Production of a Urinary Selenium Metabolite, Trimethylselenonium, by Thiopurine S-Methyltransferase and Indolethylamine N-Methyltransferase.

Selenium (Se) is an essential trace element in animals; however, the element can become highly toxic in excess amounts beyond the nutritional level. Although Se is mainly excreted into urine as a selenosugar within the nutritional level, excess amounts of Se are transformed as an alternative urinary metabolite, trimethylselenonium ion (TMSe). Se methylation appears to be an important metabolic process for the detoxification of excess Se; however, the biochemical mechanisms underlying the Se methylation have not been elucidated. In this study, we evaluated biochemical characteristics of two human methyltransferases for Se methylation, thiopurine S-methyltransferase (TPMT) and indolethylamine N-methyltransferase (INMT). The first methylation of Se, i.e., a nonmethylated to a monomethylated form, was specifically driven by TPMT, and INMT specifically mediated the third methylation, i.e., dimethylated to trimethylated form. The second methylation, i.e., a monomethylated to dimethylated form, was driven by either TPMT or INMT. Exogenous expression of TPMT, but not INMT, ameliorated the cytotoxicity of inorganic nonmethylated selenium salt, suggesting that only TPMT gave the cellular resistance against selenite exposure. TPMT was ubiquitously expressed in most mouse tissues and preferably expressed in the liver and kidneys, while INMT was specifically expressed in the lung and supplementally expressed in the liver and kidneys. Our results revealed that both TPMT and INMT cooperatively contributed to the TMSe production, enabling urinary excretion of Se and maintenance of homeostasis of this essential yet highly toxic trace element. Thus, TPMT and INMT can be recognized as selenium methyltransferases as a synonym.

Human metabolism and renal excretion of selenium compounds after oral ingestion of sodium selenate dependent on trimethylselenium ion (TMSe) status.

An in vivo metabolism study in humans was carried out to investigate the toxicokinetics and metabolism of sodium selenate differentiating by the trimethylselenium (TMSe) status. Therefore, the changes in blood plasma concentration and the urinary excretion within 24 h of seven healthy subjects after oral administration of a dietary supplement containing sodium selenate (50 µg selenium) were analyzed. Three subjects belong to the subgroup of TMSe eliminators, and four subjects were related to the non-TMSe eliminators subgroup. The concentrations of total selenium in blood plasma and urine samples were determined by inductively coupled plasma-mass spectrometry (ICP-MS). Additionally, speciation analysis of urine samples was performed using ICP-MS coupled to a liquid chromatography system. Plasma selenium concentration changed from 82.5 ± 12.5 µg Se/L before to 85.1 ± 12.0 µg Se/L 2-3 h after supplementation. Considering the individual 24-hour background amounts of renal excreted selenium, the ingestion caused an additional excretion of 15.4 ± 3.3 µg Se/24 h (≙31.1 ± 7.6 % of the administered dose) with a maximum elimination already 2 h after exposure. The differentiated analysis revealed that in all subjects, the main elimination product (30.1 ± 6.9 % of the administered dose) was unmetabolized selenate. TMSe was only detected in the urine of the TMSe eliminators. This subgroup excreted in comparison with the non-TMSe eliminators a significantly lower amount of selenate. Only one subject metabolized selenate to a larger portion to methyl-2-acetamido-2-deoxy-1-seleno-ß-D-galactopyranoside (SeSug1) and methyl-2-amino-2-deoxy-1-seleno-ß-D-galactopyranoside (SeSug3). All other subjects showed only a minor metabolism of selenate to selenium-containing carbohydrates. By individuals, which do not excrete TMSe in urine basically, selenate is metabolized only marginally and is excreted rapidly via urine generally. In contrast, a considerable portion of this inorganic selenium compound is metabolized by individuals, which eliminate TMSe basically. An elevated metabolism may also be provided by individuals, which eliminate high levels of selenium-containing carbohydrates basically. The difference in metabolism may imply a different disposition for pharmacological or toxic effects by exposure to inorganic selenium compounds.

