Identification of selenosugars and other low-molecular weight selenium metabolites in high-selenium cereal crops

Effect of inorganic arsenic in paddy soil on the migration and transformation of selenium species in rice plants

Selenium (Se) in paddy rice is one of the significant sources of human Se nutrition. However, the effect of arsenic (As) pollution in soil on the translocation of Se species in rice plants is unclear. In this research, a pot experiment was designed to examine the effect of the addition of 50 mg As/kg soil as arsenite or arsenate on the migration of Se species from soil to indica Minghui 63 and Luyoumingzhan. The results showed that the antagonism between inorganic As and Se was closely related to the rice cultivar and Se oxidation state in soil. Relative to the standalone selenate treatment, arsenite significantly (p < 0.05) decreased the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, sheaths, leaves, brans and kernels of both cultivars by 21.4%-100.0%, 40.0%-100.0%, 41.0%-100%, 5.4%-96.3%, 11.3%-100.0% and 26.2%-39.7% respectively, except for selenocystine in the kernels of indica Minghui 63 and selenomethionine in the leaves of indica Minghui 63 and the stems of indica Luyoumingzhan. Arsenate also decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenate in the roots, stems, brans and kernels of both cultivars by 34.9%-100.0%, 30.2%-100.0%, 11.3%-100.0% and 5.6%-39.6% respectively, except for selenate in the stems of indica Minghui 63. However, relative to the standalone selenite treatment, arsenite and arsenate decreased (p < 0.05) the accumulation of selenocystine, selenomethionine and selenite only in the roots of indica Minghui 63 by 45.5%-100.0%. Our results suggested that arsenite and arsenate had better antagonism toward Se species in selenate-added soil than that in selenite-added soil; moreover, arsenite had a higher inhibiting effect on the accumulation of Se species than arsenate.

Selenosugar, selenopolysaccharide, and putative selenoflavonoid in plants

Selenium (Se) is a naturally occurring essential micronutrient that is required for human health. Selenium supports cellular antioxidant defense and possesses bioeffects such as anti-inflammation, anti-cancer, anti-diabetic, and cardiovascular and liver protective effects arising from Se-enhanced cellular antioxidant activity. Past studies on Se have focused on elucidating Se speciation in foods, biofortification strategies to produce Se-enriched foods to address Se deficiency in the population, and the biochemical activities of Se in health. The bioavailability and toxicity of Se are closely correlated to its chemical forms and may exhibit varying effects on body physiology. Selenium exists in inorganic and organic forms, in which inorganic Se such as sodium selenite and sodium selenate is more widely available. However, it is a challenge for safe and effective supplementation considering inorganic Se low bioavailability and high cytotoxicity. Organic Se, by contrast, exhibits higher bioavailability and lower toxicity and has a more diverse composition and structure. Organic Se exists as selenoamino acids and selenoproteins, but recent research has provided evidence that it also exists as selenosugars, selenopolysaccharides, and possibly as selenoflavonoids. Different food categories contain various Se compounds, and their Se profiles vary significantly. Therefore, it is necessary to delineate Se speciation in foods to understand their impact on health. This comprehensive review documents our knowledge of the recent uncovering of the existence of selenosugars and selenopolysaccharides and the putative evidence for selenoflavonoids. The bioavailability and bioactivities of these food-derived organic Se compounds are highlighted, in addition to their composition, structural features, and structure-activity relationships.

Selenate triggers diverse oxidative responses in Astragalus species with diverse selenium tolerance and hyperaccumulation capacity

