Study of selenium species distribution in biological tissues by size exclusion and ion exchange chromatagraphy inductively coupled plasma–mass spectrometry

Simultaneous speciation analysis of Hg and Se in fish by high-performance liquid chromatography and inductively coupled plasma-mass spectrometry following microwave-assisted enzymatic hydrolysis

This study reports the development and validation of a new analytical method for simultaneous speciation analysis of Se and Hg in fish muscle. For this purpose, four Se species (selenite/Se(IV), selenate/Se(VI), selenomethionine/SeMet, and selenocysteine/SeCys) and two Hg species (inorganic mercury/iHg and methylmercury/MeHg) were extracted simultaneously by microwave-assisted enzymatic hydrolysis and then separated by HPLC in less than 15 min by using a column with both anion and cation exchange mechanisms and a mobile phase consisting of a mixture of methanol 5% (v/v), 45 mM HNO3, 0.015% 2-mercaptoethanol, and 1.5 mM sodium 3-mercapto-1-propanesulfonate. The separated species of Hg and Se were detected online by inductively coupled plasma-mass spectrometry (ICP-MS). The speciation analysis method was validated by means of the accuracy profile approach by carrying out three series of measurements in duplicate on three different days over a time-span of 3 weeks. The limits of quantification (LOQ) are in the range of 0.010-0.013 mg/kg wet weight (ww) for all selenium species, except for Se(IV) (0.15 mg/kg ww), while the coefficient of variation in terms of intermediate reproducibility (CVR) was < 7%. The LOQ for MeHg was 0.006 mg/kg ww, while the CVR was 3%. The method was successfully applied to the analysis of muscle samples from four different fish species: rainbow trout, tuna, swordfish, and dogfish.

A Review of Plant Selenium-Enriched Proteins/Peptides: Extraction, Detection, Bioavailability, and Effects of Processing

As an essential trace element in the human body, selenium (Se) has various physiological activities, such as antioxidant and anticancer activity. Selenium-enriched proteins/peptides (SePs/SePPs) are the primary forms of Se in plants and animals, and they are the vital carriers of its physiological activities. On the basis of current research, this review systematically describes the extraction methods (aqueous, alkaline, enzymatic, auxiliary, etc.) and detection methods (HPLC-MS/MS, GC-ICP-MS, etc.) for SePs/SePPs in plants. Their bioavailability and bioactivity, and the effect of processing are also included. Our review provides a comprehensive understanding and theoretical guidance for the utilization of selenium-enriched proteins/peptides.

Trimethylselenonium ion determination in human urine by high-performance liquid chromatography-hydride generation-atomic fluorescence spectrometry optimization of the hydride generation step

This work describes the intricacies of the determination of the trimethylselenonium ion (TMSe) in human urine via high-performance liquid chromatography-hydride generation-atomic fluorescence spectrometry (HPLC-HG-AFS). By definition, this technique requires that the separated TMSe can be online converted into a volatile compound. Literature data for the determination of TMSe via the hydride generation technique are contradictory; i.e., some authors claim that direct formation of volatile compounds is possible under reduction with NaBH₄, whereas others reported that a digestion step is mandatory prior to conversion. We studied and optimized the conditions for online conversion by varying the mobile phase composition (pyridine, phosphate, and acetate), testing different reaction coils, and optimizing the hydride generation conditions, although technically no hydride (H₂Se) is formed but a dimethylselenide (DMSe). The optimized conditions were used for the analysis of 64 urine samples of 16 (unexposed) volunteers and the determination of low amounts of TMSe (LOD = 0.2 ng mL^-1). Total (specific gravity-corrected) selenium concentrations in the urine samples ranged from 7.9 ± 0.7 to 29.7 ± 5.0 ng mL^-1 for individual volunteers. Four volunteers were characterized as TMSe producers (hINMT genotype GA) and 12 were non-producers (hINMT genotype GG). Urine of TMSe producers contained 2.5 ± 1.7 ng mL^-1 of TMSe, compared to 0.2 ± 0.2 ng mL^-1 for non-producers.

