VITAMIN E

The most active form of vitamin E, a-tocopherol, is a 6-hydroxychroman derivative with methyl groups in position 2,5,7, and 8 and a phytyl side chain attached at carbon 2. There are 8 known forms of vitamin E: a-,b-,g-, and d-tocopherols contain saturated phytol side chains and a-, b-, g-, and dd-tocotrienols have 3 double bonds in the side chain.

It seems that d-tocotrienol is the earliest member of the group to be formed in plants, methylation leading to the other tocotrienols and hydrogenation producing the respective tocopherols.
Natural a-tocopherol, termed d-a-tocopherol, may be described chemically as 2R-(4'R,8'R)-5,7,8-trimethyltocol, the term tocol being the name for the 2-ring structure basic to all vitamin E compounds.
The term "vitamin E" should be used for all tocopherol and tocotrienol derivatives exhibiting the biological activity of d-a-tocopherol. The term "tocopherol" should be used for all methyl tocols. Since tocotrienols have some vitamin E activity, "tocopherol" is not synonymous with "vitamin E".

Vitamin E was discovered in 1922 when Evans HM et al. (Science 1922, 56, 650) described a "substance X" that was essential to maintain rat fertility. After obtaining similar results, Sure B called the substance "vitamin E" because vitamins A, B, C, and D were already known (Sure B, J Biol Chem 1924, 58, 693). 
Pure a-tocopherol was isolated from wheat-germ oil in 1936 (Evans HM et al., J Biol Chem 1936, 113, 319). Evans et al. called for the first time the isolated substance "tocopherol", a name derived from the ancient Greek word phero, "to bring" and the word tocos, meaning "childbirth". 
By 1931, Cummings MJ et al. suggested that vegetable oils rich in vitamin E could prevent the loss of vitamin A in mixed diets by protecting it from oxidation and suggested that vitamin E had "anti-oxidant activity" (J Nutr 1931, 3, 421). Later, in 1937, Olcott HS et al determined that all tocopherols are effective antioxidants (J Am Chem Soc 1937, 59, 1008).
The correct structure of a-tocopherol was given in 1938 (Fernholz E, J Am Chem Soc 1938, 60, 700), and its first synthesis (all-racemic form) realized the same year (Karrer P et al., Helv Chim Acta 1938, 21, 820).

Tocotrienols were prepared for the first time in 1963 (Schudel P et al., Helv Chim Acta 1963, 46, 2517) and synthesized in 1976 (Scott JW et al.,Helv Chim Acta 1976, 59, 290).

A comprehensive history of the discovery of vitamin E may be found in a paper by Evans HM (Vitam Horm 1962, 20, 379) and Wolf G (J Nutr 2005, 135, 363).
                
A new vitamin E constituent, a-tocomonoenol, was discovered in palm oil (Matsumoto A et al., J Jap Oil Chem Soc 1995, 44, 593) and was attributed as a biosynthetic intermediate along the reductive pathway from tocotrienols to tocopherols. 

Later, an isomeric form of that compound was isolated from salmon eggs and named "marine-derived tocopherol", it was shown to have identical antioxidant activity as does a-tocopherol (Yamamoto Y et al., J Nat Prod 1999, 62, 1685). It was also shown that it is broadly distributed in the tissue of marine fish, its content being greater in cold-water fish than in tropical fish (Yamamoto Y et al., PNAS 2001, 98, 13144).