Selenium (Se) hyperaccumulators are capable of uptake and tolerate high Se dosages. Excess Se-induced oxidative responses were compared in Astragalus bisulcatus and Astragalus cicer. Plants were grown on media supplemented with 0, 25 or 75 μM selenate for 14 days. Both A. bisulcatus and A. cicer accumulated >2000 μg/g dry weight Se to the shoot but the translocation factors of A. cicer were below 1 suggesting its non hyperaccumulator nature. A. cicer showed Se sensitivity indicated by reduced seedling fresh weight, root growth and root apical meristem viability, altered element homeostasis in the presence of Se. In Se-exposed A. bisulcatus, less toxic organic Se forms (mainly MetSeCys, γ-Glu-MetSeCys, and a selenosugar) dominated, while these were absent from A. cicer suggesting that the majority of the accumulated Se may be present as inorganic forms. The glutathione-dependent processes were more affected, while ascorbate levels were not notably influenced by Se in either species. Exogenous Se triggered more intense accumulation of malondialdehyde in the sensitive A. cicer compared with the tolerant A. bisulcatus. The extent of protein carbonylation in the roots of the 75 μM Se-exposed A. cicer exceeded that of A. bisulcatus indicating a correlation between selenate sensitivity and the degree of protein carbonylation. Overall, our results reveal connection between oxidative processes and Se sensitivity/tolerance/hyperaccumulation and contribute to the understanding of the molecular responses to excess Se.

Isotopologue pattern based data mining for selenium species from HILIC-ESI-Orbitrap-MS-derived spectra

Automated and specific picking of selenium-containing molecular entities has not been an obvious option for software tools associated with electrospray high-resolution mass spectrometry (MS). In our study, a comprehensive pattern matching approach based on intra-isotopologue distance and isotopologue ratio data was critically evaluated in terms of reproducibility and selenium isotope selection on three samples, including selenized Torula yeast and the selenium hyperaccumulator plant Cardamine violifolia. Hydrophilic interaction liquid chromatography was applied to provide a one-step separation for water soluble metabolites to put an end to the need for either orthogonal setups or poor retention on reversed phase chromatography. Assistance from inductively coupled plasma-MS was taken only for chromatographic verification purposes, and the involvement of absolute mass defect (MD) data in selenometabolite-specific screening was assessed by multivariate statistical tools. High focus was placed on screening efficiency and on the validation of discovered selenized molecules to avoid reporting of artefacts. From the >1000 molecular entries detected, selenium-containing molecules were picked up with a recovery rate of >88% and a false positive rate of <10%. Isotop(ologu)e pairs of 78Se-80Se and 80Se-82Se proved to be the most performant in the detection. On the basis of accurate mass information and hypothetical deamination processes, elemental composition could be proposed for 72 species out of the 75 selenium species encountered without taking into account selenocompound databases. Absolute MD data were used to significantly differentiate a potentially sample-specific subgroup of false positive molecular entities from non-selenized and selenized entities.

Direct and indirect selenium speciation in biofortified wheat: A tale of two techniques

Wheat can be biofortified with different inorganic selenium (Se) forms, selenite or selenate. The choice of Se source influences the physiological response of the plant and the Se metabolites produced. We looked at selenium uptake, distribution and metabolization in wheat exposed to selenite, selenate and a 1:1 molar mixture of both to determine the impact of each treatment on the Se speciation in roots, shoots, and grains. To achieve a comprehensive quantification of the Se species, the complementarity of high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy was exploited. This approach allowed the identification of the six main selenium species: selenomethionine, selenocysteine, selenocystine, selenite, selenate, and elemental selenium. The three treatments resulted in similar total selenium concentration in grains, 90-150 mg Se kg^-1 , but produced different effects in the plant. Selenite enhanced root accumulation (66% of selenium) and induced the maximum toxicity, whereas selenate favored shoot translocation (46%). With the 1:1 mixture, selenium was distributed along the plant generating lower toxicity. Although all conditions resulted in >92% of organic selenium in the grain, selenate produced mainly C-Se-C forms, such as selenomethionine, while selenite (alone or in the mixture) enhanced the production of C-Se-Se-C forms, such as selenocystine, modifying the selenoamino acid composition. These results provide a better understanding of the metabolization of selenium species which is key to minimize plant toxicity and any concomitant effect that may arise due to Se-biofortification.