Characterization of Selenium Species in the Shijimi Clam

Selenium is an essential trace element for humans and animals. Fish and shellfish are known to be rich in selenium and suppose to be an effective selenium source. In this study, we characterized the selenium species in the Shijimi clam (Corbicula japonica), which is a typical clam eaten in Japan. The Shijimi clam contains a relatively high concentration of selenium (3.5 µg-selenium/g-dry Shijimi). Approximately 30% of the total selenium in the Shijimi clam meat was extractable with water, while selenium in the Shijimi clam was hardly extracted with ethanol, chloroform and hexane. Based on an ultrafiltration study, the molecular mass of the major selenium species in the Shijimi water-extract was estimated to be less than 5000. Because amphoteric selenium species were contained in the Shijimi water-extract, which was indicated by ion-exchange chromatographic separation, an ion-pair reagent was utilized to extract the ionic selenium species into an organic solvent. A matrix assisted laser desorption ionization (MALDI) time of flight (TOF)-mass spectrometric analysis revealed the selenium isotopic pattern involving one selenium atom in a molecule with the ⁸⁰Se molecular ion peak at m/z 534. This selenium species was mainly found in the visceral part of the Shijimi clam by imaging mass spectrometry.

Influence of Dietary Selenium Species on Selenoamino Acid Levels in Rainbow Trout

Two forms of selenium (Se) supplementation of fish feeds were compared in two different basal diets. A 12-week feeding trial was performed with rainbow trout fry using either a plant-based or a fish meal-based diet. Se yeast and selenite were used for Se supplementation. Total Se and Se speciation were determined in both diets and whole body of trout fry using inductively coupled plasma mass spectrometry (ICP MS) and high-performance liquid chromatography (HPLC). The two selenoamino acids, selenomethionine (SeMet) and selenocysteine (SeCys), were determined in whole body of fry after enzymatic digestion using protease type XIV with a prior derivatization step in the case of SeCys. The plant-based basal diet was found to have a much lower total Se than the fish meal-based basal diet with concentrations of 496 and 1222 μg(Se) kg(-1), respectively. Dietary Se yeast had a higher ability to raise whole body Se compared to selenite. SeMet concentration in the fry was increased only in the case of Se yeast supplementation, whereas SeCys levels were similar at the end of the feeding trial for both Se supplemented forms. The results show that the fate of dietary Se in fry is highly dependent on the form brought through supplementation and that a plant-based diet clearly benefits from Se supplementation.

Simultaneous quantitation and identification of organic and inorganic selenium in diet supplements by liquid chromatography with tandem mass spectrometry

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for selenium speciation in dietary supplements. Chromatographic separation was performed on a TSK-Gel ODS-100V column using a mixture of 5mM ammonium acetate water solution and methanol as a mobile phase. Conditions chosen for this process allowed to separate all investigated chemical compounds of selenium: seleno-l-methionine, methyl-seleno-l-cysteine, l-selenocystine, methaneseleninic acid, selenite and selenate. A tandem mass spectrometer with an ion trap operating in negative or positive ion mode according to the selenium form being determined was used as a detector. Three extraction procedures: water extraction, enzymatic hydrolysis and sequential extraction were used for preparation of samples for the determination of the actual forms of selenium in diet supplements. The developed method was used for analysis of six dietary supplements containing selenium bought in a pharmacy and supermarket. Apart from speciation analysis of selenium content in supplements total selenium content was determined using instrumental neutron activation analysis (INAA). All expected forms of selenium except for selenite were determined using LC-MS/MS technique. It should be stressed that amounts of selenate were smaller than expected.

