A pdf version of the article can be downloaded from this link http://www.ingentaconnect.com/content/stl/sciprg/2011/00000094/00000004/art00004. On here it seems to work ok if you set the "page size" to 100%.
Free Radicals – the Bad Guys.
It is widely held1
that free radicals are involved in the initiation and propagation of many and
various illnesses, including cancer, heart disease, stroke, rheumatoid arthritis,
diabetes, and multiple sclerosis (MS). The list runs on, and even the process
of ageing itself is believed to be driven by free radicals, also called Reactive
Oxygen Species (ROS) or Reactive Oxygen Intermediates (ROI). Now, the species
classified as ROS or ROI are derived from molecular oxygen (O2) which
obviously we need to breathe to stay alive. In the main, ROS are the superoxide
radical anion (O2−•), its conjugate acid, the
hydroperoxyl radical (HOO•), the hydroxyl radical (HO•), organic
peroxyl radicals (ROO•), alkoxyl radicals (RO•) as bona fide free radical (unpaired
electron) molecules, but also included on the list are molecular (especially, singlet)
oxygen (O2), organic hydroperoxides, ROOH and hydrogen peroxide
itself, H2O2. It can be said that all oxygen free
radicals are ROS/ROI but not all ROS/ROI are free radicals. As respired O2
enters living cells it is metabolised e.g. by the mitochondria to O2−•,
which is not in itself strongly oxidising, but it provides a source of other
ROS. To avoid living cells being overwhelmed by O2−•,
they contain the enzyme superoxide dismutase which catalyses the reaction
(equation 1):
2O2−• +
2H+ → H2O2 + O2 (1)
Now H2O2
is not harmless in cells since it can provide a source of HO•
radicals, particularly if there is free iron present, which promotes the Fenton
Reaction (equation 2):
Fe2+ + H2O2
→ Fe3+ + HO• + −OH (2)
HO•
radicals can attack sensitive molecules in cells, including membrane lipids,
carbohydrates and proteins, and if they are formed in the cell nucleus, DNA
bases too, potentially leading to strand-breaks and cell mutations. The attack
of HO• and other kinds of radicals on lipids can initiate the process
known as lipid peroxidation, which is responsible for the rancidification of
foodstuffs including meat. That our human “meat” does not become rancid while
we remain alive is due to the fact that living cells contain antioxidants, In
particular, catalase which is a
common enzyme
found in nearly all living organisms that are exposed to oxygen. Catalase is
able to catalyse the decomposition of hydrogen peroxide to water and oxygen, and has one of the highest turnover
numbers of all enzymes - one
catalase molecule can convert 40 million molecules of hydrogen peroxide to
water and oxygen per second. Glutathione peroxidise also catalyses the decomposition
of H2O2 via the
reaction by coupling its reduction to water with oxidation of reduced
glutathione (GSH), a thiol-containing tripeptide (glu-cys-gly) (equation 3):
H2O2
+ 2GSH → GSSG + 2H2O (3)
The product,
oxidized glutathione (GSSG), contains a disulphide bridge, and can be converted
back to GSH by glutathione reductase enzymes. It is now thought that peroxiredoxins
may be even more important2 in removing H2O2
from cells in animals, bacteria, and probably plants. There are at least three
classes of these enzymes, but in the function of all of them an cys-SH group present
on the peroxiredoxin is oxidised by H2O2 to a sulfenic
acid, cys-SOH. The interception of ROS is not perfect and around 1% of respired
O2 ends-up as ROS. Over a year this amounts to 1.7 kg of ROS, since
humans are fairly large animals and breathe substantial amounts of oxygen. To
cope with what ROS remain, there are both intrinsic and extrinsic antioxidants
present in cells, the latter being brought into the living organism and hence
its cells by ingestion, i.e. in our food and in the form of deliberately taken
dietary supplements. The effectiveness of latter is debatable, however, as
shall shortly become evident. Many molecules that are designated as
antioxidants possess phenolic groups, e.g. the vitamin-E series and compounds
present in green tea, principally epigallocatechin gallate (EGCG). It is
thought that such materials can act as chain-breaking antioxidants, in which
the chain of free radical propagation is “broken” by transfer of an H-atom from
a phenolic OH moiety to an ROO• radical (equation 4):
ArOH + ROO• → ArO•
+ ROOH (4)
This effectively deactivates
the ROO• radical from abstracting an H-atom from a lipid unit to
give a carbon-centred radical, which by the addition of O2 would
form another ROO• radical to propagate the autoxidation process
(equation 5):
ROO• + RH → ROOH
+ R• (5)
The door to the field of free-radical toxicology was set open in the
proposal by Gerschman et al. in 19543 that oxygen poisoning and the
effect of X-irradiation on animals had a common mechanism which involved the formation
of free radicals. Two years
later, Denham Harman suggested that the ageing process too was mediated by free
radicals.4 The abstracts of these two classic papers are as follows:
Abstract:
A consideration of various isolated reports in the literature has led us to the
hypothesis that oxygen poisoning and radiation injury have at least one common
basis of action, possibly through the formation of oxidizing free radicals.
