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.

Antitumor effect of selenium-rich Brazil nuts and selenomethionine dietary supplementation on pre-existing 4T1 mammary tumor growth in mice

Selenium (Se) is an essential micronutrient known to play an important role in the antioxidant system that can potentially influence tumor growth. We aimed to investigate the effects of dietary Se supplementation after detection of 4T1 mammary tumor growth in BALB/c mice. Thirty female mice received subcutaneous inoculation of 4T1 cells. After five days, all animals presenting palpable tumors were randomly assigned to three groups: a control group (Se-control) receiving a diet with adequate Se (0.15 mg/kg) and two other groups that received Se-supplemented diets (1.4 mg/kg of total Se) with either Brazilian nuts (Se-Nuts) or selenomethionine (SeMet). Data were assessed by either One or Two-way ANOVA followed by Tukey's HSD or Bonferroni's post hoc tests, respectively. Both Se-supplemented diets reduced tumor volume from the thirteenth day of feeding compared with the Se-adequate (control) diet (p < 0.05). The SeMet group presented a higher Se blood concentration (p < 0.05) than the Se-control group, with the Se-Nuts group presenting intermediate values. Selenoprotein P gene expression in the liver was higher in the Se-Nuts group than in the Se-control group (p < 0.05), while the SeMet group presented intermediate expression. Dietary Se supplementation, starting after detection of 4T1 palpable lesions, reduced tumor volume in mice.

Selenium in Brazil nuts: An overview of agronomical aspects, recent trends in analytical chemistry, and health outcomes

Se is an essential element in mammals. We review how its bioavailability in soil and the ability of plants to accumulate Se in foods depends on the soil Se profile (including levels and formats), besides to describe how the various selenoproteins have important biochemical functions in the body and directly impact human health. Owing to its favorable characteristics, the scientific community has investigated selenomethionine in most nut matrices. Among nuts, Brazil nuts have been highlighted as one of the richest sources of bioavailable Se. We summarize the most commonly used analytical methods for Se species and total Se determination in nuts. We also discuss the chemical forms of Se metabolized by mammals, human biochemistry and health outcomes from daily dietary intake of Se from Brazil nuts. These findings may facilitate the understanding of the importance of adequate dietary Se intake and enable researchers to define methods to determine Se species.

Uptake Dynamics of Ionic and Elemental Selenium Forms and Their Metabolism in Multiple-Harvested Alfalfa (Medicago sativa L.)

A pot experiment, under greenhouse conditions, was carried out aiming at investigating the agronomic biofortification of alfalfa (Medicago sativa L.) with Se and monitoring the Se uptake and accumulation dynamics within four consecutive harvests within the same growing season. Two ionic Se forms, i.e., sodium selenate (Se (VI)) and sodium selenite (Se (IV)), were applied once at a rate of 1, 10, and 50 mg kg−1 (added on Se basis), while 10 and 50 mg L−1 of a red elemental Se (red Se0) were used; all Se treatments were added as soil application. Application of Se (VI) at the rate of 50 mg kg−1 was toxic to alfalfa plants. The effect of Se forms on Se accumulation in alfalfa tissues, regardless of the applied Se concentration, follows: Se (VI) > Se (IV) > red Se0. The leaf, in general, possessed higher total Se content than the stem in all the treatments. The accumulation of Se in stem and leaf tissues showed a gradual decline between the harvests, especially for plants treated with either Se (VI) or Se (IV); however, the chemically synthesized red Se0 showed different results. The treatment of 10 mg kg−1 Se (VI) resulted in the highest total Se content in stem (202.5 and 98.0 µg g−1) and leaf (643.4 and 284.5 µg g−1) in the 1st and 2nd harvests, respectively. Similar tendency is reported for the Se (IV)-treated plants. Otherwise, the application of red Se0 resulted in a lower Se uptake; however, less fluctuation in total Se content between the four harvests was noticed compared to the ionic Se forms. The Se forms in stem and leaf of alfalfa extracted by water and subsequently by protease XIV enzyme were measured by strong anion exchange (SAX) HPLC-ICP-MS. The major Se forms in our samples were selenomethionine (SeMet) and Se (VI), while neither selenocysteine (SeCys) nor Se (IV) was detected. In water extract, however, Se (VI) was the major Se form, while SeMet was the predominant form in the enzyme extract. Yet, Se (VI) and SeMet contents declined within the harvests, except in stem of plants treated with 50 mg L−1 red Se0. The highest stem or leaf SeMet yield %, in all harvests, corresponded to the treatment of 50 mg L−1 red Se0. For instance, 63.6% (in stem) and 38.0% (in leaf) were calculated for SeMet yield % in the 4th harvest of plants treated with 50 mg L−1 red Se0. Our results provide information about uptake and accumulation dynamics of different ionic Se forms in case of multiple-harvested alfalfa, which, besides being a good model plant, is an important target plant species in green biorefining.

