Complementary use of molecular and element-specific mass spectrometry for identification of selenium compounds related to human selenium metabolism

The aim of this paper is to give an overview of analytical data on the identification of selenium compounds in biological samples with relevance for selenium metabolism. Only studies applying the combination of element-specific inductively coupled plasma mass spectrometry as well as molecular electrospray mass spectrometry detection have been included. Hence, selenium compounds are only considered identified if molecular mass spectra obtained by analysis of the authentic biological sample have been provided. Selenium compounds identified in selenium-accumulating plants and yeast are included, as extracts from such plants and yeast have been widely used for examination of the cancer-preventive effect of selenium in cell lines, animal models and human intervention trials. Hence, these selenium compounds are available for absorption and further metabolism. Identification of selenium metabolites in simulated gastric and intestinal juice, intestinal epithelial tissue, liver and urine is described. Hence, selenium metabolites identified in relation to absorption, metabolism and excretion are included.     

Characteristics of selenium in australian marine biota

The occurrence, distribution and speciation of selenium in Australian marine biota is discussed. Biochemical pathways for the accumulation of selenium by marine organisms are also postulated. Comparison of the levels of selenium in macroalgae, fish, crustaceans and molluscs indicates that preferential accumulation of selenium by particular taxa does not occur. Phaeophyta have significantly lower selenium concentrations than Rhodophyta and Chlorophyta. Fish have lower selenium contents in muscle tissues than molluscs and crustaceans. Marine animals with different dietary intake (planktonic vs herbivorous vs carnivorous) are not observed to have significantly different levels of selenium (P>0.05). Selenium in all the organisms studied was predominantly associated with free amino-acids or protein residues and was not present as characterizable inorganic selenium species (SeO₃^2?, SeO₄^2?). These results indicate that selenium is probably only incorporated into biota for specific biochemical purposes with any exces selenium being excreted or eliminated.     

The Effects of Selenium on Bone Health: From Element to Therapeutics

Osteoporosis, characterized by low bone mass and a disruption of bone microarchitecture, is traditionally treated using drugs or lifestyle modifications. Recently, several preclinical and clinical studies have investigated the effects of selenium on bone health, although the results are controversial. Selenium, an important trace element, is required for selenoprotein synthesis and acts crucially for proper growth and skeletal development. However, the intake of an optimum amount of selenium is critical, as both selenium deficiency and toxicity are hazardous for health. In this review, we have systematically analyzed the existing literature in this field to determine whether dietary or serum selenium concentrations are associated with bone health. In addition, the mode of administration of selenium as a supplement for treating bone disease is important. We have also highlighted the importance of using green-synthesized selenium nanoparticles as therapeutics for bone disease. Novel nanobiotechnology will be a bridgehead for clinical applications of trace elements and natural products.     

硒化合物诱导细胞凋亡的信号转导机制

从硒化合物诱导细胞凋亡及其机理阐明硒防治肿瘤的生物功能,是当今硒的生物无机化学与相关学科交叉的前沿。本文通过作者的研究工作及国内外有关工作介绍了活性氧（ROS）介导的硒化合物诱导细胞凋亡的信号转导机制,阐述了ROS作为信号分子的特性及硒化合物与ROS作用的化学基础。     

Analytical selenoamino acid studies by chromatography with interfaced atomic mass spectrometry and atomic emission spectral detection

