Selenium distribution and metabolic profile in relation to nutritional selenium status in rats

Effect of administration route and dose on metabolism of nine bioselenocompounds

The nutritional availability of selenium (Se) is highly dependent on its chemical form because chemical form affects absorption, distribution, metabolism, and excretion. We evaluated the effects of administration route and dose on the bioavailability of nine Se compounds found in biota, the so-called bioselenocompounds, such as selenite, selenate, selenocyanate (SeCN), Se-methylselenocysteine (MeSeCys), selenomethionine (SeMet), selenohomolanthionine (SeHLan), selenocystine (SeCys2), 1β-methylseleno-N-acetyl-d-galactosamine (SeSug1), and trimethylselenonium ion (TMSe). We determined the bioavailability of bioselenocompounds recovered as urinary selenometabolites and serum selenoproteins from urine and serum of Se-deficient rats after the administration of bioselenocompounds by speciation analysis. Urinary Se was more easily recovered than serum selenoproteins, suggesting that the speciation of urinary Se is a better tool to indicate Se status in the body. The intravenous administration of bioselenocompounds showed different Se bioavailability from the oral administration. Intestinal microflora might be involved in the bioavailability of some bioselenocompounds, such as SeCN, MeSeCys, and SeSug1.

Structural characterization and antitumor activity of a novel Se-polysaccharide from selenium-enriched Cordyceps gunnii

Selenium (Se) has been recognized as an essential element.

Detoxification of selenite to form selenocyanate in mammalian cells

When human hepatoma HepG2 cells were exposed to sodium selenite, an unknown selenium metabolite was detected in the cytosolic fraction by HPLC-inductively coupled plasma mass spectrometry (ICP-MS). The unknown selenium metabolite was also detected in the mixture of HepG2 homogenate and sodium selenite in the presence of exogenous glutathione (GSH). The unknown selenium metabolite was identified as selenocyanate by electrospray ionization mass spectrometry (ESI-MS) and ESI quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS). Because exogenous cyanide increased the amount of selenocyanate in the mixture, selenocyanate seemed to be formed by the reaction between selenide or its equivalent, the product of the reduction of selenite, and endogenous cyanide. Rhodanase, an enzyme involved in thiocyanate synthesis, was not required for the formation of selenocyanate. Selenocyanate was less toxic to HepG2 cells than selenite or cyanide, suggesting that it was formed to reduce the toxicity of selenite. However, selenocyanate could be assimilated into selenoproteins and selenometabolites in rats in the same manner as selenite. Consequently, selenite was metabolized to selenocyanate to temporarily ameliorate its toxicity, and selenocyanate acted as an intrinsic selenium pool in cultured cells exposed to surplus selenite.

Methylated Metal(loid) Species in Humans

While the metal(loid)s arsenic, bismuth, and selenium (probably also tellurium) have been shown to be enzymatically methylated in the human body, this has not yet been demonstrated for antimony, cadmium, germanium, indium, lead, mercury, thallium, and tin, although the latter elements can be biomethylated in the environment. Methylated metal(loid)s exhibit increased mobility, thus leading to a more efficient metal(loid) transport within the body and, in particular, opening chances for passing membrane barriers (blood-brain barrier, placental barrier). As a consequence human health may be affected. In this review, relevant data from the literature are compiled, and are discussed with respect to the evaluation of assumed and proven health effects caused by alkylated metal(loid) species.

Selenium speciation analysis using inductively coupled plasma-mass spectrometry

Selenium exists in several oxidation states and a variety of inorganic and organic compounds, and the chemistry of selenium is complex in both the environment and living systems. Selenium is an essential element at trace levels and toxic at greater levels. Interest in speciation analysis for selenium has grown rapidly in this last decade, especially in the use of chromatographic separation coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Complete characterization of selenium compounds is necessary to understand selenium's significance in metabolic processes, clinical chemistry, biology, toxicology, nutrition and the environment. This review describes some of the essential background of selenium, and more importantly, some of the currently used separation methodologies, both chromatographic and electrophoretic, with emphasis on applications of selenium speciation analysis using ICP-MS detection.

