Characterization of the selenocysteine-containing metabolome in selenium-rich yeast : Part II. On the reliability of the quantitative determination of selenocysteine

A critical evaluation of literature reports demonstrates the poor quality of analytical data on the presence and the concentration of selenocysteine (SeCys) in yeast. The speciation of this amino acid was revisited by studying the behaviour of a well-defined mixture of selenocysteine-containing di- and tripeptides, isolated from yeast, during derivatisation and proteolytic digestion by 2D (size-exclusion–ion-pairing reversed-phase) HPLC-ICP-MS and electrospray MS/MS. Carbamidomethyl-selenocysteine was also synthesised, isolated by cation-exchange chromatography, characterised by electrospray Q-TOF MS/MS, and used for retention time matching. SeCys-containing oligopeptides showed a remarkable resistance to usual proteolytic digestion rendering the quantitative liberation of selenocysteine impossible. The quantification of this amino acid in yeast as the sum of Se-glutathione tripeptides (till now the only family of SeCys-bearing species identified) was therefore recommended.