Selenium compounds in the fathead minnow (Pimephales promelas)--II. Quantitative approach to gastrointestinal absorption, routes of elimination and influence of dietary pretreatment

Following administration by gavage [75Se]selenate and [75Se]selenite were absorbed from the gastrointestinal tract of fathead minnows (Pimephales promelas) at 94 and 80% efficiency, respectively. Approximately 12% of the [75Se]selenate administered by i.p. injection was eliminated via the urine, and across the gill within 2 hr. The urine was the primary route of elimination followed by the gill. The bile contained significantly lower amounts of 75Se than that eliminated either across the gill or in the urine. The mucus is capable of binding significant amounts of 75Se. Dietary pretreatment with selenite reduced the retention of a subsequent [75Se]selenite dose administered by gavage.     

An essential role of s-adenosyl-L-methionine:L-methionine s-methyltransferase in selenium volatilization by plants. Methylation of selenomethionine to selenium-methyl-L-selenium- methionine,the precursor of volatile selenium

Selenium (Se) phytovolatilization, the process by which plants metabolize various inorganic or organic species of Se (e.g. selenate, selenite, and Se-methionine [Met]) into gaseous Se forms (e.g. dimethylselenide), is a potentially important means of removing Se from contaminated environments. Before attempting to genetically enhance the efficiency of Se phytovolatilization, it is essential to elucidate the enzymatic pathway involved and to identify its rate-limiting steps. The present research tested the hypothesis that S-adenosyl-L-Met:L-Met S-methyltransferase (MMT) is the enzyme responsible for the methylation of Se-Met to Se-methyl Se-Met (SeMM). To this end, we identified and characterized an Arabidopsis T-DNA mutant knockout for MMT. The lack of MMT in the Arabidopsis T-DNA mutant plant resulted in an almost complete loss in its capacity for Se volatilization. Using chemical complementation with SeMM, the presumed enzymatic product of MMT, we restored the capacity of the MMT mutant to produce volatile Se. Overexpressing MMT from Arabidopsis in Escherichia coli, which is not known to have MMT activity, produced up to 10 times more volatile Se than the untransformed strain when both were supplied with Se-Met. Thus, our results provide in vivo evidence that MMT is the key enzyme catalyzing the methylation of Se-Met to SeMM.     

Effect of byproduct,nitrogen fertilizer,and zeolite on phosphate rock dissolution and extractable phosphorus in acid soil

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO₄ (0.5 mM) and PO₄ (2 μM) similar to those found in soil solution. Concentrations up to 5 mM SO₄ and 200 μM PO₄ allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more phytotoxic than selenate, especially for shoot growth. Selenate was less toxic, and tended to preferentially inhibit root growth. Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%). A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species. In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots. Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se. This revised version was published online in June 2006 with corrections to the Cover Date.     

Selenium compounds in the fathead minnow (Pimephales promelas)--I. Uptake, distribution, and elimination of orally administered selenate, selenite and l-selenomethionine

Treatment of fathead minnows (Pimephales promelas) with either [75Se]selenate, -selenite or -l-selenomethionine by gavage at 20 ng Se/g resulted in organ uptake and early distribution patterns which differed significantly between compounds. The greatest differences in uptake between compounds was observed in liver tissue which accumulated much less [75Se]selenate than either selenite or l-selenomethionine. The 75Se burdens and relative distribution among the various organs were nearly identical during the elimination phase for [75Se]selenate and -selenite. This suggests that selenium derived from these compounds converge to a common metabolic pool. The whole body T1/2, rate of 75Se uptake and magnitude of 75Se accumulation were generally greater for [75Se]selenomethionine than the inorganic forms. Selenium-75 was present in the bile following the oral administration of each compound. The partitioning of selenate and selenite into the plasma and cellular fraction of blood differs with both the compound and time following exposure.     

