Selenium: Trace Mineral

Selenium is a trace mineral that is perhaps the least understood. I've found several sites regarding this mineral when a patient was actually diagnoses with a Selenium deficiency....

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Selenium


Selenium is a trace element that is essential in small amounts, but like all essential elements, it is toxic at high levels. Humans and animals require selenium for the function of a number of selenium-dependent enzymes, also known as selenoproteins. During selenoprotein synthesis, selenocysteine is incorporated into a very specific location in the amino acid sequence in order to form a functional protein. Unlike animals, plants do not appear to require selenium for survival. However, when selenium is present in the soil, plants incorporate it non-specifically into compounds that usually contain sulfur (1).

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Function

Selenoproteins

At least 25 selenoproteins have been identified, but the metabolic functions have been identified for only about one-half of them (2).

The selenoproteins with an identified function include:

Glutathione peroxidases

Five selenium-containing glutathione peroxidases (GPx) have been identified: cellular or classical GPx, plasma or extracellular GPx, phospholipid hydroperoxide GPx, gastrointestinal GPx, and olfactory GPx (2). Although each GPx is a distinct selenoprotein, they are all antioxidant enzymes that reduce potentially damaging reactive oxygen species (ROS), such as hydrogen peroxide and lipid hydroperoxides, to harmless products like water and alcohols by coupling their reduction with the oxidation of glutathione (diagram). Sperm mitochondrial capsule selenoprotein, an antioxidant enzyme that protects developing sperm from oxidative damage and later forms a structural protein required by mature sperm, was once thought to be a distinct selenoprotein but now appears to be phospholipid hydroperoxide GPx (3).

Thioredoxin reductase

In conjunction with the compound thioredoxin, thioredoxin reductase participates in the regeneration of several antioxidants, possibly including vitamin C. Maintenance of thioredoxin in a reduced form by thioredoxin reductase is important for regulating cell growth and viability (2, 4).

Iodothyronine deiodinases (thyroid hormone deiodinases)

The thyroid gland releases very small amounts of biologically active thyroid hormone (triiodothyronine or T3) and larger amounts of an inactive form of thyroid hormone (thyroxine or T4) into the circulation. Most of the biologically active T3 in the circulation and inside cells is created by the removal of one iodine atom from T4 in a reaction catalyzed by selenium-dependent iodothyronine deiodinase enzymes. Three different selenium-dependent iodothyronine deiodinases (types I, II, and III) can both activate and inactivate thyroid hormone by acting on T3, T4, or other thyroid hormone metabolites. Thus, selenium is an essential element for normal development, growth, and metabolism because of its role in the regulation of thyroid hormones (2, 5).

Selenoprotein P

Selenoprotein P is found in plasma and also associated with vascular endothelial cells (cells that line the inner walls of blood vessels). The primary function of selenoprotein P appears to be a transport protein for selenium (6). It also functions as an antioxidant that protects endothelial cells from damage induced by such compuonds as peroxynitrite, a reactive nitrogen species (RNS) (7).

Selenoprotein W

Selenoprotein W is found in muscle. Although its function is presently unknown, it is thought to play a role in muscle metabolism (. There is about 80% homology of this selenoprotein from six different species of animals (9). For more information on selenoprotein W, see the Linus Pauling Institute's Spring/Summer 2007 Research Newsletter.

Selenophosphate synthetase

Incorporation of selenocysteine into selenoproteins is directed by the genetic code and requires the enzyme selenophosphate synthetase. A selenoprotein itself, selenophosphate synthetase catalyzes the synthesis of monoselenium phosphate, a precursor of selenocysteine that is required for the synthesis of selenoproteins (2).

Methionine-R-sulfoxide reductase

Methionine-R-sulfoxide reductase was initially identified as selenoprotein R and selenoprotein X by two different laboratories. However, later studies revealed that the protein catalyzes stereospecific reduction of oxidized methionine residues in reactions that use thioredoxin as a reductant. There are two forms of this specific selenoprotein (2).

15 kDA selenoprotein (Sep15)

Sep15 is mammalian protein located in the endoplasmic reticulum of the cell. Here, it binds UDP-glucose:glycoprotein glucosyltransferase, an enzyme that senses protein folding. Sep 15 has a redox function and is also implicated in cancer prevention (2).

