Uptake of selenite,selenomethionine and selenate by brush border membrane vesicles isolated from rat small intestine

The uptake of selenite, selenate and selenomethionine (SeMet) was performed with brush border membrane vesicles (BBMV) prepared from rats fed selenium-deficient and supplemented diets. At equilibrium (60 min), the uptake of ⁷⁵Se from [⁷⁵Se]selenite ranged from 16.5 to 18.9 nmol mg^-1 protein. There was a curvilinear relationship in the uptake of selenite over a concentration range of 10–1000 m. About 2 nmol mg^-1 protein was obtained with selenomethionine (SeMet) which occurred between 90 and 180 s. In contrast to selenite, there was a linear relationship in the initial uptake of SeMet over a concentration range of 10–1000 m. The uptake of selenate was approximately 50-fold lower than selenite, reaching 350 pmol mg^-1 protein. Dietary selenium level had no effect on the rate of ⁷⁵Se accumulation by BBMV. Dramatic differences are found in the uptake and binding of selenium by BBMV incubated with different selenocompounds.     

Chemical forms of selenium present in rat and ram spermatozoa

In vivo and in vitro studies were conducted to investigate the chemical forms by ion-exchange chromatography of selenium (Se) present in rat and ovine spermatozoa. After injection with ⁷⁵Se-selenite, the form of ⁷⁵Se in rat sperm was selenocysteine, but selenocysteine and selenomethionine (SeMet) were present in ovine sperm. Presumably, synthesis of SeMet by rumen microbes are responsible for its presence in ovine sperm. In vitro incubation of ram sperm with selenocysteine or SeMet produced no changes, but incubation with selenite produced a compound that eluted one fraction before SeMet from the ion-exchange column. After treatment of this fraction with mercaptoethanol, it eluted in a later fraction upon rechromatography, suggesting it to be selenodicysteine. This compound is apparently formed because of high levels of cysteine in semen. Cysteine, reduced glutathione, and oxidized glutathione were also found in semen. The significance of the results is discussed.     

Uptake and Retention of Selenite and Selenomethionine in cultured K-562 cells

The selenium uptake and retention have been studied in K-562 cells exposed to selenite or selenomethionine. In the uptake experiments the cells were exposed to two doses of selenite (5 or 50 M) or selenomethionine (10 or 50 M). In the retention study the cells were treated for 2 h with the above mentioned doses of the selenocompounds before being observed at different times. The selenium uptake in cells exposed to selenite 5 M began to saturate at 8 h, but increased again between 48 and 96 h. In cells exposed to selenite 50 M the selenium uptake never reached a maximum, however, at 48 and 96 h the cell viability decreased strongly. The two doses of selenite showed different retention patterns, with a relatively small cellular decrease of selenium after treatment with selenite 5 M compared to treatment with 50 M of selenite. The selenium uptake in cells exposed to selenomethionine 10 M or selenomethionine 50 M began to saturate at 24 h and 48 h, respectively. The retention patterns were similar for both selenomethionine doses with a continuous decrease of the selenium concentration during the whole observation period. The results indicated a more controlled uptake and retention pattern of selenomethionine compared to selenite.     

Selenite or selenomethionine interaction with methylmercury on uptake and toxicity showing a weak selenite protection: studies on cultured K-562 cells

Selenium and methylmercuric chloride (MMC) interactions regarding cellular uptake and selenium protection on MMC toxicity have been studied. Human K-562 cells were pretreated or simultaneously treated with either selenite (5 or 50 μM) or selenomethionine (10 or 50 μM) together with (3.5 or 5 μM) MMC. Cells simultaneously treated with selenite or selenomethionine and 3.5 μM MMC showed a decreased mercury concentration with increased selenium dose especially seen in the selenite combinations. The simultaneous selenite and MMC 3.5 μM combinations showed growth curves with an increasing number of viable cells with increased selenite dose. All combinations with 5 μM MMC were toxic to the cells. Interactions between selenite or selenomethionine and MMC regarding cellular uptake of mercury and selenium were observed and indications of selenite protection against MMC toxicity in human K-562 cells were noticed.     