Selenium metabolism to the trimethylselenonium ion (TMSe) varies markedly because of polymorphisms in the indolethylamine N-methyltransferase gene.

BACKGROUND: Selenium is an essential element, but its metabolism in humans is not well characterized. A few small studies indicate that the trimethylselenonium ion (TMSe) is a common selenium metabolite in humans. OBJECTIVE: This study aimed to elucidate the human metabolism of selenium to TMSe. DESIGN: Study individuals constituted subsamples of 2 cohorts: 1) pregnant women (n = 228) and their 5-y-old children (n = 205) in rural Bangladesh with poor selenium status [median urinary selenium (U-Se): 6.4 µg/L in mothers, 14 µg/L in children] and 2) women in the Argentinian Andes (n = 83) with adequate selenium status (median U-Se: 24 µg/L). Total U-Se and blood selenium were measured by inductively coupled plasma mass spectrometry (ICPMS), and urinary concentrations of TMSe were measured by high-performance liquid chromatography/vapor generation/ICPMS. A genomewide association study (GWAS) was performed for 1,629,299 (after filtration) single nucleotide polymorphisms (SNPs) in the Bangladeshi women (n = 72) by using Illumina Omni5M, and results were validated by using real-time polymerase chain reaction. RESULTS: TMSe "producers" were prevalent (approximately one-third) among the Bangladeshi women and their children, in whom TMSe constituted ∼10-70% of U-Se, whereas "nonproducers" had, on average, 0.59% TMSe. The TMSe-producing women had, on average, 2-µg U-Se/L higher concentrations than did the nonproducers. In contrast, only 3 of the 83 Andean women were TMSe producers (6-15% TMSe in the urine); the average percentage among the nonproducers was 0.35%. Comparison of the percentage of urinary TMSe in mothers and children indicated a strong genetic influence. The GWAS identified 3 SNPs in the indolethylamine N-methyltransferase gene (INMT) that were strongly associated with percentage of TMSe (P < 0.001, false-discovery rate corrected) in both cohorts. CONCLUSIONS: There are remarkable population and individual variations in the formation of TMSe, which could largely be explained by SNPs in INMT. The TMSe-producing women had higher U-Se concentrations than did nonproducers, but further elucidation of the metabolic pathways of selenium is essential for the understanding of its role in human health. The MINIMat trial was registered at isrctn.org as ISRCTN16581394.

Determination of selenium urinary metabolites by high temperature liquid chromatography-inductively coupled plasma mass spectrometry.

The coupling of high temperature liquid chromatography (HTLC) and inductively coupled plasma mass spectrometry (ICPMS) for the determination of selenium metabolites in urine samples is reported for the first time. In order to achieve "ICPMS-friendly" chromatographic conditions, the retention on a graphite stationary phase of the major selenium urinary metabolites using only plain water with 2% methanol as the mobile phase was investigated. Under the optimal conditions (T=80°C, Ql=1.2 mL min(-1)), methyl 2-acetamido-2-deoxy-1-seleno-ß-d-galactopyranoside (selenosugar 1), methyl 2-acetamido-2-deoxy-1-seleno-ß-d-glucosopyranoside (selenosugar 2) and trimethylselenonium ion were efficiently separated in less than 7 min, without any interferences due to other common selenium species (selenite, selenate, selenocystine and selenomethionine) or detectable effect of the urine matrix. The limits of detection were 0.3-0.5 ng Se mL(-1), and the precision of the analytical procedure was better than 3% (RSD%, n=5). The HTLC-ICPMS method was applied to the analysis of urine samples from two volunteers before and after ingestion of Brazil nuts or selenium supplements. The developed procedure proved to be adequate for the analytical task, providing results consistent with previous studies.

