Figure 1: Free radicals possessing an unpaired electron in the outer shell.

Antioxidants are a group of substances which, when present at low concentration in relation to oxidizable substrates, significantly inhibit or delay oxidative processes, while often being oxidized themselves. Antioxidants can be better understood by having information about their nature of reactivity with reactive oxygen species or free radicals. An antioxidant can act by scavenging reactive oxygen species by inhibiting their formation, by binding transition metal ions and preventing the formation of hydroxyl and/or decomposition of lipid peroxides, by repair damage or by combination of all ².

Figure 2: antioxidant scavenging the free radical

Types of Free radicals:

Oxygen is required in many metabolic processes, particularly for the release of energy. During these processes, molecular oxygen is completely reduced and converted to water. However if the reduction of O₂is incomplete, a series of reactive radicals are formed ³.

Table 1: Biologically significant free radicals.

Reactive Oxygen Species
O₂⁻	Superoxide radical
OH	Hydroxyl radical
ROO	Peroxyl radical
H₂O₂	Hydrogen peroxide
¹O₂	Singlet oxygen
NO	Nitric oxide
ONOO⁻	Peroxynitrite
HOCl	Hypochlorous acid

Sources of Free radical:

a. Endogenous sources:

Aerobic respiration, peroxisomes and stimulation of polymorphonuclear leucocytes and macrophages, Respiratory burst, Sub-cellular organelles (mitochondria, chloroplasts, microsomes, peroxisomes, nuclei), Transition metals ions, Ischemic reperfusion injury.

b. Exogenous sources:

Drugs, Radiation, Tobacco smoking, pesticides, inorganic particles, Gases (e.g. ozone), Others (fever, pesticides, solvents, anaesthetics, exhaust fumes and aromatic hydrocarbons)

c. Physiological sources:

Stress, Emotions and diseased conditions ⁴.

Classification of Antioxidants

Ø Classification of antioxidants based on their roles:

Enzymes: Superoxide dismutase (SOD), Catalase, Glutathione Peroxidase.

Vitamins: Alpha tocopherol, Beta carotene, Ascorbic acid⁵.

Ø Classification of antioxidants based on their sources:

Table 2: Classification of antioxidants based on their sources ⁵

Source Material	Example	Antioxidant
Vegetable Oils	Soybean oil	Tocopherols
Tropical Oils	Palm oil	Tocotrienols
Plant Oils	Palm oil	Carotenoids
Herbs and Spices	Rosemary and Sage	Complex phenolics
Cereals	Wheat and buckwheat	Flavonoids
Legumes	Soybean	Isoflavones
Oil Seeds	Canola and Mustard	Phenolic acids and Phenylpropanoids
Teas	Green Tea	Catechins and Polyphenols
Fruit skin and seeds	Grape seed and skin	Polyphenols and Tannins

MECHANISM OF ACTION OF ANTIOXIDANTS:

There are four routes:

1. Chain breaking reactions e.g. alpha-tocopherol

2. Reducing the concentration of reactive oxygen species e.g. glutathione peroxidase, catalase

3. Scavenging initiating radicals e.g. superoxide dismutase

4. Chelating the transition metal catalysts e.g. transferrin, lactoferrin^5,6

THERAPEUTIC USES OF ANTIOXIDANTS:

1. Anti-cancer agents in medicinal chemistry:

a. Lanthanides as anti-cancer agents:

Text Box: • Pancreatitis
• Inflammatory bowel disease and colitis
• Aging A lot of metal–based drugs are widely used in the treatment of cancer. The clinical success of cisplatin and other platinum complexes is limited by significant. Side effects are acquired or intrinsic resistance. Therefore, much attention has focused on designing new coordination compound with improved pharmacological properties and a broader range of antitumor activity. Strategies for developing new anti-cancer agents include the incorporation of carrier groups that can target tumor cells with high specificity. Also of interest is to develop complexes that bind to DNA in a fundamentally different manner than cisplatin, in an attempt to overcome the resistance pathway that has evolved to eliminate the drug ^7,8.

b. Lycopene as a potential anti-cancer agent:

