Analysis of animal and plant selenometabolites in roots of a selenium accumulator, Brassica rapa var. peruviridis, by speciation

Abstract

Many studies have examined the metabolic pathway of selenium (Se) compounds in Se-accumulating plants (hereafter “Se accumulators”) when the plants are exposed to inorganic Se, such as selenite and selenate. However, if we were to consider Se circulation in the biosphere, the metabolism of organic Se, in particular, selenometabolites of animals and plants, in plants should be elucidated. In this study, Brassica rapa var. peruviridis, a known Se accumulator, was hydroponically cultivated and then exposed to selenometabolites of animals and plants, such as methyl-2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside (selenosugar, SeSug), trimethylselenonium (TMSe), selenomethionine (SeMet), and Se-methylselenocysteine (MeSeCys). Then, the metabolic pathway of the organic Se compounds/selenometabolites in B. rapa var. peruviridis was investigated by speciation analysis. Two selenometabolites were detected in the roots when the plant was exposed to SeMet, MeSeCys, and SeSug. They were assigned to S-(methylseleno)-glutathione and MeSeCys using electrospray tandem mass spectrometry (ESI-MS-MS) and HPLC-inductively coupled plasma mass spectrometry (ICP-MS). Contrary to SeMet, MeSeCys, and SeSug, TMSe was not metabolized even if it was more efficiently incorporated into the roots than the other Se compounds. The identified metabolites enabled us to propose a metabolic pathway for the organic Se metabolites except TMSe in the plant roots: a monomethylseleno moiety (CH₃Se–) commonly existing in SeMet, MeSeCys, and SeSug was cleaved off and conjugated with GSH, and then the CH₃Se group was transferred to O-acetylserine to form MeSeCys.