Selenosugar and trimethylselenonium among urinary Se metabolites: dose- and age-related changes

Once selenium (Se) is absorbed by the body, it is excreted mostly into the urine and the major metabolite is 1beta-methylseleno-N-acetyl-d-galactosamine (selenosugar) within the required to low-toxic range. Selenosugar plateaus with a dose higher than 2.0 microg Se/ml water or g diet, and trimethylselenonium (TMSe) starts to increase, indicating that TMSe can be a biomarker of excessive and toxic doses of Se. Here, we show dose-related changes in the two urinary Se metabolites to clarify the relationship between the dose and urinary metabolites by feeding selenite to rats. It was also examined whether the metabolites are related to age, and further whether a possible exogenous source of the N-acetyl-d-galactosamine moiety, chondroitin 4-sulfate, affects the urinary metabolites. Selenite in drinking water was fed ad libitum to male Wistar rats of 36 and 5 weeks of age, and the concentrations of Se in the urine and organs were determined together with speciation of the urinary Se metabolites. In young rats, selenosugar was always the major urinary metabolite and TMSe increased with a dose higher than 2.0 microg Se/ml drinking water. On the other hand, in adult rats, TMSe increased only marginally despite that the rats suffered much more greatly from the Se toxicity, suggesting that TMSe cannot be a biomarker of Se toxicity. The results suggest that sources of the sugar moiety of selenosugar are more abundant in adult rats than in young rats. Chondroitin 4-sulfate did not affect the ratio of the two urinary metabolites, suggesting that the sugar source is of endogenous origin and that it increases with age.

Selenium Metabolites in Urine: A Critical Overview of Past Work and Current Status

Background: Selenium is an essential trace element that also elicits toxic effects at modest intakes. Investigations of selenium metabolites in urine can help our understanding of the transformations taking place in the body that produce these beneficial and detrimental effects. There is, however, considerable discord in the scientific literature regarding the selenium metabolites thought to play important roles in these biotransformation processes.

Approach: We critically assessed the published reports on selenium urinary metabolites, from the first report in 1969 to the present, in terms of the rigor of the data on which structures have been proposed.

Content: We present and discuss data from ∼60 publications reporting a total of 16 identified selenium metabolites in urine of humans or rats, a good model for human selenium metabolism. We assessed the analytical methods used and the validity of the ensuing structural assignments.

Summary: Many of the studies of selenium metabolites in urine appear to have assigned incorrect structures to the compounds. The long-held view that trimethylselenonium ion is a major human urinary metabolite appears unjustified. On the other hand, recent work describing selenosugars as major urinary metabolites looks sound and provides a firm basis for future studies.

Metabolic pathway for selenium in the body: speciation by HPLC-ICP MS with enriched Se

Selenium (Se) is an ultramicro essential nutrient and both inorganic (selenite and selenate) and organic (selenocysteine and selenomethionine) forms of Se can be used as nutritional sources. Metabolic pathways for Se in the body were studied for selenite and selenate, with the use of enriched 82Se, by speciation with separation by gel filtration HPLC and detection by element-specific mass spectrometry with ionization with inductively coupled argon plasma (HPLC-ICP MS). The concentrations of 82Se in organs and body fluids and the distributions of their constituents depending on the dose and time after the intravenous administration of 82Se-selenite and -selenate to rats were determined. Selenite was taken up by red blood cells within several minutes, reduced to selenide by glutathione, and then transported to the plasma, bound selectively to albumin and transferred to the liver. Contrary to selenite, intact selenate was either taken up directly by the liver or excreted into the urine. The 82Se of selenite origin and that of selenate origin were detected in the forms of the two Se peak materials in the liver, A and B. The former one was methylated to the latter in vivo and in vitro. The latter one was identical with the major urinary metabolite and it was identified as Se-methyl-N-acetyl-selenohexosamine (selenosugar). The chemical species-specific metabolic pathway for Se was explained by the metabolic regulation through selenide as the assumed common intermediate for the inorganic and organic Se sources and as the checkpoint metabolite between utilization for the selenoprotein synthesis and methylation for the excretion of Se.