This article reviews the pertinent material that led to this hypothesis and
also presents the supporting evidence obtained from (i) experiments on the
protective action against oxygen poisoning by substances of varied chemical
nature known to increase resistance to irradiation, and (ii) experiments on the
survival in oxygen of mice irradiated and exposed to high oxygen tensions
simultaneously or at different intervals.3
Abstract: This paper describes a theory about mechanisms of aging that is based on free radical chemistry: "Aging and the degenerative diseases associated with it are attributed basically to the deleterious side attacks of free radicals on cell constituents and on the connective tissues. The free radicals probably arise largely through reactions involving molecular oxygen catalyzed in the cell by oxidative enzymes and in the connective tissues by traces of metals such as iron, cobalt, and manganese.4
These ideas and their broader ramifications underwent a gestation period, with periodic mention, leading to a seminal paper by Trevor Slater and his colleagues in which an explanation for the toxicity of carbon tetrachloride (CCl4), principally to the liver, was advanced in terms of a free-radical mechanism.5 For those chronically exposed to “carbon tet” over lengthly periods, damage to the liver was not infrequent and in some cases, liver failure occurred, in addition to neurotoxic effects of CCl4, and potential links to liver cancer and kidney cancer. CCl4 used to be widely employed in the dry cleaning industry and was also commonly used as an organic solvent, but due to its toxicity has been largely superseded by safer materials. The mechanism of activation involves a reductive elimination of Cl− from CCl4, (e.g. by cytochrome P450 enzymes) which forms a CCl3• radical. The CCl3• radicals can then add O2 to form CCl3OO• radicals, which are particularly reactive versions of peroxyl radicals. This enhanced reactivity can be viewed in terms of the limiting canonical structures: CCl3OO• <--> CCl3O+•O− which for common ROO• radicals, normally contribute around 50:50 to the overall structure. However, the three strongly electron withdrawing Cl-atoms tend to disfavour the second structure, with the positive charge on the O-atom adjacent to the CCl3-group, and so the unpaired electron becomes increasingly localised onto the terminal O-atom according to an increased weighting of the limiting structure CCl3OO•.6 An increased localisation or “exposure” of the unaired electron tends to engender a more reactive radical character and so the H-atom abstraction reaction (equation 5) is facilitated. Thus the lipid peroxidation process overall is encouraged, causing severe damage to the liver cells so that the organ becomes cirrhotic and ultimately fails.
Good
Free Radicals?