Effect of inorganic and organic forms of selenium supplementation on development of larval Heliothis virescens

Selenium (Se) is an essential micronutrient for vertebrates though little is known about the effects on insects. Herbivorous insect larvae acquire Se from plant tissues in the inorganic form of sodium selenate and sodium selenite, and in the organic form of selenoamino acids, selenomethionine, and selenocystine. In this study, we document the effects of dietary supplementation with sodium selenite, sodium selenate, selenocystine, selenomethionine, and selenized yeast on the developmental rate of Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae). Larvae tolerated high levels of Se (500 µg g^?1 Se) as sodium selenate and to a lesser extent as selenocystine. Lower levels of sodium selenite (>1 µg g^?1 Se) caused increased mortality, reduced rates of pupation, more pupal/adult intermediates, and reduced adult emergence. Selenomethionine proved toxic to larvae at levels above 25 µg g^?1 Se, significantly delaying pupation and raising mortality. Provision of Se as selenized yeast, which contains primarily selenomethionine, was also extremely detrimental to larval development and survival. The results indicate that the impact of dietary Se supplement for insects may differ from vertebrates.     

Selenium in human and animal nutrition: Resolved and unresolved issues. A partly historical treatise in commemoration of the fiftieth anniversary of the discovery of the biological essentiality of selenium,dedicated to the memory of Klaus Schwarz (1914–1978) on the occasion of the thirtieth anniversary of his death

Selenium (Se) is an essential trace element unevenly distributed on the Earth’s crust with low selenium regions predominating. To prevent selenium-deficiency diseases in livestock, additions of selenium to animal feed are required and were approved for all species, but the chemical form of the element to be added was not specified. Presently, sodium selenite is still widely employed, although it is not a natural nutritional form of selenium. Its use creates ecological problems and affects human selenium nutriture in as much as the meat, milk, and eggs from animals maintained on selenite contain less selenium than from animals receiving it as selenomethionine, the chief natural nutritional form of the element present in grain crops grown in selenium-adequate regions, or from high-selenium yeast added to feedstock.

Human dietary selenium intakes are sub-optimal in many countries but are considered to be adequate if they reach the currently adopted Recommended Dietary Allowances (RDAs). Their upward revision will be required if the health benefits of selenium are to be fully utilized.     

Selenoproteins from rat testis cytosol

Radioactive inorganic selenium, administered intraperitoneally at 1 mg/kg body weight to young adult rats, acculumulates in testes for 7 days or longer, whereas liver, kidney and serum levels fall more rapidly. 3–4 h after administration of [⁷⁵Se]selenite, 55–60% of the radioactivity in the testes was found in the cytosol, associated with protein. Ultragel ACA-22 chromatography of testis cytosol prepared 4 h after ⁷⁵Se treatment revealed a major selenoprotein having an apparent molecular weight of 59 000. Sodium dedecyl sulfate polyacrylamide gel electrophoresis indicated extensive heterogeneity of radioactivity with apparent molecular weights of about 57 000 and 45 000 and 15 000. Cytosol from rats treated 4 weeks earlier showed predominance of the 15 000 molecular weight [⁷⁵Se]selenoprotein. Sucrose density gradient ultracentrifugation at either low or high ionic strength demonstrated a single 7 S selenoprotein. Chromatography with Blue-Sepharose indicated that the radioactivity was not associated with albumin. Strong ⁷⁵Se binding to protein was demonstrated by overnight dialysis against water, 2 M NaCl, β-mercaptoethanol, 8 M urea, selenite. However, 85% of the ⁷⁵Se was removed by dialysis against 0.5 M NaOh. This stability contrasts with the lability of disulfide reagents of selenite-protein complexes formed in vitro. The fact that selenium is incorporated in substantial amounts into a discrete and stable protein suggests a physiological role for this essential trace element in the testes.     

