Naturally Occurring Antioxidants: The different antioxidants occurring naturally in the body of organisms are described below: Alkaloids and related compounds: Alkaloids constitute a wide variety of nitrogenous compounds. They are usually, but not always, of plant origin, heterocyclic and basic. A possible antioxidant role of alkaloids and related nitrogenous compounds could be as quenchers of singlet oxygen. Polyamines such as spermine, spermidine, and putrescine have been shown to accumulate in some plants exposed to elevated levels of UV. Boldine, an aporphine derivative, is the principal alkaloid found in the bark of the leaves of Peumus boldo, was found to inhibit autooxidation in several biological systems. Besides these several other derivatives are associated with antioxidant activity.
Amino acids and peptide derivatives: Amino acids have been variously reported to act as an antioxidant. The indole-containing amino acids and derivatives like tryptophan, melatonin, and tryptamine are known to have antioxidant activities in some systems.
Beta carotene: The carotenes are a class of terpenoid hydrocarbons found in almost all higher plants. The most abundant of these hydrocarbons beta carotene has a structure featuring wo substitute cyclohexene rings linked by a 22 carbon polyene chain. It is almost entirely insoluble in water but readily soluble in hydrophobic environments and non-polar solvents.
Carotenoids particularly beta carotene scavenges free radicals under some conditions. Many studies have demonstrated that beta carotene inhibits the autooxidation of lipids in biological tissues and food products. Peroxy radicals in particular have been shown either to add to long the chain of conjugated double bonds present in beta carotene and other carotenoids or to take part in electron transfer reactions giving rise to carbon-centered beta carbonyl free radicals. Carnosine: It is a sulfur-containing peptide that has been suggested to possess antioxidant activity. It is found in muscle tissue at levels from 1-60mM. Physiologists have believed that carnosine’s present at such high levels could be related to its buffer activity, however, it also appears to be a potent antioxidant. The addition of carnosine to meat products, leads to greatly improved storage, stability due to inhibition of lipid oxidation. Several investigators have shown that carnosine has relatively high activity with hydroxyl peroxide and other free radicals. Chalcones and catechins: Chalcones are natural polyphenolic precursors of flavonoids: They occur in plants and have shown. antioxidant activity in several investigations. Butein has shown surprisingly high activity of antioxidant activity.
Catechins or flavans, a tricyclic polyphenol related to flavonoids and condensed tannins from tea is a potential anticancer agent.
Epicatechin, a relative of catechin were found to be similar in antioxidant activity.
Curcumin and derivatives: The rhizomes of tropical gingers and turmeric are rich in curcumins, their derivatives, and other potent antioxidants. They can scavenge almost all free radicals generated in the cell. Curcumin is a very interesting substance because it generates phototoxic oxidizing species, including HO. and H₂O₂, when exposed to light, but it also protects lipid peroxidation as a radical scavenger.
Ergothioneine: The amino acid derivative ergothioneine is a major sulfur-containing constituent of some fungi. It is not by mammals but when ingested. It assimilates and its concentration. maintained to the point where it can approach concentrations in the liver, bone marrow, erythrocytes, and other tissues. Evidence suggests that is likely to provide protection against several varieties of oxidative stress like lipid peroxidation.
Flavonoids: Flavonoids represent a large and diverse group of phenolic compounds derived from higher plants. Derivatives of flavone display a wide range of substitution patterns and oxidation states including flavonols, flavanones, flavans, or catechins. The compounds appear to possess a variety of mechanisms of action which include free radical scavenging and metal ion complexation.
The reaction of flavonoid derivatives with superoxide has been thoroughly investigated. Reactions. of flavonoids with singlet oxygen have been studied. It has been found that flavonols such as quercetin and fisetin, which quench singlet oxygen by chemical reaction, were generally more reactive than those of other types such as flavones.
Anthocyanins are cationic polyphenols and are considered a class of flavonoids. Several researchers have found positive responses regarding the antioxidant activity of this group. Polyphenols, which occur naturally in many fruits, account for the majority of antioxidant activity. However, polyphenols can undergo various reactions in the course of food processing and storage, which affect their stability. At present, the probable toxicity of synthetic antioxidants has been concerned and thus there is a shift towards the use of natural antioxidants. It is strongly believed that regular consumption of plant-derived phytochemicals may drift the balance towards an adequate antioxidant status in the body.
Free phenolic acids: Metabolites of the shikimic acid pathway, and in particular compounds derived from the C6- C3 phenylpropanoid unit, are virtually universal in plant tissues and are especially abundant in seeds and barks. The basic structural unit undergoes many alterations in the biosynthesis of phenylalanine, tyrosine, tannins, flavonoids, lignin, and lignins.