Recent research7
published from King’s College in London indicates that mice deliberately bred
to possess more of an enzyme (NADPH oxidase-4) that actually produces ROS,
including free radicals, suffered less heart disease than animals in which the
enzyme had been “deleted”. This rather runs counter to the prevailing argument
espoused above but it is thought that exposure to ROS can actually “toughen-up”
an organism, so that it becomes more resistant to certain conditions like
cardiovascular disease. It is well known that ROS, including superoxide, can
act as cell-messengers, and so in concentrations that do not overwhelm the protective
antioxidative capacity of the organism may be beneficial. Some of the ROS may
act as signalling agents to operate protective pathways, for example in
enhancing myocardial angiogenesis, which is the physiological process involving
the growth of new blood vessels from pre-existing vessels. The latter is a
critical determinant of cardiac adaptation to overload stress. Several years
ago, another group of London researchers, this time from University College
(UCL), reported that the basic theory underlying the toxicity of oxygen
radicals is flawed.8 White
blood cells, or leukocytes
(also spelled "leucocytes", leuco- Ancient Greek
"white"), are cells of the immune system that participate in
defending the body against infectious
diseases
and xenobiotics (foreign agents). Five different kinds of leukocytes are known,
all of them stemming from a multipotent cell termed a haematopoietic stem
cell, which exists in bone marrow.
Leukocytes are found throughout the body, and are present in the blood and
lymphatic system,
with a typical lifetime of 3 – 4 days. Leukocytes comprise ca 1% of the blood of a healthy adult,
and the leukocyte count is often an indicator of disease, being raised (leukocytosis)
above the normal levels of 4×109 - 1.1×1010
white blood cells/litre. The name "white blood cell" derives from the
observation that after a blood sample has been centrifuged, the white cells are
found in the buffy coat, a thin layer of nucleated cells between the
sedimented red blood cells and the blood plasma, which is normally white in
appearance.
Leukocytes produce oxygen ROS, and the process by which they do so is vital for killing microbes efficiently. In some people, the process is defective, rendering them liable to chronic, severe and often fatal infections. Accordingly, the inference has been drawn that the ROS are themselves highly toxic, and must be harmful to human tissues if they are sufficiently virulent to kill organisms as robust as bacteria and fungi. In contrast, the UCL group found that it was not ROS that made white blood cells so destructive but the release of enzymes (proteases) with the power to digest foreign invading species. The enzymes are triggered by a flow of K+ cations within the cell. When the process was blocked using iberiotoxin (derived from scorpion venom) and paxilline (a fungal mycotoxin), the cells were no longer able to combat pathogens, demonstrating that the ROS are not as toxic as previously thought.8 The paper concludes: “These data have significance beyond the inherent value of defining the precise molecular mechanisms involved in a physiological process of paramount importance to survival. The perception that neutrophils kill microbes through toxic oxygen radicals and their metabolites provided much of the biological basis for the theories relating the toxicity of oxygen radicals to the pathogenesis of a wide variety of human diseases, and the development of antioxidant drugs for their treatment. These theories and treatment merit re-evaluation.”
Noteworthy too is a study9 by researchers at McGill’s Department of Biology, who tested the accepted “free radical theory of ageing” by creating mutant worms with an increased production of ROS in their bodies. It was found that in contrast to the expected outcome, the worms lived longer than regular worms. Even more significantly, when the mutant worms were treated with antioxidants, e.g Vitamin C, their lifetimes were shortened. The researchers then sought to mimic the apparent beneficial effect of the free radicals by treating regular, wild worms with Paraquat, a herbicide that generates superoxide and hence other ROS, by redox-cycling. Paraquat is so toxic to humans and animals that it is banned in the European Union and its use is restricted in many other parts of the world. Remarkably, they discovered that the worms lived longer after being exposed to paraquat. It is thought that in the genetically modified worms, the production of ROS can help to trigger the body’s general protective and repair mechanisms, thus acting to preserve life. Whether one can extrapolate these results for worms to far more complex organisms such as humans is a moot point, of course.
Antioxidant Supplements.
It
is widely held that a “Mediterranean Diet” is very healthy since the incidences
of cancer and cardiovascular diseases in the Mediterranean are lower than in
the colder northern countries. This is the basis of the “five a day” diet, in
which it is recommended that we consume five 80g portions of fruit and vegetables
daily. An explanation for this, which has entered the public consciousness, is
that a diet rich in fresh fruit and vegetables is full of antioxidants and by
mopping-up free radicals is protective against these particular maladies. This
must be qualified by a recent European study which found a relatively small
reduction in the overall cancer rate according to their intake of fruit and
vegetables in a sample of almost half a million people.10 However, in
an extension of this line of thinking, a massive multi-billion dollar industry
has uprisen which supplies pure antioxidant compounds in the form of pills and
capsules to be taken as dietary supplements. In the U.S. alone, more than half
of all adults take some form of vitamin or mineral supplement, at a cost of £23
billion/year.11 Now, not only is there precious little hard
scientific evidence that taking these compounds additionally and above what is
present in the diet actually does any good, it is quite possible that in too
high a dose some of them can have adverse effects. The pioneer protagonist of such
dietary supplementation was Linus Pauling who recommended taking Vitamin C (L-ascorbic
acid) in large amounts. He did live to be 93.