Exploring the new dimensions of selenium research to understand the underlying mechanism of its uptake, translocation, and accumulation

Selenium (Se) is a vital mineral for both plants and animals. It is widely distributed on the earth's crust and is taken up by the plants as selenite or selenate. Plants substantially vary in their physiological response to Se. The amount of Se in edible plants is genetically controlled. Its availability can be determined by measuring its phytoavailability in soil. The low concentration of Se in plants can help them in combating stress, whereas higher concentrations can be detrimental to plant health and in most cases it is toxic. Thus, solving the double-edged sword problem of nutritional Se deficiency and its elevated concentrations in environment requires a better understanding of Se uptake and metabolism in plants. The studies on Se uptake and metabolism can help in genetic biofortification of Se in plants and also assist in phytoremediation. Moreover, Se uptake and transport, especially biochemical pathways of assimilation and incorporation into proteins, offers striking mechanisms of toxicity and tolerance. These developments have led to a revival of Se research in higher plants with significant break throughs being made in the previous years. This review explores the new dimensions of Se research with major emphasis on key research events related to Se undertaken in last few years. Further, we also discussed future possibilities in Se research for crop improvement.

Selenium-Containing Proteins/Peptides from Plants: A Review on the Structures and Functions

Selenium is an essential microelement required for biological processes. Traditional selenium supplements (selenite and selenomethionine mainly) remain concerns due to toxicity and bioavailability. In recent decades, biofortification strategies have been applied to produce selenium-enriched edible plants to address the challenges of superior nutritional quality requirements. Plant-derived selenium-containing proteins/peptides offer potential health benefits beyond the basic nutritional requirements of Se. Highly nucleophilic seleno-amino acids, special peptide sequences, and favorable bioavailability contribute to the biological activities of selenium-containing proteins/peptides, such as antioxidant, antihypertensive, anti-inflammatory, and immunomodulatory effects. However, their applications on a commercial scale are insufficient owing to the complexity of purification and identification techniques and the sparse information on bioavailability and metabolism. In this review, selenium status, structural features, bioactivities, structure-activity relationships, and bioavailability, as well as the mechanisms underlying the bioactivities and metabolism of plant-derived selenium-containing proteins/peptides, are summarized and discussed for their nutraceutical use.

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.).

Selenium tolerance, accumulation, localization and speciation in a Cardamine hyperaccumulator and a non-hyperaccumulator

Cardamine violifolia (family Brassicaceae) is the first discovered selenium hyperaccumulator from the genus Cardamine with unique properties in terms of selenium accumulation, i.e., high abundance of selenolanthionine. In our study, a fully comprehensive experiment was conducted with the comparison of a non-hyperaccumulator Cardamine species, Cardamine pratensis, covering growth characteristics, chlorophyll fluorescence, spatial selenium/sulfur distribution patterns through elemental analyses (synchrotron-based X-Ray Fluorescence and ICP-OES) and speciation data through selenium K-edge micro X-ray absorption near-edge structure analysis (μXANES) and strong cation exchange (SCX)-ICP-MS. The results revealed remarkable differences in contrast to other selenium hyperaccumulators as neither Cardamine species showed evidence of growth stimulation by selenium. Also, selenite uptake was not inhibited by phosphate for either of the Cardamine species. Sulfate inhibited selenate uptake, but the two Cardamine species did not show any difference in this respect. However, μXRF derived speciation maps and selenium/sulfur uptake characteristics provided results that are similar to other formerly reported hyperaccumulator and non-hyperaccumulator Brassicaceae species. μXANES showed organic selenium, "C-Se-C", in seedlings of both species and also in mature C. violifolia plants. In contrast, selenate-supplied mature C. pratensis contained approximately half "C-Se-C" and half selenate. SCX-ICP-MS data showed evidence of the lack of selenocystine in any of the Cardamine plant extracts. Thus, C. violifolia shows clear selenium-related physiological and biochemical differences compared to C. pratensis and other selenium hyperaccumulators.