Stimulation of mucosal uptake of selenium from selenite by some thiols at various sites of rat intestine

The influence of several thiols (conc. 1 mmol/L) on mucosal uptake of⁷⁵Se from⁷⁵Se-labeled selenite (conc. 10 μmol/L) across the brush border of rat jejunum and cecum was investigated in vitro using a short-term uptake technique.l-Cysteine (l-Cys) stimulated⁷⁵Se uptake in the mid- and distal jejunum and cecum, but not in the proximal jejunum. The effect was maximal in the distal jejunum.d-Cys was less effective in the jejunum and similarly effective in the cecum.l-Leucine (l-Leu) andl-glutamic acid significantly reduced the stimulatory effect ofl-Cys on Se uptake in the distal jejunum, whereas the respective effect ofd-Cys was not diminished byl-Leu. Cysteamine stimulated mucosal⁷⁵Se uptake at all intestinal sites tested, whereas the effect of mercaptopyruvate was restricted to the distal jejunum. Thioglycolate also enhanced⁷⁵Se uptake in the distal jejunum. The stimulatory effects ofl-Cys, mercaptopyruvate, and thiologlycolate were Na⁺-dependent, whereas the effect of cysteamine also occurred in the absence of Na⁺. Mercaptosuccinate,d-penicillamine, ergothioneine, and thiosulfate did not enhance mucosa⁷⁵Se uptake. It is concluded from these findings that the reaction of some thiols with selenite results in Se compounds that are rapidly absorbed by the intestinal epithelium through various Na⁺-dependent and Na⁺-independent, mechanisms. The high bioavailability of Se from selenite found by others might thus be the result of the presence of thiols in the gastrointestinal tract.     

Selenium protection against mercury-induced apoptosis and growth inhibition in cultured K-562 cells

Selenium and mercuric chloride (MC) interactions regarding effects on cell growth and cell death have been studied. Human K-562 cells were pretreated or simultaneously treated with either selenite (5 or 50 μM) or selenomethionine (10 or 50 μM) and with MC (35 or 50 μM). The 35-μM MC treatments resulted in a clear inhibition of cell growth with no obvious difference between mercury-treated and mercury-selenium-treated cells. Furthermore, the apoptotic frequency was similar at all observations for all selenium treatments with 35 μM MC. In the simultaneously treated selenite and 50-μM MC combinations, a selenite-dependent protection was shown both by increased cell growth and by lower apoptotic frequency at 48 and 96 h of exposure. Both treatments with selenomethionine showed protection observed as an increased cell growth at 48 and 96 h and as decreased apoptotic frequency at 96 h of exposure.     

Incorporation of selenium from selenite and selenocystine into glutathione peroxidase in the isolated perfused rat liver

The erythrocyte-free, isolated perfused rat liver was used to study the incorporation of selenium into glutathione peroxidase. Gel filtration and ion exchange chromatography of liver supernatant demonstrated ⁷⁵Se incorporation into glutathione peroxidase. A 9-fold excess of unlabelled selenium as selenite or selenide very effectively reduced ⁷⁵Se incorporation from L[⁷⁵Se]-selenocystine, but a 100-fold excess of unlabelled selenium as selenocystine was relatively ineffective as compared to selenite or selenide in diluting ⁷⁵Se incorporation from [⁷⁵Se]selenite. These results indicate that selenide and selenite are more readily metabolized than is selenocysteine to the immediate selenium precursor used for glutathione peroxidase synthesis, and suggest a posttranslational modification at another amino acid residue, rather than direct incorporation of selenocysteine, as the mechanism for formation of the presumed selenocysteine moiety of the enzyme.     