Scientific opinion on the tolerable upper intake level for selenium

Following a request from the European Commission, the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the tolerable upper intake level (UL) for selenium. Systematic reviews of the literature were conducted to identify evidence regarding excess selenium intake and clinical effects and potential biomarkers of effect, risk of chronic diseases and impaired neuropsychological development in humans. Alopecia, as an early observable feature and a well-established adverse effect of excess selenium exposure, is selected as the critical endpoint on which to base a UL for selenium. A lowest-observed-adverse-effect-level (LOAEL) of 330 μg/day is identified from a large randomised controlled trial in humans (the Selenium and Vitamin E Cancer Prevention Trial (SELECT)), to which an uncertainty factor of 1.3 is applied. A UL of 255 μg/day is established for adult men and women (including pregnant and lactating women). ULs for children are derived from the UL for adults using allometric scaling (body weight0.75). Based on available intake data, adult consumers are unlikely to exceed the UL, except for regular users of food supplements containing high daily doses of selenium or regular consumers of Brazil nuts. No risk has been reported with the current levels of selenium intake in European countries from food (excluding food supplements) in toddlers and children, and selenium intake arising from the natural content of foods does not raise reasons for concern. Selenium-containing supplements in toddlers and children should be used with caution, based on individual needs.

Understanding soil selenium accumulation and bioavailability through size resolved and elemental characterization of soil extracts

Dietary deficiency of selenium is a global health threat related to low selenium concentrations in crops. Despite the chemical similarity of selenium to the two more abundantly studied elements sulfur and arsenic, the understanding of its accumulation in soils and availability for plants is limited. The lack of understanding of soil selenium cycling is largely due to the unavailability of methods to characterize selenium species in soils, especially the organic ones. Here we develop a size-resolved multi-elemental method using liquid chromatography and elemental mass spectrometry, which enables an advanced characterization of selenium, sulfur, and arsenic species in soil extracts. We apply the analytical approach to soils sampled along the Kohala rainfall gradient on Big Island (Hawaii), which cover a large range of organic carbon and (oxy)hydroxides contents. Similarly to sulfur but contrarily to arsenic, a large fraction of selenium is found associated with organic matter in these soils. However, while sulfur and arsenic are predominantly found as oxyanions in water extracts, selenium mainly exists as small hydrophilic organic compounds. Combining Kohala soil speciation data with concentrations in parent rock and plants further suggests that selenium association with organic matter limits its mobility in soils and availability for plants.

Selenium-Enriched Cardamine violifolia Increases Selenium and Decreases Cholesterol Concentrations in Liver and Pectoral Muscle of Broilers

BACKGROUND

Supernutrition of selenium (Se) in an effort to produce Se-enriched meat may inadvertently cause lipid accumulation. Se-enriched Cardamine violifolia (SeCv) contains >80% of Se in organic forms.

OBJECTIVES

This study was to determine whether feeding chickens a high dose of SeCv could produce Se-biofortified muscle without altering their lipid metabolism.

METHODS

Day-old male broilers were allocated to 4 groups (6 cages/group and 6 chicks/cage) and were fed either a corn-soy base diet (BD, 0.13-0.15 mg Se/kg), the BD plus 0.5 mg Se/kg as sodium selenite (SeNa) or as SeCv, or the BD plus a low-Se Cardamine violifolia (Cv, 0.20-0.21mg Se/kg). At week 6, concentrations of Se and lipid and expression of selenoprotein and lipid metabolism-related genes were determined in the pectoral muscle and liver.

RESULTS

The 4 diets showed no effects on growth performance of broilers. Compared with the other 3 diets, SeCv elevated (P < 0.05) Se concentrations in the pectoral muscle and liver by 14.4-127% and decreased (P < 0.05) total cholesterol concentrations by 12.5-46.7% and/or triglyceride concentrations by 28.8-31.1% in the pectoral muscle and/or liver, respectively. Meanwhile, SeCv enhanced (P < 0.05) muscular α-linolenic acid (80.0%) and hepatic arachidonic acid (58.3%) concentrations compared with SeNa and BD, respectively. SeCv downregulated (P < 0.05) the cholesterol and triglyceride synthesis-related proteins (sterol regulatory element binding transcription factor 2 and diacylglycerol O-acyltransferase 2) and upregulated (P < 0.05) hydrolysis and β-oxidation of fatty acid-related proteins (lipoprotein lipase, fatty acid binding protein 1, and carnitine palmitoyltransferase 1A), as well as selenoprotein P1 and thioredoxin reductase activity in the pectoral muscle and/or liver compared with SeNa.