Selenoprotein V

Selenoprotein V is expressed exclusively in testes and is thought to function in spermatogenesis (2).

Selenoprotein S

Selenoprotein S is involved in retrotranslocation of misfolded proteins from the endoplasmic reticulum to the cytosol. This protein may also be involved in inflammatory and immune responses (2).

http://lpi.oregonstate.edu/infocenter/minerals/selenium/

from the same article:

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Deficiency

Insufficient selenium intake results in decreased activity of the glutathione peroxidases as well as some other thioredoxin reductase and thyroid deiodinases. Even when severe, isolated selenium deficiency does not usually result in obvious clinical illness. However, selenium-deficient individuals appear to be more susceptible to additional physiological stresses (11).

Individuals at increased risk of selenium deficiency

Clinical selenium deficiency has been observed in chronically ill patients who were receiving total parenteral nutrition (TPN) without added selenium for prolonged periods of time. Muscular weakness, muscle wasting, and cardiomyopathy (inflammation and damage to the heart muscle) have been observed in these patients. TPN solutions are now supplemented with selenium to prevent such problems. People who have had a large portion of the small intestine surgically removed or those with severe gastrointestinal problems, such as Crohn's disease, are also at risk for selenium deficiency due to impaired absorption. Specialized medical diets used to treat metabolic disorders, such as phenylketonuria (PKU), are often low in selenium. Specialized diets that will be used exclusively over long periods of time should have their selenium content assessed to determine the need for selenium supplementation (11).

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Keshan disease

Keshan disease is a cardiomyopathy that affects young women and children in a selenium-deficient region of China. The acute form of the disease is characterized by the sudden onset of cardiac insufficiency, while the chronic form results in moderate to severe heart enlargement with varying degrees of cardiac insufficiency. The incidence of Keshan disease is closely associated with very low dietary intakes of selenium and poor selenium nutritional status. Selenium supplementation protects people from developing Keshan disease but cannot reverse heart muscle damage once it occurs (11, 12). Despite the strong evidence that selenium deficiency is a fundamental factor in the etiology of Keshan's disease, the seasonal and annual variation in its occurrence suggests that an infectious agent is involved in addition to selenium deficiency. Coxsackievirus is one of virus type that has been isolated from Keshan patients, and studies in selenium-deficient mice show that this virus is capable of causing an inflammation of the heart called myocarditis. Studies in mice also indicate that oxidative stress induced by selenium deficiency results in changes in the viral genome; such genomic changes are capable of converting a relatively harmless viral strain to a myocarditis-causing strain (13, 14). Though not proven in Keshan disease, selenium deficiency may result in a more virulent strain of virus with the potential to invade and damage the heart muscle. See Disease Prevention for more information on selenium and viral infection.

Kashin-Beck Disease

Kashin-Beck disease is characterized by the degeneration of articular cartilage between joints (osteoarthritis) and is associated with poor selenium status in areas of northern China, North Korea, and eastern Siberia. The disease affects children between the ages 5 and 13 years. Severe forms of the disease may result in joint deformities and dwarfism. Unlike Keshan disease, there is little evidence that improving selenium nutritional status prevents Kashin-Beck disease. Thus, the role of selenium deficiency in the etiology of Kashin-Beck disease is less certain. A number of other causative factors have been suggested for Kashin-Beck disease, including fungal toxins in grain, iodine deficiency, and contaminated drinking water (11, 12).

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The Recommended Dietary Allowance (RDA)

The RDA was revised in 2000 by the Food and Nutrition Board (FNB) of the Institute of Medicine. The most recent RDA is based on the amount of dietary selenium required to maximize the activity of the antioxidant enzyme glutathione peroxidase in plasma (15).

Life Stage Age Males (mcg/day) Females (mcg/day)

Infants 0-6 months 15 (AI) 15 (AI)

Infants 7-12 months 20 (AI) 20 (AI)
Children 1-3 years 20 20
Children 4-8 years 30 30
Children 9-13 years 40 40
Adolescents 14-18 years 55 55
Adults 19 years and older 55 55
Pregnancy all ages - 60
Breast-feeding all ages - 70