Metabolism of selenomethionine and effects of interacting compounds by mammalian cells in culture

Since differences have been found in animals, the efficacies of selenomethionine (SeMet), selenite, and selenocystine (SeCys) for glutathione peroxidase (GPx) induction and cellular incorporation were compared and some effects of interacting nutrients on SeMet utilization were examined in tissue cultures. In three cell lines, Chang liver cells, mouse myoblasts and human fibroblasts, selenite was more effective than SeMet for GPx induction. However, radiotracer studies showed that SeMet was more rapidly incorporated into all cells than either selenite or SeCys. Chromatography of acid hydrolysates of Chang liver cells grown with 75Se-labeled SeMet indicated that approximately 90% of incorporated 75Se remained as SeMet, and less than 10% was as SeCys, the form of Se in GPx. Selenite supplementation slightly reduced both the incorporation of 75SeMet and the proportion of cellular 75Se recoverable as SeCys in Chang liver cells. Supplementation with L-methionine, however, significantly reduced 75SeMet incorporation, but significantly increased the proportion of cellular 75Se recovered as SeCys. L-cystine supplementation had no effect on either the cellular incorporation of 75SeMet or the proportion of cellular 75Se recovered as SeCys. These studies of SeMet utilization and effects of interacting nutrients are reflective of observations on SeMet metabolism in whole animals and humans.     

Effect of sodium selenosulfate on restoring activities of selenium-dependent enzymes and selenium retention compared with sodium selenite in vitro and in vivo

Sodium selenosulfate has been extensively used as a precursor of selenide ions in the preparation of nano Se-containing compounds. Its biological properties remain completely unknown. Sodium selenosulfate and sodium selenite were added to the medium of HepG2 cells and administered intraperitoneally at a dose of 0.1 mg Se/kg body weight to selenium-deficient mice, respectively. Both of the selenium compounds could increase the activities of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) in a dose-dependent manner in cells and efficiently restore selenium retention and activities of GPx and TrxR in mice. All of the variables were in correlation with the Se supply. There was no distinction in elevating activities of GPx and TrxR between selenosulfate and selenite in vitro. After a 2-d supply of selenosulfate, the activity of GPx in the liver was 65% (p < 0.001) and Se accumulations in the liver, kidney and blood were 64%, 86%, and 65%, respectively, of those treated with selenite (allp < 0.01). With the 7-d selenosulfate supplementation, the activity of GPx in the kidney and activities of TrxR in the liver and kidney were 88%, 75%, and 78%, respectively, of those treated with selenite (allp < 0.01); Se retentions in the liver and kidney were 85% and 93%, respectively of those supplemented with selenite (bothp < 0.01). These facts indicated that selenosulfate could be absorbed and utilized in the biological system. No difference in vitro demonstrated that selenosulfate could be absorbed and generate reduced selenide as efficiently as selenite. The differences between the two compounds in vivo were the result of other factors that affected selenosulfate utilization in tissues.     

Chemical form of selenium greatly affects metal uptake and responses by cultured human lymphocytes

In this work, possible interference with functional activities of human lymphocytes after in vitro treatment with selenium was examined. Sodium selenite and selenomethionine compounds were tested in parallel, and their capability to inhibit or to increase the antibody production by lymphocytes was investigated. Furthermore, after incubation for 7 d, total cell-associated Se was measured by a fluorimetric method. The in vitro doses of Se employed in this study mainly reflect those measured in blood of individuals with different Se intake. Low doses of Se (0.5–2.0μM) added either as sodium selenite or selenomethionine did not alter the secretion of antibodies. When Se was added at higher levels, instead, an inhibitory effect was found using selenite, whereas a progressive increase in immunoglobulin production was observed after exposure to selenomethionine. In both cases, modifications were detected at 5 μM (395 μg Se/L), and were significant at 10 μM (789 μg Se/L). A different trend between the two chemical forms was also observed with regard to Se uptake by cells. Interestingly, both Se uptake and cell sensitivity were influenced by the density of the cells in culture. Our data suggest that the biological effects of Se in mammalian systems are strongly influenced by its chemical form, and caution should be exerted to avoid toxic effects of selenium.     

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.     