Characterization of selenium incorporation into wheat proteins by two-dimensional gel electrophoresis-laser ablation ICP MS followed by capillary HPLC-ICP MS and electrospray linear trap quadrupole Orbitrap MS

A method has been developed for a rapid and precise location of selenium-containing proteins in large two-dimensional (2D) electrophoresis gels. A sample was divided into four aliquots which were analyzed in parallel by 1D isoelectric focusing electrophoresis (IEF)-laser ablation (LA) inductively coupled plasma mass spectrometry (ICP MS), 1D sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE)-LA ICP MS, and, in duplicate, by 2D IEF-PAGE. On the basis of the 1 D electropherograms obtained, areas supposed to contain the largest concentrations of Se were subjected to LA ICP MS imaging to locate precisely the position of Se-containing proteins which were then identified in the parallel 2D gel by electrospray Orbitrap MS/MS. The method was applied to the identification and semiquantitative determination of selenium storage proteins in wheat. MS evidence is presented for the Se-S substitution in plants not only in methionine but also in cysteine.

Selenium species and their distribution in freshwater fish from Argentina

The distribution and speciation of selenium (Se) in freshwater fish (muscle and liver tissue) from lakes in Argentina was investigated. Three introduced species, brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis), and one native species, creole perch (Percichthys trucha), were investigated. Values for total selenium in muscle ranged from 0.66 to 1.61 μg/g, while in the liver, concentrations were much higher, from 4.46 to 73.71 μg/g on a dry matter basis. Separation of soluble Se species (SeCys(2), selenomethionine (SeMet), SeMeSeCys, selenite and selenate) was achieved by ion exchange chromatography and detection was performed by inductively coupled plasma-mass spectrometry. The results showed that in fish muscle, from 47 to 55 % of selenium was soluble and the only Se species identified was SeMet, which represented around 80 % of soluble Se, while in the liver, the amount of soluble Se ranged from 61 to 76 % and the percentage of species identified (SeMet and SeCys(2)) was much lower and ranged from 8 to 17 % of soluble Se.

Identification of selenosugars and other low-molecular weight selenium metabolites in high-selenium cereal crops

Several novel selenium containing compounds were characterized in staple crops (wheat, rice and maize) grown on soils naturally rich in selenium. A dedicated method based on the coupling of liquid chromatography with multiplexed detection (ICP-MS, ESI-Orbitrap MS(/MS)) was developed for the speciation of low-molecular weight (<5 kDa) selenium metabolites. Nine species present in different proportions as a function of the crop type were identified by cation-exchange HPLC-ESI-Orbitrap MS on the basis of the accurate molecular mass and MS/MS spectra. The natural origin of these species was then validated by varying extraction conditions and by using hydrophilic interaction LC (HILIC)-ESI-Orbitrap MS(/MS). Among the identified compounds, Se-containing monosaccharides (hexose moiety, m/z 317 and m/z 358) or Se-containing disaccharides (hexose-pentose moiety, m/z 407 and m/z 408) were the first selenosugars reported in edible plants. It is also the first report of the presence of 2,3-dihydroxypropionyl-selenolanthionine (m/z 345) in rice. Because these crops can be an important source of selenium in animal and human nutrition, the understanding of the origin and the fate of these species during metabolic processes will be of great interest.

