Identification of water-soluble gamma-glutamyl-Se-methylselenocysteine in yeast-based selenium supplements by reversed-phase HPLC with ICP-MS and electrospray tandem MS detection

Abstract

Identification of gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC) in water-soluble yeast fractions was accomplished by on-line reversed-phase (RP) HPLC with ICP-MS and electrospray ionisation (ESI) tandem MS. The sample was leached with water using accelerated solvent extraction (ASE) and the aqueous extracts were directly analysed using on-line RP HPLC-ICP-MS. HPLC was carried out with 0.1% formic acid in methanol–water (2 + 98, v/v) solution. Se-specific detection of the chromatographic effluent by ICP-MS allowed identification of γ-glutamyl-SeMC on the basis of comparison of retention times with a matching standard. Aqueous extracts of three different Se–yeast supplements (1291, 1550 and 1983 μg g⁻¹ Se in the dry sample) were analysed by HPLC-ICP-MS for their γ-glutamyl-SeMC content (as Se). A significant variation of the Se speciation in the water extracts appeared to occur with an increase in the total Se concentration from 1550 to 1983 μg g⁻¹. Apparently, the contribution of water-soluble SeMC and selenomethionine (SeMet) increased whereas the contribution of Se incorporated into γ-glutamyl-SeMC decreased with the increasing total Se content. Verification of the presence of γ-glutamyl-SeMC in the water-soluble yeast extract, on the basis of molecular mass determination for the [M + H]^{+ 80}Se ions (m/z 313) and detection of its product ions using on-line RP HPLC-ESI-MS/MS, is reported here for the first time. This was achieved without the need for a cleanup of the aqueous yeast extract. The presence of γ-glutamyl-SeMC might be relevant to the anticarcinogenic potential of selenised yeast since this species is believed to serve primarily as a carrier of SeMC, which appears to be easily converted in animals and possibly humans to methylselenol. This Se metabolite is thought to be an effective anticarcinogen.