Everything about Antioxidants totally explained
An
antioxidant is a
molecule capable of slowing or preventing the
oxidation of other molecules. Oxidation is a
chemical reaction that transfers
electrons from a substance to an
oxidizing agent. Oxidation reactions can produce
free radicals, which start
chain reactions that damage
cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by being oxidized themselves. As a result, antioxidants are often
reducing agents such as
thiols or
polyphenols.
Although oxidation reactions are crucial for life, they can also be damaging; hence,
plants and
animals maintain complex systems of multiple types of antioxidants, such as glutathione,
vitamin C, and
vitamin E as well as
enzymes such as
catalase,
superoxide dismutase and various
peroxidases. Low levels of antioxidants, or
inhibition of the antioxidant enzymes, causes
oxidative stress and may damage or kill cells.
As oxidative stress might be an important part of many human diseases, the use of antioxidants in
pharmacology is intensively studied, particularly as treatments for
stroke and
neurodegenerative diseases. However, it's unknown whether oxidative stress is the cause or the consequence of disease. Antioxidants are also widely used as ingredients in
dietary supplements in the hope of maintaining health and preventing diseases such as
cancer and
coronary heart disease. Although some studies have suggested antioxidant supplements have health benefits, other large
clinical trials didn't detect any benefit for the formulations tested, and excess supplementation may be harmful. In addition to these uses in medicine, antioxidants have many industrial uses, such as
preservatives in food and cosmetics and preventing the degradation of
rubber and
gasoline.
History
The term antioxidant originally was used to refer specifically to a chemical that prevented the consumption of oxygen. In the late 19th and early 20th century, extensive study was devoted to the uses of antioxidants in important industrial processes, such as the prevention of metal
corrosion, the
vulcanization of rubber, and the
polymerization of fuels in the
fouling of
internal combustion engines.
Early research on the role of antioxidants in biology focused on their use in preventing the oxidation of
unsaturated fats, which is the cause of
rancidity. Antioxidant activity could be measured simply by placing the fat in a closed container with oxygen and measuring the rate of oxygen consumption. However, it was the identification of
vitamins A,
C, and E as antioxidants that revolutionized the field and led to the realization of the importance of antioxidants in biochemistry of living organisms.
The possible
mechanisms of action of antioxidants were first explored when it was recognized that a substance with anti-oxidative activity is likely to be one that's itself readily oxidized. Research into how vitamin E prevents the process of
lipid peroxidation led to the identification of antioxidants as reducing agents that prevent oxidative reactions, often by scavenging
reactive oxygen species before they can damage cells.
The oxidative challenge in biology
A paradox in metabolism is that while the vast majority of complex life requires
oxygen for its existence, oxygen is a highly reactive molecule that damages living organisms by producing reactive oxygen species. Consequently, organisms contain a complex network of antioxidant metabolites and enzymes that work together to prevent oxidative damage to cellular components such as
DNA,
proteins and
lipids. In general, antioxidant systems either prevent these reactive species from being formed, or remove them before they can damage vital components of the cell. The hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. This species is produced from hydrogen peroxide in
metal-catalyzed redox reactions such as the
Fenton reaction. These oxidants can damage cells by starting chemical chain reactions such as lipid peroxidation, or by oxidizing DNA or proteins. while damage to proteins causes enzyme inhibition,
denaturation and
protein degradation.
The use of oxygen as part of the process for generating metabolic energy produces reactive oxygen species. In this process, the superoxide anion is produced as a by-product of several steps in the
electron transport chain. Particularly important is the reduction of
coenzyme Q in
complex III, since a highly reactive free radical is formed as an intermediate (Q
·−). This unstable intermediate can lead to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion, instead of moving through the normal series of well-controlled reactions of the electron transport chain. In a similar set of reactions in plants, reactive oxygen species are also produced during
photosynthesis under conditions of high light intensity. This effect is partly offset by the involvement of
carotenoids in
photoinhibition, which involves these antioxidants reacting with over-reduced forms of the
photosynthetic reaction centres to prevent the production of reactive oxygen species.
Metabolites
Overview
Antioxidants are classified into two broad divisions, depending on whether they're soluble in
water (
hydrophilic) or in lipids (
hydrophobic). In general, water-soluble antioxidants react with oxidants in the cell
cytoplasm and the
blood plasma, while lipid-soluble antioxidants protect
cell membranes from lipid peroxidation. The action of one antioxidant may therefore depend on the proper function of other members of the antioxidant system.