Selenium speciation in paired serum and cerebrospinal fluid samples

Se speciation was performed in 24 individual paired serum and cerebrospinal fluid (CSF) samples from neurologically healthy persons. Strong anion exchange (SAX) separation, coupled to inductively coupled plasma-dynamic reaction cell-mass spectrometry (ICP-DRC-MS), was employed. Species identification was done by standard matched retention time, standard addition and by size exclusion chromatography followed from SAX (2-D SEC-SAX-ICP-DRC-MS) and by SAX followed from CE-ICP-DRC-MS (2-D SAX-CE-ICP-DRC-MS). Limit of detection (LoD, 3×standard deviation (SD) of noise) was in the range of 0.026-0.031 μg/L for all investigated species and thus was set uniformly to 0.032 μg/L. Quality control for total Se determination was performed by analysing control materials "human serum" and "urine", where determined values met target values. Several Se species were found in both sample types having following median values (sequence: serum/CSF, each in μg Se/L): total Se, 58.39/0.86; selenoprotein P (SePP), 5.19/0.47; Se-methionine (SeM), 0.23/<LoD; glutathione peroxidase (GPx), 4.2/0.036; thioredoxinreductase (TrxR), 1.64/0.035; Se IV, 12.25/0.046; Se-human serum albumin (Se-HSA), 18.03/0.068. Other Se species, such as Se-cystine (SeC), Se VI and up to four non-identified compounds were monitored (if ever) only in very few samples usually close to LoD. Therefore, their median values were <LoD. Linear relationships based on median values provide information about Se-species passage across neural barriers (NB): SePP(-serum) is significantly correlated to total Se(-serum) when the latter was > 65 μg/L; however, SePP(-CSF) appeared independent of SePP(-serum). For Se-HSA(-serum) versus (vs.) Se-HSA(-CSF), a weak linear relationship was found (r(2)=0.1722). On the contrary, for anti-oxidative Se-enzymes, higher r (2) values were calculated: GPx(-serum) vs. GPx(-CSF), r(2)=0.3837; TrxR(-serum) vs. TrxR(-CSF), r(2)=0.6293. Q(-Se-species) values (= ratios of CSF(-Se-species)/serum(-Se-species)) were compared with the Q (-Alb) value (HSA(-CSF)/HSA(-serum)=clinical index of NB integrity) for deeper information about NB passage of Se species. The Q (-Se-HSA) value (3.8×10(-3)) was in accordance to the molecular mass dependent restriction at NB (Q(-Alb) at 5.25×10(-3)). Increased Q values were seen for TrxR (21.3×10(-3)) and GPx (8.3×10(-3)) which are not (completely) explained by molecular size. For these two anti-oxidative Se-enzymes (GPx, TrxR), we hypothesize that there might be either a facilitated diffusion across NB or they might be additionally synthesized in the brain.

Changes in selenium speciation associated with increasing tissue concentrations of selenium in wheat grain

Wheat (Triticum aestivum) collected in the Nawanshahr-Hoshiarpur Region (Punjab, India) showed the highest selenium concentrations ever recorded in cereal grains (29-185 microg g(-1)). There was a strong positive relationship between the selenium content in shoots and that in kernels, showing that grain selenium concentration can be predicted from that in the vegetative tissues of the plant. The identity and content of the selenocompounds in the grain samples and in wheat-based reference materials were investigated by HPLC-ICP-dynamic reaction cell-MS. Reversed-phase, cation exchange, and anion exchange HPLC were used to separate the selenium species after ultrasound-assisted enzymatic extraction with an ultrasonic probe. Selenomethionine and selenate accounted for 72-85% and 2-6% of the sum of the selenium species, respectively. The proportion of organic Se species varied with increasing Se content; namely, SeMet showed a relative reduction whereas the other organoselenium compounds increased up to 18-22% of the total chromatographed selenium. Se-methyl-selenocysteine was detected as a minor compound (0.2-0.5%) in high-Se wheat by both reversed-phase and cation exchange HPLC using retention time matching with the standard substance spiked to the sample extracts. Regular consumption of locally produced wheat-based food items may lead the population of the study area to an excessive intake of selenium. On the other hand, the large predominance of selenomethionine shows that local wheat can be a promising raw material for naturally enriched products to be used to supplement human and animal diets in low selenium areas.

Formation, Occurrence, Significance, and Analysis of Organoselenium and Organotellurium Compounds in the Environment

Among all environmentally-relevant trace elements, selenium has one of the most diverse organic chemistries. It is also one of the few trace elements that may biomagnify in food chains under certain conditions. Yet, the exact chemical forms of selenium involved in the uptake into organisms and transfer to higher trophic levels, as well as the biochemical mechanisms that lead to their subsequent metabolism in organisms, are still not well understood. This is in part due to the analytical challenges associated with measuring the myriad of discrete Se species occurring in organisms. While there are generalized concepts of selenium metabolism, there is a lack of conclusive analytical evidence supporting the existence of many postulated intermediates. Likewise, there is a disconnect between the major selenium species encountered in abiotic compartments (waters, soils, and sediment), and those found in organisms, which renders the qualitative and quantitative description of the bioaccumulation process uncertain. Here, we summarize the knowledge on important selenium and tellurium species in all environmental compartments, and identify gaps and uncertainties in the existing body of knowledge, with emphasis on problems associated with past and current analytical methodology.