Efficient interface for online coupling of capillary electrophoresis with inductively coupled plasma-mass spectrometry and its application in simultaneous speciation analysis of arsenic and selenium.

A simple and highly efficient online system coupling of capillary electrophoresis to inductively coupled plasma-mass spectrometry (CE-ICP-MS) for simultaneous separation and determination of arsenic and selenium compounds was developed. CE was coupled to an ICP-MS system by a sprayer with a novel direct-injection high-efficiency nebulizer (DIHEN) chamber as the interface. By using this interface, six arsenic species, including arsenite (As(III), arsenate (As(V)), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB), and arsenocholine (AsC) and five selenium species (such as sodium selenite (Se(IV)), sodium selenate (Se(VI)), selenocysteine (SeCys), selenomethionine (SeMet), and Se-methylselenocysteine (MeSeCys)) were baseline-separated and determined in a single run within 9 min under the optimized conditions. Minimum dead volume, low and steady sheath flow liquid, high nebulization efficiency, and high sample transport efficiency were obtained by using this interface. Detection limits were in the range of 0.11-0.37 µg L(-1) for the six arsenic compounds (determined as (75)As at m/z 75) and 1.33-2.31 µg L(-1) for the five selenium species (determined as (82)Se at m/z 82). Repeatability expressed as the relative standard deviations (RSD, n = 6) of both migration time and peak area were better than 2.68% for arsenic compounds and 3.28% for selenium compounds, respectively. The proposed method had been successfully applied for the determination of arsenic and selenium species in the certified reference materials DORM-3, water, urine, and fish samples.

Metabolism of selenite to selenosugar and trimethylselenonium in vivo: tissue dependency and requirement for S-adenosylmethionine-dependent methylation.

Impaired S-adenosylmethionine (SAM)-dependent transmethylation and methylation capacity feature in diseases related to obesity or aging, and selenium (Se) metabolism is altered in these states. We tested the hypothesis that SAM metabolism is required for methylation and excretion of Se in a rat model. Four hours after selenite and periodate-oxidized adenosine (POA; an inhibitor of SAM metabolism) were administered, circulating markers of single-carbon status were unchanged, except for decreased circulating phosphatidylcholine (P<.05). In contrast, liver and kidney SAM and S-adenosylhomocysteine were elevated (P<.05 for all). Concentrations of total Se were significantly elevated in both liver (P<.001) and kidney (P<.01), however the degree of accumulation in liver was significantly greater than that of kidney (P<.05). Red blood cell Se levels were decreased (P=.01). Trimethylselenonium levels were decreased in liver and kidney (P=.001 for both tissues) and Se-methyl-N-acetylselenohexosamine selenosugar was decreased in liver (P=.001). Urinary output of both trimethylselenonium (P=.001) and selenosugar (P=.01) was decreased as well. Trimethylselenonium production is more inhibited by POA than is selenosugar production (P<.05). This work indicates that low molecular weight Se metabolism requires SAM-dependent methylation, and disrupting the conversion of SAM to S-adenosylhomocysteine prevents conversion of selenite and intermediate metabolites to final excretory forms, suggesting implications for selenium supplementation under conditions where transmethylation is suboptimal, such as in the case of obese or aging individuals.

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.

Rapid speciation and quantification of selenium compounds by HPLC-ICP MS using multiple standards labelled with different isotopes.

Speciation analysis using high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP MS) is now commonly used to investigate metabolic and toxicological aspects of some metals and metalloids. We have developed a rapid method for simultaneous identification and quantification of metabolites of selenium (Se) compounds using multiple standards labelled with different isotopes. A mixture of the labelled standards was spiked in a selenised garlic extract and the sample was subjected to speciation analysis by HPLC-ICP MS. The selenised garlic contains γ-glutamyl-methylselenocysteine, methylselenocysteine, and selenomethionine and the concentrations of those Se compounds were 723.8, 414.8, and 310.7 ng Se ml(-1), respectively. The isotopically labelled standards were also applied to the speciation of Se in rat urine. Selenate, methylselenonic acid, selenosugar, and trimethyselenium ions were found to be excreted by the present speciation procedure. Multiple standards labelled with different stable isotopes enable high-throughput identification and quantitative measurements of Se metabolites.

