Selenium in biology

In humans, selenium is a trace element nutrient that functions as cofactor for glutathione peroxidases and certain forms of thioredoxin reductase.

The main selenium indicator plants are Astragalus species (including some locoweeds), prince's plume (Stanleya sp.

[13] Selenium may be measured in blood, plasma, serum or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in a forensic investigation in a case of fatal overdosage.

Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to being eliminated in the urine.

Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.

[17] In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale) have suffered from selenium toxicity.

[18] Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage.

[21] Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses.

Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands.

[24] Selenium deficiency can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and[25] in those of advanced age (over 90).

[29] The US Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for selenium in 2000.

[33] For US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV).

Brazil nuts are the richest ordinary dietary source and could cause selenium toxicity if consumed regularly – though the actual concentration of selenium (as with any plant-based food sources, such as another selenium-accumulating "paradise nut" Lecythis, belonging to the same family Lecythidaceae) is soil-dependent and may vary significantly by geographic location.

[39] "Although an inverse association between selenium exposure and the risk of some types of cancer was found in some observational studies, this cannot be taken as evidence of a causal relation, and these results should be interpreted with caution...

"[40] To date, many studies have been conducted on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level, indicating that likely selenium functions as an immunostimulator, i.e. reversing the immunosuppression in tumour microenvironment towards antitumour immunity by activating immune cells (e.g. M1 macrophages and CD8+ T-lymphocytes, the elevated number of neutrophils and activated cytotoxic NK cells) and releasing pro-inflammatory cytokines such as IFNγ and TNFα.

Whether this decline in selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.

[44] Abnormally high or low levels of dietary selenium can have an adverse effect on sperm quality, with a consequent lowering of fertility.

However, increased selenium intake can preserve the enzyme activities, reducing the adverse effects caused by mercury exposure.

The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchin, fish and chicken.

Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium, magnesium, copper, and zinc, may have been lacking in some terrestrial mineral-deficient areas.

These findings suggest that aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element.

From about 500 Mya, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (Vitamin C), polyphenols (including flavonoids), tocopherols, etc.

In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.

Selenocysteine is the main organic molecule involving selenium in humans .
Relationship between survival of juvenile salmon and concentration of selenium in their tissues after 90 days (Chinook salmon: Hamilton et al. 1990) or 45 days (Atlantic salmon: Poston et al. 1976) exposure to dietary selenium. The 10% lethality level (LC10=1.84 μg/g) was derived by applying the biphasic model of Brain and Cousens (1989) to only the Chinook salmon data. The Chinook salmon data comprise two series of dietary treatments, combined here because the effects on survival are indistinguishable.
Selenium at nutritional levels is required for cell homeostasis , playing a role as an anti-oxidant through selenoproteins , thus, act chemo-preventive against cancer. In contrast, supra-nutritional levels act as pro-oxidant toxic in tumour cells. [ 39 ]
Selenium can either play a pro-oxidant role inducing ROS , activating the Akt–NF–кB pathway or play an antioxidant role through selenoprotein synthesis such as TXNRD relocating to the nucleus and activating NF-кB , resulting in further activation of leukocytes and pro-inflammatory cytokine genes. [ 39 ]