Variation in selenium tolerance and accumulation among 19 Arabidopsis thaliana accessions

Selenium (Se) is an essential element for many organisms but also toxic at higher levels. The objective of this study was to identify accessions from the model species Arabidopsis thaliana that differ in Se tolerance and accumulation. Nineteen Arabidopsis accessions were grown from seed on agar medium with or without selenate (50 microM) or selenite (20 microM), followed by analysis of Se tolerance and accumulation. Tissue sulfur levels were also compared. The Se Tolerance Index (root length+Se/root length control) varied among the accessions from 0.11 to 0.44 for selenite and from 0.05 to 0.24 for selenate. When treated with selenite, the accessions differed by two-fold in shoot Se concentration (up to 250 mgkg(-1)) and three-fold in root Se concentration (up to 1000 mgkg(-1)). Selenium accumulation from selenate varied 1.7-fold in shoot (up to 1000 mgkg(-1)) and two-fold in root (up to 650 mgkg(-1)). Across all accessions, a strong correlation was observed between Se and S concentration in both shoot and root under selenate treatment, and in roots of selenite-treated plants. Shoot Se accumulation from selenate and selenite were also correlated. There was no correlation between Se tolerance and accumulation, either for selenate or selenite. The F(1) offspring from a cross between the extreme selenate-sensitive Dijon G and the extreme selenate-tolerant Estland accessions showed intermediate selenate tolerance. In contrast, the F(1) offspring from a cross between selenite-sensitive and -tolerant accessions (Dijon GxCol-PRL) were selenite tolerant. The results from this study give new insight into the mechanisms of plant selenium (Se) tolerance and accumulation, which may help develop better plants for selenium phytoremediation or as fortified foods.     

Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite

Selenite can be a dominant form of selenium (Se) in aerobic soils; however, unlike selenate, the mechanism of selenite uptake by plants remains unclear. Uptake, translocation and Se speciation in wheat (Triticum aestivum) supplied with selenate or selenite, or both, were investigated in hydroponic experiments. The kinetics of selenite influx was determined in short-term (30 min) experiments. Selenium speciation in the water-extractable fraction of roots and shoots was determined by HPLC-ICPMS. Plants absorbed similar amounts of Se within 1 d when supplied with selenite or selenate. Selenate and selenite uptake were enhanced in sulphur-starved and phosphorus-starved plants, respectively. Phosphate markedly increased K(m) of the selenite influx. Selenate and selenite uptake were both metabolically dependent. Selenite was rapidly converted to organic forms in roots, with limited translocation to shoots. Selenomethionine, selenomethionine Se-oxide, Se-methyl-selenocysteine and several other unidentified Se species were detected in the root extracts and xylem sap from selenite-treated plants. Selenate was highly mobile in xylem transport, but little was assimilated to organic forms in 1 d. The presence of selenite decreased selenate uptake and xylem transport. Selenite uptake is an active process likely mediated, at least partly, by phosphate transporters. Selenite and selenate differ greatly in the ease of assimilation and xylem transport.     

Route-dependent pharmacokinetics, distribution, and placental permeability of organic and inorganic selenium in hamsters

Inorganic selenium (Se) salts (selenite and selenate oxyanions) and the organic selenoamino acids (selenomethionine and seleniferous grains) are teratogenic and embryolethal in domestic and wild birds. Selenium bioaccumulation has been held responsible for reproductive failure among waterfowl at the Kesterson Reservoir (California), the Ouray and Stewart Lake Wildlife Refuges (Utah), and the Carson Sink (Nevada). Anecdotal field and controlled laboratory reports have implicated Se exposure in mammalian embryotoxicity (including human), but developmental toxicity studies in hamsters failed to demonstrate an adverse response, except at maternally toxic doses (Ferm et al., Reprod. Toxicol., in press). Uptake, distribution, and elimination of Se after a single bolus equimolar dose (60 mumol/kg) of selenate or selenomethionine by oral or intravenous administration were compared using day 8 pregnant hamsters. Intravenous selenate was eliminated ten times more rapidly from maternal plasma than oral selenate, but concentrated in liver, kidney, and placenta to the same degree. Intravenous (iv) L-selenomethionine achieved lower maximum circulating total [Se], but it was eliminated more slowly than iv selenate. Larger areas under the plasma and peripheral tissue [Se]:time curve (AUC) after oral or parenteral selenomethionine than after equimolar selenate were consistent with previous studies in rodents and in humans. Embryonic [Se] plateaued at 3 nmol/g after selenate, but embryonic [Se] after selenomethionine continued to accumulate (80 nmol/g) as gestation progressed. The lack of a teratogenic response in hamsters at doses of either selenate or selenomethionine less than those associated with maternal intoxication cannot be attributed to lack of Se accumulation in early embryonic and placental tissue.     

