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.     

Selenium uptake by Butyrivibrio fibrisolvens and Bacteroides ruminicola

Abstract The uptake and incorporation of ⁷⁵[Se]selenite by Butyrivibrio fibrisolvens and Bacteroides ruminicola were by constitutive systems. Rates of uptake were higher in chemostat culture than in batch culture and there may be some inducible component. Uptake of [⁷⁵Se]selenite was distinct from sulphate or selenate transport, since sulphate and selenate did not inhibit selenite uptake, nor could sulphate or selenate uptake be demonstrated in these organisms. Selenite uptake in B. fibrisolvens had and apparent K _m of 1.74 mM and a V _max of 109 ng Se · min⁻¹· (mg protein)⁻¹. An apparent K _m of 1.76 mM and V _max of 1.5 μg Se · min⁻¹· (mg protein)⁻¹ was obtained for B. ruminicola . [⁷⁵Se]Selenite uptake by both organisms was partially sensitive to inhibition by 2,4-DNP. Uptake by B. fibrisolvens was also partially inhibited by azide and arsenate and in B. ruminicola it was partially inhibited by fluoride. CCCP, CPZ, DCCD or quinine did not inhibit uptake in either B. fibrisolvens or B. ruminicola . Selenite transport by both organisms was sensitive to IAA and NEM and was strongly inhibited by sulphite and nitrite. [⁷⁵Se]Selenite was converted to selenocystine, selenohomocystine and selenomethionine by B. fibrisolvens. B. ruminicola did not incorporate [⁷⁵Se]selenite into organic compounds, but did reduce it to red elemental selenium.     

Diffusibility of selenate, selenite, seleno-methionine, and seleno-cystine during simulated gastrointestinal digestion

The present study was undertaken to evaluate the in vitro availability of chemically varying forms of selenium (Se), supplemented in cow's milk. Two inorganic (selenite and selenate) and two organic (seleno-methionine [Se-Met] and seleno-cystine [Se-Cys]) Se sources were evaluated. The in vitro availability was estimated by the diffusibility of Se during simulated gastrointestinal digestion. First, the diffusibility was compared after adding a constant amount of Se as either selenate, selenite, seleno-methionine, or Se-Cys in milk samples. Se-Met and selenate were found to be significantly more diffusible than selenocystine and selenite under the simulated gastrointestinal conditions. The tendency for superior in vitro availability of selenate and Se-Met compared to selenite and Se-Cys was confirmed for a supplementation range of 5–40 ng/g of Se. This study suggests that the high diffusibility of selenate and Se-Met in a simulated gastrointestinal environment may contribute to their high absorption in vivo.     

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.     

Human urinary excretion and metabolism of (82)Se-enriched selenite and selenate determined by LC-ICP-MS

Urinary excretion of selenium after ingestion of isotope labeled selenite and selenate was studied in seven healthy volunteers, 4 men and 3 women (age 28-50 years). An aqueous solution containing 330 μL (82)Se-selenate (corresponding to 74.3 μg (82)Se) was given orally and urine samples were subsequently collected during the following 24 hours. The scheme was repeated four weeks later with a 280 μL (82)Se-selenite solution (corresponding to 74.4 μg (82)Se). The amount of total Se in the urine samples was determined by inductively coupled mass spectrometry. The mean total urinary excretion of (82)Se following (82)Se-selenate administration was 33.7% (range 15.6-42.5%) while the mean total excretion of (82)Se after (82)Se-selenite administration was 3.2% (range 2.8-3.9%) of the ingested amount. LC-ICPMS analysis of the urine samples showed that the majority of the selenium excreted after selenate ingestion was unchanged selenate for 6 of the individuals while one individual had metabolized a fraction (approx. 20%) of the selenate to selenosugar. Ingestion of 10 times larger doses of selenite in two individuals resulted in 13-23% excretion primarily excreted as selenosugar. These results show that the human metabolic pathways of selenite and selenate are different and indicate that not all selenate, although well absorbed, may be available for the beneficial health effects.     

Periodate-oxidized adenosine inhibits the formation of dimethylselenide and trimethylselenonium ion in mice treated with selenite

The metabolic detoxification of selenite and many other selenium compounds involves a series of S-adenosylmethionine-dependent methylations yielding dimethylselenide (DMSe), which is exhaled, and trimethylselenonium ion (TMSe), which is excreted in the urine. This paper shows that periodate-oxidized adenosine (Adox) inhibits these methylation reactions in vivo and increases the toxicity of selenite. When Adox was injected in mice at 100 mumol/kg 30 min before injection of [75Se]selenite at 0.4 mg Se/kg the appearances of [75Se]DMSe in the breath and [75Se]TMSe in the liver were completely inhibited for 90 min. This was mediated by accumulation of S-adenosylhomocysteine, the methyltransferase inhibitor, in the livers of Adox-treated mice due to inhibition of its hydrolase enzyme. During 24 h, Adox-treated mice excreted no detectable urinary [75Se]TMSe and exhaled only 20% as much [75Se]DMSe as controls. The urine of Adox-treated mice also contained S-adenosylhomocysteine at a level (ca. 4 mM), 200 times that of untreated mice, which provided a convenient index of methylation potential in the intact animal. When three groups of three mice each were injected with 100 mumol Adox/kg, selenite at 4 mg Se/kg, or a combination of the two, the mice receiving the combination were dead within 2 days, while the mice in the other two groups all survived at least 4 days. These results verify the enzymatic nature of selenium methylation in vivo, support its importance in detoxification, and indicate the value of Adox in further studies of selenium metabolism.     

