Bio-fortification and isotopic labelling of Se metabolites in onions and carrots following foliar application of Se and Se

The aims were to bio-fortify onions by foliar application of selenium (Se) and to intrinsically label bioactive Se-metabolites in onion and carrot by enriched, stable ⁷⁷Se for use in human physiological studies. Onion bulbs and leaves were enriched in Se by repeated foliar spraying of 10 or 100 μg Se ml⁻¹ solutions of sodium selenite (Se(IV)) or sodium selenate (Se(VI)). ICP-MS analysis of onion leaves and bulbs showed that the Se concentration was enhanced by up to a factor of approximately 50 and 200 in bulbs and leaves, respectively. HPLC–ICP-MS analysis of proteolytic plant extracts showed that foliar application of Se(IV) gave rise to bio-synthesis of a higher fraction of the desired organic Se species and was better tolerated by the plants than Se(VI). Based on these findings onions and carrots were bio-fortified by foliar application of a solution of ⁷⁷Se(IV) that was enriched to 99.7% as ⁷⁷Se. The ⁷⁷Se- labelled metabolites in onions were predominantly γ-glutamyl-⁷⁷Se-selenomethyl-selenocysteine (γ-glu-Me⁷⁷SeCys), ⁷⁷Se-methylselenocysteine (Me⁷⁷SeCys) and ⁷⁷Se-selenomethionine (⁷⁷SeMet). Furthermore, we report here for the first time the finding in carrots of the bioactive Me⁷⁷SeCys, the identity of which was verified by HPLC–ESI-MS/MS.