Simultaneous analysis of mercury and selenium species including chiral forms of selenomethionine in human urine and serum by HPLC column-switching coupled to ICP-MS.

The simultaneous speciation of elements is of great concern, especially in the study of the interactions of species in living organisms. Here we report a method based on the coupling of HPLC-ICP-MS that is capable of separating and analyzing different selenium and mercury species (Se-methylselenocysteine, selenite, selenate, L-selenomethionine, D-selenomethionine, methylmercury and inorganic mercury). The proposed method uses two different mobile phases that are suitable for selenium and mercury speciation and leads to a successful determination of all the species in less than 27 min with good efficiency and resolution. The method was efficiently applied for simultaneous speciation of mercury and selenium in urine and in serum, the latter from umbilical cord samples. Selenocystine has been successfully identified in the former sample. Detection limits obtained were between 0.30 and 2.46 ng. Recovery studies of samples spiked with all species were performed to check the reliability of the method, and satisfactory recoveries (93-110%) were obtained in all cases. The relative standard deviations (RSDs) for species with ten replicate determinations of 80 µg L(-1) were between 4.5 and 9.2%. The proposed method offers a deeper insight into selenium and mercury interactions in the human body.

Marked individual variability in the levels of trimethylselenonium ion in human urine determined by HPLC/ICPMS and HPLC/vapor generation/ICPMS.

Selenium species were determined using HPLC/ICPMS and HPLC/vapor generation/ICPMS in the urine from seven human volunteers investigated at background selenium concentrations and at slightly elevated concentrations after ingestion of 200 microg Se as a selenite supplement. Trimethylselenonium ion (TMSe) was present, together with selenosugars, in the urine samples, a result that dispels recent doubts about its possible previous misidentification with a cationic selenosugar. Although TMSe was present as only a trace metabolite in urine from five of the seven volunteers (0.02-0.28 microg Se/L, equivalent to 1-5% of the sum of selenosugars and TMSe), it was a significant metabolite (up to 4.6 microg Se/L, 22%) in one volunteer, and it was the major identified metabolite (up to 15 microg Se/L, 53%) in another volunteer. This marked individual variability in the formation of TMSe was maintained in a duplicate investigation of urine from the same seven volunteers.

Availability and metabolism of 77Se-methylseleninic acid compared simultaneously with those of three related selenocompounds.

Nutritional selenocompounds are considered to be transformed into the common intermediate selenide for utilization as selenoenzymes and/or for excretion as selenosugar and trimethylselenonium (TMSe). Therefore, selenocompounds can only be traced with a labeled selenium atom. Methylseleninic (MSA(IV)) has been proposed to be a third nutritional selenium source, the other two being inorganic selenocompounds and organic selenoamino acids, and to be a proximate selenochemical for producing the assumed biologically active form methylselenol. Here we applied a new tracer method to compare the availability and metabolism of MSA(IV) with those of three related selenocompounds under exactly identical host and tracing conditions. (82)Se-Selenite, (78)Se-selenate, (77)Se-MSA(IV) and (76)Se-methylselenonic acids (MSA(VI)) were simultaneously administered orally, each at the dose of 25 microg Se/kg body weight, to rats that had been depleted of endogenous natural abundance selenium with a single stable isotope ((80)Se). Time-related changes in the concentrations and/or distributions of the four labeled isotopes in the serum, liver, kidney, pancreas, lung and urine were determined simultaneously by inductively coupled argon plasma mass spectrometry (ICP MS) and/or HPLC-ICP MS. The availability with different isotope ratios was in the decreasing order of selenate>selenite=MSA(IV)>MSA(VI). Although selenate and MSA(VI) were distributed in organs and urine partly in their intact forms, MSA(IV) and selenite were not detected in the intact forms at all. MSA(IV) and MSA(VI) but not selenite or selenate produced TMSe in organs other than the liver, suggesting the transformation of MSA(IV) into methylselenol, and then either into selenide for the synthesis of selenoproteins and selenosugar or directly into TMSe. Thus, selenosugar and TMSe were produced widely in the organs. However, TMSe was not detected in the liver. The organ- and selenium source-specific production of TMSe was discussed as to the differences in selenium sources, and demethylation and methylation activity.