L-ascorbic acid (or L-ascorbate) is an essential nutrient for humans and certain other animal species.12 In living organisms ascorbate is thought to act as an antioxidant by protecting the body against oxidative stress. Ascorbate is a cofactor in at least eight enzyme catalysed reactions, including a number involved in collagen synthesis, and when they do not function properly the disease known as scurvy arises. In animals these reactions are especially important in wound-healing and in preventing bleeding from capillaries. The nickname given by Americans to the English, “Limeys”, derives from the practice of taking lime-fruits on board ships in the British Navy so that sailors could drink the juice (which is now known to contain Vitamin C) and offset the symptoms of scurvy which had formerly beset them on long sea voyages. While the daily recommended dose of 40 – 95 mg/day is sufficient for the needs of a human adult, doses of 10 -100 times this amount have been advocated by some practitioners. There is, however, no clinical evidence that such megadoses protect against developing cancer, coronary disease or the common cold, and indeed might be harmful, e.g. in promoting kidney failure.12 Most of the excess Vitamin C is simply excreted from the body (occasioning diarrhoea) so it is unlikely to do much good.
L-ascorbic acid (or L-ascorbate) is an essential nutrient for humans and certain other animal species.12 In living organisms ascorbate is thought to act as an antioxidant by protecting the body against oxidative stress. Ascorbate is a cofactor in at least eight enzyme catalysed reactions, including a number involved in collagen synthesis, and when they do not function properly the disease known as scurvy arises. In animals these reactions are especially important in wound-healing and in preventing bleeding from capillaries. The nickname given by Americans to the English, “Limeys”, derives from the practice of taking lime-fruits on board ships in the British Navy so that sailors could drink the juice (which is now known to contain Vitamin C) and offset the symptoms of scurvy which had formerly beset them on long sea voyages. While the daily recommended dose of 40 – 95 mg/day is sufficient for the needs of a human adult, doses of 10 -100 times this amount have been advocated by some practitioners. There is, however, no clinical evidence that such megadoses protect against developing cancer, coronary disease or the common cold, and indeed might be harmful, e.g. in promoting kidney failure.12 Most of the excess Vitamin C is simply excreted from the body (occasioning diarrhoea) so it is unlikely to do much good.
The most infamous case of a dietary supplement proving actually harmful is the Beta-Carotene and Retinol Efficacy Trial (CARET) in which daily β-carotene (30 mg) and retinyl palmitate (25 000 IU) were given to 18,314 participants who were at high risk for lung cancer because of a history of smoking or asbestos exposure.13 The study was stopped ahead of schedule in January 1996 because participants who were randomly assigned to receive the active intervention were found to have a 28% increase in incidence of lung cancer, a 17% higher death-rate and a higher rate of death from cardiovascular disease compared with participants in the placebo group. The notion that beta-carotene could be protective against cancer stemmed from the observation made in the 1970s that people who ate a lot of carrots had a lower cancer rate than the average. I seem to remember that drinking carrot-juice was quite popular at this time, and that some people who overdid their consumption of it found their skin turned orange in places! However, there are many other substances present in actual plant material, which might act in some as yet unknown fashion in regard to inhibiting the development of cancer. In the early 1990s, trials of Vitamin E looked to be a resounding success in regard to preventing heart disease. In two studies involving over 127,000 people, it was found that those who consumed a diet rich in Vitamin E had a significantly (40%) lower incidence of cardiovascular disease than those who didn’t. It was found that the addition of Vitamin E to blood samples in vitro seemed to protect LDLs against oxidation, which was believed to be a central modality in the development of heart disease. Sales of Vitamin E soared, with 23 million Americans taking it by the end of the decade, and yet the results of various studies on Vitamin E supplements rather than as present naturally in the diet, are inconsistent in terms of overall health benefits.11,14