Humans and animals are unable to synthesize vitamin E, they must obtain it from plant sources. Whereas various homologs are found in plants, only a-tocopherol and a lower proportion of g-tocopherol are present in human and animal tissues including blood. Enrichment of the diet is commonly performed with synthetic all-rac-a-tocopherol acetate. The biological activities vary depending on the vitamer and the test chosen, they range from 100% for a-tocopherol to 30% for g-tocopherol and 1.4% for d-tocopherol. It must be noticed that g-tocopherol was found to be more potent than a-tocopherol in its interaction with reactive nitrogen oxide species (Cooney RV et al., Proc Natl Acad Sci USA 1993, 90, 1771). The biopotency of a-tocotrienol was found to be about 30% but, recently, it was found to be a better antioxidant than a-tocopherol (Suzuki YJ et al., Biochemistry 1993, 32, 10692). The interest in tocotrienol as a hypocholesterolemic compound began in 1986 (Qureshi AA et al. J Biol Chem 1986, 261, 10544), several subsequent studies confirmed this observation (Theriault A et al., Clin Biochem 1999, 32, 309). A review summarizing the main antioxidant and nonantioxidant effects of tocotrienols and their potential as health-maintaining compounds was released by Schaffer S et al. (J Nutr 2005, 135, 151) and by Nesaretnam K et al. (Eur J Lipid Sci Technol 2007, 109, 445). Actually, all the biological activity of vitamin E is seen and understood in the light of protection of membrane fatty acids against any oxidative attack (Traber MG et al., Free Rad Biol Med 2007, 43, 4).

To know more about vitamin E antioxidant properties, consult the VERIS web site.

Up to 1957 an International Unit was used and defined as the vitamin E activity of 1 mg of a-tocopherol acetate (the average amount of the vitamin required to prevent gestation-resorption in rats deprived of vitamin E). Now, owing to the knowledge of the molecular purity and stereochemical composition of preparations of a-tocopherol, an international reference is no longer used.

Vitamin E is mainly present in oils of seeds. Alfalfa (5 mg/100 g), corn (15 mg/100 g) and soybean (110 mg/100 g) represent the major sources for animals. While tocopherols are generally present in nuts and common vegetable oils, tocotrienols are concentrated in several plant sources such as cereal grains (rye, barley, oat) and certain oils (palm oil, rice bran oil). In human nutrition, the major sources are salad oil, dressings, shortenings and margarines.

T: tocopherol, T3: tocotrienol, all are expressed in mg/100 g

Tocopherol and tocotrienol contents of several vegetable oils and industrial fats may be found in an original article (Schwartz H et al., J Food Comp Anal 2008, 21, 152).

The vitamin E requirement for human adults is about 15 mg per day a-tocopherol equivalents.

European legislation for dietary additives accepts 4 types of tocopherols:

- E 306: natural extract enriched in tocopherols (more than 34%), added to food at a concentration from 200mg/Kg up to 1g/Kg.
- E 307: synthetic dl-a-tocopherol
- E 308: synthetic dl-g-tocopherol
- E 309: synthetic dl-d-tocopherol
- Ascorbic acid palmitate (E 304) is also used as food additive.

The global annual production of vitamin E exceeds 20 000 tons and synthetic vitamin E represents up to 90% of the total production. The predominant amount is used for animal feeding and about a quarter is used for human applications.

The practical challenge is to keep a-tocopherol stable until use. The most common approach is to use the ester a-tocopheryl acetate which needs to be cleaved in the digestive tract to recover the antioxidant properties of a-tocopherol. A second approach is phosphorylation. Evidence has been found for tocopherol phosphate in common foods as well as present in humans, indicating that phosphorylation of tocopherol is a natural process. The transfer of a phosphoryl group (-PO₃H₂) to the hydroxyl group on the chroman nucleus was shown to have a potent antioxidant effect (Rezk B et al., Biochim Biophys Acta 2004, 1683, 16). It was demonstrated that the new compound acts as a detergent forming a barrier which may inhibit the transfer of radicals from a substrate to another. This new mechanism may form the basis for a new class of antioxidant. It was also reported that pretreatment of cultured mouse skin with tocopherol phosphate provided significant protection greater than the acetate derivative against UV-B-induced skin damage characterized sunburn cell formation, and DNA degradation (Nakayama et al. J. Invest. Dermatol 2003, 121, 406).

a-Tocopherol phosphate seems now to be an ideal candidate for a number of cellular functions, such as intracellular transport, cell proliferation and inflammation (Munteanu A et al., Biochem Biophys Res Commun 2004, 318, 311). Several hypotheses have been made for its possible roles in cellular signaling (Negis Y et al., IUBMB Life 2005, 57, 23). It has been even proposed that at physiological concentrations a-tocopherol may act mainly as a ligand of not yet identified specific proteins and not as an antioxidant (Azzi A, Free Rad Biol Med 2007, 43, 16). An important role in cell signaling for a-tocopheryl phosphate has also been hypothesized.