Consumption of selenium enriched plants or yeast-based nutritional supplements has been reported to provide anticarcinogenic benefits which are selenium compound dependent. Separation and identification of these selenium compounds is critical to understand the activity. Plants and yeast convert inorganic selenium in the soil or growth media into organoselenium compounds, probably following a route similar to the sulfur assimilatory pathway. Non-volatile selenium compounds produced include selenoamino acids, some of which have shown anticarcinogenic activity. Volatile compounds produced by chemical reaction of involatile precursors have also been found. An ion pair chromatographic method with ICP-MS detection for the separation of selenoamino acid standards potentially present in real samples is given. The method allows separation of selenoamino acids including such analytes as the cis-trans isomers of Se-1-propenyl-dl-selenocysteine. The method also provides the capability of determining the presence of selenoxides and possibly selenones, and tracking of other functionalities and reactions by selective derivatization. Alternatively, selenoamino acids are treated with ethylchloroformate to produce stable volatile derivatives which are amenable to GC separation with element specific atomic emission detection (GC-AED). Results of total selenium determination and speciation of selenium enriched yeast-based nutritional supplements, selenium enriched allium vegetables and bioremediation samples are presented.     

The determination of selenium in urine

The selenium excreted in urine can be measured to assess the dietary status of selenium, an essential trace element in human nutrition. The objectives of this work were: 1) to develop a procedure, capable of high sample throughout, by which the major interferences can be reduced such that selenium concentrations can be measured in urine by Neutron Activation Analysis (NAA) using^77mSe (17.4 s; and 2) to apply the method to a human dietary selenium study in which several selenium monitors were compared. The method involves a pre-irradiation arsenic-coprecipitation separation of the selenium from urine in the presence of a high specific-activity⁷⁵Se tracer. The processed urine samples are analyzed using NAA. The procedure was applied to 58 urine specimens longitudinally collected from 12 subjects consuming three different levels of selenium. A dose-response relationship was observed in urine as well as a high correlations with both serum and whole blood selenium concentrations.     

豆腐生产过程中微量元素硒分布特性研究

为确定以不同硒含量的大豆为原料生产豆腐过程中硒分布特性,用石墨炉原子吸收分光光度法对该过程中各种产物的硒含量进行了分析研究。结果表明,以硒含量分别为0.692、2.561、2.801 mg/kg的大豆为原料生产得到的豆腐、黄浆水、豆渣的硒含量分别为:0.089、0.250、0.138 mg/kg,0.462、0.760、0.507 mg/kg和0.532、0.764、0.524 mg/kg。豆腐生产过程中,以低硒大豆为原料生产豆腐,硒总回收率为75.59%;采用富硒大豆为原料生产,硒的回收率分别达到87.37%、87.20%;生产过程中豆腐、黄浆水和豆渣中硒含量占总回收硒的比率分别为39.01%～49.97%、23.92%～29.89%、26.11%～30.92%。     

Laser ablation synthesis of selenium superoxide anion SeO4- via selenium trioxide photolysis. Time-of-flight mass spectrometry and ab initio calculations

Laser desorption/ionisation and laser ablation of solid selenium trioxide, as well as the gas-phase behaviour of selenium trioxide, were studied. Selenium trioxide undergoes photochemical decomposition and, from the mass spectra obtained by laser desorption/ionisation time-of-flight mass spectrometry (LDI-TOF-MS), the following species were identified: O-, O2-, O3-, SeO-, SeO2-, SeO3-, SeO4-, Se2O7-, Se3O11-, and Se4O14-. Formation of the selenium superoxide SeO4- anion is described in this work for the first time. In addition, low-abundance selenium species such as Se2O8H2-, Se3O11H-, and Se4O15H2- were also detected. The stoichiometry of all ions was confirmed via isotopic pattern modeling and/or post-source decay (PSD) analysis. Photolysis of selenium trioxide leads partly to ozone formation. It was found that the most likely mechanisms of selenium superoxide formation are oxidation of selenium trioxide with ozone and/or reactive oxygen radicals, or photolysis of selenium trioxide tetramer (SeO3)4. Therefore, ab initio calculations were performed to support the mass spectrometric evidence and to suggest probable geometries for selenium superoxide anion SeO4- and diselenium superoxide anion Se2O7-, as well as to provide insight into and/or predict possible formation pathways. It has been found that both cyclic and non-cyclic peroxide structures of SeO4- and Se2O7- ions are possible. In addition, the SeO4 structure was also calculated guided by thermodynamic considerations using Gaussian-2 methodology, and the inferred stability of the SeO4 neutral molecule was supported by ab initio calculations.     

