a b s t r a c tSpeciation 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.
The speciation and stability of organoselenium compounds present in the selenized yeast evaluated in National Cancer Institute human intervention trials were examined along with those of related supplements. Total selenium was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), ICP-mass spectrometry (ICP-MS) and/or electrothermal atomic absorption spectrometry (ET-AAS) after microwave digestion. Enzymatic extraction enabled selenium speciation profiles to be obtained by ion-pair reversed phase high performance liquid chromatography (HPLC) with ICP-MS detection. Extracts were also subjected to chloroformate derivatization followed by Se and S specific GC analysis with atomic emission detection (GC-AED). Currently available and archived supplements show major differences in speciation for selenomethionine, selenomethionine Se-oxide and S-(methylseleno)cysteine with respect to time and conditions of storage. The formation of the latter compound is reported under different synthetic and elevated temperature conditions.
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