Dietary chemoprevention has emerged as a cost-effective approach to control most prevalent chronic diseases including cancer. In particular, tomato and products are recognized to confer a wide range of health benefits. Epidemiology studies have provided evidence that high consumption of tomatoes effectively lowers the risk of reactive oxygen species (ROS)-mediated diseases such cancer by improving the antioxidant carotenoid which is reported to be more stable and potent singlet oxygen quenching agent compared to other carotenoids. In addition to its antioxidants properties, lycopene shows an array of biological effects including cardio-protective, anti-inflammatory, anti-mutagenic and anti-carcinogenic activities. The cancer activities of lycopene have been demonstrated in both vitro and in vivo tumour models ^7,9.

c. Selenium derivatives as cancer preventive agents:

The role of selenium in the prevention of cancer has been recently established by laboratory experiments, clinical trials and epidemiological data^10,11. Consequently, selenium supplementation has moved from the realm of correcting nutritional deficiencies to one of pharmacological intervention, especially in the clinical domain of cancer, chemoprevention and in the control of heart failure⁷.

2. Applications of lipoic acid:

Fig 4: therapeutic applications of lipoic acid as antioxidant

Lipoic acid protects against diseases of aging. This offer powerful antioxidant protection against three common afflictions (two of them potentially disastrous) association with the aging, stroke, heart attack and cataracts. It does it by suppressing the action of free radicals in the cells of the brain, heart and eyes. Lipid acid has an unusual relationship with four other important antioxidants: glutathione, coenzyme-Q₁₀, vitamin C and vitamin E. Lipoic acid not only acts as a primary antioxidant in brain cells but serves to boost glutathione levels through the antioxidant network interactions. Lipoic acid in the form of gene therapy promises to be one of the most exciting and fruitful avenues of medical practice in the twenty-first century and it offer powerful antioxidant protection against common afflictions including diabetes¹¹.

3. Acute central nervous system injury: Oxidative stress has been implicated as a potential contributor to acute central nervous system (CNS) injury by ischemic or haemorrhagic stroke or trauma. Free radicals can cause damage to cardinal cellular components such as lipids, proteins and nucleic acid e.g. DNA leading to subsequent cell death by modes of necrosis or apoptosis. The damage can become more widespread due to weakened cellular antioxidant defence systems. Moreover, acute brain injury increases the level of excitoxic amino acids (such as glutamate), which also produce ROS, thereby promoting parenchymatous destruction. Therefore, treatment with antioxidants may theoretically act as tissue damage and improve both the survival and neurological outcome. Better understanding of the pathological mechanisms of acute CNS injury would characterize the exact primary targets for drug intervention improved antioxidant design should take into consideration the relevant and specific harmful free radical ¹¹.

4. Neurodegenerative diseases:

Oxidative stress has a mechanistic role in the development of Alzheimer's dementia. Several lines of evidence previously implied that oxidative damage to lipid membranes could disrupt normal neuronal and glial cell functioning, leading to the formation of amyloid plaques and to neuronal cell death. Hence, it is found that dietary intake of antioxidants such as vitamins E, C and beta carotene might inhibit the production of free radicals and reactive oxygen species. Antioxidants are also being investigated as possible treatments for Parkinson's disease^12,13.

5. Cardiovascular diseases:

Heart disease is the leading cause of death in the United States. It is estimated that one in three Americans will eventually die from this disease ¹⁴. While several factors, such as high cholesterol levels, hypertension, cigarette smoking, and diabetes, are believed to promote atherosclerosis, a growing body of evidence suggests a critical step in its development is the oxidation of low-density lipoprotein (LDL) within the arterial wall. This theory is supported by several epidemiological studies which link low intakes of dietary antioxidants to an increased frequency of heart disease ¹⁴. Additionally, an inverse relationship between heart disease and plasma antioxidant levels has been reported ^16,17. Antioxidants have been shown to prevent LDL oxidation in vitro and retard the progression of atherosclerosis in animal models ¹⁸. Several human studies found increased vitamin E levels in LDL, increased resistance of LDL oxidation, and decreased the rate of LDL oxidation ¹⁹. In a recent retrospective study, Stampfer, et al. found that nurses who consumed higher amounts of vitamin E on a regular basis had a 41% lower incidence of heart disease than nurses who consumed the lowest level of vitamin E from their diet and supplements. It has been estimated that dietary increases in antioxidant vitamins may reduce the risk of heart disease by 20-30% ^14,18,20.