A method for low volume and low Se concentration samples and application to paired cerebrospinal fluid and serum samples

The well-known beneficial health effects of Se have demanded the development of rapid and accurate methods for its analysis. A flow injection (FI) method with inductively coupled plasma mass spectrometry (ICP-MS) as a selenium-selective detector was optimized. Flow injection was carried out using a Knauer 1100 smartline inert series liquid chromatograph coupled with a Perkin Elmer DRC II ICP-mass spectrometer. For sample injection a Perkin Elmer electronic valve equipped with a 25microL sample loop was employed. Before measurement, standards or samples were administered with 1microg/L rhodium as internal standard for correction of changes in detector response according to changes in sample electrolyte concentration. The method characterization parameters are: LOD (3sigma criterion): 26ng/L, LOQ (10sigma criterion): 86ng/L, linearity: 0.05->10microg/L, r(2)=0.9999, serial or day-to-day precision at 2microg/L: 4.48% or 5.6%. Accuracy was determined by (a) recovery experiments (CSF spiked with 2microg/L Se); (b) comparison of FI-ICP-MS measurement with graphite furnace atomic absorption (GFAAS) measurements of 1:10 diluted serum samples; (c) Se determination in urine and serum control materials. Recovery (a) was 101.4%, measurement comparison with GFAAS (b) showed 98.8% (5 serum samples, 1:10 diluted in the range of 0.5-1.3microg/L, compared to GFAAS determination, which was set to 100%), and accuracy was 96.8% or 105.6% for the serum or urine control material. Analysis time per sample was short and typically below 2min for the complete measurement, including sample introduction, sample-line purge and quadruplicate Se determination. This method was used to determine Se in cerebrospinal fluid (CSF) and plasma (here parallel to GFAAS) in 35 paired serum and CSF samples. Se determination gave values in the range of 42-130microg/L for serum and 1.63-6.66microg/L for CSF. The median for Se in 35 individual CSF samples was 3.28microg/L, the mean (+/-SD) was 3.67 (1.35)microg/L, whilst for individual serum samples the median was 81microg/L and the mean (+/-SD) was 85 (26)microg/L. When relating the paired Se concentrations of CSF samples to respective serum samples it turned out that Se-CSF (behind blood brain barrier (BBB)) is independent on Se-serum concentration (before BBB).

Organic and inorganic selenium speciation in environmental and biological samples by nanometer-sized materials packed dual-column separation/preconcentration on-line coupled with ICP-MS

A novel, fast, and cheap nonchromatographic method for direct speciation of dissolved inorganic and organic selenium species in environmental and biological samples was developed by flow injection (FI) dual-column preconcentration/separation on-line coupled with ICP-MS determination. In the developed technique, the first column packed with nanometer-sized Al(2)O(3) could selectively adsorb the inorganic selenium [Se(IV), Se(VI)], and the retained inorganic selenium could be eluted by 0.2 mol l(-1) NaOH, while the organic Se [selenocystine (SeCys(2)) and selenomethionine (Se-Met)] was not retained. On the other hand, the second column packed with mesoporous TiO(2) chemically modified by dimercaptosuccinic acid (DMSA) could selectively adsorb Se(IV) and SeCys(2) and barely adsorb Se(VI) and Se-Met. When the sample solution was passed through the column 1, separation of inorganic selenium and organic selenium could be achieved first. Then, the effluent from column 1 was successively introduced into the column 2 and the speciation of organic selenium could be attained due to the different adsorption behaviors of Se-Met and SeCys(2) on DMSA modified TiO(2). After that, the eluent from column 1 contained Se(IV), and Se(VI) was adjusted to desired pH and injected into column 2, and the speciation of Se(IV) and Se(VI) could also be realized thanks to their different retention on column 2. The parameters affecting the separation were investigated systematically and the optimal separation conditions were established. The detection limits obtained for Se(IV), Se(VI), Se-Met and SeCys(2) were 45-210 ng l(-1) with precisions of 3.6-9.7%. The proposed method has been successfully applied for the speciation of dissolved inorganic and organic selenium in environmental and biological samples. In order to validate the methodology, the developed method was also applied to the speciation of selenium in certified reference material of SELM-1 yeast, and the determined values were in good agreement with the certified values.