Selenium heterocycles XIV. 2,6-diaryltetrahydroselenopyran-4-ones

In our previous communications, we have reported that selenium heterocycles could be easily obtained either by using selenium dioxide (3) or by selenium (4). In continuation of our studies on the synthesis and chemistry of selenium heterocycles, it was of interest to use aluminum selenide as the precursor of selenium in the preparation of selenium heterocycles.     

Analytical selenoamino acid studies by chromatography with interfaced atomic mass spectrometry and atomic emission spectral detection

Consumption of selenium enriched plants or yeast-based nutritional supplements has been reported to provide anticarcinogenic benefits which are selenium compound dependent. Separation and identification of these selenium compounds is critical to understand the activity. Plants and yeast convert inorganic selenium in the soil or growth media into organoselenium compounds, probably following a route similar to the sulfur assimilatory pathway. Non-volatile selenium compounds produced include selenoamino acids, some of which have shown anticarcinogenic activity. Volatile compounds produced by chemical reaction of involatile precursors have also been found. An ion pair chromatographic method with ICP-MS detection for the separation of selenoamino acid standards potentially present in real samples is given. The method allows separation of selenoamino acids including such analytes as the cis-trans isomers of Se-1-propenyl-dl-selenocysteine. The method also provides the capability of determining the presence of selenoxides and possibly selenones, and tracking of other functionalities and reactions by selective derivatization. Alternatively, selenoamino acids are treated with ethylchloroformate to produce stable volatile derivatives which are amenable to GC separation with element specific atomic emission detection (GC-AED). Results of total selenium determination and speciation of selenium enriched yeast-based nutritional supplements, selenium enriched allium vegetables and bioremediation samples are presented. Received: 16 February 1998 / Revised: 4 June 1998 / Accepted: 9 June 1998     

Uptake and speciation of selenium in garlic cultivated in soil amended with symbiotic fungi (mycorrhiza) and selenate

The scope of the work was to investigate the influence of selenate fertilisation and the addition of symbiotic fungi (mycorrhiza) to soil on selenium and selenium species concentrations in garlic. The selenium species were extracted from garlic cultivated in experimental plots by proteolytic enzymes, which ensured liberation of selenium species contained in peptides or proteins. Separate extractions using an aqueous solution of enzyme-deactivating hydroxylamine hydrochloride counteracted the possible degradation of labile selenium species by enzymes (such as alliinase) that occur naturally in garlic. The selenium content in garlic, which was analysed by ICP–MS, showed that addition of mycorrhiza to the natural soil increased the selenium uptake by garlic tenfold to 15 μg g⁻¹ (dry mass). Fertilisation with selenate and addition of mycorrhiza strongly increased the selenium content in garlic to around one part per thousand. The parallel analysis of the sample extracts by cation exchange and reversed-phase HPLC with ICP–MS detection showed that γ-glutamyl-Se-methyl-selenocysteine amounted to 2/3, whereas methylselenocysteine, selenomethionine and selenate each amounted to a few percent of the total chromatographed selenium in all garlic samples. Se-allyl-selenocysteine and Se-propyl-selenocysteine, which are selenium analogues of biologically active sulfur-containing amino acids known to occur in garlic, were searched for but not detected in any of the extracts. The amendment of soil by mycorrhiza and/or by selenate increased the content of selenium but not the distribution of detected selenium species in garlic. Finally, the use of two-dimensional HPLC (size exclusion followed by reversed-phase) allowed the structural characterisation of γ-glutamyl-Se-methyl-selenocysteine and γ-glutamyl-Se-methyl-selenomethionine in isolated chromatographic fractions by quadrupole time-of-flight mass spectrometry.     