The determination of total Se in urine and serum by graphite furnace atomic absorption spectrometry using Ir as permanent modifier and in situ oxidation for complete trimethylselenonium recovery.

The present work evaluated the use of iridium (Ir) as permanent modifier for the determination of total selenium in urine and serum by graphite furnace atomic absorption spectrometry. Concerning urine, the presence of trimethylselenonium (TMSe(+)) was especially considered. Pyrolysis and atomization temperatures of 1,000 and 2,100 degrees C, respectively, were used. For nondigested urine and serum samples, 0.2% v/v HNO(3) and Triton X-100 were used as diluents, respectively, and the same initial platform Ir treatment was effective for up to 1,100 atomization cycles. Good precision [less than 5% relative standard deviation (RSD)] can be achieved with the proposed method. Low TMSe(+) recovery was observed for nondigested urine samples. Thus, if this species is to be considered in urine analysis, a previous external mineralization step was found to be necessary. Alternatively, an in situ oxidation treatment was developed. Detection limits of 8, 10, and 7 mug l(-1) were obtained after dilution, microwave-assisted digestion, and in situ oxidation procedures, respectively. The accuracy of the method was validated by the analysis of certified reference or commercial quality control materials and spiked samples.

Selenium metabolites in human urine after ingestion of selenite, L-selenomethionine, or DL-selenomethionine: a quantitative case study by HPLC/ICPMS.

To obtain quantitative information on human metabolism of selenium, we have performed selenium speciation analysis by HPLC/ICPMS on samples of human urine from one volunteer over a 48-hour period after ingestion of selenium (1.0 mg) as sodium selenite, L-selenomethionine, or DL-selenomethionine. The three separate experiments were performed in duplicate. Normal background urine from the volunteer contained total selenium concentrations of 8-30 microg Se/L (n=22) but, depending on the chromatographic conditions, only about 30-70% could be quantified by HPLC/ICPMS. The major species in background urine were two selenosugars, namely methyl-2-acetamido-2-deoxy-1-seleno-beta-D-galactopyranoside (selenosugar 1) and its deacylated analog methyl-2-amino-2-deoxy-1-seleno-beta-D-galactopyranoside (selenosugar 3). Selenium was rapidly excreted after ingestion of the selenium compounds: the peak concentrations (approximately 250-400 microg Se/L, normalized concentrations) were recorded within 5-9 hours, and concentrations had returned to close to background levels within 48 hours, by which time 25-40% of the ingested selenium, depending on the species ingested, had been accounted for in the urine. In all experiments, the major metabolite was selenosugar 1, constituting either approximately 80% of the total selenium excreted over the first 24 hours after ingestion of selenite or L-selenomethionine or approximately 65% after ingestion of DL-selenomethionine. Selenite was not present at significant levels (<1 microg Se/L) in any of the samples; selenomethionine was present in only trace amounts (approximately 1 microg/L, equivalent to less than 0.5% of the total Se) following ingestion of L-selenomethionine, but it constituted about 20% of the excreted selenium (first 24 hours) after ingestion of DL-selenomethionine, presumably because the D form was not efficiently metabolized. Trimethylselenonium ion, a commonly reported urine metabolite, could not be detected (<1 microg/L) in the urine samples after ingestion of selenite or selenomethionine. Cytotoxicity studies on selenosugar 1 and its glucosamine isomer (selenosugar 2, methyl-2-acetamido-2-deoxy-1-seleno-beta-D-glucosopyranoside) were performed with HepG2 cells derived from human hepatocarcinoma, and these showed that both compounds had low toxicity (about 1000-fold less toxic than sodium selenite). The results support earlier studies showing that selenosugar 1 is the major urinary metabolite after increased selenium intake, and they suggest that previously accepted pathways for human metabolism of selenium involving trimethylselenonium ion as the excretionary end product may need to be re-evaluated.