Cycloaddition products of a-tocopherol with carotenoids have been described. They were named pittosporumxanthins. 

The natural metabolites of  a- and g-tocopherol have been also determined . It was demonstrated that an excess of ingested tocopherols leads to an excretion of parent molecules mainly as glucuronide or sulfate conjugates in the urine (Traber MG et al., FEBS Lett 1998, 437, 145; Stahl W et al., Anal Biochem 1999, 275, 254). These molecules have the common methyl hydroxychroman nucleus but with a short side chain (3 carbon atoms) with a carboxyl group instead of a phytyl tail (see below). a-Tocopherol is thus oxidized by the cytochrome P450 complex which leads to the shortening of the phytyl chain and to the transformation into 2,5,7,8-tetramethyl-2-(2'-carboxy-ethyl)-6-hydroxychroman (a-CEHC). This enzyme system is, to date, the only catabolic pathway which acts on vitamin E, some differences being observed according to the oxidized substrate (Sontag TJ et al., J Lipid Res 2007, 48, 1090). Furthermore, it seems possible that these molecules can be used as a measure of the body vitamin E status.

Hydroxychromanols with a phytyl chain of 3, 5 or 9 carbon atoms were shown to be produced from tocopherols and tocotrienols in various cancerous cell types (from lung or liver) (You CS et al., J Nutr 2005, 135, 227).

It has been shown that sulfated long-chain carboxychromanols were generated in cultivated cells from man and rats (Jiang Q et al., J Lipid Res 2007, 48, 1221). These studies demonstrated that sulfation occurred at long-chain carboxychromanols, the intermediate metabolites generated from b-oxidation of the phytyl chain, the sulfation rate varying between vitamin E forms.

While glycosylated derivatives of tocopherols have not been yet detected in plant tissues, glycosylation of tocopherols has been demonstrated in cultured suspension cells of Eucalyptus perriniana (Shimoda K et al., Phytochemistry 2007, 68, 2678). a-Tocopherol was shown to be converted mainly into a mono-glucoside derivative (a-tocopheryl 6-0-b-D-glucopyranoside) and a di-glucoside derivative (a-tocopheryl 6-0-(6-0-b-D-glucopyranosyl)-b-D-glucopyranoside or a-tocopheryl b-gentiobioside). Small amounts of a-tocopheryl 6-0-(6-0-b-L-rhamnopyranosyl)-b-D-glucopyranoside (or a-tocopheryl b-rutinoside) were also formed. d-Tocopherol was also converted into mono- and di-glycosylated derivatives.

a-tocopheryl 6-0-b-D-glucopyranoside

a-tocopheryl 6-0-(6-0-b-D-glucopyranosyl)-b-D-glucopyranoside

These new di-glycosylated derivatives may act as potent anti-allergic agents as synthetically synthesized mono-glycosylated derivatives.

The absorption peaks for the various substituted tocols are given in the following table.

	MW	l max (nm)	mol. abs.
a-tocopherol	430.7	294	3056
g-tocopherol	416.7	298	3867
d-tocopherol	402.7	298	3673
a-tocotrienol	424.7	292	3865
b-tocotrienol	410.7	296	3573
d-tocotrienol	396.7	292	3293
a-tocopherol quinone	446.7	262	19500
a-tocopherol acetate	472.7	285	2034

Tocopherols (except acetate and quinone) fluoresce strongly, emitting at 340 nm when excited around 295 nm.