Comparative effects of selenite and selenate on growth and selenium accumulation in lettuce plants under hydroponic conditions

The effects of different concentrations of selenite (2–30 μM) and selenate (2–60 μM) on biomass production, leaf area, and concentrations of photosynthetic pigments in lettuce plants were investigated. On the basis of the obtained results, the threshold of toxicity for the selenite and selenate has been designated. The toxicity thresholds for selenite and selenate were determined at concentrations of 15 and 20 μM, respectively. Next, four selenium (Se) concentrations (2, 4, 6 or 15 μM), below or near the toxicity boundary, have been selected for the lettuce biofortification experiment. In the biofortified plants, the oxidant status (levels of lipid peroxidation and H₂O₂ concentrations), as well as Se and sulphur (S) accumulation were analysed. In the edible parts of the lettuce, the Se concentration was higher for selenate presence compared to selenite; however, this difference was not as obvious as it was noted in the case of the roots, where selenite application caused the high accumulation of Se. An application of 15 μM Se as selenite caused a decline in the biomass and an intensification of prooxidative processes in the plant’s tissues and as toxic should be excluded from further biofortification experiments. These results indicate that an application of either selenate or selenite to the nutrient solution at concentrations below 15 μM can be used for biofortification of lettuce with Se, evoking better plant growth and not inducing significant changes in the oxidant status, the concentration of assimilation pigments and S accumulation.     

Comparative effects of selenite and selenite on the glutathione-related enzymes activity in pig blood platelets

The effects of inorganic selenium (Se) compounds (sodium selenite and selenate) on the activities of glutathione-related enzymes (glutathione peroxidase, glutathione-S-transferase [GST] and glutathione reductase [GR]) in pig blood platelets were investigated in vitro. GST activity in blood platelets treated with 10⁻⁴ M of selenite was reduced to 50%, whereas no decrease GST activity was observed after the treatment of platelets with the same dose of selenate. In platelets incubated with physiological doses (10⁻⁷, and 10⁻⁶ M) of Se compounds, the activity of glutathione peroxidase (GSH-Px) was enhanced (about 20%). GR activity after the exposure of platelets to tested Se compounds was unaffected.     

Evaluation of the Potential of Peas (Pisum sativum L.) to Be Used in Selenium Biofortification Programs Under Mediterranean Conditions

Selenium (Se), which has antioxidant, anticancer, and antiviral properties, is an essential micronutrient for humans and animals. This micronutrient is found in high quantity in legumes. Peas have an ever-increasing importance in Spain, and to increase their nutritional value, two foliar Se fertilizers: sodium selenate and sodium selenite, at five different rates: 0, 10, 20, 40, 80 g?ha^?1, were studied during the 2010/2011 crop season on semiarid Mediterranean conditions. Sodium selenate was much more effectively taken up by plants compared to sodium selenite. There was a strong linear relationship between the total Se content and Se rate in both sodium selenate and selenite. For each gram of Se fertilization as either sodium selenate or sodium selenite, the increase of total Se concentration in the grain was 148 and 19 μg Se?kg^?1 dry weight, respectively. Ingestion of 100 g of peas previously fertilized with 10 g of sodium selenate per hectare would result in an intake of 179 μg of Se. This is almost 90 % of the daily recommended dose needed to reduce the chance of some cancers and about 179 % of the minimum concentration required to prevent Se deficiency diseases in animals. The pea has shown to have a strong ability to uptake and accumulate Se under Mediterranean conditions; therefore, this would make it a very strong candidate for inclusion in biofortification programs aiming to increase Se in the food chain.     

Selenium accumulation in lettuce germplasm

Selenium (Se) is an essential micronutrient for animals and humans. Increasing Se content in food crops offers an effective approach to reduce the widespread selenium deficiency problem in many parts of the world. In this study, we evaluated 30 diverse accessions of lettuce (Lactuca sativa L.) for their capacity to accumulate Se and their responses to different forms of Se in terms of plant growth, nutritional characteristics, and gene expression. Lettuce accessions responded differently to selenate and selenite treatment, and selenate is superior to selenite in inducing total Se accumulation. At least over twofold change in total Se levels between cultivars with high and low Se content was found. Synergistic relationship between Se and sulfur accumulation was observed in nearly all accessions at the selenate dosage applied. The change in shoot biomass varied between lettuce accessions and the forms of Se used. The growth-stimulated effect by selenate and the growth-inhibited effect by selenite were found to be correlated with the alteration of antioxidant enzyme activities. The different ability of lettuce accessions to accumulate Se following selenate treatment appeared to be associated with an altered expression of genes involved in Se/S uptake and assimilation. Our results provide important information for the effects of different forms of Se on plant growth and metabolism. They will also be of help in selecting and developing better cultivars for Se biofortification in lettuce.     