The bioavailability of various selenium compounds to a marine wading bird

The uptake of dietary selenium (about 3.5 mg/kg AF dry wt) as selenomethionine, selenocystine, selenite, selenate, and fish selenium in the plasma and red blood cells (RBC) of the oystercatcher has been investigated. The birds received the various selenium compounds subsequently, for at least 9 wk. After dietary supplementation of selenocystine, selenite, and selenate, plasma selenium was about 350 μg/L and RBC selenium 2.1 mg/kg dry wt. After supplementation of selenomethionine, the plasma concentration increased to 630 μg/L, and the RBC concentration to 4.1 mg/kg dry wt. When the fodder contained 3.1 mg/kg fish Se, an average plasma and RBC concentration of 415 μg/L and 14.4 mg/kg dry wt, respectively, was measured. The maximal increase of the selenium concentration in the plasma was attained at first sampling, 14 d after a change in dietary selenium (selenomethione or fish Se); the uptake seemed to be a concentration-regulated process. RBC concentrations (γ in mg/kg dry wt) increased with time (X in d) according toY=a?be^?cX . Fifty percent of the total increase was attained within 17d, suggesting that diffusion into the RBC played a role. The selenium concentration in the plasma was positively correlated with the (fish) Se concentration in the fodder; the RBC concentration (60 d after the change in diet) was positively correlated with the plasma concentration. When the diet contained fish Se, the blood selenium concentrations of the captive birds were similar to the concentrations measured in field birds. Fish Se is a yet undetermined selenium compound. The present experiment showed that fish Se differed from selenomethionine, selenocystine, selenite, or selenate in uptake from the food and uptake in the RBC.     

Mode of in vitro interaction of mercuric mercury with selenite to form high-molecular weight substance in rabbit blood

Mode of interaction of mercuric mercury and selenite in rabbit blood was investigated in vitro. After the incubation of rabbit blood with 10^?5 M each of ²⁰³HgCl₂ and Na₂⁷⁵SeO₃, the amounts of both ²⁰³Hg and ⁷⁵Se incorporated into erythrocytes were markedly larger than the case where the blood was treated separately with one of these compounds. Most of ²⁰³Hg and ⁷⁵Se distributed into plasma and erythrocytes were found in high-molecular weight substance(s) (HMWS) fractionated by gel filtration at a molar ratio of 1:1. The ²⁰³Hg and ⁷⁵Se in HMWS found in plasma and erythrocytes were hardly diffusable through the erythrocytes membrane. The formation of the HMWS containing mercury and selenium was observed in stroma-free hemolysate incubated with mercuric chloride and selenite, but not in plasma. Addition of reduced glutathione (GSH) to the plasma, however, gave the HMWS as reaction products containing equimolar amounts of mercury and selenium. Further the binding properties of selenium to proteins were studied in the plasma incubated with selenodiglutathione (GSSeSG) or with selenite in the presence of GSH. The results indicated that GSH, a cellular component, is essential for the formation of an active selenium compound from selenite and that the interaction of mercuric mercury and selenite in plasma in the presence of GSH may occur through the other mechanism than the formation of GSSeSG.     

Selenium and Glutathione Peroxidase Levels in Lambs Receiving Feed Supplemented with Sodium Selenite or Selenomethionine

Twenty-one 6 months old female lambs were divided into 7 groups and fed a basal diet containing 0.13 mg Se/kg. The basal diet was further supplemented with 0, 0.1, 0.5 or 1.0 mg Se/kg either as sodium selenite or as selenomethionine, and was fed for 10 weeks. Both feed additives produced an increase in the selenium concentration in the tissues analysed. Significant correlations were found between the concentrations of selenomethionine or sodium selenite added to the feed and the subsequent tissue levels. However, the selenium levels seemed to plateau at approximately 0.5 mg Se/kg of supplemented sodium selenite. The total glutathione peroxidase (GSH-Px) activity of the tissues increased when the selenium supplementation increased from 0 to 0.1 mg/kg for both selenium compounds. With further increase in selenium supplementation the GSH-Px activity in the tissues plateaued except in the blood where the activity continued to rise with increasing selenomethionine supplementation. The selenium dependent GSH-Px activity in the liver rose with increasing selenomethionine supplementation, but approached a plateau when 0.1 mg Se/kg as sodium selenite was added to the feed. The selenium concentration in whole blood responded more rapidly to the selenium supplementation than did GSH-Px activity. The experiment indicates that the optimal selenium concentration in the feed is considerably higher than 0.1 mg Se/kg, and that selenium levels of 1.0 mg/kg in the feed do not result in any risk for the animals or the consumers of the products.     