The Alpha-Tocopherol, Beta-Carotene (ATBC) Trial14 was a cancer prevention study conducted by the U.S. National Cancer Institute (NCI) and the National Public Health Institute of Finland from 1985 to 1993. It’s aim was to determine whether certain vitamin supplements would prevent lung cancer and other cancers in a group of 29,133 male smokers in Finland. The participants (aged 50-69) took a pill daily over a period of 5-8 years containing either: 50 milligrams (mg) alpha-tocopherol (a form of Vitamin E), 20 mg of beta-carotene (a precursor of vitamin A), both, or a placebo. The main results were as follows, and might be described as “mixed” in their benefits:
• Men
who took beta-carotene had an 18% increased incidence of lung cancer and an
overall death rate of 8%. Vitamin E had no effect on the incidence of lung
cancer or overall mortality. Similar results were found for taking both
supplements to those taking beta-carotene alone.
• The
effects of beta-carotene appeared more adverse in men with a relatively modest
alcohol intake (more than 11 grams per day; 15 grams of alcohol is equivalent
to one drink) and in those smoking at least 20 cigarettes daily.
• Those
taking vitamin E had 32% fewer cases of prostate cancer and the death-rate from
prostate cancer was reduced by 41%. However, death from hemorrhagic stroke was increased
by 50% in men taking alpha-tocopherol supplements, primarily among those with
high blood pressure.
• The
results of both the trial and post-trial follow-up of the ATBC Study, in
conjunction with results from the CARET Study (Beta-Carotene and Retinol
Efficacy Trial) completed in 1996, continue to support the
recommendation that beta-carotene supplementation should be avoided by smokers.
The possible preventive effects of alpha-tocopherol on prostate cancer require
confirmation in other ongoing trials.14
Can Vitamin Supplements Cut the Benefits of Exercise?
It is well known that exercise can promote a reduction in insulin resistance, which is a precursor condition to type 2 diabetes. However, when Ristow's team measured the effects of exercise on insulin sensitivity, they found no reduction in those volunteers who were taking antioxidants, but a significant decrease in those not taking them. Thus it might be concluded that antioxidants are preventing the health effects of exercise, though it should be noted that not all vitamin supplements contain such high doses of vitamin C and E, which are also far higher than would be obtained from eating the recommended amount of fruit and vegetables. The positive effect on health from eating fruit and vegetables may be because they contain other protective compounds, and taking vitamin supplements is no substitute for them. Malcolm Jackson at the University of Liverpool is reported as commenting9: "These data are fully in accord with recent work on the actions of ROS in cells, although clearly at odds with the popular concept that dietary antioxidants are inevitably beneficial."
Antioxidant Therapies?
The
issue of antioxidants acting as defenders of the body against ROS has been
extended to their use in medical therapies.15 If antioxidants
present in the diet can protect against damage to the organism by ROS and the
development of various diseases, it might be plausible to treat various
illnesses with antioxidants. This at least goes the line of reasoning, which is
similar to the case for taking dietary antioxidant supplements, although as we
have seen this is a fairly weak case at best. However, few antioxidants
including edaravone (to treat ischaemic stroke in Japan) have found accepted
clinical use. Moreover, many well-known substances including antioxidant vitamins
(A, C and E), and more recently developed materials like nitrones (also used as
spin-traps for radicals in Electron Spin Resonance investigations16)
have not unanimously passed the scrutiny of clinical trials that they are
effective in the prevention and treatment of various diseases. To date, there
have been several large (>7,000 participants) clinical trials aimed to test
the effectiveness of antioxidants as cancer prevention agents specifically,
none of which have been convincing.17 A recent review18
emphasises the complexity of cancer and its development and the importance of
eliminating as far as possible exposure to environmental carcinogens including
carcinogenic metals, concluding that “prevention, as in all threatening aspects
of life, being better than cure.”
