الفهرس 
Selenium Properties and PhysiologyOnly 14 pages are availabe for public view
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Abstract
Selenium has long been a source of concern as a toxic element but research over time has shown that it is vital for human health. Selenium is the chief component of selenoproteins which exert their major role as antioxidants. An adequate level of body Se status is essential for exerting its beneficial effects. Different biomarkers have been evaluated for the measurement of selenium in the body. Selenium occurs in both organic and inorganic forms. The organic form includes selenomethionine and selenocysteine which are found predominantly in plants and animals respectively. Population Selenium levels are different in different geographic areas of the world because of variability in selenium content of soil. Selenium compounds are generally very efficiently absorbed by humans. Although, the gastrointestinal tract is a major determinant, tissue utilization of the absorbed nutrient and renal conservation (Se) are also important factors influencing bioavailability. The main route of selenium excretion is urine, and very small amount is excreted through expired air and feces. Selenium is an essential element presenting a very narrow range between deficient, essential and toxic doses. Excess of everything is bad; same is shown in case of Se toxicity.
Selenium is the key component of the active site of several Se-proteins having essential biological functions. Twenty-five Se-proteins have been identified in the human proteome. Most Se-proteins exhibit antioxidant activities, but other specific processes have been linked with Se-proteins, including reduction of oxidized proteins and membranes, redox regulation of transcription factors, regulation of apoptosis, immunomodulation, regulation of thyroid hormones, selenium transport and storage, protein folding and degradation of misfolded proteins in the endoplasmatic reticulum.
Under normal physiological conditions, a homeostatic balance exists between the formation of reactive oxygen species and their removal by endogenous antioxidant scavenging compounds. Severe sepsis is characterized by an increase in ROS and low endogenous antioxidant capacity. Systemic inflammatory response syndrome (SIRS) is a syndrome related to critical illness in which Se status may play a key role in clinical outcome. SIRS is characterized by an early decrease in Se status as demonstrated by low plasma Se, GPx3 activity, and SELP levels. It was demonstrated that there was a correlation between initial plasma Se levels and clinical outcome, with patients with SIRS exhibiting lower Se levels presenting with a higher rate of nosocomial pneumonia, organ system failure, and mortality.
Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H2O2 produced for thyroid hormone biosynthesis. The Se content in endocrine tissues is higher than in many other organs. Selenium has also a role in autoimmune thyroid diseases. Also, there is an association between selenium and type 2 diabetes. The evidence supporting selenium on the risk of diabetes is variable occasionally conflicting, and limited to very few human studies.
Selenium is an essential dietary component for humans and is regarded as a protective agent against cancer. Although the mode of anticancer action of Se is not fully understood yet, several mechanisms, such as antioxidant protection by selenoenzymes, specific inhibition of tumor cell growth by Se metabolites, modulation of cell cycle and apoptosis, and effect on DNA repair have all been proposed. Moreover, recent studies suggest that Se has a potential to be used not only in cancer prevention but also in cancer treatment where in combination with other anticancer drugs or radiation, it can increase efficacy of cancer therapy. In combating cancer cells, Se acts as pro-oxidant rather than antioxidant, inducing apoptosis through the generation of oxidative stress. Thus, the inorganic Se compound, sodium selenite (SeL), due to its prooxidant character, represents a promising alternative for cancer therapy.
Selenium levels can affect the viruses as Coxsackie B virus, HIV and Polio virus. Se, also participates in the immune response through several actions. Negative correlations were observed between serum Se level and rheumatoid arthritis, asthma, and immune activation in Crohn’s disease. There are important roles for Se and Se-proteins in the brain, which is also highly exposed to oxidative stress due to elevated oxygen consumption. Damage from ROS takes place in several neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, ischemic damage, brain tumors, multiple sclerosis and epilepsy. Low plasma Se concentrations have been associated with increased cardiovascular disease mortality. Following a severe burn injury, patients lose significant amounts of selenium through wound exudate. Selenium deficiency is linked with immune compromise and given that burn patients are prone to nosocomial infection it is important to ensure that they receive supplemental selenium during their course in hospital. Plasma GPx3 activity and Se level in RBCs, whole blood or plasma were found to be significantly lower in patients with chronic renal failure (CRF) and in hemodialyzed (HD) uremic patients compared to healthy controls, in some cases it has also been associated with progression of the disease.