Vitamin E refers to a group of compounds that include both tocopherols and tocotrienols. Of the many different forms of vitamin E, γ-tocopherol is the most common form found in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine, and dressings. α-tocopherol, the most biologically active form of vitamin E, is the second-most common form of vitamin E in the diet. This variant can be found most abundantly in wheat germ oil, sunflower, and safflower oils. As a fat-soluble antioxidant, it interrupts the propagation of reactive oxygen species that spread through biological membranes or through a fat when its lipid content undergoes oxidation by reacting with more-reactive lipid radicals to form more stable products. Regular consumption of more than 1,000 mg (1,500 IU) of tocopherols per day may be expected to cause hypervitaminosis E, with an associated risk of vitamin K deficiency and consequently of bleeding problems.
FormsThe nutritional content of vitamin E is defined by α-tocopherol activity. The molecules that contribute α-tocopherol activity are four tocopherols and four tocotrienols, identified by the prefixes alpha- (α-), beta- (β-), gamma- (γ-), and delta- (δ-). Natural tocopherols occur in the RRR-configuration only. The synthetic form contains eight different stereoisomers and is called 'all-rac'-α-tocopherol. Water-soluble forms such as d-alpha-tocopheryl succinate are used as food additive.
α-Tocopherolalpha-Tocopherol is an important lipid-soluble antioxidant. It performs its functions as antioxidant in the glutathione peroxidase pathway, and it protects cell membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction. This removes the free radical intermediates and prevents the oxidation reaction from continuing. The oxidized α-tocopheroxyl radicals produced in this process may be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol. Other forms of vitamin E have their own unique properties; for example, γ-tocopherol is a nucleophile that can react with electrophilic mutagens.
Tocotrienols[[File:Tocotrienols.svg|350px|right|thumb|General chemical structure of tocotrienols. alpha(α)-Tocotrienol: R1 = Me, R2 = Me, R3 = Me; beta(β)-Tocotrienol: R1 = Me, R2 = H, R3= Me; gamma(γ)-Tocotrienol: R1 = H, R2 = Me, R3= Me; delta(δ)-Tocotrienol: R1 = H, R2 = H, R3= Me]] Tocotrienols are members of the vitamin E family: four tocotrienols (alpha, beta, gamma, delta) similar in structure to the four tocopherols, with the critical difference is that tocopherols have saturated side chains whereas tocotrienols have unsaturated isoprenoid side chains with three double bonds. Preliminary clinical trials on dietary supplement tocotrienols indicate potential for anti-disease activity. Tocotrienols have lower bioavailability in blood, potential for anticoagulant effects, and appear to be safe and well-tolerated.
FunctionsVitamin E has many biological functions, including its role as a fat-soluble antioxidant.
- As an antioxidant, vitamin E acts as a peroxyl radical scavenger, disabling the production of damaging free radicals in tissues, by reacting with them to form a tocopheryl radical, which will then be reduced by a hydrogen donor (such as vitamin C) and thus return to its reduced state. As it is fat-soluble, it is incorporated into cell membranes, which protects them from oxidative damage. Vitamin E has also found use as a commercial antioxidant in ultra high molecular weight polyethylene ( UHMWPE) used in hip and knee implants by resisting oxidation.
- As an enzymatic activity regulator, for instance, protein kinase C (PKC), which plays a role in smooth muscle growth, can be inhibited by α-tocopherol. α-Tocopherol has a stimulatory effect on the dephosphorylation enzyme, protein phosphatase 2A, which in turn, cleaves phosphate groups from PKC, leading to its deactivation, bringing the smooth muscle growth to a halt.
- Vitamin E also has an effect on gene expression. Macrophages rich in cholesterol are found in atherosclerotic tissue. Scavenger receptor CD36 is a class B scavenger receptor found to be up-regulated by oxidized low density lipoprotein (LDL) and binds it. Treatment with α-tocopherol was found to downregulate the expression of the CD36 scavenger receptor gene and the scavenger receptor class A (SR-A) and modulates expression of the connective tissue growth factor (CTGF). The CTGF gene, when expressed, is responsible for the repair of wounds and regeneration of the extracellular tissue lost or damaged during atherosclerosis.
- Vitamin E also plays a role in eye and neurological functions, and inhibition of platelet coagulation.
- Vitamin E also protects lipids and prevents the oxidation of polyunsaturated fatty acids.
DeficiencyVitamin E deficiency can cause:
- spinocerebellar ataxia
- peripheral neuropathyInstitute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press, 2000.
- skeletal myopathy
- impairment of the immune response
- red blood cell destruction