MECHANISM OF ANTIOXIDANTS IN THE BODY

Antioxidants provide protection against oxidative attack by decreasing oxygen concentration, intercepting singlet oxygen, preventing first chain initiation by scavenging initial radicals, binding of metal ion catalysts, decomposing the primary products of oxidation to non radical compounds and chain breaking to prevent continuous hydrogen removal from substrates.
Antioxidants react with radicals and other reactive species faster than biological substrates, thus protecting biological targets from oxidative damage. Further more the resulting anti oxidant radical possess high stability that is the antioxidant radical interrupts (rather than propagate) a chain reaction.
2.4. EXOGENOUS ANTIOXIDANT SYSTEM:
The body gets some of its antioxidants from the environment, more specifically from the food that is consumed (exogenous antioxidants). While the enzymatic antioxidants are intrinsic to the organism, the non-enzymatic components are both of intrinsic and exogenous nature. The non-enzymatic antioxidants consist of nutrient and non nutrient compounds.
2.4.1ENZYMATIC ANTIOXIDANTS:
Among the enzymatic antioxidants mainly three groups of enzymes play significant roles in protecting cells from oxidant stress:
Superoxide Dismutases (SOD) are enzymes that catalyze the conversion of two super oxides into hydrogen peroxide and oxygen. The benefit here is that hydrogen peroxide is substantially less toxic that superoxide. SOD accelerates this detoxifying reaction roughly 10,000-fold over the non-catalyzed reaction.
SODs are metal-containing enzymes that depend on bound manganese, copper or zinc for their antioxidant activity. In mammals, the manganese-containing enzyme is most abundant in mitochondria, while the zinc or copper forms predominant in cytoplasm. Interestingly, SODs are inducible enzymes - exposure of bacteria or vertebrate cells to higher concentrations of oxygen results in rapid increases in the concentration of SOD.
Catalase is found in peroxisomes in eucaryotic cells. It degrades hydrogen peroxide to water and oxygen, and hence finishes the detoxification reaction started by SOD.
Glutathione peroxidase is a group of enzymes, the most abundant of which contain selenium. These enzymes, like catalase, degrade hydrogen peroxide. They also reduce organic peroxides to alcohols, providing another route for eliminating toxic oxidants.In addition to these enzymes, glutathione transferase, ceruloplasmin, hemoxygenase and possibly several other enzymes may participate in enzymatic control of oxygen radicals and their products.
2.4.2NON –ENZYMATIC ANTIOXIDANTS:

NUTRIENT COMPOUNDS:
Vitamin C:

Ascorbate, an essential vitamin found in fruits and vegetables, has been particularly well studied in its role as an antioxidant and is suggested to serve several physiological functions including (1) preventing free-radical-induced damage to DNA, (2) quenching oxidants which can lead to the development of cataracts, (3) improving endothelial cell dysfunction, and (4) decreasing LDL induced leukocyte adhesion. Vitamin C readily scavenges reactive oxygen and nitrogen species and may thereby prevent oxidative damage to important biological macromolecules such as DNA, lipids, and proteins. Vitamin C also reduces redox active transition metal ions in the active sites of specific biosynthetic enzymes.Ascorbic acid, or vitamin C, has the potential to protect both cytosolic and membrane components of cells from oxidant damage. In the cytosol, ascorbate acts as a primary antioxidant to scavenge free radical species that are generated as by-products of cellular metabolism. For cellular membranes, it may play an indirect antioxidant role to reduce the a-tocopheroxyl radical to a-tocopherol. The erythrocyte results indicate that ascorbate can interact directly with the plasma membrane as an antioxidant. Excellent sources of vitamin C include: parsley, broccoli, bell pepper, strawberries, oranges, lemon juice, papaya, cauliflower, kale, mustard greens.
Fig3. Structure of Ascorbate

Vitamin E (α-tocopherol):

Vitamin E (tocopherol) is a fat-soluble vitamin which functions solely as a membrane bound antioxidant that prevents cell membrane damage by inhibiting per oxidation of membrane phospholipids and disrupting free radical chain reactions induced by formation of lipid peroxides. Vitamin E also increases the bioavailability of vitamin A by inhibiting its intestinal oxidation . As the only membrane-bound lipid-soluble antioxidant, Vitamin E plays a key role in preventing cellular injury from oxidative stress associated with premature aging, cataracts, uncontrolled diabetes, cardiovascular disease, inflammation, and infection. Exogenous supplementation of functionally efficient antioxidants like vitamin E reactivates the enzymatic antioxidant system and guards against the insult caused by ROS during the pathogenesis of the diseases.Increased production of reactive oxygen species secondary to phagocyte respiratory burst occurs in pulmonary tuberculosis (TB) vitamin E and selenium supplementation reduces oxidative stress and enhances total antioxidant status in patients with pulmonary TB treated with standard chemotherapy. Vitamin E is found only in foods of plant origin. Wheat germ is the richest source of the vitamin. Vegetable oils and whole grains are additional rich sources of this nutrient. Nuts, peanut butter, salad dressings and vegetable oils are also good sources of vitamin E.
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Fig4.The chemical structure of alpha-tocopherol

