ammonium Toxicity in Salvia splendens ‘Vista Red’ with Silicon Supplementation

Ammonium (NH₄⁺) toxicity seriously hampers the yield and quality of salvia plants because most varieties or sub-species are highly sensitive to NH₄⁺. Silicon (Si) is an alternative that is used to minimize these disturbances and maintain better growth under NH₄⁺ toxicity. Nevertheless, the mitigatory effects of Si on NH₄⁺-stressed salvia are unknown. Therefore, this study was carried out to determine how Si assists to alleviate the NH₄⁺ toxicity degree in salvia. To this end, salvia plants were cultivated in a controlled environment supplied with a constant N (nitrogen) level (13 meq·L⁻¹) in the form of three NH₄⁺:NO₃⁻ ratios (0:100, 50:50, 100:0), each with (1.0 meq·L⁻¹) or without Si. Physiological disorders and typical NH₄⁺ toxicity symptoms, as well as interrupted photosynthesis, were observed in the 100% NH₄⁺-treated plants. Furthermore, cation uptake inhibition and oxidative damage were also imposed by the 100% NH₄⁺ supply. In contrast, in the presence of Si, the NH₄⁺ toxicity degree was attenuated and plant growth was ensured. Accordingly, the NH₄⁺ toxicity appearance ratio decreased significantly. Furthermore, Si-treated plants showed an ameliorated photosynthetic ability, elevated internal K and Ca levels, and enhanced antioxidative capacity, as reflected by improved major antioxidant enzyme activities, as well as diminished accumulation of ROS (reactive oxygen species) and MDA (malondialdehyde). Our findings enlightened the agronomic importance of additional Si to nutrient solutions, especially pertaining to bedding plants at risk of NH₄⁺ toxicity.

Molecular Evolution of Maize Ascorbate Peroxidase Genes and Their Functional Divergence

Ascorbate peroxidase (APX) is an important antioxidant enzyme. APXs in maize are encoded by multiple genes and exist as isoenzymes. The evolutionary history and functional divergence of the maize APX gene family were analyzed through comparative genomic and experimental data on the Internet in this paper. APX genes in higher plants were divided into classes A, B, and C. Each type of APX gene in angiosperms only had one ancestral gene that was duplicated along with the genome duplication or local (or tandem) duplication of the angiosperm. A total of eight genes were retained in maize and named APXa1, APXa2, APXa3, APXb1, APXb2, APXc1.1, APXc1.2, and APXc2. The APX genes of class A were located in the chloroplasts or mitochondria, and the class B and C genes were localized in the peroxisomes and cytoplasm, respectively. The expression patterns of eight APXs were different in vegetative and reproductive organs at different growth and development stages. APXa1 and APXb1 of maize may participate in the antioxidant metabolism of vegetative organs under normal conditions. APXa2, APXb2, APXc1.1, and APXc1.2 may be involved in the stress response, and APXb2 and APXc2 may participate in the senescence response. These results provide a basis for cultivating high-yield and resistant maize varieties.

Comparison of Galdieria growth and photosynthetic activity in different culture systems

In the last years, the acidothermophilic red microalga Galdieria sulphuraria has been increasingly studied for industrial applications such as wastewater treatment, recovery of rare earth elements, production of phycobilins. However, even now it is not possible an industrial cultivation of this organism because biotechnological research on G. sulphuraria and allied species is relatively recent and fragmented. Having in mind a possible scale-up for commercial applications, we have compared the growth and photosynthetic performance of G. sulphuraria in four suspended systems (Inclined bubble column, Decanter Laboratory Flask, Tubular Bioreactor, Ultra-flat plate bioreactor) and one immobilized system (Twin Layer Sytem). The results showed that G. sulphuraria had the highest growth, productivity and photosynthetic performance, when grown on the immobilized system, which also offers some economics advantages.

