Selenium as antioxidant - Functions of selenium

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Selenium antioxidant function is brought about by the Glutathione peroxidase selenoproteins.
In the biochemical processes in the body certain oxidative free radicals are produced. Though these are required in smaller quantities for the biological process, excess of them can be self-hurting, damaging our cells and membranes and also triggering oxidation of LDL and plaque deposit on the arterial walls.
Antioxidant (catalyzed by selenium compounds) neutralizes these free radicals turning them into harmless products.
We get some of the antioxidant forms from our food, whereas many of them are endogenously synthesized by our body.
Many of their function are catalyzed by enzymes and among them selenoproteins (obviously containing selenium) like glutathione peroxidases (GPX) and thioredoxin reductases are very important.

Selenium as antioxidant

This mineral in its rare elemental form is not absorbed as such by our body and does not have any biological function except being toxic in excess exposure and inhalation.
However the water soluble selenite and selenate salts of selenium, the amino acid selenomethionine and food forms are absorbed by the body.
In protein building, selenomethionine can be used by the body instead of the amino acid methionine, resulting in some of the selenium incorporated in tissues and unavailable in plasma.
Though selenium compound as such is not antioxidant, it has the vital action in catalyzing the reduction activity as an enzyme.
The cellular and sub cellular membrane integrity depends totally on glutathione peroxidases.
Further the protective catalyst function of glutathione peroxidase itself depends on the presence of selenium.

Selenoproteins as selenium enzymes

This mineral is a constituent of the selenoproteins enzymes glutathione peroxidases (GPXs) and thioredoxin reductases (TrxRs).
GPXs main function is in catalysing the oxidation of glutathione (GSH), the most important endogenous antioxidant produce by the body cells.
Glutathione (GSH) in the presence of selenium enzymes, participates directly to neutralize the harmful free radicals and other reactive oxygen compounds.
It also keeps the exogenous polyphenols, vitamin C and vitamin E in their active reduced forms.
Thioredoxin reductases (TrxRs) catalyse the reduction of the redox protein thioredoxin (Trx); the reaction is NADPH-dependent.

Types of selenium antioxidant enzymes and their functions

There are several isozymes of the selenium containing selenoprotein enzymes glutathione peroxidases.
They vary in their substrate specificity and their cellular location.
In humans, so far eight isoforms of glutathione peroxidases have been identified.
The most abundant form Glutathione peroxidase 1 (GPX1) is found in the cytoplasm of nearly all cells with hydrogen peroxide (H2O2) as preferred substrate.
Glutathione peroxidase 2 (GPX2) is an intestinal and extracellular enzyme.
Glutathione peroxidase 3 (GPX3) is extracellular enzyme and abounds plasma.
Glutathione peroxidase 4 (GPX4) is present in every cell and its preferred substrates are lipid hydroperoxides.
The antioxidant catalyst process is as follows.
2GSH +H2O2 → GS-SG + 2H2O
(GSH represents reduced monomeric glutathione, and GS–SG represents glutathione disulfide.)
The function and process is at the selenium-cysteine site, wherein it is in a Se(-) form as resting state.
This is oxidised by peroxide to SeOH which then reacts with GSH to form GS-Se and water.
Then Gs-Se reacts with another GSH molecule to form Se(-) again, releasing GS-SG as the by-product.
Glutathione reductase then reduces the oxidized glutathione to form the GSH again.
GS–SG + NADPH + H+ → 2 GSH + NADP+
Thioredoxin reductases (TrxRs) are a family of selenium-containing pyridine nucleotide-disulphide oxidoreductases.
The main function of TrxRs is catalysis of the NADPH-dependent reduction of the redox protein thioredoxin (Trx), as well as of other exogenous and endogenous compounds.
Presently two confirmed forms have been identified.
Again The availability of Selenium is a key factor for TrxR activity.
Related topics on selenium health benefits:
Treatment of Keshan disease
Heart disease
Health benefits
Prostate cancer
Effect on thyroid hormones
SELECT for cancer prevention
Supplements and dosage
Overdose and side effects
Selenium antioxidant - selenoproteins - function (current topic)

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