Identification of a novel selenium metabolite, Se-methyl-N-acetylselenohexosamine, in rat urine by high-performance liquid chromatography--inductively coupled plasma mass spectrometry and--electrospray ionization tandem mass spectrometry

The major urinary metabolite of selenium (Se) in rats was identified by HPLC-inductively coupled argon plasma mass spectrometry (ICP-MS) and--electrospray tandem mass spectrometry (ESI-MS/MS). As the urine sample was rich in matrices such as sodium chloride and urea, it was partially purified to meet the requirements for ESI-MS. The group of signals corresponding to the Se isotope ratio was detected in both the positive and negative ion modes at m/z 300 ([M+H]+) and 358 ([M+CH3COO]-) for 80Se, respectively. These results suggested that the molecular mass of the Se metabolite was 299 Da for 80Se. The Se metabolite was deduced to contain one methylselenyl group, one acetyl group and at least two hydroxyl groups from the mass spectra of the fragment ions. The spectrum of the Se metabolite was completely identical to that of the synthetic selenosugar, 2-acetamide-1,2-dideoxy-beta-D-glucopyranosyl methylselenide. However, the chromatographic behavior of the Se metabolite was slightly different from that of the synthetic selenosugar. Thus, the major urinary Se metabolite was assigned as a diastereomer of a selenosugar, Se-methyl-N-acetyl-selenohexosamine.

Effects of dietary selenium species on Se concentrations in hair, blood, and urine

The effects of the chemical species and concentration of selenium (Se) in diets on the concentrations of Se in hair, blood serum, red blood cells (RBCs), and urine were studied to gain an insight into the toxicological and nutritional significance of different chemical forms of Se. Male Wistar rats were fed an Se-deficient diet (Se, less than 0.03 microgram/g) for 3 weeks, and then an Se-adequate (Se, 0.2 microgram/g) or Se-excess diet (Se, 2.0 micrograms/g), including seleno-L-methionine (SeMet) or selenite for up to 12 weeks. Hair, blood, and urine specimens were obtained every two weeks, and the concentrations of Se and its distribution in serum and urine on a size-exclusion column were determined. The concentrations of Se in hair, serum, and urine attained constant levels 2 weeks after a change of in the dietary Se concentration irrespective of the chemical species, the levels being dependent on the chemical species and the concentration. Specifically, in hair and serum, selenite gave the lowest constant levels irrespective of the dose, while SeMet resulted in higher levels than selenite in a dose-dependent manner. The two major selenoproteins in serum exhibited comparable concentrations. On the other hand, in urine, the concentration of Se was dependent on the dose but not on the chemical species. The results could be explained by regulated metabolism of selenite, and both nonregulated and regulated aspects of the metabolism of SeMet.

Binding of selenium (administered as selenite) to albumin after efflux from red blood cells

The role of albumin in the metabolism of inorganic selenium (Se) was studied in vivo and in vitro using a HPLC-ICP-MS method. Although Se injected in the form of selenite binds selectively to albumin after being reduced to selenide and then being effluxed into the plasma, Se was shown to be metabolized normally in the absence of albumin. The reduced form of Se, selenide, bound selectively to albumin but only to a percentage of it. The thiol group and the intermolecular disulfide group at the 34th cysteinyl residue of albumin were not responsible for the selective binding of Se to albumin. Selenide was suggested to be bound to a disulfide not a thiol group, i.e., to one of the 17 disulfide bonds in a conformationally different isoform of albumin.

Recent applications of high-performance liquid chromatography to the analysis of metal complexes

Interest in metal complexes in modern inorganic chemistry has resulted in increasing demands for the analysis of these compounds. This paper reviews the most recent applications of high-performance liquid chromatography (HPLC) to the analysis of metal complexes. The review centres on the use of the technique in metal complex syntheses, reactions, characterizations and complexations and retention behaviour of these compounds, as reported in the literature since 1994.

Metabolism of selenite labelled with enriched stable isotope in the bloodstream

The metabolism of selenium (Se) in the bloodstream of rats was studied using HPLC-ICP-MS with an enriched Se stable isotope, and the results were used as Se-specific indicators for Se nutritional status. Concentration of endogenous Se in plasma depended on dietary Se, while changes in concentrations and distributions of exogenous Se revealed its metabolic pathway. Namely, selenite was taken up by red blood cells and reduced to selenide, and then reappeared in plasma in a form bound selectively to albumin within 10 min, disappeared from plasma again within 30 min after injection. Then, the concentration of labelled Se started to increase slowly as selenoprotein P and extracellular glutathione peroxidase, and attained a maximum level at about 6 h after injection. The isotope ratio of endogenous to exogenous Se concentrations in plasma after 48 h post-injection was proposed to represent the Se-specific indicator in plasma reflecting the nutritional status of Se.