A peroxidase mimetic protects skeletal muscle cells from peroxide challenge and stimulates insulin signaling

Reactive oxygen species such as hydrogen peroxide have been implicated in causing metabolic dysfunction such as insulin resistance. Heme groups, either by themselves or when incorporated into proteins, have been shown to scavenge peroxide and demonstrate protective effects in various cell types. Thus, we hypothesized that a metalloporphyrin similar in structure to heme, Fe(III)tetrakis(4-benzoic acid)porphyrin (FeTBAP), would be a peroxidase mimetic that could defend cells against oxidative stress. After demonstrating that FeTBAP has peroxidase activity with reduced nicotinamide adenine dinucleotide phosphate (NADPH) and NADH as reducing substrates, we determined that FeTBAP partially rescued C2C12 myotubes from peroxide-induced insulin resistance as measured by phosphorylation of AKT (S473) and insulin receptor substrate 1 (IRS-1, Y612). Furthermore, we found that FeTBAP stimulates insulin signaling in myotubes and mouse soleus skeletal muscle to about the same level as insulin for phosphorylation of AKT, IRS-1, and glycogen synthase kinase 3β (S9). We found that FeTBAP lowers intracellular peroxide levels and protects against carbonyl formation in myotubes exposed to peroxide. Additionally, we found that FeTBAP stimulates glucose transport in myotubes and skeletal muscle to about the same level as insulin. We conclude that a peroxidase mimetic can blunt peroxide-induced insulin resistance and also stimulate insulin signaling and glucose transport, suggesting a possible role of peroxidase activity in regulation of insulin signaling.

Oxidative stress induced by cadmium in lettuce (Lactuca sativa Linn.): Oxidative stress indicators and prediction of their genes

Environmental contamination with heavy metals is of concern as plants have the ability to absorb chemical toxicants facilitating the entry of toxic metals into the food chain. Lettuce (Lactuca sativa Linn.) was cultured in four nutrient solutions containing different concentrations of cadmium (0, 3, 6, and 9 mmol). The impact of heavy metal on the morphological features, antioxidant properties and antioxidant enzymes activity were investigated with primary focus on superoxide dismutase, ascorbate peroxidase, peroxidase and catalase enzymes. In silico methods were utilized in the study of the genes of these enzymes. Significant changes were observed in the morphological features of the plant with plants appearing stunted, more spherical and yellow in colour. A decrease in the dry mass of the plant was also detected. The Translocation factor (TF) for cadmium was significantly high in lettuce. Enhanced antioxidant enzymatic activity suggests that these enzymes are integrally involved in the defense mechanism of the plant to heavy metal stress. Also observed was an increase in total soluble protein, and total phenolic content. Total flavonoid content was not significantly affected. Fourteen genes encoding for ascorbate peroxidase and nineteen genes for superoxide dismutase were identified in lettuce. These enzymes varied from each other with regards to the number of exons and amino acids present, as well as their location within the cell. Plants exhibit various response mechanisms to combat heavy metal contamination.

Oxidative damage and antioxidative system in algae

Reactive oxygen species (ROS) typically produce in algae and act as secondary messengers in numerous cellular processes. Under abiotic stresses, the balance between production and suppression of ROS disappears and causes increase of ROS. Increasing excessive ROS can cause damage to various cellular components comprising cell membranes, proteins and lipids. Algae have an antioxidant defense system to overcome on oxidative damage. Antioxidant defense mechanisms are of two types, namely enzymatic and non-enzymatic antioxidants. The enzymatic antioxidants include superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase. The non-enzymatic antioxidants include carotenoids, tocopherol, ascorbic acid, glutathione, flavonoids and phenolic compounds. In this review, we describe the various types of ROS and their production, and antioxidant defense mechanisms for ROS suppression.

Myoglobin as a versatile peroxidase: Implications for a more important role for vertebrate striated muscle in antioxidant defense

Myoglobins (Mb) are ubiquitous proteins found in striated muscle of nearly all vertebrate taxa. Although their function is most commonly associated with facilitating oxygen storage and diffusion, Mb has also been implicated in cellular antioxidant defense. The oxidized (Fe³⁺) form of Mb (metMB) can react with hydrogen peroxide (H₂O₂) to produce ferrylMb. FerrylMb can be reduced back to metMb for another round of reaction with H₂O₂. In the present study, we have shown that horse skeletal muscle Mb displays peroxidase activity using 2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) and 3,3',5,5'-tetramethylbenzidine (TMB) as reducing substrates, as well as the biologically-relevant substrates NADH/NADPH, ascorbate, caffeic acid, and resveratrol. We have also shown that ferrylMb can be reduced by both ethanol and acetaldehyde, which are known to accumulate in some vertebrate tissues under anaerobic conditions, such as anoxic goldfish and crucian carp, implying a potential mechanism for ethanol detoxification in striated muscle. We found that metMb peroxidase activity is pH-dependent, increasing as pH decreases from 7.4 to 6.1, which is biologically relevant to anaerobic vertebrate muscle when incurring intracellular lactic acidosis. Finally, we found that metMb reacts with hypochlorite in a heme-dependent fashion, indicating that Mb could play a role in hypochlorite detoxification. Taken together, these data suggest that Mb peroxidase activity might be an important antioxidant mechanism in vertebrate cardiac and skeletal muscle under a variety of physiological conditions, such as those that might occur in contracting skeletal muscle or during hypoxia.