β-carotenoids:
Carotenoids are nature’s most widespread pigments and have also received substantial attention because of both their provitamin and antioxidant roles. More than 600 different carotenoids have been identified in nature. Carotenoids have a 40-carbon skeleton of isoprene units. The structure may be cyclized at one or both ends, may have various hydrogenation levels, or may possess oxygen-containing functional groups. Lycopene and ß-carotene are examples of acyclized and cyclized carotenoids, respectively. Carotenoid compounds most commonly occur in nature in the all-trans form. Mixtures of carotenoids or associations with others antioxidants (e.g. vitamin E) can increase their activity against free radicals. Carotenoids are found in colored fruits and vegetables. Apricots, antaloupe, carrots, pumpkin and sweet potato are sources of a-carotene and b-carotene; pink grapefruit, tomatoes and watermelon are sources of lycopene, z-carotene, b-carotene, phytofluene and phytoene. Mango, papaya, peaches, prunes, squash and oranges are sources of lutein, zeaxanthin, aand b-cryptoxanthin, a-, b- and z-carotene, phytofluene and phytoene, whereas green fruits and vegetables such as green beans, broccoli, brussel sprouts, cabbage, kale, kiwi, lettuce, peas and spinach are sources of lutein, zeaxanthin, a- and b-carotene. Carotenoid concentrations in fruits and vegetables vary with plant variety, degree of ripeness, time of harvest, and growing and storage conditions.

Fig5.Different types of Carotenoids
2.4.3NON NUTRIENTCOMPOUNDS:
Polyphenols - the potent antioxidants in plant foods:

Phenolic antioxidants, a specific group of secondary metabolites play the important role of protecting organism against harmful effects of oxygen radicals and other highly ROS. Their formation in human organisms is closely connected with the development of a wide range of degenerative diseases, mainly arteriosclerosis and other associated complications ,cancer and aging.Among natural antioxidants plant poly phenols play a very important role. Flavonoids are a class of poly phenolic compounds is widely and ubiquitously found in fruits , vegetables , grains ,nuts, and medicinal plants. High consumption of plant phenolics in the daily diet has been found to provide their ability to low density lipo proteins, platelet aggregation, growth of tumour cells and inflammation reactions. They are one of the major groups of nonessential dietary components appearing in vegetable foods. They are a wide chemical compounds group that are considered as secondary plant metabolites, with different activity and chemical structure,including more than 8,000 different compounds.

Fig6. Different Polyphenolic compounds
Flavonoids:

Flavonoids are a group of phenolic compounds with antioxidant activity that have been identified in fruits, vegetables, and other plant foods and that have been linked to reducing the risk of major chronic diseases. More than 4000 distinct flavonoids have been identified. Flavonols (quercetin, kaempferol, and myricetin), flavones (luteolin and apigenin), flavanols (catechin, epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate), flavanones (naringenin), anthocyanidins, and isoflavonoids (genistein) are common flavonoids in the diet. Flavonoids are most frequently found in nature as conjugates in glycosylated or esterified forms but can occur as aglycones, especially as a result of the effects of food processing.

Fig7.Types of Flavonoids

Phenolic acids:

Phenolic acids can be subdivided into two major groups, hydroxybenzoic acids and hydroxycinnamic acids .Hydroxybenzoic acid derivatives include p-hydroxybenzoic, protocatechuic, vannilic, syringic, and gallic acids. They are commonly present in the bound form and are typically a component of a complex structure like lignins and hydrolyzable tannins. They can also be found in the form of sugar derivatives and organic acids in plant foods. Food processing, such as thermal processing, pasteurization, fermentation, and freezing, contributes to the release of these bound phenolic acids Hydroxycinnamic acid derivatives include p-coumaric, caffeic, ferulic, and sinapic acids. They are mainly present in the bound form, linked to cell-wall structural components, such as cellulose, lignin, and proteins through ester bonds. Ferulic acids occur primarily in the seeds and leaves of plants, mainly covalently conjugated to mono- and disaccharides, plant-cell-wall polysaccharides, glycoproteins, polyamines, lignin, and insoluble carbohydrate biopolymers. Wheat bran is a good source of ferulic acids, which are esterified to hemicellulose of the cell walls. Free, soluble-conjugated, and bound ferulic acids in grains are present in the ratio of 0.1:1:100. Food processing, such as thermal processing, pasteurization, fermentation, and freezing, contributes to the release of these bound phenolic acids.




2.5ANTIOXIDANT ACTIVITY VERSUS ANTIOXIDANTS:

Due to the chemical diversity of antioxidant compounds present in foods, complete databases on food antioxidant content are not yet available. In addition, levels of single antioxidants in food do not necessarily reflect their total antioxidant capacity (TAC); this also depends on the synergic and redox interactions among the different molecules present in the food. The combined activity of the antioxidants in food or plasma is termed as AOA, which provides an integrated parameter rather than the simple sum of measurable antioxidants. In AOA the capacity of known and unknown antioxidants and their synergistic interaction is assessed, thus giving an insight into the delicate balance invivo, between oxidants, antioxidants, allied substances and metabolites.