Application of X-ray absorption near edge spectroscopy to the study of the effect of sulphur on selenium uptake and assimilation in wheat seedlings

Selenium (Se) is an essential trace element for humans and animals. A hydroponic experiment was performed to study the effects of sulphur (S) on Se uptake, translocation, and assimilation in wheat (Triticum aestivum L.) seedlings. Sulphur starvation had a positive effect on selenate uptake and the form of Se supplied greatly influenced Se speciation in plants. Compared with the control plants, Se uptake by the S-starved plants was enhanced by 4.81-fold in the selenate treatment, and selenate was readily transported from roots to shoots. By contrast, S starvation had no significant effect on selenite uptake, and selenite taken up by roots was rapidly converted to organic forms and tended to accumulate in roots. X-ray absorption near edge spectroscopy (XANES) analysis showed that organic forms of selenium, including selenocystine, Se-methyl-selenocysteine (MeSeCys), and selenomethionine-Se-oxide, were dominant in the plants exposed to selenite and accounted for approximately 90 % of the total Se. Whereas selenate remained as the dominant species in the roots and shoots exposed to selenate, with little selenate converted to selenite and MeSeCys. Besides, sulphur starvation increased the proportion of inorganic Se species in the selenate-supplied plants, but had no significant effects on Se speciation in plants exposed to selenite. The present study provides important knowledge to understand the associated mechanism of Se uptake and metabolism in plants.     

Toxicity of selenium to Lemna minor in relation to sulfate concentration

The aquatic plant Lemna minor L. was treated with sodium selenite or sodium selenate to test the toxicity of these salts in relation to high or low levels of sulfate in the culture medium. Several morphophysiological aspects, such as multiplication rate (MR), ratio of the number of fronds to number of colonies (Nfr/Ncol), frond size, cell ultrastructure, pigment content and guaiacol peroxidase (EC 1.11.1.7) activity were evaluated. Their variations might be an indirect means of evaluating the degree of susceptibility or tolerance of this plant to selenium (Se). Sodium selenite or sodium selenate treatments at concentrations ranging from 1 to 256 μ M generally decreased the investigated parameters. Moreover, the sulfate concentration influenced the toxicity of both Se salts. In general, with treatments in a medium containing a high sulfate (HS) content, sodium selenite appeared more toxic than sodium selenate, whereas in a low sulfate (LS) medium, sodium selenate seemed more toxic. MR was significantly increased at 1–4 μ M selenite and LS or 8 μ M selenate and HS levels, suggesting that Se may be an essential nutrient for this plant.     

Laboratory-scale continuous reactor for soluble selenium removal using selenate-reducing bacterium,Bacillus sp. SF-1

A model continuous flow bioreactor (volume 0.5 L) was constructed for removing toxic soluble selenium (selenate/selenite) of high concentrations using a selenate-reducing bacterium, Bacillus sp. SF-1, which transforms selenate into elemental selenium via selenite for anaerobic respiration. Model wastewater contained 41.8 mg-Se/L selenate and excess lactate as the carbon and energy source; the bioreactor was operated as an anoxic, completely mixed chemostat with cell retention time between 2.2-95.2 h. At short cell retention times selenate was removed by the bioreactor, but accumulation of selenite was observed. At long cell retention times soluble selenium, both selenate and selenite, was successfully reduced into nontoxic elemental selenium. A simple mathematical model is proposed to evaluate Se reduction ability of strain SF-1. First-order kinetic constants for selenate and selenite reduction were estimated to be 2.9 x 10(-11) L/cells/h and 5.5 x 10(-13) L/cells/h, respectively. The yield of the bacterial cells by selenate reduction was estimated to be 2.2 x 10(9) cells/mg-Se.     

Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallidurans CH34 exposed to selenite and selenate

Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se(0)). We have studied the kinetics of selenite (Se(IV)) and selenate (Se(VI)) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se(0) and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se(0). Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se(0) was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. Se(IV) was detected as a transient species in the first 12 h after selenate introduction, Se(0) also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.     