Antioxidant and anti-proliferative properties of extracts from heterotrophic cultures of Galdieria sulphuraria

This study explores the possibility to use the extremophilic microalga Galdieria sulphuraria (strain 064) as a source of natural biomolecules with beneficial and protective effects on human health. Galdieria was cultivated in heterotrophy conditions and cells extracts for their antioxidant and anti-proliferative properties were tested. Galdieria extracts showed high antioxidant power tested through ABTS assay and revealed high glutathione and phycocyanin contents. Based on Annexin-V FITC/propidium iodide and MTT analysis, algae extracts inhibited the proliferation of human adenocarcinoma A549 cells (51.2% inhibition) through the induction of apoptosis without cell cycle arrest. Besides, cytotoxicity and cytometry assays showed a positive pro-apoptotic mechanism. On these bases, we suggest that G. sulphuraria from heterotrophic culture, for its therapeutic potential, could be considered a good candidate for further studies with the aim to isolate bioactive anti-cancer molecules.

Thicker three-dimensional tissue from a "symbiotic recycling system" combining mammalian cells and algae

In this paper, we report an in vitro co-culture system that combines mammalian cells and algae, Chlorococcum littorale, to create a three-dimensional (3-D) tissue. While the C2C12 mouse myoblasts and rat cardiac cells consumed oxygen actively, intense oxygen production was accounted for by the algae even in the co-culture system. Although cell metabolism within thicker cardiac cell-layered tissues showed anaerobic respiration, the introduction of innovative co-cultivation partially changed the metabolism to aerobic respiration. Moreover, the amount of glucose consumption and lactate production in the cardiac tissues and the amount of ammonia in the culture media decreased significantly when co-cultivated with algae. In the cardiac tissues devoid of algae, delamination was observed histologically, and the release of creatine kinase (CK) from the tissues showed severe cardiac cell damage. On the other hand, the layered cell tissues with algae were observed to be in a good histological condition, with less than one-fifth decline in CK release. The co-cultivation with algae improved the culture condition of the thicker tissues, resulting in the formation of 160 μm-thick cardiac tissues. Thus, the present study proposes the possibility of creating an in vitro “symbiotic recycling system” composed of mammalian cells and algae.

Evolution of alternative biosynthetic pathways for vitamin C following plastid acquisition in photosynthetic eukaryotes

Ascorbic acid (vitamin C) is an enzyme co-factor in eukaryotes that also plays a critical role in protecting photosynthetic eukaryotes against damaging reactive oxygen species derived from the chloroplast. Many animal lineages, including primates, have become ascorbate auxotrophs due to the loss of the terminal enzyme in their biosynthetic pathway, l-gulonolactone oxidase (GULO). The alternative pathways found in land plants and Euglena use a different terminal enzyme, l-galactonolactone dehydrogenase (GLDH). The evolutionary processes leading to these differing pathways and their contribution to the cellular roles of ascorbate remain unclear. Here we present molecular and biochemical evidence demonstrating that GULO was functionally replaced with GLDH in photosynthetic eukaryote lineages following plastid acquisition. GULO has therefore been lost repeatedly throughout eukaryote evolution. The formation of the alternative biosynthetic pathways in photosynthetic eukaryotes uncoupled ascorbate synthesis from hydrogen peroxide production and likely contributed to the rise of ascorbate as a major photoprotective antioxidant.

DOI:http://dx.doi.org/10.7554/eLife.06369.001

Animals, plants, algae and other eukaryotic organisms all need vitamin C to enable many of their enzymes to work properly. Vitamin C also protects plant and algal cells from damage by molecules called reactive oxygen species (ROS), which can be produced when these cells harvest energy from sunlight in a process called photosynthesis. Photosynthesis occurs inside structures called chloroplasts, and has evolved on multiple occasions in eukaryotes when non-photosynthetic organisms acquired chloroplasts from other algae and then had to develop improved defences against ROS.