Selenium and
zinc are commonly referred to as antioxidant nutrients, but these
chemical elements have no antioxidant action themselves and are instead required for the activity of some antioxidant enzymes, as is discussed below.
| Antioxidant metabolite |
Solubility |
Concentration in human serum (μM) |
Concentration in liver tissue (μmol/kg) |
| Ascorbic acid (vitamin C) |
Water |
50 – 60 |
260 (human) |
| Glutathione |
Water |
325 – 650 |
6,400 (human) |
4 – 5 (rat) |
| Uric acid |
Water |
200 – 400 |
1,600 (human)retinol (vitamin A): 1 – 3
|
5 (human, total carotenoids) |
| α-tocopherol (vitamin E) |
Lipid |
10 – 40 |
200 (human) |
Ascorbic acid
Ascorbic acid or "vitamin C" is a
monosaccharide antioxidant found in both animals and plants. As it can't be synthesised in humans and must be obtained from the diet, it's a vitamin. Most other animals are able to produce this compound in their bodies and don't require it in their diets. In cells, it's maintained in its reduced form by reaction with glutathione, which can be catalysed by
protein disulfide isomerase and
glutaredoxins. Ascorbic acid is a reducing agent and can reduce and thereby neutralize reactive oxygen species such as hydrogen peroxide. In addition to its direct antioxidant effects, ascorbic acid is also a
substrate for the antioxidant enzyme
ascorbate peroxidase, a function that's particularly important in stress resistance in plants.
Glutathione
Glutathione is a
cysteine-containing
peptide found in most forms of aerobic life. It isn't required in the diet and is instead synthesized in cells from its constituent
amino acids. Glutathione has antioxidant properties since the thiol group in its cysteine moiety is a reducing agent and can be reversibly oxidized and reduced. In cells, glutathione is maintained in the reduced form by the enzyme
glutathione reductase and in turn reduces other metabolites and enzyme systems as well as reacting directly with oxidants. Unlike other antioxidants, melatonin doesn't undergo
redox cycling, which is the ability of a molecule to undergo repeated
reduction and
oxidation. Redox cycling may allow other antioxidants (such as vitamin C) to act as
pro-oxidants and promote free radical formation. Melatonin, once oxidized, can't be reduced to its former state because it forms several stable end-products upon reacting with free radicals. Therefore, it has been referred to as a terminal (or suicidal) antioxidant.
Tocopherols and tocotrienols (vitamin E)
Vitamin E is the collective name for a set of eight related tocopherols and
tocotrienols, which are fat-soluble vitamins with antioxidant properties. Of these, α-tocopherol has been most studied as it has the highest
bioavailability, with the body preferentially absorbing and metabolising this form.
It has been claimed that the α-tocopherol form is the most important lipid-soluble antioxidant, and that it protects membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction. This removes the free radical intermediates and prevents the propagation reaction from continuing. This reaction produces oxidised α-tocopheroxyl radicals that can be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol.
However, the roles and importance of the various forms of vitamin E are presently unclear, and it has even been suggested that the most important function of α-tocopherol is as a
signaling molecule, with this molecule having no significant role in antioxidant metabolism. The functions of the other forms of vitamin E are even less well-understood, although γ-tocopherol is a
nucleophile that may react with
electrophilic mutagens,
Pro-oxidant activities
Antioxidants that are reducing agents can also act as pro-oxidants. For example, vitamin C has antioxidant activity when it reduces oxidizing substances such as hydrogen peroxide, however, it'll also reduce metal ions that generate free radicals through the
Fenton reaction.
» 2 Fe
3+ + Ascorbate → 2 Fe
2+ + Dehydroascorbate
:2 Fe
2+ + 2 H
2O
2 → 2 Fe
3+ + 2 OH
· + 2 OH
−
The relative importance of the antioxidant and pro-oxidant activities of antioxidants are an area of current research, but vitamin C, for example, appears to have a mostly antioxidant action in the body.
Enzyme systems
Overview
As with the chemical antioxidants, cells are protected against oxidative stress by an interacting network of antioxidant enzymes.
Superoxide dismutase, catalase and peroxiredoxins
Superoxide dismutases (SODs) are a class of closely related enzymes that catalyse the breakdown of the superoxide anion into oxygen and hydrogen peroxide. SOD enzymes are present in almost all aerobic cells and in extracellular fluids. Superoxide dismutase enzymes contain metal ion cofactors that, depending on the isozyme, can be
copper, zinc,
manganese or
iron. In humans, the copper/zinc SOD is present in the
cytosol, while manganese SOD is present in the
mitochondrion. The mitochondrial isozyme seems to be the most biologically important of these three, since mice lacking this enzyme die soon after birth. In contrast, the mice lacking copper/zinc SOD are viable but have lowered fertility, while mice without the extracellular SOD have minimal defects. In plants, SOD isozymes are present in the cytosol and mitochondria, with an iron SOD found in
chloroplasts that's absent from
vertebrates and
yeast.