Measurement of isotopic composition of selenium

Conclusions Measurement of the isotopic composition of selenium as the average of values of its isotopic abundance, determined as selenium hexafluoride and as metallic selenium, leads to a distortion of the amount of heavy isotopes due to the formation of hydrides, which are observed during the analysis of metallic selenium.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 150–152, January, 1973.     

Nondispersive X-ray fluorescence in the study of rapid radiochemical separation of selenium from arsenic,germanium and zinc

A rapid radiochemical separation of selenium from arsenic, germanium and zinc based on the precipitation of elemental selenium by NaHSO₃ is discussed. Yields of 90% and greater are achieved for selenium in 1 to 2 min. The kinetics, separation characteristics of the process and chemical yield of Se and trace contaminants such as Zn, Ge and As were studied using nondispersive X-ray fluorescence techniques. Work supported by USAEC.     

Utilization of two different chemical forms of selenium during lactation using stable isotope tracers: an example of speciation in nutrition.

The bioavailability and metabolism of different chemical species of mineral nutrients in the diet are receiving much attention from research nutritionists. In order to make scientifically based recommendations for mineral intakes, the chemical form of the mineral, with its specific absorption, utilization and retention, needs to be considered. Selenium is an example of an essential nutrient that is consumed in several different chemical forms, hence information is needed on the bioavailability and metabolism of each form before recommendations for dietary intakes can be made. A valuable tool for research on bioavailability and metabolism in humans is stable isotope tracers. When there are more than two stable isotopes available, as with selenium, stable isotope methodology allows the comparison of the utilization of different chemical forms of the nutrient simultaneously in in vivo studies. As an example of speciation questions addressed by nutritionists, a study is described that simultaneously evaluated utilization (absorption, retention and appearance in milk and blood) of two different chemical forms of selenium (selenite and selenomethionine) in lactating, non-lactating and never pregnant women using stable isotope tracers. All three groups of women had similar selenium status at the start of the study. Significantly more selenium from selenomethionine than from selenite was absorbed and appeared in the plasma in all groups. Milk contained more selenium from apparently absorbed selenomethionine than from selenite. All groups retained significantly more selenium from selenomethionine than from selenite; lactating women retained more selenium from selenite than did the other two groups, suggesting that milk losses may be partially compensated by enhanced retention of dietary selenium as selenite.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selenium Determination in Serum by Graphite Furnace Atomic Absorption Spectrometry

Selenium concentration in serum has been proposed as the most valuable indicator of human selenium status. In this work, the concentration of selenium in serum is measured by GFAAS using longitudinal Zeeman-effect background correction, a highly sensitive technique that can be used to determine selenium at sub-ppb level. Standard reference materials are analyzed to verify the accuracy and precision of this technique. The direct method proposed in this study is used for the determination of selenium in human serum collected from healthy people of 19–25 years and women of 21–34 years in the First trimester of pregnancy. The method developed in this study, which can quickly and accurately measure serum selenium, is suitable for applications in clinical diagnosis or biological monitoring.     

蛋氨酸硒络合反应条件的响应曲面法优化

以蛋氨酸和亚硒酸钠为原料合成了蛋氨酸硒;采用基于3因素5水平的中心组合设计实验的响应曲面法,分析了温度(30℃~80℃)、酸度(pH 4~9)、时间以及它们之间的交互作用对蛋氨酸硒产率的影响,确定了合成蛋氨酸硒的最佳工艺条件.通过典型分析确定优化的络合反应条件为:温度50℃,pH 6.93,时间90 min;相应的蛋氨酸硒的产率为51.58%.相关研究结果表明,采用响应曲面法优化确定的蛋氨酸硒合成工艺合理有效、切实可行.     

Mass spectra of tetraselenafulvalenes,diselenadithiafulvalenes and tetrathiafulvalenes

The mass spectra of 13 heterofulvalenes are reported. The spectra show great similarities within the selenium and within the sulphur series. The main difference between the selenium and the sulphur compounds results from the more facile loss of selenium compared with sulphur, and from the first fragmentation of the molecular ion, as the selenium fulvalenes lose an alkyne molecule, whereas the sulphur fulvalenes first lose an (S?R) radical. An important feature of the spectra of the simple heterofulvalenes is the formation of a rearrangement ion by migration of a heteroatom. The mechanism was elucidated by ¹³C labelling and is discussed in detail.     