There are several steps involved in the production of vitamin C. In many animals, an enzyme called GULO carries out the final step by converting a molecule known as an aldonolactone into vitamin C; this reaction also produces ROS as a waste product. The GULO enzyme is missing in humans, primates and some other groups of animals, so these organisms must get all the vitamin C they need from their diet.

Plants and algae use a different enzyme—called GLDH—to make vitamin C from aldonolactone. GLDH is very similar to GULO, but it does not produce ROS as a waste product. It is not clear how the different pathways have evolved, or why some animals have lost the ability to make their own vitamin C.

Here, Wheeler et al. used genetics and biochemistry to investigate the evolutionary origins of vitamin C production in a variety of eukaryotic organisms. This investigation revealed that although GULO is missing from the insects and several other groups of animals, it is present in the sponges and many other eukaryotes. This suggests that GULO evolved in early eukaryotic organisms and has since been lost by the different groups of animals. On the other hand, GLDH is only found in plants and the other eukaryotes that can photosynthesize.

Wheeler et al.'s findings suggest that GULO has been lost and replaced by GLDH in all plants and algae following their acquisition of chloroplasts. GDLH allows plants and algae to make vitamin C without also producing ROS, which could explain why vitamin C has been able to take on an extra role in these organisms. The results allow us to better understand the functions of vitamin C in photosynthetic organisms and the processes associated with the acquisition of chloroplasts during evolution.

DOI:http://dx.doi.org/10.7554/eLife.06369.002

Metal/metalloid stress tolerance in plants: role of ascorbate, its redox couple, and associated enzymes

The enhanced generation of reactive oxygen species (ROS) under metal/metalloid stress is most common in plants, and the elevated ROS must be successfully metabolized in order to maintain plant growth, development, and productivity. Ascorbate (AsA) is a highly abundant metabolite and a water-soluble antioxidant, which besides positively influencing various aspects in plants acts also as an enigmatic component of plant defense armory. As a significant component of the ascorbate-glutathione (AsA-GSH) pathway, it performs multiple vital functions in plants including growth and development by either directly or indirectly metabolizing ROS and its products. Enzymes such as monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) and dehydroascorbate reductase (DHAR, EC 1.8.5.1) maintain the reduced form of AsA pool besides metabolically controlling the ratio of AsA with its oxidized form (dehydroascorbate, DHA). Ascorbate peroxidase (APX, EC 1.11.1.11) utilizes the reduced AsA pool as the specific electron donor during ROS metabolism. Thus, AsA, its redox couple (AsA/DHA), and related enzymes (MDHAR, DHAR, and APX) cumulatively form an AsA redox system to efficiently protect plants particularly against potential anomalies caused by ROS and its products. Here we present a critical assessment of the recent research reports available on metal/metalloid-accrued modulation of reduced AsA pool, AsA/DHA redox couple and AsA-related major enzymes, and the cumulative significance of these antioxidant system components in plant metal/metalloid stress tolerance.

Tc-cAPX, a cytosolic ascorbate peroxidase of Theobroma cacao L. engaged in the interaction with Moniliophthora perniciosa, the causing agent of witches' broom disease

The level of hydrogen peroxide (H2O2) in plants signalizes the induction of several genes, including that of ascorbate peroxidase (APX-EC 1.11.1.11). APX isoenzymes play a central role in the elimination of intracellular H2O2 and contribute to plant responses to diverse stresses. During the infection process in Theobroma cacao by Moniliophthora perniciosa oxidative stress is generated and the APX action recruited from the plant. The present work aimed to characterize the T. cacao APX involved in the molecular interaction of T. cacao-M. perniciosa. The peroxidase activity was analyzed in protein extracts from cocoa plants infected by M. perniciosa and showed the induction of peroxidases like APX in resistant cocoa plants. The cytosolic protein of T. cacao (GenBank: ABR68691.2) was phylogenetically analyzed in relation to other peroxidases from the cocoa genome and eight genes encoding APX proteins with conserved domains were also analyzed. The cDNA from cytosolic APX was cloned in pET28a and the recombinant protein expressed and purified (rTc-cAPX). The secondary structure of the protein was analyzed by Circular Dichroism (CD) displaying high proportion of α-helices when folded. The enzymatic assay shows stable activity using ascorbate and guaiacol as an electron donor for H2O2 reduction. The pH 7.5 is the optimum for enzyme activity. Chromatographic analysis suggests that rTc-cAPX is a homodimer in solution. Results indicate that the rTc-cAPX is correctly folded, stable and biochemically active. The purified rTc-cAPX presented biotechnological potential and is adequate for future structural and functional studies.

