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Vitamin E

Definition:
Vitamin E is a generic term used for a group of chemically-similar compounds sharing the tocopherol and tocotrienol structures, which are lipid-soluble; hence, vitamin E is a fat-soluble vitamin that exists in eight different forms. Each form has its own biological activity, which is the measure of potency or functional use in the body. Alpha-tocopherol (α-tocopherol) is the name of the most active form of vitamin E in humans.
Vitamin E is also a powerful biological antioxidant which protects against lipid peroxidation (which could contribute to cell membrane weakness).
Vitamin E in supplements is usually sold as alpha-tocopheryl acetate, a form that protects its ability to function as an antioxidant.
The synthetic form is labeled "D, L" while the natural form is labeled "D". The synthetic form is only half as active as the natural form.

Vitamin E - Structures and Chemistry:

Vitamin E chemical structure Vitamin E chemical structure Nomenclature
Vitamin E is the generic term used for all of the compounds in this group.

The vitamin can exist as two types of structures:

The tocopherol and tocotrienol structures. Both structures are similar except the tocotrienol structure has double bonds on the isoprenoid units. There are many derivatives of these structures due to the different subsistent possible on the aromatic ring at positions 5, 6, 7, and 8.
Notice that there are three chiral centers, at positions 2', 4', and 8', in the phytyl tail. There is thus the possibility of eight stereoisomer's. The most abundant of the naturally-occurring forms is the R,R,R form.
The tocotrienols share the same ring structure, but have an unsaturated tail.
 



Position of methyl groups on aromatic ring Tocopherol structure Tocotrienol structure
5,7,8 alpha-Tocopherol alpha-Tocopherol
5,8 beta-Tocopherol beta-Tocopherol
7,8  tau-Tocopherol tau-Tocopherol
8 delta-Tocopherol delta-Tocopherol

Chemistry
Rersonance-stabilized radicalTocopherols (Vitamin E) are equipped to perform a unique function. They can interrupt free radical chain reactions by capturing the free radical; this imparts to them their antioxidant properties. The free hydroxyl group on the aromatic ring is responsible for the antioxidant properties. The hydrogen from this group is donated to the free radical, resulting in a relatively stable free radical form of the vitamin.

Physiological Aspects of Vitamin E:
Absorption:
The process of absorption is passive and does not require the use of a protein carrier to bring in the vitamin. Absorption occurs in the small intestine and the vitamin can only be absorbed if it has been cleaved by esterase located in the stomach lining. The vitamin is then packaged into very low-density lipoproteins (v LDL) by the addition of lipid-like substances. vLDLs enter the lymphatic cells and are eventually released into the bloodstream. The alpha-tocotrienol appears to be better absorbed than the other tocopherol forms.

Transport:
Vitamin E does not have a specific carrier protein in the bloodstream but it is transferred by hepatic and lymphatic mechanisms. When it is first absorbed into the hepatic portal vein it is contained inside a lipid like structure called a "chylomicron". This structure is then converted hepatically to three distinct lipoprotein structures: high-density lipoprotein)(HDL), low-density lipoprotein (LDL), and vLDLs.

The alpha-tocotrienol is the form most rapidly secreted into the plasma after uptake.
The transport process is an important aspect in the delivery of vitamin E. Without the protective barrier of the lipoproteins (HDL, LDL, and VLDL), the vitamin would be exposed to the oxidative radicals circulating through the blood. Additionally, vitamin E will prevent the oxidation of LDL; oxidized LDL is thought to be a factor in atherogenesis, so adequate amounts of vitamin E will help protect against hardening of the arteries, heart attacks, and stroke.

Tissue Uptake:
Tissue uptake occurs by one of two ways: by lipases digesting the lipoprotein constituents or by "receptor mediated uptake" by binding of the lipoprotein to a specific tissue receptor site. This allows for the vitamin to enter the tissue. 
Vitamin E enters a variety of different tissue types, with adipose and the adrenal gland having the highest levels. It is found primarily in mitochondria. It is thought to play a role there in either stabilizing ubiquinone, or in helping ubiquinone transfer electrons.
The body's capacity for storage of the vitamin is worth noting. The vitamin can be stored in tissue for long periods of time (years) due to its exceedingly slow turnover rate. Interestingly, the natural R,R,R-alpha-tocopherol form of the vitamin is stored up to six times longer than synthetic versions.

Metabolism:
Vitamin E is considered to be metabolized after it has performed its antioxidant function (see Vitamin E Chemistry). It is converted from a tocopherol to a tocopherylquinone. The elimination of this end product is primarly through the feces but a small fraction is removed by urine (less than 1 percent). 
In order for tocopherylquinone to be excreted, it first has to be converted to tocopherylhydroquinone, a partially reduced form. This form can then combined with glucuronic acid so that it can mix with bile. Bile is removed from the body through feces.

Sources of vitamin E from diet:
The human can get vitamin E from 4 main groups:
Vegetable oils:
Corn, canola, sunflower, sesame, cottonseed, peanuts, rice bran, and palm oils.

Nuts:
Almonds, walnuts, peanuts, pistachios and hazelnuts.

Oil seeds, legumes and grains:
Corn, lentils, wheat, rice, northern beans, chickpeas, Barley Grassand oats.

Wheat

If you don't want to take vitamin E supplements then consider wheat germ and wheat germ oil . These are great sources of natural vitamin E. Because they are rich in vitamin E you can raise your intake without taking in too much fat.

Wheat germ oil Extracted from the germ of wheat, Wheat germ oil has been used since the '20s as a vitamin E supplement. It offers a good combination of tocopherols plus tocotrienols.

Foods containing wheat are a good source of vitamin E. These foods vary in their content of vitamin E based on the particular source and processing involved. Wheat germ oil is the richest source of natural vitamin E. If the wheat product is processed to make other foods such as margarine, the content of vitamin E is reduced due to the methods involved in formulation and exposure to chemicals (acids and bases) that can destroy vitamin E.

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