NRT1.1B improves selenium concentrations in rice grains by facilitating selenomethinone translocation

Selenium (Se) is an essential trace element for humans and other animals, yet approximately one billion people worldwide suffer from Se deficiency. Rice is a staple food for over half of the world's population that is a major dietary source of Se. In paddy soils, rice roots mainly take up selenite. Se speciation analysis indicated that most of the selenite absorbed by rice is predominantly transformed into selenomethinone (SeMet) and retained in roots. However, the mechanism by which SeMet is transported in plants remains largely unknown. In this study, SeMet uptake was found to be an energy‐dependent symport process involving H⁺ transport, with neutral amino acids strongly inhibiting SeMet uptake. We further revealed that NRT1.1B, a member of rice peptide transporter (PTR) family which plays an important role in nitrate uptake and transport in rice, displays SeMet transport activity in yeast and Xenopus oocyte. The uptake rate of SeMet in the roots and its accumulation rate in the shoots of nrt1.1b mutant were significantly repressed. Conversely, the overexpression of NRT1.1B in rice significantly promoted SeMet translocation from roots to shoots, resulting in increased Se concentrations in shoots and rice grains. With vascular‐specific expression of NRT1.1B, the grain Se concentration was 1.83‐fold higher than that of wild type. These results strongly demonstrate that NRT1.1B holds great potential for the improvement of Se concentrations in grains by facilitating SeMet translocation, and the findings provide novel insight into breeding of Se‐enriched rice varieties.     

In Vitro Incorporation of Selenomethionine into Protein by Vigna radiata Polysomes

Vigna radiata polysomes efficiently incorporated [⁷⁵Se]selenomethionine, [¹⁴C]methionine, and [¹⁴C]leucine in vitro. The optimal conditions for translation were determined to be 4.8 millimolar Mg²⁺, 182 millimolar K⁺, and pH 7.4. The rates of incorporation of [⁷⁵Se]selenomethionine and [¹⁴C]methionine were similar when measured separately, but [⁷⁵Se]selenomethionine incorporation was 35% less than [¹⁴C]methionine incorporation when both amino acids were present in equal molar concentrations. Polyacrylamide gel electrophoresis of the hot trichloroacetic acid precipitable translation products demonstrated synthesis of high molecular weight labeled proteins in the presence of [⁷⁵Se]selenomethionine or [³⁵S]methionine. No major differences in molecular weights could be detected in the electrophoretic profiles. Utilization of selenomethionine during translation by Vigna radiata polysomes establishes a route for the assimilation of selenomethionine by plants susceptible to selenium toxicity.     

Effects of glutathione and of cysteine on intestinal absorption of selenium from selenite

The influence of glutathione (1 mmol/L) (GSH) on in vitro mucosal uptake and in vivo absorption of⁷⁵Se-labeled selenite (10 μmol/L) was investigated in rat jejunum. For comparison, the effect ofl-cysteine (1 mmol/L) on in vivo absorption of⁷⁵Se-labeled selenite was also studied. In the in vitro, uptake experiments, only the mucosal surface was exposed to the incubation medium for 3 min. For the in vivo experiments, a luminal perfusion technique was employed. GSH inhibited in vitro mucosal Se uptake, whereas absorption in vivo was stimulated by GSH.l-Cysteine also stimulated in vivo Se absorption, confirming former in vitro mucosal uptake experiments. Thus, unlikel-cysteine, GSH affected in vitro and in vivo absorption of Se from selenite differently. Enzymatic cleavage of products of the reaction of selenite with GSH occuring more efficiently under in vivo than in vitro conditions may be a prerequisite for the stimulatory effect of GSH on Se absorption. This apparently does not apply to the stimulatory effect of cysteine. Since, GSH occurs in the intestinal lumen under physiological conditions, it may contribute to the high bioavailability of Se from selenite.