In plants, the free phenolic acids occur as substituted benzoic and cinnamic types. Most fests of the antioxidant effects of these compounds have shown that the cinnamle derivatives are superior to benzoic derivatives. Antioxidant activities of caffeic and ferulic acids are also positive in their responses
Glutathione: The cysteine-containing tripeptide glutathione. It is one of the most important biological antioxidants. It occurs in high concentrations in the cytosol of many types of cells including human blood plasma. In addition, is also present in organelles like chloroplasts. It is the key component of a variety of cellular mechanisms including detoxification of foreign metabolites, maintenance of growth rates, and protection against gamma radiation damage. Glutathione is poorly absorbed if ingested, and most animals synthesize it within their body. The antioxidant biochemistry of glutathione has been summarized. Like other antioxidants, it is readily oxidized. thiols such as GSH, in particular, react rapidly with many one-electron oxidants to form thiol radicals
Hydroquinones and quinines: Arbutin is a simple naturally occurring hydroquinone derivative found in some plants and has shown to have antioxidant properties. Ubiquinol, another derivative of hydroquinone is found in the heart, kidney, and liver. The compound is a good inhibitor of free radical generators and scavengers of the same in lipid systems. Quinines are known to react with superoxide to remove them. Ubiquinone can react readily with peroxy radicals generated from lipids and singlet oxygen.
Isoflavonoids: Isoflavonoids are of restricted distribution in the plant kingdom. Only one family Leguminosae commonly contains them. It is less effective than flavonoids in its antioxidant activity. Genistein isolated from soybean has been reported to inhibit the activities of several enzymes, and also to promote the synthesis of antioxidant enzymes like catalase.
Lignans: C6-C3 dimers of varying degrees of complexity are antioxidants. Their activity tends to be correlated with the number of phenoxy or alkoxy substituents in the compound. Kadsurin isolates from Kadsuraheteroclita and was found to inhibit lipid peroxidation.
Lipoic acid: This compound is synthesized from linoleic acid and occurs naturally in many organisms like microorganisms, plants, and animals. It acts as an important coenzyme and growth factor. It sometimes occurs as an amide derivative. It is a potent antioxidant, so it is used in liver disorders and as an antidote for poisoning.
Ovothiol: The non-protein aminoacidovothiol is a thiol derivative of histidine that is found in marine animals and parasitic protozoa. The combinations of readily oxidized functional groups in this compound make it an extremely effective antioxidant.
Retinol and derivatives: They share many of the structural features of carotenoids and the assumption has been that they could also exhibit antioxidant activities.
Tetrapyrroles: Bilirubin is a linear tetrapyrrolic bile pigment found in the blood that inhibits several free-radical induced oxidation reactions, probably because of its reactivity with peroxy radicals. They are also well-known sensitizers of singlet oxygen formation and highly effective physical quenchers. It also reacts with superoxide Chlorophyll-a, chlorophyll-b and their related compounds are considered as potential antioxidants, of which Chlorophyll-a, chlorophyll-b shows maximum scavenging potential.
Uric acid and other purines: Uric acid is the most-studied and the most active antioxidant having a purine structure. Uric acid occurs in high concentrations in excretory products of many animals and was considered a total waste product with no biological functions. But researchers have found that uric acid is an effective antioxidant in biological systems containing DNA and lipids. Uric acid is a potential scavenger of Hydroxyl and peroxy radicals. It is also effective in reducing the blood plasma concentration of ozone.
Vitamin C: Vitamin C is found in some fruits, aqueous fractions of animal tissues including the spinal cord, lung, eye, blood plasma, etc. Although most organisms can synthesize it, a few have to obtain it in their diets. Ascorbic acid is a potent antioxidant against hydroxyl radicals, peroxy radicals, and singlet oxygen. However, the reaction efficiency of ascorbate is partially ameliorated by its ability to produce superoxide upon its oxidation by molecular oxygen.
Vitamin E and related compounds: Tocopherols and related substances are compounds found in high concentrations in certain vegetable oils, grains, and other plant products and at much lower concentrations in animal tissues. This most potent antioxidant agent is very important to maintain cell membranes and other cell parts. The concentration of vitamin E, even if low is adequate to prevent most instances of antioxidative damage in normally functioning cells. There are several isomers of tocopherol, of which α, β, γ, and
δ are most abundant. The order of antioxidant activity among the tocopherols is α→ β→ γ→ δ. Vitamin E analogs like prunusols A and B are also good antioxidants.
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