Catalases are enzymes that catalyse the conversion of hydrogen peroxide to water and oxygen, using either an iron or manganese cofactor. This protein is localized to
peroxisomes in most
eukaryotic cells. Catalase is an unusual enzyme since, although hydrogen peroxide is its only substrate, it follows a
ping-pong mechanism. Here, its cofactor is oxidised by one molecule of hydrogen peroxide and then regenerated by transferring the bound oxygen to a second molecule of substrate. Despite its apparent importance in hydrogen peroxide removal, humans with genetic deficiency of catalase — "
acatalasemia" — or mice genetically engineered to lack catalase completely, suffer few ill effects.
Peroxiredoxins are peroxidases that catalyze the reduction of hydrogen peroxide,
organic hydroperoxides, as well as
peroxynitrite. They are divided into three classes: typical 2-cysteine peroxiredoxins; atypical 2-cysteine peroxiredoxins; and 1-cysteine peroxiredoxins. These enzymes share the same basic catalytic mechanism, in which a redox-active cysteine (the peroxidatic cysteine) in the active site is oxidized to a
sulfenic acid by the peroxide substrate. Peroxiredoxins seem to be important in antioxidant metabolism, as mice lacking peroxiredoxin 1 or 2 have shortened lifespan and suffer from
hemolytic anaemia, while plants use peroxiredoxins to remove hydrogen peroxide generated in chloroplasts.
Thioredoxin and glutathione systems
The
thioredoxin system contains the 12-k
Da protein thioredoxin and its companion
thioredoxin reductase. Proteins related to thioredoxin are present in all sequenced organisms, with plants such as
Arabidopsis thaliana having a particularly great diversity of isoforms. The active site of thioredoxin consists of two
neighboring cysteines, as part of a highly-conserved CXXC
motif, that can cycle between an active dithiol form (reduced) and an oxidized
disulfide form. In its active state, thioredoxin acts as an efficient reducing agent, scavenging reactive oxygen species and maintaining other proteins in their reduced state. After being oxidized, the active thioredoxin is regenerated by the action of thioredoxin reductase, using
NADPH as an electron donor.
The glutathione system includes glutathione, glutathione reductase,
glutathione peroxidases and glutathione
S-transferases. Glutathione peroxidase 1 is the most abundant and is a very efficient scavenger of hydrogen peroxide, while glutathione peroxidase 4 is most active with lipid hydroperoxides. Surprisingly, glutathione peroxidase 1 is dispensable, as mice lacking this enzyme have normal lifespans, but they're hypersensitive to induced oxidative stress. In addition, the
glutathione S-transferases are another class of glutathione-dependent antioxidant enzymes that show high activity with lipid peroxides. These enzymes are at particularly high levels in the liver and also serve in
detoxification metabolism.
Oxidative stress in disease
Oxidative stress is thought to contribute to the development of a wide range of diseases including
Alzheimer's disease,
Parkinson's disease, the pathologies caused by
diabetes,
rheumatoid arthritis, and
neurodegeneration in
motor neurone diseases. In many of these cases, it's unclear if oxidants trigger the disease, or if they're produced as a consequence of the disease and cause the disease
symptoms;
A
low calorie diet extends median and maximum lifespan in many animals. This effect may involve a reduction in oxidative stress. While there's good evidence to support the role of oxidative stress in aging in model organisms such as
Drosophila melanogaster and
Caenorhabditis elegans, the evidence in mammals is less clear. Diets high in fruit and vegetables, which are high in antioxidants, promote health and reduce the effects of aging, however antioxidant vitamin supplementation has no detectable effect on the aging process, so the effects of fruit and vegetables may be unrelated to their antioxidant contents. One reason for this might be the fact that consuming antioxidant molecules such as polyphenols and vitamin E will produce changes in other parts of metabolism, so it may be these other non-antioxidant effects that are the real reason they're important in human nutrition. Consequently, antioxidants are commonly used as
medications to treat various forms of brain injury. Here, superoxide dismutase mimetics,
sodium thiopental and
propofol are used to treat
reperfusion injury and
traumatic brain injury, while the experimental drug
NXY-059 and
ebselen are being applied in the treatment of stroke. These compounds appear to prevent oxidative stress in neurons and prevent
apoptosis and neurological damage. Antioxidants are also being investigated as possible treatments for neurodegenerative diseases such as
Alzheimer's disease,
Parkinson's disease, and
amyotrophic lateral sclerosis, and as a way to prevent
noise-induced hearing loss.
