Free Radicals and Antioxidants

Vitamin E (Lipid Soluble Anti-oxidant Vitamin)

Manoj Sigdel

Department of Biochemistry

Manipal College of Medical Sciences

Email: manoj.sigdel@hotmail.com

Vitamin E- An Antioxidant Vitamin

Vitamin E is a naturally occuring antioxidant

Essential for normal reproduction, so known as anti-sterility vitamin

Known as vitamin in search of a disease (due to the lack of any specific deficiency disease in humans)

Chemistry

Vitamin E is a name given to a group of tocopherols and tocotrienols

Includes eight naturally occuring tocopherols and tocotrienols

The most active one is D--tocopherol

Chemically, they are isoprenoids with 6-hydroxylchromane ring

D--tocopherol

For the four tocopherols the side chain is saturated, whereas for the next four tocotrienols the side chain contains three double-bonds, all of which adjoin a methyl group

tocopherols

tocotrienols

The four tocopherols and the four tocotrienols have an , , and form named on the basis of the number and position of the methyl groups on the chromanol ring

Tocopherol (8- methyltocol)

Absorption, transport and storage

Absorbed along with fat in the small intestine

Bile salts are necessary for absorption

In the liver, it is incorporated into lipoproteins and transported

Stored in adipose tissue, liver and muscle

1) Membrane antioxidant

Cell membranes and plasma lipoproteins contain tocopherol which function as a chain breaking antioxidant

Attached to the hydrophobic structure of alpha tocopherol is a hydroxyl group which hydrogen is easy to remove

Biochemical Functions

When peroxyl and alkoxyl radicals are generated during lipid peroxidation they will combine preferably with tocopherol rather than with an adjacent fatty acid

tocopherol is converted to tocopheryl( tocopherol-O.) free radical which is poorely reactive and will not attack an adjacent fatty acid side chain

tocopheryl free radical now is reoxidised to tocopherol by dehydroascorbic acid

Or, tocopheryl free radical reacts further with peroxyl free radical so that the chromane ring and the side chain are oxidised to the non free radical product

Vit E serves as a chain-breaking, free radical trapping antioxidant in cell membranes

hydroperoxide product ROOH,

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This oxidation product is conjugated with glucuronic acid and excreted in the bile

If it reacts in this second manner, tocopherol is not recycled

The anti-oxidant action of tocopherol is effective at high oxygen concn

Concentrated at those lipid structures that are exposed to highest O2 partial pressures eg;

erythrocyte membrane,

membranes of the respiratory tree, and

retina

This oxidation product is conjugated with glucuronic acid via the 2-hydroxyl group and excreted in the bile

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2) Preserves and maintain germinal epithelium of gonads. So associated with reproductive function and prevents sterlity

3) Increases the synthesis of heme by enhancing the activity of aminolevulinic acid (ALA) synthase

4) Prevents the oxidation of LDL

5) Works in association with vitamins A, C and carotene to delay the onset of cataract

Vegetable oils are rich sources of vitamin E, whereas liver and eggs contain moderate amounts

Sources of Vitamin E

Daily Requirement

The RDA for -tocopherol is 10 mg for men and 8 mg for women

Requirement increases as the intake of polyunsaturated fatty acid (PUFA) increases

Deficiency of vitamin E

Vitamin E deficiency is almost entirely restricted to premature infants.

When observed in adults, it is usually associated with defective lipid absorption or transport.

The signs of human vitamin E deficiency include sensitivity of erythrocytes to peroxide free radical (ROO.-), and the appearance of abnormal cellular membranes

Toxicity of vitamin E

Vitamin E is the least toxic of the fat-soluble vitamins, and no toxicity has been observed at doses of 300 mg/day for 23 years

Vitamin E and Selenium

Selenium is found in the enzyme glutathione peroxidase that destroys free radicals

Antioxidant function of Selenium and vit E are similar and both act synergistically

Selenium can spare the requirement of Vit E and vice versa

Interaction between antioxidants in the cell membranes and the cytosol

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