CONCLUSIONS

Compared with SeNa, SeCv effectively raised Se and reduced lipids in the liver and muscle of broilers. The effect was mediated through the regulation of the cholesterol and triglyceride biosynthesis and utilization-related genes.

Selenium in cereals: Insight into species of the element from total amount

Selenium (Se) is a trace mineral micronutrient essential for human health. The diet is the main source of Se intake. Se-deficiency is associated with many diseases, and up to 1 billion people suffer from Se-deficiency worldwide. Cereals are considered a good choice for Se intake due to their daily consumption as staple foods. Much attention has been paid to the contents of Se in cereals and other foods. Se-enriched cereals are produced by biofortification. Notably, the gap between the nutritional and toxic levels of Se is fairly narrow. The chemical structures of Se compounds, rather than their total contents, contribute to the bioavailability, bioactivity, and toxicity of Se. Organic Se species show better bioavailability, higher nutritional value, and less toxicity than inorganic species. In this paper, we reviewed the total content of Se in cereals, Se speciation methods, and the biological effects of Se species on human health. Selenomethionine (SeMet) is generally the most prevalent and important Se species in cereal grains. In conclusion, Se species should be considered in addition to the total Se content when evaluating the nutritional and toxic values of foods such as cereals.

Water soluble selenometabolome of Cardamine violifolia

Low molecular weight selenium containing metabolites in the leaves of the selenium hyperaccumulator Cardamine violifolia (261 mg total Se per kg d.w.) were targeted in this study. One dimensional cation exchange chromatography coupled to ICP-MS was used for purification and fractionation purposes prior to LC-Unispray-QTOF-MS analysis. The search for selenium species in full scan spectra was assisted with an automated mass defect based filtering approach. Besides selenocystathionine, selenohomocystine and its polyselenide derivative, a total number of 35 water soluble selenium metabolites other than selenolanthionine were encountered, including 30 previously unreported compounds. High occurrence of selenium containing hexoses was observed, together with the first assignment of N-glycoside derivatives of selenolanthionine. Quantification of the most abundant selenium species, selenolanthionine, was carried out with an ion pairing LC - post column isotope dilution ICP-MS setup, which revealed that this selenoamino acid accounted for 30% of the total selenium content of the leaf (78 mg (as Se) per kg d.w.).

Potential of Fourier Transform Mass Spectrometry (Orbitrap and Ion Cyclotron Resonance) for Speciation of the Selenium Metabolome in Selenium-Rich Yeast

The evolution of the field of element speciation, from the targeted analysis for specific element species toward a global exploratory analysis for the entirety of metal- or metalloid-related compounds present in a biological system (metallomics), requires instrumental techniques with increasing selectivity and sensitivity. The selectivity of hyphenated techniques, combining chromatography, and capillary electrophoresis with element-specific detection (usually inductively coupled plasma mass spectrometry, ICP MS), is often insufficient to discriminate all the species of a given element in a sample. The necessary degree of specificity can be attained by ultrahigh-resolution (R >100,000 in the m/z < 1,000 range for a 1 s scan) mass spectrometry based on the Fourier transformation of an image current of the ions moving in an Orbitrap or an ion cyclotron resonance (ICR) cell. The latest developments, allowing the separate detection of two ions differing by a mass of one electron (0.5 mDa) and the measurement of their masses with a sub-ppm accuracy, make it possible to produce comprehensive lists of the element species present in a biological sample. Moreover, the increasing capacities of multistage fragmentation often allow their de novo identification. This perspective paper critically discusses the potential state-of-the-art of implementation, and challenges in front of FT (Orbitrap and ICR) MS for a large-scale speciation analysis using, as example, the case of the metabolism of selenium by yeast.

Selenium in plant foods: speciation analysis, bioavailability, and factors affecting composition

Interest in selenium has been increasing over the past few decades with growing knowledge of its importance to overall health. The ability of several plants to accumulate and transform inorganic selenium forms into its bioactive organic compounds has important implications for human nutrition and health. In this review, we present the studies carried out during the last decade to characterize selenium species produced by different plant foods. Attention is also paid to the effect of selenium treatment on chemical composition and antioxidant properties of plants.