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 approximately 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.

Bioavailability of selenium from fish, yeast and selenate: a comparative study in humans using stable isotopes.

OBJECTIVE: To measure the bioavailability of selenium from cooked and raw fish in humans by estimating and comparing apparent absorption and retention of selenium in biosynthetically labelled fish with labelled selenate and biosynthetically labelled selenium in brewers yeast. DESIGN: The intervention study was a parallel, randomised, reference substance controlled design carried out at two different centres in Europe. SETTING: The human study was carried out at the Institute of Food Research, Norwich, UK and at TNO Nutrition and Food Research, Zeist, The Netherlands. SUBJECTS: In all, 35 male volunteers aged 18-50 y were recruited; 17 subjects were studied in Norwich (UK) and 18 in Zeist (Netherlands). All of the recruited subjects completed the study. INTERVENTIONS: Biosynthetically labelled trout fish (processed by two different methods), biosynthetically labelled brewers yeast and isotopically labelled selenate were used to estimate selenium apparent absorption and retention by quantitative analysis of stable isotope labels recovered in faeces and urine. Subjects consumed the labelled foods in four meals over two consecutive days and absorption was measured by the luminal disappearance method over 10 days. Urinary clearance of isotopic labels was measured over 7 days to enable retention to be calculated. RESULTS: Apparent absorption of selenium from fish was similar to selenate and there was no difference between the two processing methods used. However, retention of fish selenium was significantly higher than selenate (P<0.001). Apparent absorption and retention of yeast selenium was significantly different (P<0.001) from both fish selenium and selenate. CONCLUSION: Fish selenium is a highly bioavailable source of dietary selenium. Cooking did not affect selenium apparent absorption or retention from fish. Selenium from yeast is less bioavailable.

Quantitative significance of measuring trimethylselenonium in urine for assessing chronically high intakes of selenium in human subjects.

The purpose of the present study was to investigate the effects of Se restriction on the excretion of Se in men who had consumed high levels of this element during their entire lives. With the use of stable isotopes of Se as selenite, the excretion of methylated Se in urine was investigated in Chinese men (n 10) who had habitual chronic high intakes of this element. The relationship between either urine Se or trimethylselenonium (TMSe) to the estimated long-term Se intake was not linear over the entire range of intake, which was also true for the infusion of labelled selenite. A non-linear relationship was also found between urine TMSe and urine Se both for TMSe arising from catabolism of endogenous body Se and that from infused selenite. The data suggest a close precursor-product relationship of urine Se and its TMSe component based on the nearly identical specific activities for these two selenocompounds. Although dimethylselenide in breath was not measured in the present study, combining urinary TMSe with this breath test may be more useful in the assessment of long-term Se status.

Speciation of metabolites of selenate in rats by HPLC-ICP-MS.

