Bioaccessibility of Selenium in Wheat, Rice and Mustard Grown in Seleniferous Soils and its Bioactivity

Abstract

Selenium is an essential element for both humans and animals. Selenium enters into the body as a dietary supplement through food. In humans, low dietary Se intakes are associated with many oxidative stress diseases, which could be prevented by dietary supplementation of selenium through inorganic selenium salts or selenium rich food crops. Most of the cultivated crops possess the ability to absorb, metabolize, and store significant amounts of Se in their tissues when grown on selenium containing soil. Selenium prevents these crops from the deteriorating effect of many environmental stresses by improving many defense mechanisms (enzymatic and non-enzymatic). Present doctoral work was focused on selenium determination, its bioaccessibility and study of different bioactive properties of wheat, rice and mustard seeds grown in seleniferous soils. Selenium levels in these crops were significantly higher when compared to the control (collected from non-seleniferous regions) and the observed selenium content in selenium-rich wheat, rice and mustard seeds were 115.4 ± 0.83, 19.14 ± 0.19 and 110.0 ± 3.04 µg/g respectively whereas, in their respective controls it was 0.48 ± 0.01, below detection limit and 0.48 ± 0.02 µg/g. Selenium content was also determined in processed samples obtained from these selenium rich crops, for example, chapati (112.3 ± 1.72 µg/g), cooked rice (18.62 ± 0.35 µg/g), different wheat storage proteins (albumin, 444.9 ± 14.59 µg/g; globulin, 377.2 ± 6.57 µg/g; glutelin, 527.8 ± 4.9 µg/g and prolamin, 495.8 ± 9.58 µg/g), wheatgrass (cultivated under visible light, 148.1 ± 4.05 µg/g and UV-B light, 151.3 ± 2.46 µg/g), mustard oil (3.50 ± 0.66 µg/g) and mustard cake (143.0 ± 5.18 µg/g). Bioaccessibility of selenium from processed and unprocessed grains were determined by simulated in-vitro gastrointestinal digestion. The obtained results indicated that, thermal processing of grains (chapati, 96.26% and cooked rice, 81.53%) were significantly increased the selenium bioaccissibilty when compared to their respective raw grains (wheat, 82.20% and rice, 63.37%). From wheat storage proteins, bioaccessibility of selenium was 93.66, 87.97, 76.77 and 69.68% for albumin, globulin, glutelin and prolamin respectively. Lowest selenium bioaccessibility was observed from wheatgrass samples (30-32%). The doctoral work was also aimed to evaluate the effect of selenium and light quality (visible and UV-B) on different in-vitro biochemical and enzymatic assays of wheatgrass samples. These assays include total phenolic content (TPC), total flavonoid content (TFC), DPPH radical scavenging, Trolox equivalent antioxidant capacity (TEAC), ferric reducing antioxidant power (FRAP), iron (Fe2+) Chelating capacity and lipid peroxidation (TBARS assay), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), ascorbate peroxidase (APx) and guaiacol peroxidase (GuPx). Obtained results indicated that, selenium and UV-B radiation independently enhanced the different antioxidant properties whereas, selenium and UV-B light showed synergistic affect and significantly increases antioxidant status of wheatgrass. The aim of present work was also to determine the concentration and characterization of different free (alcohol extracted) and bound forms (alkali extracted) of phenolic compounds from wheatgrass samples and also to evaluate their in-vitro bioaccessibility by using HPLC. Results showed that, Selenium independently and along with UV-B light, significantly increases the levels of many free and bound phenolics, and bioaccessibility of free phenolics were high when compared to bound phenolics. Selenium rich mustard cake was used for the Isolation, quantification and characterization of isoselenocyanates. Oily residue obtained after myrosinase based enzymatic auto hydrolysis of selenium-rich mustard cake powder was subjected to GC-MS analysis and obtained results confirmed the presence of butenyl isothiocyanate (92%), allyl isothiocyanate (4.48%), phenethyl isothiocyanate (2.8%) and allyl thiocyanate (0.43%) along with very small fractions of allyl selenocyanate (0.13%) and allyl isoselenocyanate (0.17%). Mustard cake residue left after extraction of isothiocyanate/isoselenocyanates was further used for the extraction of selenium-rich storage protein. Isolated protein was subjected to quantification of selenium and total selenium content recorded was 582.3 ± 6.23 µg/g. Further the cytoprotective effect of selenium-rich mustard protein on tert-butyl hydroperoxide (TBHP) induced cytotoxicity was studied in mouse melanoma cell line (B16-F10). Protein extracted from selenium-rich mustard was observed to protect the melanoma cells from organic peroxide induced oxidative stress with significant increase in expression of GPx enzyme.