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Antioxidants, nature and chemistry
Submitted by
Dr. Tamer Fouad, M.D.
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Antioxidants are substances
that protect other chemicals of the body from damaging
oxidation reactions
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Non-Enzymatic antioxidants
Alpha tocopherol (vitamin E)
Nomenclature:
Is the major lipid
soluble antioxidant found in cells. The name originated in the
early 1920s when vegetable oil was discovered to restore fertility
in rats. This
unknown substance was designated vitamin E by Sure in 1924.
The term tocopherol was first used by Evans. Because
this compound permitted an animal to have offspring, he named
it tocopherol from the Greek word tokos, meaning
childbirth, and added the verb phero, meaning to bring
forth. To indicate the alcohol nature of the molecule, ol
was added to the ending.
Vitamin E is a generic term that includes all entities that
exhibit the biological activity of natural vitamin E, d-alpha-tocopherol.
In nature, eight substances have been found to have vitamin
E activity:
d-alpha-, d-beta-,
d-gamma- and d-delta-tocopherol (which differ in methylation
site and side-chain saturation (Kellof et al. 1996);
and d-alpha-, d-beta-, d-gamma- and d-delta-tocotrienol. Also,
the acetate and succinate derivatives of the natural tocopherols
have vitamin E activity, as do synthetic tocopherols and their
acetate and succinate derivatives.
Of all these, d-alpha-tocopherol has the highest biopotency,
and its activity is the standard against which all the others
must be compared. It is the
predominant isomer in plasma.
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Source and Nature:
Vitamin
E is an essential nutrient that functions as an antioxidant
in the human body. It is essential, by definition, because the
body cannot manufacture its own vitamin E and thus it must be
provided by foods and supplements.
Tocopherols are present in oils, nuts, seeds, wheat germ and
grains. Absorption is believed
to be associated with intestinal fat absorption. Approximately
40% of the ingested tocopherol is absorbed. Most tocopherols
enter the blood via lymph where they are associated with chylomicrons.
Vitamin E was shown to be stored in adipose tissue. Phospholipids
of the mitochondria & endoplasmic reticulum & plasma membranes
possess affinities for alpha tocopherol & the vitamin tends
to concentrate in these sites.
Mechanisms of Action:
Vitamin E is more appropriately described as an antioxidant
than a vitamin. This is because, unlike most vitamins, it does
not act as a co-factor for enzymatic reactions.
Also, deficiency of vitamin E does not produce a disease with
rapidly developing symptoms such as scurvy or beriberi. Overt
symptoms due to vitamin E deficiency occur only in cases involving
fat malabsorption syndromes, premature infants and patients
on total parenteral nutrition. The effects of inadequate vitamin
E intake usually develop over a long time, typically decades,
and have been linked to chronic diseases such as cancer and
atherosclerosis.
Hence, its main function is to prevent the peroxidation of membrane
phospholipids, and avoids cell membrane damage through its antioxidant
action. The lipophilic character of tocopherol enables it to
locate in the interior of the cell membrane bilayers (Halliway
and Getteridge, 1992; Borg, 1993). Tocopherol-OH can transfer
a hydrogen atom with a single electron to a free radical, thus
removing the radical before it can interact with cell membrane
proteins or generate lipid peroxidation. When tocopherol-OH
combines with the free radical, it becomes tocopherol-O·, itself
a radical. When ascorbic acid is available, tocopherol-O· plus
ascorbate (with its available hydrogen) yields semidehydroascorbate
(a weak radical) plus tocopherol-OH (Halliway and Gutteridge,
1992). By this process, an aggressive ROI is eliminated
and a weak ROI (dehydroascorbate) is formed, and tocopherol-OH
is regenerated. Despite this complex defence system, there are
no known endogenous enzymatic antioxidant systems for the hydroxyl
radical.
Alpha tocopherol + LOO˙ ®
Alpha tocopherol˙+
LOOH
Alpha tocopherol˙+
LOO˙
® LOO-alpha tocopherol
Vitamin E also stimulates the immune response. Some studies
have shown lower incidence of infections when vitamin E levels
are high, and vitamin E may inhibit cancer initiation through
enhanced immunocompetence.
Vitamin E also has a direct chemical function. It inhibits the
conversion of nitrites in smoked, pickled and cured foods to
nitrosamines in the stomach. Nitrosamines are strong tumour
promoters.
Alpha-tocopherol has been shown to be capable of reducing ferric
iron to ferrous iron (i.e. to act as a pro-oxidant). Moreover,
the ability of alpha-tocopherol to act as a pro-oxidant (reducing
agent) or antioxidant depends on whether all of the alpha-tocopherol
becomes consumed in the conversion from ferric to ferrous iron
or whether, following this interaction, residual alpha-tocopherol
is available to scavenge the resultant ROI (Yamamoto and
Nike, 1988).
Possible therapeutic effects:
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Vitamin E decreases the incidence of
ischaemic heart disease (Gey et al. 1991).
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Decreases the incidence of cataract
(Packer, 1991; 1992).
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Decreases the incidence of osteoarthritis
(Blankenhorn, 1986).
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Decreases the incidence of rheumatoid
arthritis (Kheir El-dein et al. 1992).
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