Selenium and Other Elements in Wheat (Triticum aestivum) and Wheat Bread from a Seleniferous Area

The objective of the present study was to assess the levels of Se, as well as other essential and toxic trace elements in wheat grains and traditional Roti-bread from whole-grain flour in a seleniferous area of Punjab (India) using inductively-coupled plasma mass-spectrometry. Wheat grain and bread selenium levels originating from seleniferous areas exceeded the control values by a factor of more than 488 and 179, respectively. Se-rich wheat was also characterized by significantly increased Cu and Mn levels. Se-rich bread also contained significantly higher levels of Cr, Cu, I, Mn, and V. The level of Li and Sr was reduced in both Se-enriched wheat and bread samples. Roti bread from Se-enriched wheat was also characterized by elevated Al, Cd, and Ni, as well as reduced As and Hg content as compared to the respective control values. Se intake with Se-rich bread was estimated as more than 13,600% of RDA. Daily intake of Mn with both Se-unfortified and Se-fortified bread was 133% and 190% of RDA. Therefore, Se-rich bread from wheat cultivated on a seleniferous area of Punjab (India) may be considered as a potent source of selenium, although Se status should be monitored throughout dietary intervention.

Bioavailability Assessment of Copper, Iron, Manganese, Molybdenum, Selenium, and Zinc from Selenium-Enriched Lettuce

Cu, Fe, Mn, Mo, Selenium (Se), and Zn bioavailability from selenate- and selenite-enriched lettuce plants was studied by in vitro gastrointestinal digestion followed by an assay with Caco-2 cells. The plants were cultivated in the absence and presence of two concentrations (25 and 40 µmol/L of Se). After 28 days of cultivation, the plants were harvested, dried, and evaluated regarding the total concentration, bioaccessibility, and bioavailability of the analytes. The results showed that biofortification with selenate leads to higher Se absorption by the plant than biofortification with selenite. For the other nutrients, Mo showed high accumulation in the plants of selenate assays, and the presence of any Se species led to a reduction of the plant uptake of Cu and Fe. The accumulation of Zn and Mn was not strongly influenced by the presence of any Se species. The bioaccessibility values were approximately 71%, 10%, 52%, 84%, 71%, and 86% for Cu, Fe, Mn, Mo, Se, and Zn, respectively, and the contribution of the biofortified lettuce to the ingestion of these minerals is very small (except for Se and Mo). Due to the low concentrations of elements from digested plants, it was not possible to estimate the bioavailability for some elements, and for Mo and Zn, the values are below 6.9% and 3.4% of the total concentration, respectively. For Se, the bioavailability was greater for selenite-enriched than selenate-enriched plants (22% and 6.0%, respectively), because selenite is biotransformed by the plant to organic forms that are better assimilated by the cells.

Sourdough Fermentation Favorably Influences Selenium Biotransformation and the Biological Effects of Flatbread

Although selenium is of great importance for the human body, in several world regions the intake of this essential trace element does not meet the dietary reference values. To achieve optimal intake, fortification of bread by using selenium-enriched flour has been put forward. Less is known on the potential effect of sourdough fermentation, which might be worth exploring as the biological effects of selenium strongly depend on its chemical form and sourdough fermentation is known to cause transformations of nutrients and phytochemicals, including the conversion of inorganic selenium into organic selenocompounds. Here we investigated the bio transformation of selenium by sourdough fermentation in a typical Italian flatbread (piadina) made with standard (control) or selenium-enriched flour. The different piadina were submitted to in vitro digestion, and the biological activity of the resulting hydrolysates was tested by means of cultured human liver cells exposed to an exogenous oxidative stress. The use of selenium-enriched flour and sourdough fermentation increased the total content of bioaccessible selenium in organic form, compared to conventional fermentation, and led to protective effects counteracting oxidative damage in cultured cells. The present study suggests that selenium-rich, sourdough-fermented bakery products show promise for improving human selenium nutrition whenever necessary.

