Research Advances in Antioxidant Composition Of Botanical Extracts And Their Action Mechanisms

 

 

Abstract： Oxidative damages to organisms caused by free radical might lead to the occurrence of many diseases, while many natural substances have the ability to scavenge free radicals. Plants are the most important source of the human body's exogenous antioxidants. This article summarizes previously reported studies on the antioxidant composition of botanical extracts mainly consisting of polyphenols, vitamins, alkaloids, saponins, polysaccharides, bioactive peptides plants, and so on, and their action mechanisms.

Keywords：free radical;botanical extract;antioxidant ingredient;mechanism

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Free radicals refer to atoms, molecules or groups with unpaired electrons in their outer orbits. They are intermediate metabolites of many biochemical reactions in the life activities of aerobic organisms. Under normal circumstances, free radicals in the human body are in a dynamic balance of continuous generation and removal. If too many free radicals are produced or too few are removed, the excessive free radicals will cause oxidative stress to cell tissues, resulting in oxidative damage to the body and may lead to the occurrence of many diseases such as atherosclerosis, hypertension, diabetes, tumors, Parkinson's disease, Alzheimer's disease, etc. [1-2]. As the most important source of exogenous antioxidant substances in the human body, the antioxidant research and development of plant extracts has received increasing attention and has achieved many results. This article briefly reviews the antioxidant components extracted from plants and their possible antioxidant mechanisms.

 

1 Research on antioxidant components of plant extracts

At present, most of the research on plant antioxidants is focused on Chinese herbal medicines, spices, vegetables, fruits, plant drinks and grains. The antioxidant active components of plant extracts mainly include polyphenols, vitamins, alkaloids, saponins, polysaccharides, polypeptides, etc.

 

1.1 Polyphenols

Plant polyphenol antioxidants can be divided into three major categories based on their chemical structures: flavonoids, phenolic acids, and tannins.

 

1.1.1 Flavonoids

Flavonoids, also known as flavonoid compounds, are the most diverse of polyphenols and are found in almost all tissues of plants. It refers to a series of compounds consisting of two benzene rings (A- and B-rings) connected by a central triple carbon bond. It can be further divided into sub-families such as flavonoids, flavonols, flavanones, dihydroflavonols, flavan-3-ols (also known as catechins), isoflavones, chalcones, and anthocyanidins.
Flavonoids have different antioxidant activities, and the magnitude of their antioxidant activity is closely related to the structure of the compound. The position and number of phenolic hydroxyl groups and their substituents (such as 4-carbonyl, hydroxyl glycosides, hydroxymethylation, and Δ2(3) double bonds) are important factors in determining their antioxidant activity [3-4]. It is generally believed that the ortho-diphenolic hydroxyl group on the B ring plays a major role in the antioxidant activity of flavonoids; the ortho-dihydroxyl group on one ring and the para-dihydroxyl group on the other ring produce very promising antioxidant activity, and the addition of hydroxyl groups at the 5, 7, and 8 positions on the A ring can increase the antioxidant capacity to varying degrees [5].
Many flavonoid compounds show significant antioxidant properties, representative examples of which include acaciatin, quercetin, naringenin, cypermethrin, tea polyphenols, soy isoflavones, trihydroxychalcone, cyanidin, etc.

 

1.1.2 Phenolic acid substances

Phenolic acid refers to a class of compounds with several phenolic hydroxyl groups on the same benzene ring. Phenolic acid substances with antioxidant properties found in natural plants can be divided into three categories: the first category is hydroxybenzoic acid and its derivatives, such as protocatechuic acid, gallic acid, syringic acid, etc.; the second category is ellagic acid and its derivatives, such as 3-hydroxyphenylacetic acid; the third category is hydroxycinnamic acid (hydroxyphenylacrylic acid) and its derivatives, such as chlorogenic acid, ferulic acid, caffeic acid, rosmarinic acid, coumaric acid, sinapinic acid, etc.
The antioxidant capacity of phenolic acid substances follows the same rules in chemical structure as flavonoids, that is, those with adjacent phenolic hydroxyl groups on the benzene ring are much stronger than those without them. For example, gallic acid and its various derivatives with a pyrogallol structure are stronger than those with only two hydroxyl groups. The antioxidant capacity of catechol-containing substances such as chlorogenic acid, caffeic acid, and rosmarinic acid is much stronger than that of ferulic acid and sinapic acid, which have only one hydroxyl group [1].

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1.1.3 Tannins

Tannins, also known as tannins, are widely distributed in plants and usually refer to plant polyphenols with a relative molecular mass of 500 to 3000. According to their different molecular structures and difficulty in hydrolysis, they can be divided into three categories: hydrolyzed tannins (such as gallic tannins and ellagitannins), condensed tannins (such as proanthocyanidins and oligomeric proanthocyanidins), and complex tannins formed by carbon bonds between condensed tannins and glucose in hydrolyzed tannins (such as camellia tannins and guava tannins). There are three factors that affect the antioxidant activity of tannins [6-7]: the bonding mode of the units; whether the hydroxyl group is free; the type and number of hexahydroxydibenzoyl (HHDP), galloyl (gall), and dehydrohexahydroxydibenzoyl (DHHDP) groups. When the tannin binding units (such as catechins) are bound by hydrolyzable ester bonds or glycosidic bonds, the antioxidant capacity of the molecule is enhanced, while when they are bound by carbon-carbon bonds to form a condensed form, the antioxidant capacity of the molecule is greatly reduced; when the phenolic hydroxyl group is free, it is conducive to the increase of activity; the activity of HHDP, gall, and DHHDP groups is in the order of HHDP > gall > DHHDP. In the binding unit, the more these three groups are, the greater the activity.