Detoxification of mercury by selenium by binding of equimolar Hg-Se complex to a specific plasma protein

Toxicity of mercury (Hg) can be reduced by coadministration with selenium (Se), and this has been explained by the formation of a complex between a specific plasma protein and the two elements, which are bound to the protein at an equimolar ratio. The purpose of the present study was to characterize the specific binding protein in order to clarify the detoxification mechanism. The coadministration of 82Se-enriched selenite and mercuric chloride into a rat produced a 82Se- and Hg-binding peak on a gel filtration column as measured by high-performance liquid chromatography with detection by inductively coupled argon plasma-mass spectrometry (ICP-MS). The specific binding protein was also detected in vitro by incubating 82Se-enriched selenite and mercuric chloride in serum in the presence of glutathione. The molar ratio of Se/Hg = 1 was maintained in binding not only to the specific protein but also to other proteins under any condition. In in vitro experiments, it was shown that although the two elements could bind to many plasma proteins, the affinity to the specific protein was extremely high and it showed a binding capacity of 500 nmol Hg or Se/the specific protein in 1 ml of serum. These results suggest that the two elements form an equimolar complex at first and then bind specifically to the protein. Further, the binding of the two elements to the protein was inhibited by the addition of polylysine to the reaction mixture, suggesting that the two elements interact with the protein through basic amino acids in the molecule and also that the protein may be one of the heparin-binding proteins since the heparin-binding sites mainly consist of basic amino acids.

Equimolar Hg-Se complex binds to selenoprotein P

Binding of equimolar mercury (Hg) and selenium (Se) to a specific plasma protein in the detoxification of Hg was studied in vitro by the HPLC/inductively coupled argon plasma-mass spectrometry (ICP-MS) method with use of an enriched stable isotope. Hg and 82Se became co-eluted with endogenous 78Se on a size exclusion column by incubation of 0-200 microM HgCl2 and 82Se-enriched selenite with rat serum in the presence of glutathione at 37 degrees C for 10 min. The endogenous 78Se peak was the most abundant plasma Se-containing protein, and it showed the affinity to heparin, indicating it to be selenoprotein P (Sel P). The 82Se/endogenous Se ratio of (Hg-Se)-Sel P complex changed with doses of HgCl2 and 82Se-enriched selenite and amounted to more than 100, suggesting that more than 1,000 units of (Hg-Se) bind to Sel P based on the fact that there are 10 selenocysteinyl residues per Sel P. These results indicate that equimolar Hg and Se bind to Sel P to form the {(Hg-Se)n}m-Sel P complex, where n is the number of Hg-Se complexes and m the number of binding sites in Sel P.

Simultaneous speciation of endogenous and exogenous elements by HPLC/ICP-MS with enriched stable isotopes

High performance liquid chromatography (HPLC)/ inductively coupled argon plasma-mass spectrometry (ICP-MS) was introduced to investigate the distributions of selenium (Se) in biological fluids. The method was to determine both the natural abundance of Se and an enriched stable isotope of Se used as a tracer. The distributions of Se in plasma and in urine specimens were determined in Wistar rats on various Se diets with and without an intravenous injection of 82Se-selenite. Although the distribution of natural abundance Se (endogenous Se) in the plasma was affected little by the nutritional status of Se, that in the urine gave a Se peak depending on the nutritional status of Se, and the peak was identified as methylselenol. When 82Se-selenite was injected in excess into rats given three different Se diets (Se-deficient, Se-adequate, Se-excessive), three Se peaks occurred in the HPLC chromatogram of the urine samples, corresponding to selenite, methylselenol and trimethylselenonium ion in the order of elution, and the intensities of the tracer peaks reflected the nutritional status. These results indicate that the HPLC/ICP-MS method is a powerful analytical tool for specifying Se-containing biological constituents, both natural abundance and enriched stable isotopes. Methylselenol in urine is proposed to be a sensitive and Se-specific biological indicator for diagnosing the nutritional status of Se. Furthermore, it was shown that an enriched stable isotope such as 82Se-selenite was shown to be used for the same purpose, and that 82Se-methylselenol and 82Se-trimethylselenonium ion in urine were more sensitive indicators of the Se status of the rats.