Speciation of Selenium in Brown Rice Fertilized with Selenite and Effects of Selenium Fertilization on Rice Proteins

Foliar Selenium (Se) fertilizer has been widely used to accumulate Se in rice to a level that meets the adequate intake level. The Se content in brown rice (Oryza sativa L.) was increased in a dose-dependent manner by the foliar application of sodium selenite as a fertilizer at concentrations of 25, 50, 75, and 100 g Se/ha. Selenite was mainly transformed to organic Se, that is, selenomethionine in rice. Beyond the metabolic capacity of Se in rice, inorganic Se also appeared. In addition, four extractable protein fractions in brown rice were analyzed for Se concentration. The Se concentrations in the glutelin and albumin fractions saturated with increasing Se concentration in the fertilizer compared with those in the globulin and prolamin fractions. The structural analyses by fluorescence spectroscopy, Fourier transform infrared spectrometry, and differential scanning calorimetry suggest that the secondary structure and thermostability of glutelin were altered by the Se treatments. These alterations could be due to the replacements of cysteine and methionine to selenocysteine and selenomethionine, respectively. These findings indicate that foliar fertilization of Se was effective in not only transforming inorganic Se to low-molecular-weight selenometabolites such as selenoamino acids, but also incorporating Se into general rice proteins, such as albumin, globulin glutelin, and prolamin, as selenocysteine and selenomethionine in place of cysteine and methionine, respectively.

Comparative transcriptomics provides novel insights into the mechanisms of selenium tolerance in the hyperaccumulator plant Cardamine hupingshanensis

Selenium (Se) is an essential mineral element for animals and humans. Cardamine hupingshanensis (Brassicaceae), found in the Wuling mountain area of China, has been identified as a novel Se hyperaccumulator plant. However, the mechanism for selenium tolerance in Cardamine plants remains unknown. In this study, two cDNA libraries were constructed from seedlings of C. hupingshanensis treated with selenite. Approximately 100 million clean sequencing reads were de novo assembled into 48,989 unigenes, of which 39,579 and 33,510 were expressed in the roots and leaves, respectively. Biological pathways and candidate genes involved in selenium tolerance mechanisms were identified. Differential expression analysis identified 25 genes located in four pathways that were significantly responsive to selenite in C. hupingshanensis seedlings. The results of RNA sequencing (RNA-Seq) and quantitative real-time PCR (RT-qPCR) confirmed that storage function, oxidation, transamination and selenation play very important roles in the selenium tolerance in C. hupingshanensis. Furthermore, a different degradation pathway synthesizing malformed or deformed selenoproteins increased selenium tolerance at different selenite concentrations. This study provides novel insights into the mechanisms of selenium tolerance in a hyperaccumulator plant, and should serve as a rich gene resource for C. hupingshanensis.

Selenolanthionine is the major water-soluble selenium compound in the selenium tolerant plant Cardamine violifolia

BACKGROUND

Selenium hyperaccumulation in plants often involves the synthesis of non-proteinaceous methylated selenoamino acids serving for the elimination of excess selenium from plant metabolism to protect plant homeostasis.

METHODS

Our study aimed at the identification of the main selenium species of the selenium hyperaccumulator plant Cardamine violifolia (Brassicaceae) that grows in the wild in the seleniferous region of Enshi, China. A sample of this plant (3.7 g Se kg^-1 d.w.) was prepared with several extraction methods and the extracted selenium species were identified and quantified with liquid chromatography mass spectrometry set-ups.

RESULTS

The Cardamine violifolia sample did not contain in considerable amount any of the organic selenium species that are often formed in hyperaccumulator plants; the inorganic selenium content (mostly as elemental selenium) accounted only for <20% of total Se. The most abundant selenium compound, accounting for about 40% of total Se was proved to be selenolanthionine, a selenium species that has never been unambiguously identified before from any selenium containing sample. The identification process was completed with chemical synthesis too. The molar ratio of lanthionine:selenolanthionine in the water extract was ca. 1:8.

CONCLUSIONS

Finding selenolanthionine as the main organic selenium species in a plant possibly unearths a new way of selenium tolerance. This article is part of a Special Issue entitled Selenium research in biochemistry and biophysics - 200 year anniversary issue, edited by Dr. Elias Arnér and Dr. Regina Brigelius-Flohe.

