Showing papers on "Selenium published in 1999"

Bacterial respiration of arsenic and selenium.

Abstract: Oxyanions of arsenic and selenium can be used in microbial anaerobic respiration as terminal electron acceptors. The detection of arsenate and selenate respiring bacteria in numerous pristine and contaminated environments and their rapid appearance in enrichment culture suggest that they are widespread and metabolically active in nature. Although the bacterial species that have been isolated and characterized are still few in number, they are scattered throughout the bacterial domain and include Gram-positive bacteria, beta, gamma and epsilon Proteobacteria and the sole member of a deeply branching lineage of the bacteria, Chrysiogenes arsenatus. The oxidation of a number of organic substrates (i.e. acetate, lactate, pyruvate, glycerol, ethanol) or hydrogen can be coupled to the reduction of arsenate and selenate, but the actual donor used varies from species to species. Both periplasmic and membrane-associated arsenate and selenate reductases have been characterized. Although the number of subunits and molecular masses differs, they all contain molybdenum. The extent of the environmental impact on the transformation and mobilization of arsenic and selenium by microbial dissimilatory processes is only now being fully appreciated.

Inhibitory sites in enzymes: Zinc removal and reactivation by thionein

Abstract: Thionein (T) has not been isolated previously from biological material. However, it is generated transiently in situ by removal of zinc from metallothionein under oxidoreductive conditions, particularly in the presence of selenium compounds. T very rapidly activates a group of enzymes in which zinc is bound at an inhibitory site. The reaction is selective, as is apparent from the fact that T does not remove zinc from the catalytic sites of zinc metalloenzymes. T instantaneously reverses the zinc inhibition with a stoichiometry commensurate with its known capacity to bind seven zinc atoms in the form of clusters in metallothionein. The zinc inhibition is much more pronounced than was previously reported, with dissociation constants in the low nanomolar range. Thus, T is an effective, endogenous chelating agent, suggesting the existence of a hitherto unknown and unrecognized biological regulatory system. T removes the metal from an inhibitory zinc-specific enzymatic site with a resultant marked increase of activity. The potential significance of this system is supported by the demonstration of its operations in enzymes involved in glycolysis and signal transduction.

Solvothermal Elemental Direct Reaction to CdE (E = S, Se, Te) Semiconductor Nanorod

Abstract: The solvothermal reaction of sulfur, selenium, or tellurium with cadmium metal powder in different organic solvents in the temperature range 120−200 °C was investigated systematically to prepare ca...

Reduction of selenite and detoxification of elemental selenium by the phototrophic bacterium Rhodospirillum rubrum.

Abstract: The effect of selenite on growth kinetics, the ability of cultures to reduce selenite, and the mechanism of detoxification of selenium were investigated by using Rhodospirillum rubrum. Anoxic photosynthetic cultures were able to completely reduce as much as 1.5 mM selenite, whereas in aerobic cultures a 0.5 mM selenite concentration was only reduced to about 0.375 mM. The presence of selenite in the culture medium strongly affected cell division. In the presence of a selenite concentration of 1.5 mM cultures reached final cell densities that were only about 15% of the control final cell density. The cell density remained nearly constant during the stationary phase for all of the selenite concentrations tested, showing that the cells were not severely damaged by the presence of selenite or elemental selenium. Particles containing elemental selenium were observed in the cytoplasm, which led to an increase in the buoyant density of the cells. Interestingly, the change in the buoyant density was reversed after selenite reduction was complete; the buoyant density of the cells returned to the buoyant density of the control cells. This demonstrated that R. rubrum expels elemental selenium across the plasma membrane and the cell wall. Accordingly, electron-dense particles were more numerous in the cells during the reduction phase than after the reduction phase.

Plant availability of selenite and selenate as influenced by the competing ions phosphate and sulfate

Abstract: 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 (05 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

Effect of selenate as a feed supplement to dairy cows in comparison to selenite and selenium yeast.

