As discussed earlier, selenium is an essential nutrient for several biochemical pathways, in the form of various selenoproteins, whose functioning is strictly selenium-dependent. Their synthesis is affected in case of selenium deficiency/restricted dietary intake. Selenium is present in the form of selenocysteine in selenoproteins (Briefing Paper, British Nutrition Foundation, 2001).
Glutathione peroxidase (GSHPx) enzymes are the best known selenoproteins. In 1973, influence of selenium on the role of these enzymes helped to recognize biological effects of this trace element. And these enzymes are known to protect cells against different peroxide-induced free radical damage. Additionally, selenium exhibits compensatory functions with other antioxidants in the body like vitamin E to protect from free radical damage, ultimately guarding the body against risk of developing number of degenerative diseases.
Regulation of Thyroid Hormone Metabolism
Selenium is found to play an essential role in controlling metabolism of thyroid hormone. An enzyme called iodothyronine deiodinase is a selenoprotein, which is responsible for the conversion of the hormone thyroxine (T4) to its active form, triiodothyronine (T3). Selenium also protects thyroid gland from hydroperoxide damage as it is known to regulate iodine metabolism as well.
Substantial evidence is available on the role of selenium in supporting the functioning of immune system. Selenium supplementation is found to boost cell-mediated immune responses, improved antibody production and protection from oxidative damage.
Selenium plays a key role in maintaining normal reproductive performance, as phospholipid GSHPx (formerly known as sperm capsule selenoprotein), a structural selenoprotein found in the sperm tail. And studies have shown impaired sperm motility in case of selenium deficiency due to morphological abnormalities. Selenium is required for normal testosterone metabolism and testicular morphology as well.