Vitamin E
The Fat Antioxidant
The main form of vitamin E is alpha-tocopherol. Tocopherols were discovered at the University of California at Berkeley in 1922, where they were found to be essential to maintain fertility. Pure alpha-tocopherol was first isolated from wheatgerm oil in 1936. The word tocopherol means “to bear offspring” and derives from the Greek root phero, which means “to bring forth,” and the Greek root tos, which means “childbirth.” Tocopherols are a family of eight fat-soluble alcohols. The final “ol” in the name tocopherol indicates that it is an alcohol.
Antioxidant Activity
The main role of vitamin E is as an antioxidant to neutralize free radicals in cell membranes, in mitochondrial membranes, and in LDL. The fats in cell membranes are susceptible to oxidation by free radicals. The interior of the cell membrane is inaccessible to water-soluble antioxidants. The fat-soluble tocopherols are perfectly suited to protecting cell membranes from free radicals. The fat-soluble tail of the tocopherol can reach in and neutralize the free radicals.
Vitamin E also protects polyunsaturated fatty acids and vitamin Afrom free radical damage.
Vitamin E neutralizes free radicals by donating a hydrogen atom from the hydroxyl group (HO) on the hexagonal head of vitamin E.
Donation of hydrogen to a free radical is easiest for the alpha-tocopherol and gamma-tocopherol forms and slightly harder for the beta-tocopherol and delta tocopherol forms.
In cell membranes, the hexagonal head of vitamin E stays near the surface of the membrane. The tail of vitamin E is deeply implanted into the cell membrane. When the tail of vitamin E is oxidized by a free radical, it moves to the surface where the head of the vitamin E can donate hydrogen. In this way, vitamin E pulls free radicals out of fatty membranes for neutralization. This vitamin E cannot perform its antioxidant function a second time until it is regenerated.
The ability of vitamin E to perform as an antioxidant can be restored by the ascorbate form of vitamin C or by coenzyme Q. After restoring the antioxidant activity of vitamin E, the vitamin C then needs glutathione to restore its own antioxidant activity, as seen in Figure 5-5. In reactivating these antioxidants, both niacin and lipoic acid may play a role. Vitamin E has the potential to act as a free radical rather than as an antioxidant when co-antioxidants such as vitamin C are not available.
Vitamin E also protects polyunsaturated fatty acids and vitamin Afrom free radical damage.
Vitamin E neutralizes free radicals by donating a hydrogen atom from the hydroxyl group (HO) on the hexagonal head of vitamin E.
Donation of hydrogen to a free radical is easiest for the alpha-tocopherol and gamma-tocopherol forms and slightly harder for the beta-tocopherol and delta tocopherol forms.
In cell membranes, the hexagonal head of vitamin E stays near the surface of the membrane. The tail of vitamin E is deeply implanted into the cell membrane. When the tail of vitamin E is oxidized by a free radical, it moves to the surface where the head of the vitamin E can donate hydrogen. In this way, vitamin E pulls free radicals out of fatty membranes for neutralization. This vitamin E cannot perform its antioxidant function a second time until it is regenerated.
The ability of vitamin E to perform as an antioxidant can be restored by the ascorbate form of vitamin C or by coenzyme Q. After restoring the antioxidant activity of vitamin E, the vitamin C then needs glutathione to restore its own antioxidant activity, as seen in Figure 5-5. In reactivating these antioxidants, both niacin and lipoic acid may play a role. Vitamin E has the potential to act as a free radical rather than as an antioxidant when co-antioxidants such as vitamin C are not available.
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