Bimodal actions of selenium essential for antioxidant and toxic pro-oxidant activities: the selenium paradox (Review)

Selenium is an essential biological trace element. Adult daily intake of selenium should be approximately 100 µg per day. This compound has a two-sided effect depending on its concentration. A selenium-deficient diet is associated with various endemic diseases, including cardiomuscular malfunctions, osteoarthritis, cancer and viral infections that lead to premature death. These defects are prevented when dietary intake of selenium is adequate. The preventive biological effect of selenium is considered to be due to the antioxidant function of selenoproteins with a selenocysteine in the active site of the catalytic domain. Antioxidant selenoproteins maintain the intracellular redox status and, as a result, normal physiological processes in the cell. Conversely, an overdose of selenium generates oxygen radicals and leads to apoptotic cell death by inducing oxidation and cross-linking of protein thiol groups essential for cell survival. A lower redox state caused by selenium may be implicated in toxic diseases, such as alkali disease and blind staggers. Collectively, selenium seems to have both harmful and beneficial attributes. The aim of this review is to summarize the various biological functions of selenium and to illustrate its opposite roles as a pro-oxidant and an antioxidant.