Simple method for determination of selenium in biological materials by flameless atomic-absorption spectrometry using a carbon-tube atomizer

A method for determination of selenium in biological materials by flameless atomic-absorption spectrometry using a carbon-tube atomizer is described. The sample is burned by an oxygen-flask combustion procedure, the resulting solution is treated with a cation-exchange resin to eliminate interfering cations, the selenium is extracted with dithizone in carbon tetrachloride and the resulting selenium dithizonate is combined with nickel nitrate in the carbon tube to enhance the sensitivity for selenium and avoid volatilization losses. The method measures selenium concentrations as low as 0.01 mug/g with a relative standard deviation of 8%.     

TISSUE SELENIUM LEVELS AND GROWTH RESPONSES OF MICE FED SELENOMETHIONINE,SE-METHYLSELENOCYSTEINE OR SODIUM SELENITE

Abstract

Mice fed diets containing selenomethionine at a level of 20 ppm selenium and raised to 30 ppm selenium at 3 weeks on experiment showed (1) delayed response to selenium toxicity, (2) slow recovery from the toxicity after removal of selenium from the diet and (3) relatively high deposition and retention of tissue selenium. These data suggest that selenomethonine initially becomes incorporated in to the primary structure of proteins and as such is not particularly toxic. However, upon its slow removal from protein, selenomethionine becomes toxic by forming selenium IV compounds through a pathway similar to that followed by methionine.

Mice fed diets containing sodium selenite or Se-methylselenocysteine at the same level of selenium as the selenomethionine diet showed (1) immediate response to selenium toxicity (2) rapid recovery from the toxicity after removal of selenium from the diet and (3) relatively low deposition and relatively rapid depletion of tissue selenium. These data suggest that sodium selenite and Se-methylselenocysteine ultimately follow similar metabolic pathways and do not become part of the primary structure of proteins. A possible metabolic route for Se-methylselenocysteine is that it is oxidized to toxic selenium IV compounds through an oxidative pathway similar to that followed by S-methylcysteine.     

Differential Pulse Voltammetric Determination of Selenocystine Using Selenium‐gold Film Modified Electrode

Differential pulse voltammetric determination of selenocystine (SeC) using selenium‐gold film modified glassy carbon electrode ((Se‐Au)/GC) is presented. In 0.10 mol?L^?1 KNO₃ (pH 3.20) solution, SeC yields a sensitive reduction peak at ?740 mV on (Se‐Au)/GC electrode. The peak current has a linear relationship with the concentration of SeC in the range of 5.0×10^?8–7.0×10^?4 mol?L^?1, and a 3σ detection limit of SeC is 3.0×10^?8 mol?L^?1. The relative standard deviation of the reduction current at SeC concentration of 10^?6 mol?L^?1 is 3.88% (n=8) using the same electrode, and 4.19% when using three modified electrodes prepared at different times. The content of SeC in the selenium‐enriched yeast and selenium‐enriched tea is determined. The total selenium in ordinary or selenium‐enriched tea is determined by DAN fluorescence method. The results indicate that in selenium‐enriched yeast about 20% of total selenium is present as SeC and in selenium‐enriched tea SeC is the major form of selenoamino acids. The total selenium content in selenium‐enriched tea soup is 0.09 μgSe/g accounting by 7% compared with that in selenium‐enriched tea. Hence, only a little amount of selenium is utilized by drinking tea, and most selenium still stay in tealeaf. Uncertainty are 22.4% and 16.1% for determination of SeC in selenium‐enriched yeast and selenium‐enriched tea by differential pulse voltammetry (DPV) on (Se‐Au)/GC electrode, respectively.