NEBRASKA REDOX BIOLOGY CENTER EDUCATIONAL PORTAL

Nebraska Redox Biology Center Educational Portal


α-Tocopherol or Vitamin E

α-tocopherol or Vitamin E is a mamber of tocoferol family of 8 similar fat-based antioxidant compounds. These various compounds differ from each other in the identity of the substituents on the chromanol core. Tocotrienols differ from the corresponding tocopherols only in their aliphatic tail by presence of three double bonds. All eight tocopherols are active antioxidants, but only α-tocopherol is specifically retained in the body. α-tocopherol wan named as a vitamin E because it was found to be essential for fertility in rats [ 1, 2, 3 ].

Biological forms of tocopherol [ 4 ].

Serum concentrations of α-tocopherol is regulated by liver. The liver retain and resecretes only α-tocopherol via the hepatic α-tocopherol transfer protein and the liver metabolizes other forms of vitamin E forms [ 2, 5, 6 ]. Recommended Dietary Allowances for Vitamin E (α-tocopherol) is 4 mg/day for 0-6 month old, 7 mg/day for 4-8 year old, 11 mg/day for 9-13 year old and 11 mg/day for 14+ year old. vitamin E deficiency is rare and deficiency symptoms have not been found in healthy people who obtain vitamin E from their diets [ 5, 6 7 ]. Plant food is a major source of α-tocopherol. Nuts, seeds, and vegetable oils are among the best sources of α-tocopherol. Significant amount of α-tocopherol is available in green leafy vegetables and fortified cereals [ 5, 6 ].

The antioxidant network showing the interaction among vitamin E, vitamin C and thiol redox cycles [ 3 ].

α-tocopherol forms a stable tocopheroxyl radical as product of its redox reactions related to quenching lipid radicals. α-tocopherol cannot work alone in isolotaion from other antioxidants. The tocopheroxyl radical can be reduced to regenerate α-tocopherol by ubiquinol, ascorbic acid, glutathione and disulfide oxidorexductases. This catalysis occurs through the interactions between water and lipid-soluble substances by both nonenzymatic and enzymatic mechanisms. Vitamin C can regenerate vitamin E directly, and thiol antioxidants, such as glutathione and lipoic acid, can regenerate vitamin E indirectly via vitamin C [ 3, 8, 9 ].

Lipid radcals scavenging by α-tocopherol [ 5 ].

Several studies shown that α-tocopherol could help prevent or delay coronary heart disease and might also help prevent the formation of blood clots that could lead to a heart attack or venous thromboembolism [ 5, 10 ]. However, several randomized clinical trials have nort confirmed the efficacy of α-tocopherol supplements to prevent coronary heart disease and found that α-tocopherol provided no significant protection against heart attacks, strokes, unstable angina, or deaths from cardiovascular disease. All-cause mortality was significantly higher in the women taking the supplements. Furthermore, use of vitamin E was associated with a significantly increased risk of hemorrhagic stroke [ 11, 12, 13, 14 ].

HOPE-TOO clinical trial and Women's Health Study demonstrated that α-tocopherol supplement did not reduce the risk of developing any form of cancer [ 15 ]. Another clinical trial showed that α-tocopherol supplementation give 17 percent increased risk of prostate cancer [ 16 ]. AREDS trial demonstrated that α-tocopherol supplementation had no effect on the development or progression of cataracts and age-related macular degeneration [ 17 ].

One study showed that consumption of α-tocopherol from foods or supplements was associated with less cognitive decline over 3 years in a prospective cohort study of individuals aged 65-102 years [ 18 ]. However, another clinical trials showed that α-tocopherol supplements provided no apparent cognitive benefits and is not influence the progression rate of Alzheimer's disease [ 19, 20 ]. Finally, most results do not support the use of vitamin E supplements by healthy individuals.

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