ABSTRACT
Selenium (Se) is an essential trace element in animals. Se is part of the active center of selenoproteins (Suzuki and Ogra, 2002; Whanger, 2002; Böck et al., 2007). Specifically, Se is required in the formation of the selenol group (–SeH), the active center of a selenocysteinyl (SeCys) residue in selenoprotein sequences (Lu and Holmgren, 2009; Reeves and Hoffmann, 2009). In addition, a selenomethionine (SeMet) residue is found in general protein sequences because animals are unable to distinguish methionine (Met) from SeMet in the protein translation process. A protein containing SeMet is called a Se-containing protein to distinguish it from a selenoprotein. Thus, SeCys and SeMet are considered the major nutritional source of Se from animals, and selenometabolites are the minor source. A proposed metabolic pathway of Se in animals is depicted in Figure 2.1. In plants, on the other hand, Se is not essential and exists as a “bystander” mineral. However, its beneficial effects on plant growth have been reported (Freeman et al., 2007). It is known that certain plants, including Allium plants belonging to the family Liliaceae, such as garlic, onion, wild leek, and shallot, and Brassica plants, such as Indian mustard, broccoli, and radish, are able to accumulate Se (Ip et al., 2000; Ravn-Haren et al., 2008; Reid et al., 2008). In those plants, some unique Se-containing amino acids, such as methyl selenocysteine (MeSeCys), γ-glutamyl methylselenocysteine (GluMeSeCys), and selenohomolanthionine (SeHLan), are biosynthesized (Montes-Bayón et al., 2002; Ogra et al., 2005, 2007). Hence, those unique selenoamino acids are considered the nutritional source of Se from plants. A proposed metabolic pathway of Se in plants is depicted in Figure 2.2. Microorganisms, and in particular, fungi, are another possible nutritional source of Se. Yeast accumulates Se as SeMet and its
2.1 Introduction .................................................................................................... 19 2.2 Nutritional Sources of Selenium ..................................................................... 21
2.2.1 Animal Metabolites ............................................................................22 2.2.2 Plant Metabolites ................................................................................23 2.2.3 Other Biological Sources of Selenium ................................................24 2.2.4 Elemental Selenium and Inorganic Salts of Selenium ........................25
