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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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Other antioxidants
Retinoids:
Retinol, retinoic acid
but not retinyl palmitate or retinyl acetate all have antioxidant
properties (Prasad, 1989).
However, retinoids in general are not classified as antioxidants
as they mainly function as antiproliferatives.
Glutathione (GSH):
GSH is synthesized intracellularly
from cysteine, glycine, and glutamate.
In addition to its role as a substrate in GSH redox cycle, GSH
is also a scavenger of hydroxyl radicals and singlet oxygen.
It is capable of either directly scavenging
ROI or enzymatically via glutathione peroxidase, as described
previously. In addition, GSH is crucial to the maintenance of
enzymes and other cellular components in a reduced state.
GSH also has an
important role in xenobiotic metabolism and leukotriene synthesis.
It is found in millimolar concentration in all human cells (Halliwell,
1994).
The majority of GSH
is synthesized in the liver, and approximately 40% is secreted
in the bile. The biologic role of GSH in bile is believed to
be defence against dietary xenobiotics and lipid peroxidation
in the lumen of the gut and protection of the intestinal epithelium
from oxygen radical attack (Aw,
1994).
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CoQ10:
CoQ10 (Coenzyme Q10)
is also known as ubiquinone. It is found in almost every living
cell (hence the name "ubiquitous") and is essential to energy
production by the mitochondria. Far beyond producing energy,
CoQ10 can protect the body from destructive free radicals and
enhance immune defences.
Uric acid:
Acts as an endogenous radical scavenger and antioxidant. It
is present in about 0.5 mmol/L in body's fluids and is the end
product of purine metabolism. Uric acid is a powerful scavenger
of singlet oxygen, peroxyl radical (ROO?)
and ?OH
radical (Halliwell,
1994).
Albumin:
Depending on the fact that albumin has one sulfhydryl group
per molecule, it itself scavenges several free radicals (Halliwell,
1994) and thus can be considered as one of the primary extracellular
defense systems.
Albumin is an additional sacrificial antioxidant that can bind
copper tightly and iron weakly to its surface. The bound metals
would still be on its surface. The bound metals would still
be available for participation in Haber-Weiss reaction, but
any generated ?OH would
immediately react with and be scavenged by albumin. The resultant
protein damage is biologically insignificant because of the
large amount of available albumin and free radicals would be
inactivated before reacting with other more vital protein structures.
Other plasma proteins namely ceruloplasmin and transferrin have
also shown antioxidant activity.
Drugs:
Several pharmaceutical
agents have been found to exert an antioxidant effect (Reilly
et al. 1991):
?
Xanthine oxidase inhibitors:
e.g. allopurinol, folic acid.
?
NADPH inhibitors: e.g. adenosine,
calcium channel blockers.
?
Superoxide dismutases.
?
Catalases.
?
Albumin.
?
Inhibitors of iron redox cycling:
deferoxamine, apotransferin and ceruloplasmin
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