Abstract: The main aim of this trial was to define the possible differences between selenite and selenate in their ability to increase the selenium (Se) concentration of milk, in comparison with organic Se. Dairy cows (n = 42) were fed a basal diet containing .10 to .12 mg Se/kg DM for 5 mo and were then divided into four groups of 10 or 11, as similar as possible in age and stage of lactation. During the next 84 d, the cows in three of the groups were supplemented with 3 mg of Se daily, whereas the cows in one control group remained unsupplemented. The Se supplement was given as sodium selenite, sodium selenate, or a Se yeast product. The total Se concentration of the diets varied with the cows' stage of lactation and was for the supplemented groups .24 to .31 mg/kg DM, but remained between .10 and .12 mg/kg in the control group. At the end of the trial, the mean whole blood Se concentrations in the selenite, selenate, yeast, and control groups were 138, 141, 165, and 104 microg/L, respectively. The Se concentration in plasma apparently reached a plateau level within 4 wk, at approximately 75 microg/L in the selenite group, 80 microg/L in the selenate group, and 90 microg/L in the yeast group. In the control group the mean concentration in plasma remained at approximately 50 microg/L. The increase of the activity of glutathione peroxidase (GSH-Px) in the erythrocytes was significantly higher in the supplemented groups than in the control group. The mean concentrations of Se in milk in the selenite, selenate, and yeast groups were 16.4, 16.4, and 31.2 microg/L, respectively, whereas the concentration remained at approximately 14 microg/L in the control group. The milk Se concentration reached a plateau within 1 wk after the start of Se supplementation. Dietary supplementation with selenite and selenate, thus, had only a limited effect on the Se concentration in milk, and there was no significant difference between the two inorganic compounds in any variable measured. Organic Se was much more effective than inorganic Se in increasing the concentration of Se in milk.

Organoselenium chemistry : a practical approach

Abstract: Preface 1. Organoselenium chemistry: an overview 2. Selenoxide eliminations 3. Addition of Electrophilic Organoselenium Reagents to Carbon-Carbon Double Bonds 4. Reactions of selenium nucleophiles 5. Oxidations with selenium reagents 6. Reductions with selenium reagents 7. Reactions of selenium-stabilized carbanions 8. Reactions of selenium stabilized carbocations 9. Free-radical reactions and reductive deselenizations 10. Selenium extrusion reactions 11. Sigmatropic rearrangements of organoselenium compounds 12. Synthesis and reactions of carbonyl selenide and isoselenocyanates 13. Reactions of selenones 14. Preparation of organic conductors Bibliography Appendix Index

Sample preparation and HPLC separation approaches to speciation analysis of selenium in yeast by ICP-MS

Abstract: Eight solid-liquid extraction procedures were evaluated for the recovery of selenium species from yeast. Speciation of Se in the extracts was characterized by different types of HPLC, including size-exclusion, anion-exchange and reversed-phase chromatography with ICP-MS detection. The results obtained depended critically on the sample preparation procedure used. Leaching with water and with methanol led only to 10-20% recoveries of Se, split into eight compounds, among which Se(IV) and selenomethionine could be identified. Leaching with pectinolytic enzymes released an additional 20% of selenomethionine. Leaching with sodium dodecyl sulfate solution allowed the solubilization of a selenoprotein that accounted for ca. 30% of the total Se present. Leaching with proteolytic enzymes led to recoveries of Se above 85%, the majority as selenomethionine. Hydrolysis of the yeast with tetramethylammonium hydroxide solubilized the sample completely but the Se species present were entirely degraded to selenomethionine and inorganic selenium. A sequential leaching procedure is proposed for the evaluation of selenium speciation in yeast without the need for a coupled technique.

Simultaneous reduction of nitrate and selenate by cell suspensions of selenium-respiring bacteria.

