Speciation, quantification and stability of selenomethionine, S-(methylseleno)cysteine and selenomethionine Se-oxide in yeast-based nutritional supplements

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.