The involvement of ATP sulfurylase in Se(VI) and Cr(VI) reduction processes in the fission yeast<Emphasis Type="Italic"> Schizosaccharomyces pombe</Emphasis>

The response of Schizosaccharomyces pombe towards the oxyanions selenate [Se(VI)] and dichromate [Cr(VI)] was investigated in order to establish the involvement of the yeast ATP sulfurylase in their reduction. An ATP sulfurylase-defective/selenate-resistant mutant of S. pombe (B-579 Se(R) -2) and an ATP sulfurylase-active/selenate-sensitive strain of S. pombe (B-579 Se(S)) were included in this study. The inhibitory effect of Se(VI) and Cr(VI) oxyanions on growth and bioaccumulation was measured. The sensitive strain showed natural sensitivity to selenate while the resistant mutant tolerated a 100-fold higher concentration of selenate. These results indicate that selenate toxicity to microorganisms is connected with the reduction of selenate to selenite. Both strains showed similar sensitivity to Cr(VI) and in this study there was no evidence that ATP sulfurylase participates in the reduction process of Cr(VI).     

Speciation and metabolism of selenium injected with 82Se-enriched selenite and selenate in rats

Selenate and selenite injected intravenously into rats were speciated by the HPLC–ICP MS method with use of an enriched stable isotope as the tracer. In dose–relation experiments, ⁸²Se-enriched selenate or selenite was injected intravenously into male Wistar rats of 8 weeks of age (three rats/group) at single doses of 10, 25, 50, 100 and 200 μg/kg body weight for the selenate group, and 2, 5, 10, 25 and 50 μg/kg body weight for the selenite group. The animals were sacrificed 1 or 24 h later, and the concentrations and distributions of ⁸²Se in the liver, kidneys, serum, and urine remaining in the bladder or 24-h urine were determined. In time-course experiments, ⁸²Se-enriched selenate and selenite were injected at doses of 50 and 10 μg/kg body weight, respectively, and the animals were sacrificed 5, 15, 30, 60 and 180 min later. It was suggested that selenate is directly taken up by the liver with an efficiency of approximately 1/2 compared with selenite, the latter being taken up by the liver after being metabolized to selenide in red blood cells. Although selenate and selenite were metabolized differently in the bloodstream, and also a part of only selenate was excreted directly into the urine, the ⁸²Se taken up by the liver was shown to be metabolized in a manner indistinguishable between selenate and selenite. ⁸²Se of selenite origin but not of selenate origin was suggested to undergo redox reaction in the bloodstream. These results suggest that although parenteral selenate is utilized less efficiently by the body, it is utilized in the liver in a similar manner to selenite much more safely.     

Isolation, Growth, and Metabolism of an Obligately Anaerobic, Selenate-Respiring Bacterium, Strain SES-3

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO₂ with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Se⁰). Hence, SES-3 can carry out a complete reduction of selenate to Se⁰. Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [¹⁴C]lactate to ¹⁴CO₂ coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m-chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Se⁰ by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [⁷⁵Se]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se⁰. This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Se⁰ may proceed, in part, by some of the components of a dissimilatory system for sulfur oxyanions.     

An <Emphasis Type="Italic">Azospira oryzae</Emphasis> (syn <Emphasis Type="Italic">Dechlorosoma suillum</Emphasis>) Strain That Reduces Selenate and Selenite to Elemental Red Selenium

A bacterium that reduces the soluble selenium oxyanions, selenate and selenite, to insoluble elemental red selenium (Se⁰) was isolated from a laboratory reactor developed to remove selenate from groundwater. Gene sequence alignment of the 16S rRNA allowed identification of the isolate as Azospira oryzae. Biochemical and morphologic characterization confirm the identification. The isolate reduces selenate and selenite to Se⁰ under microaerophilic and denitrifying conditions but not under aerobic conditions. It does not use selenate or selenite as terminal eˉ donors. Se oxyanion reduction causes the formation of Se nanospheres that are 0.25 ± 0.04 μm in diameter. Nanospheres may be associated with the cells or free in the medium. The enzymatic activity associated with the reduction of selenate has a molecular mass of approximately 500 kD, and the enzymatic activity associated with the reduction of selenite has a mass of approximately 55 kD. Selenite reduction was inhibited by tungsten. The molecular masses of these activities were different from those associated with the reduction of dimethylsulfoxide, sulfate, and nitrite. This bacterium, or perhaps its enzymes or DNA, might be useful for the remediation of waters contaminated with Se oxyanions.