Abstract: Washed-cell suspensions of Sulfurospirillum barnesiireduced selenate [Se(VI)] when cells were cultured with nitrate, thiosulfate, arsenate, or fumarate as the electron acceptor. When the concentration of the electron donor was limiting, Se(VI) reduction in whole cells was approximately fourfold greater in Se(VI)-grown cells than was observed in nitrate-grown cells; correspondingly, nitrate reduction was ∼11-fold higher in nitrate-grown cells than in Se(VI)-grown cells. However, a simultaneous reduction of nitrate and Se(VI) was observed in both cases. At nonlimiting electron donor concentrations, nitrate-grown cells suspended with equimolar nitrate and selenate achieved a complete reductive removal of nitrogen and selenium oxyanions, with the bulk of nitrate reduction preceding that of selenate reduction. Chloramphenicol did not inhibit these reductions. The Se(VI)-respiring haloalkaliphile Bacillus arsenicoselenatis gave similar results, but its Se(VI) reductase was not constitutive in nitrate-grown cells. No reduction of Se(VI) was noted for Bacillus selenitireducens, which respires selenite. The results of kinetic experiments with cell membrane preparations of S. barnesii suggest the presence of constitutive selenate and nitrate reduction, as well as an inducible, high-affinity nitrate reductase in nitrate-grown cells which also has a low affinity for selenate. The simultaneous reduction of micromolar Se(VI) in the presence of millimolar nitrate indicates that these organisms may have a functional use in bioremediating nitrate-rich, seleniferous agricultural wastewaters. Results with75Se-selenate tracer show that these organisms can lower ambient Se(VI) concentrations to levels in compliance with new regulations proposed for release of selenium oxyanions into the environment.

Selenium redox biochemistry of zinc–sulfur coordination sites in proteins and enzymes

Abstract: Selenium has been increasingly recognized as an essential element in biology and medicine. Its biochemistry resembles that of sulfur, yet differs from it by virtue of both redox potentials and stabilities of its oxidation states. Selenium can substitute for the more ubiquitous sulfur of cysteine and as such plays an important role in more than a dozen selenoproteins. We have chosen to examine zinc–sulfur centers as possible targets of selenium redox biochemistry. Selenium compounds release zinc from zinc/thiolate-coordination environments, thereby affecting the cellular thiol redox state and the distribution of zinc and likely of other metal ions. Aromatic selenium compounds are excellent spectroscopic probes of the otherwise relatively unstable functional selenium groups. Zinc-coordinated thiolates, e.g., metallothionein (MT), and uncoordinated thiolates, e.g., glutathione, react with benzeneseleninic acid (oxidation state +2), benzeneselenenyl chloride (oxidation state 0) and selenocystamine (oxidation state −1). Benzeneseleninic acid and benzeneselenenyl chloride react very rapidly with MT and titrate substoichiometrically and with a 1:1 stoichiometry, respectively. Selenium compounds also catalyze the release of zinc from MT in peroxidation and thiol/disulfide-interchange reactions. The selenoenzyme glutathione peroxidase catalytically oxidizes MT and releases zinc in the presence of t-butyl hydroperoxide, suggesting that this type of redox chemistry may be employed in biology for the control of metal metabolism. Moreover, selenium compounds are likely targets for zinc/thiolate coordination centers in vivo, because the reactions are only partially suppressed by excess glutathione. This specificity and the potential to undergo catalytic reactions at low concentrations suggests that zinc release is a significant aspect of the therapeutic antioxidant actions of selenium compounds in antiinflammatory and anticarcinogenic agents.

Selenium volatilization and accumulation by twenty aquatic plant species

Abstract: Aquatic plants can remove selenium (Se) from agricultural or industrial wastewater through Se accumulation and volatilization, that is, the conversion of selenate or selenite to volatile forms of Se. To identify aquatic plant species that will be good at removing Se from contaminated water, 20 plant species were screened for their ability to accumulate and volatilize Se when supplied with 20 μM selenate or selenite under controlled conditions. There was at least a 50-fold variation in Se accumulation and volatilization capacity among the plant species tested, regardless of the form of Se supplied. The best plant species for Se volatilization from selenate were also the best species for selenite volatilization. Selenium volatilization rates were twofold higher from selenite than from selenate but more selenate-Se was translocated into harvestable plant tissues than selenite-Se. Several aquatic plant species showed Se volatilization and accumulation rates (per unit surface area) that were comparable with Indian mustard [Brassica juncea (L.)], the best-known terrestrial plant species for Se phytoremediation. Therefore, many wetland species identified in this study, for example, parrot's feather (Myriophyllum brasiliense Camb.), iris-leaved rush (Juncus xiphioides), cattail (Typha latifolia L.), saltmarsh bulrush (Scirpus robustus), etc., have a great potential for Se phytoremediation in wetlands.

