BPB Reports

Selenium plays critical roles in various physiological processes in the human body.1) It had been only recognized as a toxic element since discovered by Berzelius until its requirement for animals was reported by Schwarz and Foltz.2) The occurrence of Keshan disease and Kashin-Beck disease (an endemic cardiomyopathy and osteoarthropathy) in China are invariably associated with a selenium deficiency.3,4) In areas that the diseases occurred, the selenium concentrations in the soil (0.1−0.2 mg/kg) were half of the values in normal areas or less.5) Since selenium is not uniformly distributed in solid earth, the selenium concentrations in the soil of certain areas in Europe and Oceania are also relatively low, which causes the decreases in the blood selenium concentration of the local residents.6) Currently, selenium is known to be involved in various physiological processes in organisms, particularly as a critical factor of antioxidative systems.1,7,8) Many studies have also demonstrated the relationships between the selenium status in the human body and various diseases such as cancer, cardiac disease, infectious disease and neurodegenerative disease.9–11) The physiological functions of selenium in organisms are thought to be exhibited by selenoproteins in which selenium is incorporated as selenocysteine (SeCys).12) SeCys is referred to as the 21st proteinogenic amino acid that contains the selenium atom in place of the sulfur atom in cysteine. SeCys in selenoproteins is coded by UGA, which usually acts as a stop codon and requires specific translational mechanisms composed of the SeCys insertion sequence (SECIS) in the 3’ untranslated region of mRNA, selenocysteine specific tRNA and several proteins. Recently, Ingold et al. reported that selenium-dependent glutathione peroxidase (GPx) 4 is a vital enzyme for cells to protect from ferroptosis, which is a nonapoptotic cell death dependent on cytosolic iron.13) Selenium needed for human health is only obtained from foods. The bioavailability of selenium is dependent on the chemical form of selenium. For example, inorganic selenious acid (SA) is highly bioavailable and utilized for the treatment and prevention of selenium deficiency, although its level to cause acute toxicity is generally much lower than those of organic selenium species.14) Selenomethionine (SeMet) is one of the major organic selenium species. Although the acute toxicity of SeMet is lower than that of SA, a large scale clinical trial revealed that the long-term daily intake of SeMet (400 μgSe/day) significantly increased the risk of type 2 diabetic mellitus.15) Therefore, it is important to assess the toxicity and efficiency of selenium species to be consumed. Regular Article