Speciation of selenomethionine metabolites in wheat germ extract

Selenometabolites transformed from selenomethionine (SeMet) in wheat germ extract (WGE) were identified by complementary use of HPLC-ICP-MS and HPLC-ESI-MS/MS. Three selenium (Se)-containing peaks tentatively named WGE1, WGE2, and WGE3 were detected by HPLC-ICP-MS. WGE1 had [M]+ at m/z 212 on HPLC-ESI-MS analysis, and its fragment ions indicated that WGE1 is selenomethionine methylselenonium (MeSeMet). WGE2 and WGE3 exhibited absorption at 254 nm and molecular ions at m/z 433 and 447, respectively. Their fragment ions revealed that WGE2 and WGE3 are Se-adenosylselenohomocysteine (AdoSeHcy) and Se-adenosylselenomethionine (AdoSeMet), respectively. Their structures were coincident with the absorption of WGE2 and WGE3 at 254 nm. In addition, a trace amount of AdoSeMet was suggested to also exist in rabbit reticulocyte lysate, a mammalian in vitrotranslation system, because the transformation of AdoSeMet from SeMet was completely inhibited by (2S)-2-amino-4,5-epoxypentanoic acid (AEPA), a potent inhibitor of AdoMet synthetase. These results suggest that SeMet and methionine (Met) share a common metabolic pathway, i.e., SeMet is not only incorporated into proteins in place of Met but also metabolized to AdoSeMet in higher eukaryotes and MeSeMet in plants.