New insights into Chlamydomonas reinhardtii hydrogen production processes by combined microarray/RNA-seq transcriptomics

Hydrogen production with Chlamydomonas reinhardtii induced by sulphur starvation is a multiphase process while the cell internal metabolism is completely remodelled. The first cellular response is characterized by induction of genes with regulatory functions, followed by a total remodelling of the metabolism to provide reduction equivalents for cellular processes. We were able to characterize all major processes that provide energy and reduction equivalents during hydrogen production. Furthermore, C. reinhardtii showed a strong transcript increase for gene models responsible for stress response and detoxification of oxygen radicals. Finally, we were able to determine potential bottlenecks and target genes for manipulation to increase hydrogen production or to prolong the hydrogen production phase. The investigation of transcriptomic changes during the time course of hydrogen production in C. reinhardtii with microarrays and RNA-seq revealed new insights into the regulation and remodelling of the cell internal metabolism. Both methods showed a good correlation. The microarray platform can be used as a reliable standard tool for routine gene expression analysis. RNA-seq additionally allowed a detailed time-dependent study of gene expression and determination of new genes involved in the hydrogen production process.

Ascorbate as seen through plant evolution: the rise of a successful molecule?

Ascorbate is a widespread and efficient antioxidant that has multiple functions in plants, traditionally associated with the reactions of photosynthesis. This review aims to look at ascorbate from an evolutionary perspective. Cyanobacteria, algae, and bryophytes contain lower concentrations of ascorbate than higher plants, where the molecule accumulates in high concentrations in both photosynthetic and non-photosynthetic organs and tissues. This increase in ascorbate concentration is paralleled by an increase in the number of isoforms of ascorbate peroxidase and the ascorbate regenerating enzymes mono- and dehydroascorbate reductase. One way of understanding the rise in ascorbate concentrations is to consider ascorbate as a molecule among others that has been subject to selection pressures during evolution, due to its cost or benefit for the cell and the organism. Ascorbate has a low cost in terms of synthesis and toxicity, and its benefits include protection of the glutathione pool and proper functioning of a range of enzymes. The hypothesis presented here is that these features would have favoured increasing roles for the molecule in the development and growth of multicellular organisms. This review then focuses on this diversity of roles for ascorbate in both photosynthetic and non-photosynthetic tissues of higher plants, including fruits and seeds, as well as further functions the molecule may possess by looking at other species. The review also highlights one of the trade-offs of domestication, which has often reduced or diluted ascorbate content in the quest for increased fruit growth and yield, with unknown consequences for the corresponding functional diversity, particularly in terms of stress resistance and adaptive responses to the environment.

Plant responses to stresses: Role of ascorbate peroxidase in the antioxidant protection

When plants are exposed to stressful environmental conditions, the production of Reactive Oxygen Species (ROS) increases and can cause significant damage to the cells. Antioxidant defenses, which can detoxify ROS, are present in plants. A major hydrogen peroxide detoxifying system in plant cells is the ascorbate-glutathione cycle, in which, ascorbate peroxidase (APX) enzymes play a key role catalyzing the conversion of H(2)O(2) into H(2)O, using ascorbate as a specific electron donor. Different APX isoforms are present in distinct subcellular compartments, such as chloroplasts, mitochondria, peroxisome, and cytosol. The expression of APX genes is regulated in response to biotic and abiotic stresses as well as during plant development. The APX responses are directly involved in the protection of plant cells against adverse environmental conditions. Furthermore, mutant plants APX genes showed alterations in growth, physiology and antioxidant metabolism revealing those enzymes involvement in the normal plant development.

Purification and kinetic characterization of the liverwort Pallavicinia lyelli (Hook.) S. Gray. cytosolic ascorbate peroxidase