Disease prevention
Antioxidants can cancel out the cell-damaging effects of free radicals. and there's evidence that some types of vegetables, and fruits in general, probably protect against a number of cancers. These observations suggested that antioxidants might help prevent these conditions. There is some evidence that antioxidants might help prevent diseases such as
macular degeneration, suppressed
immunity due to poor nutrition, and neurodegeneration. However, despite the clear role of oxidative stress in cardiovascular disease, controlled studies using antioxidant vitamins have observed no reduction in either the risk of developing heart disease, or the rate of progression of existing disease. This suggests that other substances in fruit and vegetables (possibly
flavonoids), or a complex mix of substances, may contribute to the better cardiovascular health of those who consume more fruit and vegetables.
It is thought that oxidation of low density lipoprotein in the blood contributes to heart disease, and initial observational studies found that people taking Vitamin E supplements had a lower risk of developing heart disease. Consequently, at least seven large clinical trials were conducted to test the effects of antioxidant supplement with Vitamin E, in doses ranging from 50 to per day. However, none of these trials found a statistically significant effect of Vitamin E on overall number of deaths or on deaths due to heart disease. It isn't clear if the doses used in these trials or in most dietary supplements are capable of producing any significant decrease in oxidative stress.
While several trials have investigated supplements with high doses of antioxidants, the "
Supplémentation en Vitamines et Mineraux Antioxydants" (SU.VI.MAX) study tested the effect of supplementation with doses comparable to those in a healthy diet. Over 12,500 French men and women took either low-dose antioxidants (of ascorbic acid, of vitamin E, of beta carotene, 100
g of selenium, and of zinc) or
placebo pills for an average of 7.5 years. The investigators found there was no statistically significant effect of the antioxidants on overall survival, cancer, or heart disease. However, a subgroup analysis showed a 31% reduction in the risk of cancer in men, but not women.
Many
nutraceutical and health food companies now sell formulations of antioxidants as dietary supplements and these are widely used in industrialized countries. These supplements may include specific antioxidant chemicals, like resveratrol (from grape seeds), combinations of antioxidants, like the "ACES" products that contain beta carotene (provitamin
A), vitamin
C, vitamin
E and
Selenium, or herbs that contain antioxidants - such as
green tea and
jiaogulan. Although some levels of antioxidant vitamins and minerals in the diet are required for good health, there's considerable doubt as to whether antioxidant supplementation is beneficial, and if so, which antioxidant(s) are beneficial and in what amounts. However, the suggestion that increased life expectancy comes from increased oxidative stress conflicts with results seen in the yeast
Saccharomyces cerevisiae, and the situation in mammals is even less clear.
Physical exercise
During exercise, oxygen consumption can increase by a factor of more than 10. This leads to a large increase in the production of oxidants and results in damage that contributes to muscular fatigue during and after exercise. The
inflammatory response that occurs after strenuous exercise is also associated with oxidative stress, especially in the 24 hours after an exercise session. The immune system response to damage done by exercise peaks 2 to 7 days after exercise, the period during which adaptation resulting in greater fitness is greatest. During this process, free radicals are produced by
neutrophils to remove damaged tissue. As a result, excessive antioxidant levels have the potential to inhibit recovery and adaptation mechanisms.
The evidence for benefits from antioxidant supplementation in vigorous exercise is mixed. There is strong evidence that one of the adaptations resulting from exercise is a strengthening of the body's antioxidant defenses, particularly the glutathione system, to deal with the increased oxidative stress. It is possible that this effect may be to some extent protective against diseases which are associated with oxidative stress, which would provide a partial explanation for the lower incidence of major diseases and better health of those who undertake regular exercise.
However, no benefits to athletes are seen with vitamin A or E supplementation. For example, despite its key role in preventing lipid membrane peroxidation, 6 weeks of vitamin E supplementation had no effect on muscle damage in ultramarathon runners. Although there appears to be no increased requirement for vitamin C in athletes, there's some evidence that vitamin C supplementation increased the amount of intense exercise that can be done and vitamin C supplementation before strenuous exercise may reduce the amount of muscle damage. However, other studies found no such effects, and some research suggests that supplementation with amounts as high as inhibits recovery.
Adverse effects
Relatively strong reducing acids can have anti-nutritional effects by binding to dietary minerals such as iron and zinc in the gastrointestinal tract and preventing them from being absorbed. Notable examples are
oxalic acid,
tannins and phytic acid, which are high in plant-based diets.
Calcium and iron deficiencies are not uncommon in diets in
developing countries where less meat is eaten and there's high consumption of phytic acid from beans and unleavened
whole grain bread.
Further Information
Get more info on 'Antioxidants'.
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