6. Pulmonary Disorders:

Because of its large surface area, the respiratory tract is a major target for free radical insult and also air pollution is a major source of ROS. Recent studies suggest that free radicals are involved in the development of pulmonary disorders such as asthma ²¹. Cellular damage caused by free radicals is thought to be partly responsible for the bronchial inflammation characteristic of this disease. It has been suggested that increasing antioxidant intake may help to reduce oxidant stress and help to prevent or minimize the development of asthmatic symptoms. Some evidence suggests glutathione or possibly N-acetyl cysteine, which is a precursor to glutathione, may be helpful in protecting against pulmonary damage as well ^22,15,23.

7. Neurological and ophthalmic disorders:

Other major pathologies that may involve free radicals include neurological disorders and cataracts. Neural tissue may be particularly susceptible to oxidative damage because it receives a disproportionately large percentage of oxygen and it has a high concentration of polyunsaturated fatty acids which are highly prone to oxidation ³⁰. Formation of cataracts is believed to involve damage to lens protein by free radicals, causing the lens to lose its transparency. Some evidence suggests that cataract progression might be slowed with regular consumption of supplemental antioxidants in particular vitamin E, vitamin C, and the carotenoids. It is estimated that if cataract development were delayed by 10 years as a result of increased antioxidant protection, the number of cataract surgeries performed in the U.S. would decrease by more than half ¹⁵.

8. Prevention of ageing:

Free radicals are impurities or harmful substances that cause damages to the skin. They are usually brought about by direct contact with ultraviolet radiation from exposure to sunlight. Free radicals cause chain reactions that lead to disruptions of living cells. Logically, they speed up or worsen visible signs of inevitable aging. Antioxidants help to prevent the unlikely effects of free radicals to the skin and to the entire body. Thus, antioxidants in food sources and skincare products could be considered essential in keeping our skin and body healthier. Japanese men and women have a very youthful and flawless skin. Their secret is logically their regular consumption of Phytessence Wakame, which they eat fresh or dried. Effective anti-aging skin care products in the country usually contain the sea-algae as an active ingredient. Hyaluronic acid combines with collagen and elastin to help keep elasticity and smoothness of the skin, which is very necessary especially when aging. Thus, products with this natural active ingredient could help protect our skin against potential damages brought about by harmful ultraviolet rays and unlikely environmental pollution. Skin-care formulations obviously aim to make products healthy and effective in maintaining youthfulness of the skin²⁴.

9. Uses in technology:

a. Food preservatives:

Exposure to oxygen and sunlight are the two main factors in the oxidation of food, so food is preserved by keeping in the dark and sealing it in containers or even coating it in wax, as with cucumbers. However, as oxygen is also important for plant respiration, storing plant materials in anaerobic conditions produces unpleasant flavours and unappealing colours. Consequently, packaging of fresh fruits and vegetables contains an approximately 8% oxygen atmosphere. Antioxidants are an especially important class of preservatives as, unlike bacterial or fungal spoilage, oxidation reactions still occur relatively rapidly in frozen or refrigerated food. These preservatives include natural antioxidants such as ascorbic acid (AA, E300) and tocopherols (E306), as well as synthetic antioxidants such as propyl gallate (PG, E310), tertiary butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA, E320) and butylated hydroxytoluene (BHT). Since oxidized lipids are often discolored and have unpleasant tastes, it is important to avoid oxidation in fat-rich foods. Even less fatty foods such as fruits are sprayed with sulfurous antioxidants prior to air drying. Antioxidant preservatives are also added to fat-based cosmetics such as lipstick and moisturizers to prevent rancidity ²⁵.

b. Industrial uses:

Antioxidants are frequently added to industrial products. A common use is as stabilizers in fuels and lubricants to prevent oxidation and in gasolines to prevent the polymerization that leads to the formation of engine-fouling residues. They are widely used to prevent the oxidative degradation of polymers such as rubbers, plastics and adhesives that causes a loss of strength and flexibility in these materials. Polymers containing double bonds in their main chains such as natural rubber and polybutadiene are especially susceptible to oxidation and ozonolysis. They can be protected by anti-ozonants ²⁵.