The metabolic pathway for and metabolites of selenium (Se) administered intravenously to rats in the form of selenate at a dose of 0.3 mg Se kg-1 body weight were studied by speciating Se in the bloodstream, liver and urine by HPLC-inductively coupled argon plasma mass spectrometry. Selenate was not taken up by red blood cells (RBCs) and disappeared from the bloodstream much faster than selenite, without any change in its chemical form before it disappeared from the plasma. Selenium excreted into the urine after the administration of selenate showed different patterns from those of selenite in both amounts and chemical forms. With the selenate group, the concentration of Se in urine was highest at 0-6 h and the chemical species of Se was selenate at 0-6 h; thereafter a monomethylselenol-related Se compound (MMSe*) and trimethylselenonium ions (TMSe) appeared, selenate not being excreted after 6 h. On the other hand, in the selenite group, the concentration of Se peaked at 6-12 h, and the chemical species of Se were MMSe* and TMSe. Selenate was reduced in vitro on incubation in either a liver homogenate or supernatant fraction, although much more slowly than in the whole body. Selenate was not reduced by glutathione or dithiothreitol. The results suggest that in contrast to selenite, which is taken up by and reduced in RBCs, and then transferred to the liver, approximately 20% of the selenate administered to rats was excreted into the urine without any change in its chemical form with the present dose, and the major portion of selenate was taken up by the liver, reduced and then utilized for the synthesis of selenoproteins or excreted into the urine after being methylated.

Long-term supplementation with selenate and selenomethionine: urinary excretion by New Zealand women.

Thirty-six New Zealand women aged between 18 and 23 years received daily for 32 weeks, 200 micrograms Se as Se-enriched yeast (selenomethionine, SeMet), or brewer's yeast mixed with selenate, or no added Se (placebo) in a double-blind trial. Mean daily Se excretion increased with both supplements; the selenate group excreted more than the SeMet group, 123 v. 66 micrograms/d respectively at week 2, equivalent to 57 v. 27% of the dose. Thereafter Se output increased for the SeMet group reaching a plateau at about 100 micrograms/d at week 16, when plasma Se had also plateaued at 190 ng/ml. The selenate group had reached an earlier plateau of 110 ng Se/ml at week 7. There was a close relationship between 24 h urine and plasma Se for the SeMet group but not for the selenate group. Renal plasma clearances showed two distinctly different responses; the clearance of 0.4 ml/min reached by the SeMet group at week 2 plateaued as plasma Se increased almost 2-fold; whereas for the selenate group the clearance varied between 0.8 and 1.1 ml/min whilst plasma Se remained almost constant at 110 ng/ml. Previous studies, also of 200 micrograms Se/d as Se-rich bread, in New Zealand (NZ) and elsewhere showed similar responses to Se-yeast; the selenite response was intermediate between selenate and Se-yeast (SeMet). The full significance of these studies awaits identification of Se components in plasma, glomerular filtrate and urine; meanwhile renal clearances serve as a pointer to changes in the distribution of Se-containing fractions in the plasma. Trimethylselenonium was detected in basal urines, and was a minor component in urines of supplemented NZ subjects at about 1% of the total Se.

Effects of dose on the methylation of selenium to monomethylselenol and trimethylselenonium ion in rats.

Mechanisms and metabolic significance in rats of methylation to the reduced form of selenium (Se), i.e., selenide (Se2-), were studied by dose- and time-related experiments with injection of selenite. Urinary Se-metabolites were determined by HPLC using an inductively coupled argon plasma-mass spectrometer as an in-line detector (HPLC/ICP-MS method). Although only monomethylselenon (MMSe) has been detected in urine of normal rats even in those fed a Se-excess diet, the three types of Se-metabolites - MMSe, trimethylselenonium ion (TMSe), and inorganic Se, were detected in urine of Wistar rats injected with selenite (0, 0.1, 0.3, 0.5 and 1.0 mg Se/kg body weight) into the tail vein. The amount of the three Se-metabolites was plotted against the total urinary Se concentration and shown to change dose- and time-dependently. The monomethylated metabolite, i.e., MMSe, increased in urine rapidly at first and was slowly followed by linear dose-dependent excretion of the trimethylated metabolite, TMSe. The new methylation pathway of MMSe leading to TMSe was assumed to be induced or activated when the dose of Se exceeds the limit of the normal capacity for monomethylation. Progressive methylation reactions were suggested to be regulated enzymatically.