The small-molecule antioxidants neutralize the ROS in a process called radical scavenging and carry them away. To understand the mechanism of action of antioxidants, it is necessary to understand the generation of free radicals and their damaging reactions. This review elaborates the generation and damages that free radicals create, mechanism of action of the natural antioxidant compounds and assays for the evaluation of their antioxidant properties. The reaction mechanisms of the antioxidant assays are discussed. The scope of this article is limited to the natural antioxidants and the in vitro assays for evaluation of their antioxidant properties.
These ROS can act by either of the two oxygen dependent mechanisms resulting in the destruction of the microorganism or other foreign matter. The reactive species can also be generated by the myeloperoxidase—halide—H 2 O 2 system. The enzyme myeloperoxidase MPO is present in the neutrophil cytoplasmic granules.
In presence of the chloride ion, which is ubiquitous, H 2 O 2 is converted to hypochlorous HOCl, eqn 3 , a potent oxidant and antimicrobial agent. Peroxynitrite reacts with the aromatic amino acid residues in the enzyme resulting in the nitration of the aromatic amino acids. Such a change in the aminoacid residue can result in the enzyme inactivation. However, nitric oxide is an important cytotoxic effector molecule in the defense against tumor cells, various protozoa, fungi, helminthes, and mycobacteria.
The peroxyl radicals are the carriers of the chain reactions. The peroxyl radicals can further oxidize PUFA molecules and initiate new chain reactions, producing lipid hydroperoxides LOOH eqn 10 and 11 that can break down to yet more radical species. Lipid hydroperoxides always break down to aldehydes. Many of these aldehydes are biologically active compounds, which can diffuse from the original site of attack and spread the attack to the other parts of the cell.
This type of oxidative damage to DNA is highly correlated to the physiological conditions such as mutagenesis, carcinogenesis, and aging. However, the Chydroxy-adduct radical of guanine is converted to the 8-hydroxyguanine upon oxidation reaction. The oxidation reaction of these adduct radicals with water followed by deprotonation results in the formation of the cytosine glycol and thymine glycol, respectively. The reactions of carbon-centered sugar radicals result in the DNA strand breaks and base-free sites by a variety of mechanisms.
The amino acid's lysine, proline, histidine, and arginine have been found to be the most sensitive to oxidative damage. Recent studies indicate that, a wide range of residue modifications can occur including formation of peroxides, 27,28 and carbonyls. Thus, the oxidative damage to tissue results in the increased amount of oxidized protein. A detailed review by Cooke et al.
Low levels of antioxidants have been associated with the heart disease and cancer. The other disorders to which antioxidants provide protection are cataract, cerebral ischemia, diabetes mellitus, eczema, gastrointestinal inflammatory diseases, genetic disorders. The different expression profiles, subcellular locations, and substrates of the antioxidant enzymes reveal the complex nature of the ROS biology. Clearly, the antioxidant enzymes play a major role in the prevention of oxidative damage. The enzymatic antioxidants and their mechanism of antioxidant activity has been explained in details in several review articles.
The nonenzymatic antioxidants are of two types, the natural antioxidants and the synthetic antioxidants. However, the scope of this article is limited to the natural antioxidants; hence the synthetic antioxidants will not be considered for the discussion.
The resultant tocopheroxyl radical is relatively stable and in normal circumstances, insufficiently reactive to initiate lipid peroxidation itself, which is an essential criterion of a good antioxidant. Vitamin C or ascorbic acid 2 , is a water-soluble free radical scavenger. Moreover, it regenerates vitamin E in cell membranes in combination with GSH or compounds capable of donating reducing equivalents.
The pairs of ascorbate radicals react rapidly to produce one molecule of ascorbate and one molecule of dehydroascorbate. The dehydroascorbate does not have any antioxidant capacity. Hence, dehydroascorbate is converted back into the ascorbate by the addition of two electrons. The last stage of the addition of two electrons to the dehydroascorbate has been proposed to be carried out by oxidoreductase.
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Antioxidant potential of vitamin A 3 was first described by Monaghan and Schmitt, 52 who reported that vitamin A can protect lipids against rancidity. Several reviews have appeared to outline the basic structural and metabolic characteristics of vitamin A and information about its potential as antioxidants in relation to the heart diseases. It has been reported that the bioflavonoids have a protective effect on the DNA damage induced by the hydroxyl radicals.
The flavonoids complexed with the copper or iron prevent the generation of the ROS. Therefore, it is very important to consider the concentration of the chelating metal ions, such as copper or iron while evaluating the protective or degenerative effects of quercetin and other bioflavonoids.
