Investigation of seleniummetabolites in Se-enriched kale, Brassica oleracea A, viaHPLC-ICPMS and nanoESI-ITMS

Abstract

Kale (Brassica oleracea A.) can accumulate high concentrations of Se, as well as lutein and β-carotene, which have important human health benefits and possess strong antioxidant properties. In the present study, kale plants were cultivated in soil supplemented with sodium selenite or sodium selenate. The plants accumulated over 20 times higher concentration of selenium in the selenate-enriched soil compared with selenite-enriched soil. Selenium accumulation did not suppress the plant's uptake of the essential metals, Fe, Cu and Zn. Speciation of selenium in the plant was performed via size-exclusion chromatography and ion-pairing reversed phase chromatography using inductively coupled plasma mass spectrometry (ICPMS) and nano-electrospray ion trap mass spectrometry (nanoESI-ITMS) for detection. In ion-pairing reversed phase chromatography, 0.02% HFBA was found to facilitate separating the selenium metabolites without hampering ESI-ITMS ionization. MS²/MS³ spectra were successfully obtained for identification of Se-methylselenocysteine (MeSeCys). MeSeCys was the prominent selenium metabolite formed in kale, which has been suggested as a chemopreventive food supplement. The Se species in leaf are significantly different from those in stem. The leaf shows higher content of high molecular weight selenium than the stem, while the stem produced higher amounts of MeSeCys than the leaf, indicating the metabolism to MeSeCys competes with the production of high molecular weight selenium forms. The plant leaf, when supplemented with Se(IV), contained the highest amount of high molecular weight selenium (49%) as well as the lowest amount of MeSeCys (4%).