Selenium Bioaccessibility and Speciation in Selenium-Enriched Lettuce: Investigation of the Selenocompounds Liberated after in Vitro Simulated Human Digestion Using Two-Dimensional HPLC-ICP-MS

The evaluation of selenium-enriched vegetables as potential dietary sources of selenium, an essential element for humans, requires an assessment of the plant's accumulation ability as well as of the bioaccessibility and speciation of the accumulated selenium, which influence its biological effects in humans. Lettuce hydroponically grown at three selenite (SeVI)/selenate (SeIV) amendment levels was characterized accordingly. Selenium accumulation in lettuce leaves was greatest with Se(VI) amendment, whereas bioaccessibility was 70% on average in both cases. Selenium speciation in gastrointestinal hydrolysates, characterized by anion and cation exchange HPLC-ICP-MS, showed that Se(IV) was largely biotransformed into organoselenium metabolites, with selenomethionine accounting for 1/3 of the total detected species, whereas Se(VI) was incorporated as such in the edible portion of the plant, with only a small fraction (∼20%) converted into organic species. Taking into account both nutritional quality and safety, the Se(IV)-enriched lettuce appeared more favorable as a potential selenium source for human consumption.

Mechanisms of Selenium Enrichment and Measurement in Brassicaceous Vegetables, and Their Application to Human Health

Selenium (Se) is an essential micronutrient for human health. Se deficiency affects hundreds of millions of people worldwide, particularly in developing countries, and there is increasing awareness that suboptimal supply of Se can also negatively affect human health. Selenium enters the diet primarily through the ingestion of plant and animal products. Although, plants are not dependent on Se they take it up from the soil through the sulphur (S) uptake and assimilation pathways. Therefore, geographic differences in the availability of soil Se and agricultural practices have a profound influence on the Se content of many foods, and there are increasing efforts to biofortify crop plants with Se. Plants from the Brassicales are of particular interest as they accumulate and synthesize Se into forms with additional health benefits, such as methylselenocysteine (MeSeCys). The Brassicaceae are also well-known to produce the glucosinolates; S-containing compounds with demonstrated human health value. Furthermore, the recent discovery of the selenoglucosinolates in the Brassicaceae raises questions regarding their potential bioefficacy. In this review we focus on Se uptake and metabolism in the Brassicaceae in the context of human health, particularly cancer prevention and immunity. We investigate the close relationship between Se and S metabolism in this plant family, with particular emphasis on the selenoglucosinolates, and consider the methodologies available for identifying and quantifying further novel Se-containing compounds in plants. Finally, we summarize the research of multiple groups investigating biofortification of the Brassicaceae and discuss which approaches might be most successful for supplying Se deficient populations in the future.

Inventory of metal complexes circulating in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection

Description of metal species in plant fluids such as xylem, phloem or related saps remains a complex challenge usually addressed either by liquid chromatography-mass spectrometry, X-ray analysis or computational prediction. To date, none of these techniques has achieved a complete and true picture of metal-containing species in plant fluids, especially for the least concentrated complexes. Here, we present a generic analytical methodology for a large-scale (> 10 metals, > 50 metal complexes) detection, identification and semiquantitative determination of metal complexes in the xylem and embryo sac liquid of the green pea, Pisum sativum. The procedure is based on direct injection using hydrophilic interaction chromatography with dual detection by elemental (inductively coupled plasma mass spectrometry) and molecular (high-resolution electrospray mass spectrometry) mass spectrometric detection. Numerous and novel complexes of iron(II), iron(III), copper(II), zinc, manganese, cobalt(II), cobalt(III), magnesium, calcium, nickel and molybdenum(IV) with several ligands including nicotianamine, citrate, malate, histidine, glutamine, aspartic acid, asparagine, phenylalanine and others are observed in pea fluids and discussed. This methodology provides a large inventory of various types of metal complexes, which is a significant asset for future biochemical and genetic studies into metal transport/homeostasis.

Selenium accumulation by plants

BACKGROUND

Selenium (Se) is an essential mineral element for animals and humans, which they acquire largely from plants. The Se concentration in edible plants is determined by the Se phytoavailability in soils. Selenium is not an essential element for plants, but excessive Se can be toxic. Thus, soil Se phytoavailability determines the ecology of plants. Most plants cannot grow on seleniferous soils. Most plants that grow on seleniferous soils accumulate <100 mg Se kg(-1) dry matter and cannot tolerate greater tissue Se concentrations. However, some plant species have evolved tolerance to Se, and commonly accumulate tissue Se concentrations >100 mg Se kg(-1) dry matter. These plants are considered to be Se accumulators. Some species can even accumulate Se concentrations of 1000-15 000 mg Se kg(-1 )dry matter and are called Se hyperaccumulators.