Positive Roles for ROS?
Conclusions.
Humans
have evolved in an atmosphere containing 21% O2, and derived therefrom,
ROS are ubiquitous in ourselves and other animals, and in plants and aerobic bacteria. Over the long human lifespan, continual and
accumulated damage by ROS may contribute to the age-related development of
cancer, neurodegenerative diseases, and many other disorders which ultimately
urge our decline and demise. As we age, the repair of this damage seems to
become less efficient. It is interesting that the concentration of oxidised
protein taken from different human tissues and from rats and flies, creatures
of far shorter longevity than humans, is almost constant up to about half the
life-span of the species, whereupon it accumulates rapidly, and dramatically so
during the last third of the lifespan.19 In terms of a human
lifespan it would seem that after around the age of 40 we oxidise profoundly
and inexorably. Whether this is a cause or a consequence of ageing is arguable,
since as we have seen that elevated levels of ROS appeared to extend the lives
of worms while treatment with antioxidants shortened them. It is likely that ROS act as agents to kill
microbes and protect us against infection, although we have noted one study
that showed it was the release of proteolytic enzymes rather than ROS from
white blood cells that enabled them to combat pathogens.8 ROS also
play an essential and exquisite role in cell signalling mechanisms. Thus ROS
may help to preserve us until our own reproductive years are concluded and the
next generation has reached maturity, i.e. after the age when severe oxidation
sets-in at around 40. Evolution is thoroughly pragmatic and unsentimental about
such matters. The evidence is poor20,21 that taking vitamin
supplements unequivocally protects us against diseases and that therapies
against cancer and other diseases using antioxidants is effective. Indeed,
smokers should be very careful about taking some supplements, particularly
beta-carotene, which appears to increase the incidence of lung-cancer.13,14
When people are actually deficient in a vitamin, giving them extra quantities
up to the recommended daily amount appears beneficial, but this may have
nothing to do with the antioxidant activity of the compound which may serve a
variety of biological functions.
Although there is convincing evidence from a study of nearly 500,000 subjects that consuming more than 200g of fruit and vegetables per day does protect us against developing cancer, the effect is quite small (3%).10 This, nonetheless, translates into around 7,200 cancer cases each year just in the U.K. which if prevented represents a considerable saving to the N.H.S. especially in these stringent times. It is possible that the effect of eating a diet rich in fruit and vegetables may offer some protection against cancer by some other means than the antioxidant content of these foods.20,21 Moreover, perhaps it is the “Mediterranean Lifestyle” overall that matters, and not only the diet. It is notable that much higher intakes of ca 600g/day appeared to give a protection of as much as 11% against developing cancer.10 However, the sample was much smaller and it seems likely that the lifestyle of anyone with such eating patterns differed in other respects too: less smoking and less drinking alcohol, less meat and less saturated fat, less body fat, higher dietary fibre, more exercise, and possibly a less stressful approach to life. It is likely that the human body has been adapted by evolution to adjust the balance between ROS and antioxidants so finely that the intake of additional antioxidants has but a minor influence, and so the degree of oxidative damage is little reduced. In a way, it is reminiscent of the concept of “inbuilt obsolescence”, that we cannot live forever and are designed not too, to make way for the newer and fresher generation on whom we may place our hopes.
Although there is convincing evidence from a study of nearly 500,000 subjects that consuming more than 200g of fruit and vegetables per day does protect us against developing cancer, the effect is quite small (3%).10 This, nonetheless, translates into around 7,200 cancer cases each year just in the U.K. which if prevented represents a considerable saving to the N.H.S. especially in these stringent times. It is possible that the effect of eating a diet rich in fruit and vegetables may offer some protection against cancer by some other means than the antioxidant content of these foods.20,21 Moreover, perhaps it is the “Mediterranean Lifestyle” overall that matters, and not only the diet. It is notable that much higher intakes of ca 600g/day appeared to give a protection of as much as 11% against developing cancer.10 However, the sample was much smaller and it seems likely that the lifestyle of anyone with such eating patterns differed in other respects too: less smoking and less drinking alcohol, less meat and less saturated fat, less body fat, higher dietary fibre, more exercise, and possibly a less stressful approach to life. It is likely that the human body has been adapted by evolution to adjust the balance between ROS and antioxidants so finely that the intake of additional antioxidants has but a minor influence, and so the degree of oxidative damage is little reduced. In a way, it is reminiscent of the concept of “inbuilt obsolescence”, that we cannot live forever and are designed not too, to make way for the newer and fresher generation on whom we may place our hopes.