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.     

Bioconcentration mechanism of selenium by a coccolithophorid, Emiliania huxleyi

We investigated the uptake and bioconcentration of the essential element selenium by a coccolithophorid, Emiliania huxleyi, using [75Se]selenite. The time course of 75Se uptake showed a biphasic pattern, namely a primary phase and a subsequent secondary phase. The primary and secondary phases are due to a rapid selenite uptake process that attained a stationary level within 2 min and a slow Se-accumulation process that continued at a constant rate for 4 h or longer, respectively. Kinetic analysis revealed that the selenite uptake process consists of two components, one saturable and one linearly related to substrate concentration. The Km of the saturable component was 29.8 nM selenite; the uptake activity of this component was suppressed by inhibitors of ATP biogenesis, suggesting that selenite uptake is driven by a high-affinity, active transport system. During a 6-h incubation of cells with [75Se]selenite, 70% of the intracellular 75Se was incorporated into low-molecular-mass compounds (LMCs), and 17% was incorporated into proteins, but [75Se]selenite was barely detectable. A pulse-chase experiment demonstrated that the 75Se that had accumulated in LMCs was transferred into proteins. When the syntheses of amino acids and proteins were each separately inhibited, 75Se incorporation into LMCs and proteins was decreased. These results suggest that E. huxleyi rapidly absorbs selenite, filling a small intracellular pool. Then, Se-containing LMCs are immediately synthesized from the selenite, creating a pool of LMCs that are then metabolized to selenoproteins.     

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.     

Uptake of selenate and selenite by isolated intestinal brush border membrane vesicles from pig,sheep, and rat

Selenate and selenite uptakes by isolated intestinal brush border membrane vesicles (BBMV) from pig, sheep, and rat were investigated. Selenate uptake into jejunal and ileal, but not duodenal, BBMV from pig was stimulated by an inwardly directed transmembrane Na⁺ gradient (Na _out ⁺ >Na _in ⁺ ). Selenate transport into rat ileal and sheep jejunal BBMV was also enhanced in the presence of a Na⁺ gradient. Unlike selenate uptake, selenite uptake was not Na⁺ dependent, neither in pig small intestine nor in sheep jejunum and rat ileum. Uptake of selenate represented real uptake into the vesicular lumen, whereas selenite uptake was a result of an extensive binding of⁷⁵Se to the membranes. Thiosulfate at a 250-fold concentration of selenate completely inhibited Na⁺-dependent selenate uptake into pig jejunal BBMV. Furthermore, Na⁺-dependent sulfate uptake was totally inhibited in the presence of a 250-fold selenate concentration. The results clearly show that selenate transport across the BBM of pig jejunum and ileum, sheep jejunum, and rat ileum is partially energized by a transmembrane Na⁺ gradient. Moreover, it is concluded from the results that there exists a common transport mechanism for sulfate and selenate in the BBM. The extensive binding of⁷⁵Se from⁷⁵Se-labeled selenite to the membranes could be from a spontaneous reaction of selenite with membrane-associated SH groups.     

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.     

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.     

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.     

Accumulation of selenium in a model freshwater microbial food web.

The transfer of selenium between bacteria and the ciliated protozoan, Paramecium putrinum, was examined in laboratory cultures. The population growth of the ciliate was not inhibited in the presence of the highest concentrations of dissolved selenite or selenate tested (10(3) micrograms liter-1). Experiments with radioactive 75selenite or 75selenate indicated that accumulation of selenium by ciliates through time was low when feeding and metabolism were reduced by incubating at 0 degrees C. However, selenium accumulated in ciliate biomass during incubation with dissolved 75Se and bacteria at 24 degrees C and also when bacteria prelabeled with 75Se were offered as food in the absence of dissolved selenium. When 75Se-labeled bacterial food was diluted by the addition of nonradioactive bacteria, the amount of selenite and selenate in ciliates decreased over time, indicating depuration by the ciliates. In longer-term (> 5-day) fed-batch incubations with 75selenite-labeled bacteria, the selenium concentration in ciliates equilibrated at approximately 1.4 micrograms of Se g (dry weight)-1. The selenium content of ciliates was similar to that of their bacterial food on a dry-weight basis. These data indicate that selenium uptake by this ciliate occurred primarily during feeding and that biomagnification of selenium did not occur in this simple food chain.     

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.     