AN OVERVIEW OF ANTIOXIDANT ACTIVITY OF HERBAL EXTRACTS BY DIFFERENT SCIENTISTS:

Table 3: Recent work done by different scientists

Sr. no	Name of Plant	Active constituent	Author name	Activities	Reference. No
1	Nigella sativa	Thymoquinone	N. Ilaiyaraja, F. Khanum	Anti-diabetic, Anti-microbial, Anti-cancerous, Anti-inflammatory, Gastroprotective	28
2	Dorstenia mannii, Burkea Africana	Flavonoids	S. Atawodi	Hepatoprotective, Antioxidant	29
3	Coleus forskohlii	SOD, peroxidase, polyphenol, oxidase, catalase, Lycopene, carotene	S. Khatun, N. Chandra, K. Chatterjee, U.Akilciglu	Anticancer, Antioxidant	30
4	Ageratum conyzoides	flavonoids, alkaloids, chromenes, benzofurans, terpenoids, tannins	N. Nyemb, M. Dicoum, B. Adèle, N. Njikaman, E. Abdennebi	Bacteriocide, Antidysenteric, Antilithic, Antidiabetic	31
5	Anisopus mannii	phenols	A. Aliyu, H. Ibrahim, A. Musa, M. Ibrahim, A. Oyewale, J. Amupitan	Antimicrobial, Antioxidant	32
6	Ocimum gratissimum, Ocimum americanum	Phenols	F. Lukmanul Hakkim, G.Arivazhag, R. Boopathy	Antioxidative, Antimicrobial	33
7	Artemisia anomala	Butylated hydroxytoluene, eupatilin, arteanoflarone	H. Guangrog, J. Jiaxin, D. Dehui	Antioxidative, Antibacterial	34
8	Artocarpus lakoocha	phenolic, flavonoids, tannins, glutathione, malondialdehyde	S. Singhatong, D.Leelarungry, C. Chaiyasut,	Anti-inflammatory, Antiplatelet, Skin-whitening agent	35
9	Azadirachta Indica	Azadirachtinan, nimbin	A. Ghmeray, B. Kumar, D. Cho	Antiviral, Antibacterial Antifungal, Anti-inflammatory, Antipyretic, Antiseptic, Antiparalitic	36
10	Camellia sinensis	Caffeine, xanthines, theobromine, tannins	T. Mahmood, N. Akhtar, B. Khan,	Antimutagenic, Antitumour, Antioxidant, Anticoagulant, Antiviral, blood pressure, Antihyperlipidemic	37
11	Cassia sophera	butylated hydroxyl anisole, butylated hydroxytoluene	A. Rahman, M. Rahman, Md. Sheik	Prevention of pityriasis, psoriasis, asthma, acute bronchitis, cough, diabetes, Convulsions	38
12	Echinacea purpurea	flavonoids, phenols acid, isoflavonoid, anthocyanins	T. Lee, C. Chen, Z. Shieh, B. Yu,	Antioxidant, Antihyperlipidemic	39
13	Garcinia indica	Citric acid, malic acid, polyphenols, carbohydrate, anthocyanin, pigments, ascorbic acid	A. Mishra, M. Bapat, J. Tilak	Antiallergic, Antidiarrhoeal, Appetizer, Liver tonic	40
14	Globularia alypum	tannins, flavonoids, coumarins, sterols	A. Neffati, I. Skandrani, E. Maaloul, L. Chekir Ghedira	hypoglycaemic agent, laxative, cholagogue, stomachic, purgative, sudorific	41
15	Hibiscus esculentus	Silymarin, superoxide dismutase	P. Jayaraj, M. Syam Mohan, R. Varatharajan	Antidiabetic, Laxative, Antijaundice	42
16	Murray koenigii, Alstonia scholaris, Ficusbenjamia, Sapindus trifoliatus	Phenols, flavonoids	A. Kumar, R. Kaur, S. Arora	Astringent, malaria, fever, diarrhoea, hypertension, cancer, diabetes, coughs, intestinal parasitism	43
17	Jatrophacurcas	phenolics, saponins	S. Ahmad, W. Kuan, N. Azlina Zolkifli, R.Hendra	Antioxidant, Anti-inflammatory, Anticancer	44