References.
(1) Halliwell,
B. and Gutteridge, J.M.C. (2007), Free Radicals in Biology and Medicine, 4th
Edition, Oxford University Press U.S.A.
(2) Halliwell,
B. (2006) Reactive Species and Antioxidants. Redox Biology Is a Fundamental
Theme of Aerobic Life. Plant
Physiology 141, 312-322.
(3) R. Gerschman, D. L. Gilbert, S. W. Nye, P. Dwyer, W. O. Fenn,
Oxygen Poisoning and X-irradiation: A Mechanism in Common. Science 119, 623-626
(1954).
(4) Denham Harman, Aging: A Theory
Based on Free Radical and Radiation Chemistry. J. Gerontol. 11, 298-300
(1956).
(5) Slater, T.F. (1966) In vitro effects of carbon
tetrachloride on rat-liver microsomes. Biochem. J. 101, 16p.
(6) Rhodes, C.J. and Dintinger, T.C.
(1999) ESR Studies of Lipids, in Spectral Analysis of Lipids, R.Hamilton
and J.Cast, eds., Sheffield Academic Press, Sheffield.
(7) Zhang, M. et al. (2010) NADPH oxidase-4 mediates protection against chronic
overload load-induced stress on mouse hearts by enhancing angiogenesis. PNAS, 107, 18121-18126.
(8) Ahluwalla, J et al. (2004) The large-conductance Ca2+-activated
K+ channel is essential for innate immunity. Nature, 427, 853-857.
(9) Free Radicals Good For You?
Banned Herbicide Makes Worms Live Longer. http://www.sciencedaily.com/releases/2010/12/101220084442.htm
(10) Boffetta, P. et al. (2010),
Fruit and Vegetable Intake and Overall Cancer Risk in the European Prospective
Investigation Into cancer and Nutrition. JNCI,
102, 529-437.
(11) Melton, L. (2006) The
antioxidant myth: a medical fairy tale. New
Scientist, August 5th, 40-43. http://dcscience.net/The%20antioxidant%20myth.pdf
(12) http://en.wikipedia.org/wiki/Vitamin_C
(13) http://www.ncbi.nlm.nih.gov/pubmed/15572756
(14) http://www.cancer.gov/newscenter/qa/2003/atbcfollowupqa
(15) Firuzi, O. et al. (2011) Antioxidant
Therapy: Current Status and Future Prospects. Curr. Med. Chem., 18, in
press.
(16) Rhodes, C.J. (2011) Electron
spin resonance. Part one: A diagnostic method in the biomedical sciences. Sci. Prog., 94, 16-96.
(17) Goodman, M. et al. (2011) Clinical
trials of antioxidants as cancer prevention agents: past, present and future. Free Rad. Biol. Med., 51, 1068-1084.
(18) Valko, M. et al. (2006) Free
radicals, metals and antioxidants in oxidative stress-induced cancer. Chemico-Biological
Interactions, 160, 1-40.
(19) Levine, R.L. and Stadtman, E.R.
(2001) Oxidative modification of proteins during aging. Experimental Gerontology, 36,
1495-1502.
(20) Halliwell, B. (2007) Dietary
polyphenols: Good, bad, or indifferent to your health? Cardiovscular Research, 73,
341-347.
(21) Gutteridge, J.M.C and
Halliwell, B. (2010) Antioxidants: Molecules, medicines and myths. Biochem. Biophys. Res. Comm., 393, 561-56
1 comment:
Thanks for tips about harmful radicals: why are free radicals harmful
Post a Comment