Ascorbate peroxidase (APX) of the liverwort Pallavicinia lyelli was extracted and purified through ammonium sulfate precipitation, Butyl-Toyopearl, DEAE-Cellulofine and Sephadex G-75 chromatography. The purification factor for APX was 285 with 7.9% yield. The enzyme was characterized for thermal stability, pH and kinetic parameters. The molecular mass of APX was approximately 28 kDa estimated by SDS-PAGE. The purity was checked by native PAGE, showing a single prominent band. The optimum pH was 6.0. The enzyme had a temperature optimum at 40 degrees C and was relatively stable at 60 degrees C, with 54% loss of activity. 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. Spectral analysis and inhibitor studies were consistent with the presence of a heme moiety. When compared with ascorbate peroxidase activity derived from ruptured intact chloroplasts, the purified enzyme was found to have a higher stability, a broader pH optimum for activity and the capacity to utilize alternate electron donors. p-Chloromercuribenzoate (pCMB), hydroxyurea and salicylic acid (SA) significantly inhibited APX activity. Ascorbate (AsA) and pyrogallol were found to be efficient substrates for Pallavicinia APX, considering the Vmax/Km ratio. We detected the activity of monodehydroascorbate reductase (MDHAR) involved in the regeneration of ascorbate, but failed to detect the dehydroascorbate reductase (DHAR) activity. The data obtained in this study may help to understand desiccation tolerance mechanism in the liverwort.

Euglena gracilis ascorbate peroxidase forms an intramolecular dimeric structure: its unique molecular characterization

Euglena gracilis lacks a catalase and contains a single APX (ascorbate peroxidase) and enzymes related to the redox cycle of ascorbate in the cytosol. In the present study, a full-length cDNA clone encoding the Euglena APX was isolated and found to contain an open reading frame encoding a protein of 649 amino acids with a calculated molecular mass of 70.5 kDa. Interestingly, the enzyme consisted of two entirely homologous catalytic domains, designated APX-N and APX-C, and an 102 amino acid extension in the N-terminal region, which had a typical class II signal proposed for plastid targeting in Euglena. A computer-assisted analysis indicated a novel protein structure with an intramolecular dimeric structure. The analysis of cell fractionation showed that the APX protein is distributed in the cytosol, but not the plastids, suggesting that Euglena APX becomes mature in the cytosol after processing of the precursor. The kinetics of the recombinant mature FL (full-length)-APX and the APX-N and APX-C domains with ascorbate and H2O2 were almost the same as that of the native enzyme. However, the substrate specificity of the mature FL-APX and the native enzyme was different from that of APX-N and APX-C. The mature FL-APX, but not the truncated forms, could reduce alkyl hydroperoxides, suggesting that the dimeric structure is correlated with substrate recognition. In Euglena cells transfected with double-stranded RNA, the silencing of APX expression resulted in a significant increase in the cellular level of H2O2, indicating the physiological importance of APX to the metabolism of H2O2.

Ultraviolet irradiation induced oxidative stress and response of antioxidant system in an intertidal macroalgae Corallina officinalis L

The response of the antioxidant defense system of an intertidal macroalgae Corallina officinalis L. to different dosages of UV-B irradiation was investigated. Results showed that superoxide dimutase (SOD) and peroxidase (POX) increased and then maintained at a relatively stable level when subjected to UV-B irradiation. Catalase (CAT) activity under medium dosage of UV-B irradiation (Muv) and high dosage of UV-B irradiation (Huv) treatments were significantly decreased. Ascorbate peroxidase (APX) activity first remained unaltered and then increased in Huv treatment. In addition, the assay on isozymes was carried out using non-denaturing polyacrylamide gel electrophoresis (PAGE). The activities of some SOD isoforms were altered by UV-B. Two new bands (POX V and POX VII) appeared upon exposure to all three UV-B dosages. CAT III activity was increased by low dosage of UV-B irradiation (Luv), whereas CAT III and CAT IV disappeared when the alga was exposed to Muv and Huv. Two bands of APX (APX VI and APX VII) were increased and a new band (APX X) was observed under Huv exposure. H2O2 and thiobarbituric acid reacting substance (TBARS) increased under Muv and Huv treatments. Overall, UV-B protection mechanisms are partly inducible and to a certain extent sufficient to prevent the accumulation of damage in C. officinalis.

The genome of the thermoacidophilic red microalga Galdieria sulphuraria encodes a small family of secreted class III peroxidases that might be involved in cell wall modification

We report the identification of a small family of secreted class III plant peroxidases (Prx) from the genome of the unicellular thermoacidophilic red alga Galdieria sulphuraria (Cyanidiaceae). Apart from two class I ascorbate peroxidases and one cytochrome c peroxidase, the red algal genome encodes four class III plant peroxidases, thus complementing the short list of algal cell wall peroxidases (Passardi et al. in Genomics 89:567-579, 2007). We have characterized the family gene structure, analyzed the extracellular space and cell wall fraction of G. sulphuraria for the presence of peroxidase activity and used shotgun proteomics to identify candidate extracellular peroxidases. For a detailed enzymatic characterization, we have purified a secreted peroxidase (GsPrx04) from the cell-free medium using hydrophobic interaction chromatography. The enzyme proved heat and acid-stable and exhibited an apparent molecular mass of 40 kDa. Comparative genomics between endolithically growing G. sulphuraria and a close relative, the obligatory aquatic, cell wall-less Cyanidioschyzon merolae, revealed that class III peroxidases only occur in the terrestrial microalga, thus supporting the key function of these enzymes in the process of land colonization.

