Speciation of selenium dietary supplements; formation of S-(methylseleno)cysteine and other selenium compounds.

Speciation of selenium is of interest because it is both essential and toxic to humans, depending on the species and the amount ingested. Following indications that selenium supplementation could reduce the incidence of some cancers, selenium-enriched yeast and other materials have been commercialized as supplements. Most dramatically however, the SELECT trial that utilized l-selenomethionine as the active supplement was terminated in 2008 and there is much debate regarding both the planning and the results of efficacy studies. Further, since dietary supplements are not regulated as pharmaceuticals, there are concerns about the quality, storage conditions, stability and selenium content in selenium supplements. Enzymatic hydrolysis enabled selenium speciation profiles to be obtained by high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and following derivatization gas chromatography with atomic emission detection (GC-AED). Coated fiber solid phase microextraction (SPME) was used to extract volatile selenium species for determination by GC-AED and GC-MS. Similar speciation patterns were observed between yeast-based supplements subject to extended storage and those heated briefly at elevated temperatures. All the yeast-based supplements and one yeast-free supplement formed S-(methylseleno)cysteine on heating. Evidence was obtained in support of the hypotheses that S-(methylseleno)cysteine is formed from a reaction between dimethyldiselenide and cysteine or cystine.

Heterologous expression, purification, and characterization of recombinant rat cysteine dioxygenase.

Cysteine dioxygenase (CDO, EC 1.13.11.20) catalyzes the oxidation of cysteine to cysteine sulfinic acid, which is the first major step in cysteine catabolism in mammalian tissues. Rat liver CDO was cloned and expressed in Escherichia coli as a 26.8-kDa N-terminal fusion protein bearing a polyhistidine tag. Purification by immobilized metal affinity chromatography yielded homogeneous protein, which was catalytically active even in the absence of the secondary protein-A, which has been reported to be essential for activity in partially purified native preparations. As compared with those existing purification protocols for native CDO, the milder conditions used in the isolation of the recombinant CDO allowed a more controlled study of the properties and activity of CDO, clarifying conflicting findings in the literature. Apo-protein was inactive in catalysis and was only activated by iron. Metal analysis of purified recombinant protein indicated that only 10% of the protein contained iron and that the iron was loosely bound to the protein. Kinetic studies showed that the recombinant enzyme displayed a K(m) value of 2.5 +/- 0.4 mm at pH 7.5 and 37 degrees C. The enzyme was shown to be specific for l-cysteine oxidation, whereas homocysteine inhibited CDO activity.

Chromatographic speciation of anionic and neutral selenium compounds in Se-accumulating Brassica juncea (Indian mustard) and in selenized yeast.

Selenium-accumulating plants such as Brassica juncea (Indian mustard) concentrate the element in plant shoots and roots. Such behavior may provide a cost-effective technology to clean up contaminated soils and waters that pose major environmental and human health problems (phytoremediation). Such ability to transform selenium into bioactive compounds has important implications for human nutrition and health. Element selective characterization of B. juncea grown in the presence of inorganic selenium under hydroponic conditions provides valuable information to better understand selenium metabolism in plants. The present work determines both previously observed organoselenium species such as selenomethionine and Se-methylselenocysteine and for the first time detects the newly characterized S-(methylseleno)cysteine in plant shoots and roots when grown in the presence of selenate or selenite as the only selenium source. A key feature of this study is the complementary role of selenium and sulfur specific chromatographic detection by HPLC with interfaced inductively coupled plasma mass spectrometry (ICP-MS) detection and by derivatization GC with interfaced atomic spectral emission. HPLC-ICP-MS limits of detection for such species were in the range 5-50 ng Se mL(-1) in the injected extracts. Speciation profiles are compared with those of selenium-enriched yeast by both HPLC-ICP-MS and GC-AED.

Selective detection and identification of Se containing compounds--review and recent developments.

The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine.

Extraction of arsenic species from spiked soils and standard reference materials.

The abilities of various extractants to recover four arsenic species [As(iii), As(v), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA)] from soils spiked with 20 micro g g(-1) As were investigated. The extractants were water, buffer solutions (citrate and ammonium dihydrogen phosphate), acidic solutions (phosphoric acid and acetic acid), a basic solution (sodium hydroxide) and household chemicals (vinegar and Coca Cola). Gentle shaking at room temperature with each extractant for 24 h gave different recoveries for the different arsenic species. With 0.1 M NaOH solution 46% As(iii), 53% DMA, 100% MMA and 84% As(v) were recovered. A rapid extraction procedure using a sonicator probe has been developed to obtain higher extraction efficiencies. Extracts of arsenic-spiked soil, SRM 2711 Montana soil and SRM 2709 San Joaquin soil were analyzed by HPLC-ICP-MS. In the SRM water extracts, DMA and MMA were identified in addition to inorganic arsenic. The solution detection limits (3s) were 0.1, 0.12, 0.13 and 0.15 ng mL(-1) for As(iii), DMA, MMA and As(v), respectively for HPLC-ICP-MS.

Identification and synthesis of a novel selenium-sulfur amino acid found in selenized yeast: Rapid indirect detection NMR methods for characterizing low-level organoselenium compounds in complex matrices.

After proteolytic digestion, aqueous extraction, and derivatization with diethyl pyrocarbonate or ethyl chloroformate, HPLC-inductively coupled plasma (ICP)-MS, GC-atomic emission detection (AED), and GC-MS analysis of high-selenium yeast stored at room temperature for more than 10 years showed selenomethionine as the major Se product along with substantial amounts of selenomethionine selenoxide hydrate and the previously unreported selenoamino acid having a Se-S bond, S-(methylseleno)cysteine. The identity of the latter compound was confirmed by synthesis. The natural product was shown to be different from a synthetic sample of the isomeric compound Se-(methylthio)selenocysteine. Selenium-specific NMR spectroscopic methods were developed to directly analyze the aqueous extracts of the hydrolyzed selenized yeast without derivatization or separation. Selenomethionine and S-(methylseleno)cysteine were identified by 77Se-1H HMQC-TOCSY experiments.

Modern trends in the speciation of selenium by hyphenated techniques.

The complexity of selenium speciation in the environment and in living organisms results in broad analytical challenges. The importance of the selective determination of the particular species of this element, to understand its metabolism and biological significance in clinical chemistry, biology, toxicology, and nutrition, calls for state-of-the-art analytical techniques. In this paper hyphenated techniques are evaluated with particular emphasis on interfaced separation with element-selective detection and identification of the selenium compounds detected.