LC-ICP-MS and LC-ESI-(MS)n identification of Se-methylselenocysteine and selenomethionine as metabolites of methylseleninic acid in rat hepatocytes

The metabolism of methylseleninic acid in isolated rat hepatocytes was investigated. Selenium containing metabolites excreted from the cells were detected in the supernatant of the incubation sample by LC-ICP-MS. After pre-treatment of the supernatant by preparative chromatography and pre-concentration by lyophilisation, a major metabolite was identified by molecular mass spectrometry as Se-methylselenocysteine by LC-ESI-MS, MS2 and MS3 and a minor metabolite was identified as selenomethionine by LC-ESI-MS2 and MS3 and LC-ESI-MS2(SRM). This is the first time these metabolites have been identified in hepatocytes. Complementary data from ion trap and triple quadrupole MS instruments provided solid proof of metabolite identities. A time course study showed that S-(methylseleno)cysteine and S-(methylseleno)glutathione were intermediates in the formation of the major metabolite. It is questioned if methylseleninic acid is a relevant model compound for methylated Se-amino acids in vitro.