Selenium Transport in Root Systems of Tomato

Selenate and selenite transport through tomato root systemswere followed for periods up to 4 h after removal of the planttops, using ⁷⁵Se as a tracer. With selenate, ⁷⁵Se concentrations in the xylem exudate were6 to 13 times higher than in the external solution, and chromatographicanalysis showed that the selenium was transported as inorganicselenate ( ). With selenite, ⁷⁵Se concentrations in the exudate were alwayslower than in the external solution. Analyses of exudate samplesshowed that negligible amounts of selenium were transportedas inorganic selenite ( except at very high external selenite concentrations (500 ?M), when up to 7 percent was transported as selenite. Most of the selenium transportin selenite-fed plants was as selenate or as an unknown seleniumcompound, the relative proportions of these two forms varyingboth with time and with external selenite concentration. Additionof a 5-fold excess of sulphate over selenite had no detectableeffect on the concentrations of selenate in the exudate, butcaused substantial decreases in the maximum concentrations ofboth total selenium (c. 47 per cent decrease) and the unknownselenium compound (c. 69 per cent decrease). Addition of a 5-foldexcess of sulphite decreased the concentration of the unknown(c. 39 per cent) but caused a large (2.7-fold) increase in themaximum total selenium concentration in the exudate and a 7.9-foldincrease in the maximum concentration of selenate. The resultssuggest metabolic involvement in the uptake and long distancetransport of solenium supplied as selenite, despite lower ⁷⁵Seconcentrations in the xylem exudate than in the external solution.An attempt is made to incorporate the new and existing informationinto a selenium transport model.     

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.     

Specific incorporation of selenium into lysine- and glutamate- accepting tRNAs from Escherichia coli

Amino acid transfer nucleic acids (tRNAs) that contain selenium-modified bases are synthesized by Escherichia coli in the presence of low levels (0.1-0.5 microM) of [75Se]selenite or [75Se]selenate. The amount of selenium incorporated (1-2 g atoms/100 mol of tRNA) was unchanged by 10-20-fold variations in selenium or sulfate concentrations or by the addition of 1 mM cysteine, sulfide, or sulfite. Specific incorporation of selenium (as opposed to nonspecific substitution for sulfur) was further indicated by the different reversed phase chromatographic elution patterns of 35S- and 75Se-labeled tRNAs isolated from cells labeled with 35SO2-4 and 75SeO2-4. Also, E. coli mutants unable to synthesize an abundant sulfur-modified base, 4-thiouracil, nevertheless produced normal levels of selenium-modified tRNAs. Two different methods of distinguishing between aminoacylated and nonaminoacylated tRNA, one which examined mobility during reversed phase chromatography and another which employed anti-AMP antibodies, indicated that over 50% of the selenium-containing tRNA had lysine or glutamate acceptor activity.     

In vitro synthesis of glutathione peroxidase from selenite. Translational incorporation of selenocysteine

The synthesis of glutathione peroxidase from [75Se]selenite was studied in slices and cell-free extracts from rat liver. The incorporation of [75Se]selenocysteine at the active site was detected by carboxymethylation and hydrolysis of partially purified glutathione peroxidase (glutathione:hydrogen peroxide oxidoreductase, EC 1.11.1.9) in the presence of [3H]selenocysteine and subsequent amino acid analysis. The synthesis of glutathione peroxidase in slices was inhibited by cycloheximide or puromycin and 75Se was incorporated from [75Se]selenite into free selenocysteine and selenocysteyl tRNA. Increasing concentrations of selenocystine caused a progressive dilution of the 75Se and a corresponding decrease in glutathione peroxidase labeling. In cell-free systems, [75Se]selenocysteyl tRNA was the best substrate for glutathione peroxidase synthesis. These results indicate the existence in rat liver of the de novo synthesis of free selenocysteine and a translational pathway of selenocysteine incorporation into glutathione peroxidase.     

Selenite uptake and incorporation by Selenomonas ruminatium

Selenite uptake and incorporation in Selenomonas ruminantium was constitutive with an inducible component. It was distinct from sulphate or selenate transport, since sulphate and selenate did not inhbit uptake, nor could sulphate or selenate uptake be demonstrated. Selenite uptake had an apparent K _m of 1.28 mM and a V _max of 148 ng Se min^-1 mg^-1 protein. Uptake was sensitive to inhibition by 2,4-dinitrophenol (DNP), carbonyl cyanide m-chlorophenyl hydrazone (CCCP), azide, iodoacetic acid (IAA) and N-ethylmaleimide (NEM), but not chloropromazine (CPZ), N,N-dicyclohexyl-carbodiimide (DCCD), quinine, arsenate, or fluoride. Treatment of cells accumulating ⁷⁵[Se]-Selenite with 2,4,DNP inhibited uptake, but did not cause efflux. Transport of selenite was inhibited by sulphite and nitrite, but not by nitrate, phosphate, sulphate of selenate. ⁷⁵[Se]-Selenite was incorporated into selenocystine, selenoethionine, selenohomocysteine, and selenomethionine and was also reduced to red elemental selenium.