Prevention of cytotoxic effects of arsenic by short-term dietary supplementation with selenium in mice in vivo.

Abstract: Interaction between selenium and arsenic has been used to protect against the genotoxic effects of sodium arsenite through dietary intervention by an equivalent amount (1/10 LD50) of sodium selenite. The two salts were administered by gavaging to laboratory bred Swiss albino mice sequentially and in combination. Cytogenetic endpoints, including chromosomal aberrations (CA) and damaged cells (DC) were recorded 24 h after exposure from chromosome spreads in bone marrow cells. Administration of sodium selenite 1 h before sodium arsenite reduced the clastogenic effects of the latter significantly. The protection was less when the salts were given together and negative when arsenite was given before selenite. Histological changes were recorded. Such reduction of arsenic toxicity through dietary intervention by selenium is of significance in protecting against the widespread toxicity observed in human populations exposed to arsenic through drinking water from contaminated deep tubewells in West Bengal and Bangladesh.

The influence of dietary selenium as selenium yeast or sodium selenite on the concentration of selenium in the milk of Suckler cows and on the selenium status of their calves.

Abstract: The aim of this trial was to determine whether the selenium status of suckling calves could be improved by supplementing their dams' diet with organic Se instead of sodium selenite. A herd of 103 Hereford cows, which were on grass paddocks all year round, was divided into two groups. Both groups had free access to a mineral supplement that contained 30 mg of Se/kg; for one group the source of the Se was a Se yeast product, and for the other group the source was sodium selenite. The basal feed contained .02 mg of Se/kg DM. During the trial, the mean daily consumption of the mineral supplement was approximately 110 g/cow. The calving season started in the middle of March and ended in the middle of May. Blood samples were taken from 11 cows and their calves in the yeast group and from nine in the selenite group at the end of April and again at the beginning of June, and milk samples were taken at the same times. At both samplings, the concentration of Se in whole blood and the activity of glutathione peroxidase (GSH-Px) in the erythrocytes of the cows and calves in the yeast group were higher than in the samples from the animals in the selenite group. The same pattern was seen for plasma, except for the cows at the first sampling. The mean concentrations of Se in whole blood from calves in the yeast and selenite groups were 130 and 84 microg/L, respectively, and plasma concentrations were 48 and 34 microg/ L, respectively. Mean Se concentration in the milk from the yeast group (17.3 microg/L) was higher than that in milk from the selenite group (12.7 microg/L). There were significant correlations (r = .59 to .68) between the concentrations of Se in the cow's milk or cow's whole blood compared with Se concentrations in the calves whole blood and plasma or with the erythrocyte GSH-Px activity of the calves. The Se status of the calves in the selenite group was considered to be marginal, but the status of the calves in the yeast group was considered to be adequate. Supplementation of the suckler cows' diet with organic Se in the form of Se yeast rather than sodium selenite improved the Se status of their calves when the Se was mixed into a mineral supplement containing 30 mg of Se/kg. In practice, such supplementation would probably eliminate the risk of nutritional muscular degeneration in suckling calves.

Identification of the principal selenium compounds in selenium-enriched natural sample extracts by ion-pair liquid chromatography with inductively coupled plasma- and electrospray ionization-mass spectrometric detection

Abstract: Selenium-enriched garlic and yeast sample extracts and digests were analyzed using ion-pair high performance liquid chromatography (HPLC) with on-line inductively coupled plasma-mass spectrometric (ICP-MS) and electrospray ionization-mass spectrometric (ESI-MS) detection. The principal selenium compounds in these samples were identified as selenomethionine, and Se-adenosyl-selenohomocysteine in yeast, and γ-glutamyl-Se-methyl-selenocysteine and possibly γ-glutamyl-selenomethionine in garlic. The compounds identified account for 85 and 90% of the total selenium content of the yeast and the garlic samples, respectively. On-line HPLC-ESI-MS selected ion chromatograms (SIC) and mass spectra of selenium compounds extracted from selenium enriched samples are presented. Limits of quantification (LOQ, defined as S/N = 10) for HPLC-ICP-MS were in the range 10–50 ng mL–1 Se in the injected extracts. LOQ values for HPLC-ESI-MS were ca. 100 times higher than those of HPLC-ICP-MS.