Two rice cytosolic ascorbate peroxidases differentially improve salt tolerance in transgenic Arabidopsis

In order to determine the different roles of rice (Oryza sativa L.) cytosolic ascorbate peroxidases (OsAPXa and OsAPXb, GenBank accession nos. D45423 and AB053297, respectively) under salt stress, transgenic Arabidopsis plants over-expressing OsAPXa or OsAPXb were generated, and they all exhibited increased tolerance to salt stress compared to wild-type plants. Moreover, transgenic lines over-expressing OsAPXb showed higher salt tolerance than OsAPXa transgenic lines as indicated by root length and total chlorophyll content. In addition to ascorbate peroxidase (APX) activity, antioxidant enzyme activities of catalase (CAT), superoxide dismutase (SOD) and glutathione reductase (GR), which are also involved in the salt tolerance process, and the content of H2O2 were also assayed in both transgenic and wild-type plants. The results showed that the overproduction of OsAPXb enhanced and maintained APX activity to a much higher degree than OsAPXa in transgenic Arabidopsis during treatment with different concentrations of NaCl, enhanced the active oxygen scavenging system, and protected plants from salt stress by equilibrating H2O2 metabolism. Our findings suggest that the rice cytosolic OsAPXb gene has a more functional role than OsAPXa in the improvement of salt tolerance in transgenic plants.

An inserted loop region of stromal ascorbate peroxidase is involved in its hydrogen peroxide-mediated inactivation

Ascorbate peroxidase isoforms localized in the stroma and thylakoid of higher plant chloroplasts are rapidly inactivated by hydrogen peroxide if the second substrate, ascorbate, is depleted. However, cytosolic and microbody-localized isoforms from higher plants as well as ascorbate peroxidase B, an ascorbate peroxidase of a red alga Galdieria partita, are relatively tolerant. We constructed various chimeric ascorbate peroxidases in which regions of ascorbate peroxidase B, from sites internal to the C-terminal end, were exchanged with corresponding regions of the stromal ascorbate peroxidase of spinach. Analysis of these showed that a region between residues 245 and 287 was involved in the inactivation by hydrogen peroxide. A 16-residue amino acid sequence (249-264) found in this region of the stromal ascorbate peroxidase was not found in other ascorbate peroxidase isoforms. A chimeric ascorbate peroxidase B with this sequence inserted was inactivated by hydrogen peroxide within a few minutes. The sequence forms a loop that binds noncovalently to heme in cytosolic ascorbate peroxidase of pea but does not bind to it in stromal ascorbate peroxidase of tobacco, and binds to cations in both ascorbate peroxidases. The higher susceptibility of the stromal ascorbate peroxidase may be due to a distorted interaction of the loop with the cation and/or the heme.

Photoinactivation of ascorbate peroxidase in isolated tobacco chloroplasts: Galdieria partita APX maintains the electron flux through the water-water cycle in transplastomic tobacco plants

We evaluated the H2O2-scavenging activity of the water-water cycle (WWC) in illuminated intact chloroplasts isolated from tobacco leaves. Illumination under conditions that limited photosynthesis [red light (>640 nm), 250 micromol photons m(-2) s(-1) in the absence of HCO3-] caused chloroplasts to take up O2 and accumulate H2O2. Concomitant with the O2 uptake, both ascorbate peroxidase (APX) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) lost their activities. However, superoxide dismutase (SOD), monodehydroascorbate radical reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) activities remained unaffected. The extent to which the photosynthetic linear electron flow decreased was small compared with the decline in APX activity. Therefore, the loss of APX activity lowered the electron flux through the WWC, as evidenced by a decrease in relative electron flux through PSII [Phi(PSII)xPFD]. To verify these interpretations, we created a transplastomic tobacco line in which an H2O2-insensitive APX from the red alga, Galdieria partita, was overproduced in the chloroplasts. In intact transplastomic chloroplasts which were illuminated under conditions that limited photosynthesis, neither O2 uptake nor H2O2 accumulation occurred. Furthermore, the electron flux through the WWC and the activity of GAPDH were maintained. The present work is the first report of APX inactivation by endogenous H2O2 in intact chloroplasts.