18	Malaysian Rubiaceae	phenols	R.Ahmad, E. Mahbob, Z. Noor, N. Ismail	Anti-inflammatory, Antioxidant	45
19	Melissa officinalis	rosmarinic acid, rosamirinic acid	E. Bursal, E. Dikici, F. Tozoglu, I. Gulcin	Antioxidant	46
20	Cosmos caudatus, Oenanthe javanica, Centella asiatica	phenolic compounds	N. Huda-Faujan, A. Noriham, A. Norrakiah , A. Babji	Antioxidant	47
21	Bruceasimim, Intsiabijuga, Laporteam eyeniana	tannins, triterpenes, steroids, anthraquinones, anthrones, flavonoid, glycosides.	N. Peteros, M. Mylene	Bacteriostatic, Antidepressant, Antimicrobial, Astringent, Antioxidant	48
22	Punica granatum	Phenols, Complex polysaccharides	M. Miguel, M. Neves, M. Antunes	Antimicrobial, Antioxidant, Anticancer, Anti-inflammatory	49
23	Sargassum boveanum	Phenols	R. Zahra, M. Mehrnaz, V. Farzaneh, S. Kohzad	Antioxidant	50
24	black tea	Theaflavins	C. Wang, L. Yongquan	Antioxidant, Anticancer, Antimicrobial	51
25	Tragia Plukenetii	Phenols	M. Sundaram, S. Dorairaj, P. Rangarajan	Antitumor, Antioxidant	52
26	Achilla schischkinii, Achillatertifolia	essential oils	I. Turkoglu, S. Turkoglu, S. Celik, M. Kahyaoglu	Antioxidant, antimicrobial	53
27	Tinospora cordifolia, Stachytarpheta indica, Calotropis procera, Mimosa pudica	Phenols, flavonoids	Z. Zafar, C. Bandopadhyay, M. Sinha, J. Sarkhel	Antioxidant, immunomodulator	54
28	Caesalpinia coriaria, Hiptagea benghalens, Gloriosa superb	nitrogen compounds, vitamins, terpenoids	P. Amudha, P. Shanthi	Prevention of cancer, Cardiovascular diseases, Diabetes	55
29	Gynura Procumben, Achyranth saspera, Polygonum tomentosum	cyanogenetic glycoside, alkaloid, phenolic compounds, flavonoids, steroids	S. Maw, M. Mon, Z. Oo	Antioxidant, Antitumor, Antidiabetic, Anti-inflammatory, Anti-allergic agents	56
30	Withania somnifera, gossypium herbasceum	wadeolocton, eclalbasaponin, stigmasterol, luteolin-glucoside	R. Zaman, M. Ghaffar, T. Fayyaz, S. Mehdi	Anti-inflammatory, Antihemorrhagic, Antihyperlipidemic, Antihyperglycemic	57

10. Atherosclerosis:

Extensive volume of basic, clinical and experimental research proving involvement of reactive oxygen species (ROS) in the pathogenesis of atherosclerosis suggest that antioxidant therapy could be beneficial during atherosclerosis in postmenopausal women, given that decreased production of estrogens in postmenopausal period is one of the important pathogenetic links of atherosclerosis and contributes to its development via direct (effecting the intensity of oxidative processes) and indirect (effecting the hormonal homeostasis) mechanisms. For the treatment of atherosclerosis we offer the complex of antioxidant preparations - vitamin E and triovit and the preparation “Phenovin” synthesized in Georgia out of phenol compounds of red grapes peel (“Sapheravi”) characterized by antioxidant, hypolipidemic, fibrinolytic and thrombolytic properties^{26, 27}.

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Received on 18.11.2011

Modified on 24.11.2011

Accepted on 27.11.2011

Research J. Pharmacology and Pharmacodynamics. 4(1):Jan. - Feb., 2012, 55-61