Trace Element Contamination of Total Parenteral Nutrition. 1. Contribution of Component Solutions

Abstract: Background: Trace elements have been shown to contaminate total parenteral nutrition (TPN) solutions. Methods: This study used the multi-elemental technology of inductively coupled plasma—mass spectrometry to demonstrate the extent to which trace elements were present in amounts above (ie, as contaminants) or below expected levels in eight TPN component solutions. Results: Of the 66 trace elements scanned, there were 12 trace element contaminants in amounts >1 μg/L (zinc, copper, manganese, chromium, selenium, boron, aluminum, titanium, barium, vanadium, arsenic, and strontium) in the eight component solutions studied. Trace element contaminants were present in all solutions, and different trace elements contaminated the solutions at various concentrations. Component solutions of amino acid, potassium chloride, calcium gluconate, and sodium chloride contained the greatest numbers of trace element contaminants, whereas the lowest numbers were present in sterile water and magnesium sulfate. Interlot and int...

Chemopreventive mechanisms of selenium.

Abstract: □ The element selenium (Se) was recognized only 40 years ago as being essential in the nutrition of animals and humans.It is recognized as being an essential component of a number of enzymes in which it is present as the amino acid selenocysteine (SeCys). Selenium compounds have also been found to inhibit tumorigenesis in a variety of animal models and recent studies indicate that supplemental Se in human diets may reduce cancer risk. Anti-tumorigenic activities have been associated with Se intakes that are more than sufficient to correct nutritionally deficient status; that is, Se appears to be anti-tumorigenic at intakes that are substantially greater than those associated with maximal expression of the known SeCys-containing enzymes. Therefore, while some cancer protection may involve one or more Se-enzymes, it is probable that anti-tumorigenic functions of Se are discharged by certain Se-metabolites produced in significant amounts at relatively high Se intakes.

Photomelting of selenium at low temperature

Abstract: We report on a photoinduced phenomenon in solids, namely, photomelting at low temperature. We have found that both trigonal and amorphous selenium can be molten by illumination with light at a temperature of ∼77 K. This phenomenon is pure optical (athermal) and it is associated with light-induced breaking of the interchain (intermolecular) bonds in selenium. The photomelting is important for basic science (as an example of photoinduced phase transition in condensed matter and as a key photoinduced phenomenon in selenium and related materials) and for applications (as a tool for fine manipulation with shape of solids by light at low temperatures).

Speciation of Soluble Selenium in Agricultural Drainage Waters and Aqueous Soil-Sediment Extracts Using Hydride Generation Atomic Absorption Spectrometry

Abstract: There are few methods to effectively measure organic selenium [Se(−II)] in natural water and soil-sediment extracts. A method has been developed to determine organic Se(−II) in soil-sediment extracts and agricultural drainage water by using persulfate to oxidize organic Se(−II) and using manganese oxide as an indicator for oxidation completion. This method was used to determine Se speciation in eleven soil-sediments and four agricultural drainage water samples collected from the western United States. Results showed that organic Se(−II) can be quantitatively oxidized to selenite without changing the selenate concentration in the soil-sediment extract and agricultural drainage water and then quantified by hydride generation atomic absorption spectrometry. Recoveries of spiked organic Se(−II) and selenite were 96−105% in the soil-sediment extracts and 96−103% in the agricultural drainage water. Concentrations of soluble Se in the soil-sediment extracts were 0.0534−2.45 μg/g, of which organic Se(−II) account...

Serum selenium levels and blood glutathione peroxidase activities in vitiligo

Abstract: It has recently been shown that patients with vitiligo can accumulate epidermal hydrogen peroxide (H2O2) in association with low catalase levels This study examined serum selenium levels and blood glutathione peroxidase activities in 61 patients and controls The results showed high serum selenium levels in 56% of the patients As at least one isoform of glutathione peroxidase requires selenium for its activity, enzyme activities were also evaluated The overall results were not significantly different compared with controls, but further age-related analysis of the data indicated significantly lower activities in patients up to 46 years As glutathione peroxidase can also efficiently degrade H2O2, the results of this study could indicate an additional impaired H2O2 metabolism in vitiligo