Analysis of the molecular evolutionary history of the ascorbate peroxidase gene family: inferences from the rice genome

Ascorbate peroxidase (APx) is a class I peroxidase that catalyzes the conversion of H(2)O(2) to H(2)O and O(2) using ascorbate as the specific electron donor. This enzyme has a key function in scavenging reactive oxygen species (ROS) and the protection against toxic effects of ROS in higher plants, algae, and Euglena. Here we report the identification of an APx multigene family in rice and propose a molecular evolutionary relationship between the diverse APx isoforms. In rice, the APx gene family has eight members, which encode two cytosolic, two putative peroxisomal, and four chloroplastic isoforms, respectively. Phylogenetic analyses were conducted using all APx protein sequences available in the NCBI databases. The results indicate that the different APx isoforms arose by a complex evolutionary process involving several gene duplications. The structural organization of APx genes also reflects this process and provides evidence for a close relationship among proteins located in the same subcellular compartment. A molecular evolutionary pathway, in which cytosolic and peroxisomal isoforms diverged early from chloroplastic ones, is proposed.

Defense potentials to NaCl in a mangrove, Bruguiera parviflora: differential changes of isoforms of some antioxidative enzymes

In order to assess the role of the antioxidative defense system against salt treatment, the activities of some antioxidative enzymes and levels of antioxidants were monitored in a true mangrove, Bruguiera parviflora, subjected to varying levels of NaCl under hydroponic culture. In the leaves of B. parviflora, salt treatment preferentially enhanced the content of H2O2 as well as the activity of ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), and superoxide dismutase (SOD), whereas it induced the decrease of total ascorbate and glutathione (GSH+GSSG) content as well as catalase (CAT) activity. Analysis of isoforms of antioxidative enzymes by native PAGE and activity staining revealed that leaves of B. parviflora had one isoform each of Mn-SOD and Cu/Zn-SOD and three isoforms of Fe-SOD. Expression of Mn-SOD and Fe-SOD-2 was preferentially elevated by NaCl. Similarly, out of the six isoforms of GPX, the GPX-1, 2, 3 and 6 were enhanced by salt treatment but the levels of GPX-4 and -5 changed minimally as compared to those of a control. Activity staining gel revealed only one prominent isoform of APX and two isoforms of GR (GR-1 and GR-2), all of these isoforms increased upon salt exposure. Four CAT-isoforms were identified, among which the prominent CAT-2 isoform level was maximally reduced, suggesting differential down regulation of CAT isoforms by NaCl. The concentrations of malondialdehyde (MDA), a product of lipid peroxidation, remained unchanged in leaves of the plant treated with different concentrations of NaCl. This suggests that plants are protected against activated oxygen species by the elevated levels of certain antioxidative enzymes, thus avoiding lipid peroxidation during salt exposure. The differential changes in the levels of the isoforms due to NaCl treatment may be useful as markers for recognizing salt tolerance in mangroves.

Trypanosoma cruzi expresses a plant-like ascorbate-dependent hemoperoxidase localized to the endoplasmic reticulum

In most aerobic organisms hemoperoxidases play a major role in H(2)O(2)-detoxification, but trypanosomatids have been reported to lack this activity. Here we describe the properties of an ascorbate-dependent hemoperoxidase (TcAPX) from the American trypanosome Trypanosoma cruzi. The activity of this plant-like enzyme can be linked to the reduction of the parasite-specific thiol trypanothione by ascorbate in a process that involves nonenzymatic interaction. The role of heme in peroxidase activity was demonstrated by spectral and inhibition studies. Ascorbate could saturate TcAPX activity indicating that the enzyme obeys Michaelis-Menten kinetics. Parasites that overexpressed TcAPX activity were found to have increased resistance to exogenous H(2)O(2). To determine subcellular location an epitope-tagged form of TcAPX was expressed in T. cruzi, which was observed to colocalize with endoplasmic reticulum resident chaperone protein BiP. These findings identify an arm of the oxidative defense system of this medically important parasite. The absence of this redox pathway in the human host may be therapeutically exploitable.