Anthocynidine, a class of flavonoids are potential antioxidants and their effectiveness in the inhibition of the lipid oxidation is related to their metal ion-chelating activity Scheme 8 and free-radical scavenging activity Scheme 9. Three structural groups are important determinants of the radical-scavenging activity of anthocynidines 18— Second, the 2,3 double bond in conjugation. Third, the 4-oxofunction in the C-ring.
Flavonoids form complexes with the metal ions by using the 3- or 5-hydroxyl and 4-ketosubstituents or hydroxyl groups in ortho position in the B-ring. As shown in the Scheme 9 , the anthocynidins cynidin 19 can donate an electron accompanied by a hydrogen nucleus to a free radical from —OH groups attached to the phenolic rings. The Scheme 12a demonstrate the mechanism of the radical-scavenging activity of the allicin.
The radical-scavenging activity of allicin involves H-atom transfer to a peroxyl radical from the methylene of the allyl group on the divalent sulfur. Scheme 12b demonstrate an alternative mechanism, where the radical-scavenging activity of allicin can be accounted for 2-propenesulfenic acid, which is produced from allicin by Cope elimination.
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Piperine 1-piperoylpiperidine 37 , is an alkaloid present in fruits of black pepper Piper nigrum , long pepper Piper longum , and other piper species family: Piperaceae. Piperine possesses many pharmacological activities, including anti-inflammatory and analgesic effect, anti-ulcer activities, antidepressant effect, cognitive enhancing effect, cytoprotective effect, and antioxidant activity. Whereas, in low concentrations piperine acts as an antioxidant. The free radical can undergo electron transfer or abstract H-atom from either of these two sites. However, pulse radiolysis and other biochemical methods credited the antioxidant activity of curcumin to its phenolic OH group.
The Scheme 14 depicts the mechanism for the autoxidation of curcumin initiated by hydrogen abstraction from one of the phenolic hydroxyl groups.
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The methide radical performs a 5-exo-cyclization with the double bond to give the cyclopentadione ring and generating the carbon-centered radical. The reaction of curcumin with the molecular oxygen O 2 results in the peroxyl radical. The peroxyl radical is then reduced to the hydroperoxide by abstracting a hydrogen atom from another curcumin molecule, propagating the autoxidation chain reaction. Subsequently, the hydroperoxide loses water and rearranges into the spiro-epoxide.
The hydrolysis of the epoxide by the water-derived hydroxyl group results in the formation of the final bicyclopentadione product. Then these phenoxyl radicals can generate new products or react with reduced copper ions of the complex resulting in the regeneration of the complex.
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Uric acid 39 in plasma possesses strong radical scavenging activity. It contributes for as much as two-thirds of all free radical scavenging activities in the plasma. However, it loses it's radical scavenging activity within lipid membranes. Moreover, uric acid requires the presence of ascorbic acid Scheme 16 and thiols for the complete scavenging of peroxynitrites. GSH 40 in cell cytosol, together with its related enzymes, comprises a system that maintains the intracellular reducing environment, which acts as primary defense against excessive generation of harmful ROS.
As shown in the Scheme 17 , three groups of enzymes can be identified in the GSH catalytic cycle: glutathione oxidase, glutathione reductase, and GSHPx. However, the de nova synthesis of glutathione from its amino acid constituents is required for the elevation of glutathione as an adaptive response to oxidative stress. The presence of the sulfhydryl group in glutathione allows it to serve as an antioxidant. The synthetic antioxidants are the second type of nonenzymatic antioxidants. Cinnamic acid derivatives , 41 , 42 , melatonin 43 , selegiline 44 , are the few examples of the synthetic antioxidants.
Due to its odd electron, the methanolic solution of DPPH shows a strong absorption band at nm. As shown in the Scheme 19 , the DPPH radical reacts with suitable reducing agent producing new bond, thus changing the color of solution.
The solution loses color with the increase in the concentration of antioxidant as the electrons taken up by DPPH radical from the antioxidant. As shown in the Scheme 20 , the AAPH undergoes thermal decomposition in solution to produce two carbon-centered amidino propane AP radicals, which can add O 2 to form peroxyl radicals. However, the carbon-centered radicals usually predominate. The activity of test compound to inhibit peroxidation of membrane lipids at pH 7. The interference of the test drug with color development is determined by adding a previously determined concentration of the test compound to the TBA reagents and used to determine the extent of peroxidation of animal phospholipids.
In general, the in vivo assays for testing potential antioxidants are more expensive because they require complex cellular testing systems or full clinical trials. However, it is very important to proceed to cellular assays after screening antioxidant activity with an in vitro method in order to obtain information on some aspects like uptake, bioavailability, and metabolism. There are several other reports, which elaborate the advantages and disadvantages of various methods for the evaluation of antioxidant activity.
Many investigators found that, increasing the level of defense mechanisms against oxidative stress could extend an organism's health span.