Is selenium affecting body condition and reproduction in boreal breeding scaup, scoters, and ring-necked ducks?

Elevated levels of selenium (Se) have been detected in wintering and spring-staging lesser scaup. Here, we compared spring scaup Se and mercury (Hg) levels to those of ring-necked ducks and white-winged scoters, species exhibiting increasing and decreasing boreal populations, respectively. Mercury concentrations were low in all three species. Geometric mean (95%CI) liver Se concentrations were 6.2 (5.5-7.0), 4.6 (4.0-5.4), and 32.6 (28.4-37.3)mg/kg dry weight (dw) in scaup, ringnecks and scoters, respectively. Only scoter livers (66%) were above 33 mg/kgdw Se. Scaup and ringneck Se levels were unrelated to breeding status or lipid and protein levels; breeding scoters and females with greater lipid mass had higher Se than non-breeders. Egg and follicle concentrations in scaup and scoters were normal (mean [95%CI]=2.3 [1.9-2.6] and 2.4 [2.1-2.7]mg/kgdw, respectively). Overall, we found no support for a relationship between selenium and boreal scaup and scoter declines, and discuss current Se threshold concentrations.

Selenomethionine biotransformation and incorporation into proteins along a simulated terrestrial food chain

Selenium is an essential trace element in vertebrates, but there is a narrow concentration range between dietary requirement and toxicity threshold. Although a great deal is known about the biochemistry of Se from a nutritional perspective, considerably less attention has been focused on the specific biochemistry of Se as an environmental toxicant. Recent advances in hyphenated analytical techniques have provided the capability of quantifying specific chemical forms of Se in biological tissues as well as the distribution of Se among macromolecules. We applied liquid chromatography coupled to inductively coupled plasma mass spectrometryto investigate biotransformations of selenomethionine along a simulated terrestrial food chain consisting of selenomethionine exposed crickets (Acheta domesticus) fed to western fence lizards (Sceloporus occidentalis). Evidence was obtained for selenomethionine biotransformation as well as for sex-specific differences in the metabolism of Se compounds and their subsequent incorporation into proteins in the lizard. The results demonstrate the complexities involved in trophic transfer of Se due to the potential for extensive biotransformation and the species- and even sex-specific nature of these biotransformations.

Selenium speciation from food source to metabolites: a critical review

Especially in the last decade, a vast number of papers on Se and its role in health issues have been published. This review gives a brief, critical overview of the main analytical findings reported in these papers. Of particular interest is the Se content in different food sources worldwide and the extent to which their consumption is reflected in the Se content of human tissues and body fluids. Several food sources, both natural (Brazil nuts, garlic, Brassica juncea) and Se-enriched (yeast-based supplements), are discussed as to origin, characteristics, Se metabolism and impact of their consumption on the human body. The continuous development of new and improvement of existing analytical techniques has provided different powerful tools to unravel the Se species and their function. An up-to-date literature study on Se speciation analysis is given, illustrating how analytical chemistry in its different facets aids in the identification of Se compounds and provides insight into the complete metabolic pathway of Se throughout the human body. This review includes a detailed image of the current state-of-the-art of Se speciation analysis in these food sources and in human tissues and body fluids.

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.