Selenium Accumulation, Speciation and Localization in Brazil Nuts (Bertholletia excelsa H.B.K.)

More than a billion people worldwide may be selenium (Se) deficient, and supplementation with Se-rich Brazil nuts may be a good strategy to prevent deficiency. Since different forms of Se have different nutritional value, and Se is toxic at elevated levels, careful seed characterization is important. Variation in Se concentration and correlations of this element with other nutrients were found in two batches of commercially available nuts. Selenium tissue localization and speciation were further determined. Mean Se levels were between 28 and 49 mg kg⁻¹, with up to 8-fold seed-to-seed variation (n = 13) within batches. Brazil nut Se was mainly in organic form. While present throughout the seed, Se was most concentrated in a ring 1 to 2 mm below the surface. While healthy, Brazil nuts should be consumed in moderation. Consumption of one seed (5 g) from a high-Se area meets its recommended daily allowance; the recommended serving size of 30 g may exceed the allowable daily intake (400 μg) or even its toxicity threshold (1200 μg). Based on these findings, the recommended serving size may be re-evaluated, consumers should be warned not to exceed the serving size and the seed may be sold as part of mixed nuts, to avoid excess Se intake.

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.

Brazil nuts: Nutritional composition, health benefits and safety aspects

Brazil nuts are among the richest selenium food sources, and studies have considered this Amazonian nut as an alternative for selenium supplementation. Besides selenium, Brazil nuts present relevant content of other micronutrients such as magnesium, copper, and zinc. The nutritional composition of nuts, also characterized by adequate fatty acid profile and high content of protein and bioactive compounds, has many health benefits. In the present review, we examine the nutritional composition of Brazil nuts, comparing it with other nuts, and describe the relevance of possible contaminants and metal toxicants observed in this nut for human health. Furthermore, we report different trials available in the literature, which demonstrate positive outcomes such as modulation of the lipid serum profile, enhancement of the antioxidant system and improvement of anti-inflammatory response. These effects have been assessed under different conditions, such as cognitive impairment, dyslipidemia, cancer, and renal failure.

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.

Effective selenium detoxification in the seed proteins of a hyperaccumulator plant: the analysis of selenium-containing proteins of monkeypot nut (Lecythis minor) seeds

A shotgun proteomic approach was applied to characterize the selenium (Se)-containing proteins of the selenium hyperaccumulator monkeypot nut (Lecythis minor) seeds. The exceptionally high Se content (>4,000 mg kg(-1)) of the sample enabled a straightforward procedure without the need for multiple preconcentration and fractionation steps. The proteins identified were sulfur-rich seed proteins, namely, 11S globulin (Q84ND2), 2S albumin (B6EU54), 2S sulfur-rich seed storage proteins (P04403 and P0C8Y8) and a 11S globulin-like protein (A0EM48). Database directed search for theoretically selenium-containing peptides was assisted by manual spectra evaluation to achieve around 25% coverage on sulfur analogues. Remarkable detoxification mechanisms on the proteome level were revealed in the form of multiple selenomethionine-methionine substitution and the lack of selenocysteine residues. The degree of selenomethionine substitution could be characterized by an exponential function that implies the inhibition of protein elongation by selenomethionine. Our results contribute to the deeper understanding of selenium detoxification procedures in hyperaccumulator plants.

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.

Validation of the 2,3-dihydroxy-propionyl group in selenium speciation by chemical synthesis and LC-MS analyses

Research on 2,3-dihydroxy-propionyl derivatives of selenocysteine has now been facilitated by providing a method for synthesis and clean-up.