SCOPE

This article provides an overview of Se uptake, translocation and metabolism in plants and highlights the possible genetic basis of differences in these between and within plant species. The review focuses initially on adaptations allowing plants to tolerate large Se concentrations in their tissues and the evolutionary origin of species that hyperaccumulate Se. It then describes the variation in tissue Se concentrations between and within angiosperm species and identifies genes encoding enzymes limiting the rates of incorporation of Se into organic compounds and chromosomal loci that might enable the development of crops with greater Se concentrations in their edible portions. Finally, it discusses transgenic approaches enabling plants to tolerate greater Se concentrations in the rhizosphere and in their tissues.

CONCLUSIONS

The trait of Se hyperaccumulation has evolved several times in separate angiosperm clades. The ability to tolerate large tissue Se concentrations is primarily related to the ability to divert Se away from the accumulation of selenocysteine and selenomethionine, which might be incorporated into non-functional proteins, through the synthesis of less toxic Se metabilites. There is potential to breed or select crops with greater Se concentrations in their edible tissues, which might be used to increase dietary Se intakes of animals and humans.

Speciation and identification of tellurium-containing metabolites in garlic, Allium sativum

Tellurium (Te) is a widely used metalloid in industry because of its unique chemical and physical properties. However, information about the biological and toxicological activities of Te in plants and animals is limited. Although Te is expected to be metabolized in organisms via the same pathway as sulfur and selenium (Se), no precise metabolic pathways are known in organisms, particularly in plants. To reveal the metabolic pathway of Te in plants, garlic, a well-known Se accumulator, was chosen as the model plant. Garlic was hydroponically cultivated and exposed to sodium tellurate, and Te-containing metabolites in the water extract of garlic leaves were identified using HPLC coupled with inductively coupled plasma mass spectrometry (ICP-MS) or electrospray tandem mass spectrometry (ESI-MS-MS). At least three Te-containing metabolites were detected using HPLC-ICP-MS, and two of them were subjected to HPLC-ESI-MS-MS for identification. The MS spectra obtained by ESI-MS-MS indicated that the metabolite was Te-methyltellurocysteine oxide (MeTeCysO). Then, MeTeCysO was chemically synthesized and its chromatographic behavior matched with that of the Te-containing metabolite in garlic. The other was assigned as cysteine S-methyltellurosulfide. These results suggest that garlic can assimilate tellurate, an inorganic Te compound, and tellurate is transformed into a Te-containing amino acid, the so-called telluroamino acid. This is the first report addressing that telluroamino acid is de novo synthesized in a higher plant.

Fish toxicity testing with selenomethionine spiked feed--what's the real question being asked?

The US Environmental Protection Agency and several U.S. states and Canadian provinces are currently developing national water quality criteria for selenium that are based in part on toxicity tests performed by feeding freshwater fish a selenomethionine-spiked diet. Using only selenomethionine to examine the toxicity of selenium is based in part on the limitations of the analytical chemistry methods commonly used in the 1990s and 2000s to speciate selenium in freshwater biota. While these methods provided a good starting point, recent improvements in analytical chemistry methodology have demonstrated that selenium speciation in biota is far more complex than originally thought. Here, we review the recent literature that suggests that there are numerous additional selenium species present in freshwater food chains and that the toxicities of these other selenium species, both individually and in combination, have not been evaluated in freshwater fishes. Evidence from studies on birds and mammals suggests that the other selenium forms differ in their metabolic pathways and toxicity from selenomethionine. Therefore, we conclude that toxicity testing using selenomethionine-spiked feed is only partly addressing the question "what is the toxicity of selenium to freshwater fishes?" and that using the results of these experiments to derive freshwater quality criteria may lead to biased water quality criteria. We also discuss additional studies that are needed in order to derive a more ecologically relevant freshwater quality criterion for selenium.