The main inorganic dietary form of selenium is sodium selenite (Na2SeO3). Selenium does not function directly in the body in its elemental or ionic form. In the organic forms selenomethionine and selenocysteine, a selenium atom is present in the position occupied by a sulfur atom in the amino acids methionine and cysteine.There are nearly 30 known selenoproteins containing selenocysteine. Selenocysteine is the active site of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase.Glutathione peroxidase is the most abundant selenoprotein in mammals.Vitamin E, Vitamin C and Co-Enzyme Q10 neutralize free radicals by becoming free radicals that are less harmful than the ones they neutralize.

The glutathione and thioredoxin antioxidant systems function to regenerate other antioxidants while themselves providing the primary antioxidant defense in the water phase of the body. Glutathione peroxidase and thioredoxin reductase are two natural antioxidant enzymes that contain selenium and depend upon selenium activity for their antioxidant functionality. Both the glutathione and thioredoxin systems enhance their own antioxidant activity by inducing the production of other natural antioxidant enzymes.Glutathione peroxidase protects cell membranes from damage from free radicals released from hydrogen peroxide formed during normal metabolism in the cells' mitochondria.Glutathione peroxidase converts hydrogen peroxide into water before it can produce damaging free radicals.

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another question can explain for me how the free radical for Bleomycin compound mechanism that inhibit or kill cancer cells,,,,,,,, what is the mechanism effect on cancer cells and can effect on normal cells

Bleomycin binds ferrous iron and oxygen and after reduction in vivo produces an activated intermediate, a Fe3+ hydroperoxide [BLM-Fe(III)-OOH, ferric peroxide complex] that cleaves DNA by hydrogen abstraction. Bleomycin destroys malignant cells based on a metal-dependent prooxidant free radical mechanism leading to DNA fragmentation.