 

1.2 Vitamins

Vitamins are not only indispensable food nutrients, but also the most important antioxidant substances for the human body. The antioxidant vitamins in plants are mainly VE, VC, and carotenoids, but they can also become pro-oxidants under certain circumstances [8].

 

1.2.1 VE

VE is a general term for various tocopherols, among which α-tocopherol has the greatest biological activity. Taking α-tocopherol as a benchmark, the physiological activities of β-tocopherol, γ-tocopherol and δ-tocopherol are 40%, 8% and 20% respectively, and the activities of the others are extremely weak [9]. In most cases, the antioxidant effect of VE is to react with lipid oxygen free radicals or lipid peroxyl free radicals, provide them with hydrogen ions, and interrupt the lipid peroxidation chain reaction. It is the most important fat-soluble chain-breaking antioxidant [10].

 

 

1.2.2 VC

VC, also known as ascorbic acid, is an acidic polyhydroxy compound containing α-ketolactone with 6 carbon atoms. It has an enol-type hydroxyl group that can dissociate hydrogen ions and is the most important water-soluble capturing antioxidant. It can scavenge reactive oxygen free radicals by supplying electrons step by step; it can also protect VE and promote the regeneration of VE [1 ].

 

1.2.3 Carotenoids

There are more than 600 kinds of carotenoids, all of which have an isoprenoid structure with 11 double bonds. β-carotene is a typical representative. Studies have found that lycopene, astaxanthin, lutein and zeaxanthin also have significant antioxidant properties.
β-carotene is the precursor of VA. It is composed of four isoprene double bonds connected end to end. There is a β-ionone ring at both ends of the molecule. There are mainly all-trans, 9-cis, and l3- There are 4 forms: cis and l5- cis. It has very good antioxidant properties and can inhibit the generation of reactive oxygen species by providing electrons.
To scavenge free radicals [11].
Lycopene is an acyclic carotenoid with a non-cyclic, linear all-trans structure containing 11 conjugated double bonds and 2 non-conjugated double bonds. It can accept the excitation of different electrons to generate ground state oxygen or triplet oxygen lycopene. One triplet oxygen lycopene can quench thousands of singlet oxygen free radicals, and its antioxidant capacity is 100 times that of VE and VC. 1,000 times more powerful and is nature's most powerful antioxidant that delays aging [12].
Astaxanthin is a special oxidized carotenoid. It not only has a long conjugated double bond in the molecule like other carotenoids, but also has a hydroxyl group at the 3 and 4 positions of its two violet rings. and unsaturated ketone groups. This adjacent hydroxyl group and ketone group can constitute α-hydroxyketone. These knots
The structure has a relatively active electronic effect, which can provide electrons to free radicals or attract unpaired electrons of free radicals, and can easily capture free radicals. Therefore, astaxanthin has stronger antioxidant properties than ordinary carotenoids [13].
There are 8 isomers of lutein, which are mainly found in dark green vegetables such as cabbage and spinach, and flowers such as marigolds and marigolds. Zeaxanthin is found primarily in foods such as goji berries, corn, spinach, and Asian persimmons. Lutein and zeaxanthin always exist together, and their functions are very similar.
In terms of antioxidant, it can reduce the damage of oxidative stress to the eyes, that is, it has the ability to resist the oxidation induced by light in the macula of the retina, and can prevent aging caused by the degradation of the visual spot [1 4]. In addition, it can prevent the oxidation of proteins and lipids in the lens, thereby reducing the occurrence of senile cataracts [15].

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1.2.2 VC

VC, also known as ascorbic acid, is an acidic polyhydroxy compound containing α-ketolactone with 6 carbon atoms. It has an enol-type hydroxyl group that can dissociate hydrogen ions and is the most important water-soluble capturing antioxidant. It can scavenge reactive oxygen free radicals by supplying electrons step by step; it can also protect VE and promote the regeneration of VE [1 ].

 

1.2.3 Carotenoids

There are more than 600 kinds of carotenoids, all of which have an isoprenoid structure with 11 double bonds. β-carotene is a typical representative. Studies have found that lycopene, astaxanthin, lutein and zeaxanthin also have significant antioxidant properties.
β-carotene is the precursor of VA. It is composed of four isoprene double bonds connected end to end. There is a β-ionone ring at both ends of the molecule. There are mainly all-trans, 9-cis, and l3- There are 4 forms: cis and l5- cis. It has very good antioxidant properties and can inhibit the generation of reactive oxygen species by providing electrons.
To scavenge free radicals [11].
Lycopene is an acyclic carotenoid with a non-cyclic, linear all-trans structure containing 11 conjugated double bonds and 2 non-conjugated double bonds. It can accept the excitation of different electrons to generate ground state oxygen or triplet oxygen lycopene. One triplet oxygen lycopene can quench thousands of singlet oxygen free radicals, and its antioxidant capacity is 100 times that of VE and VC. 1,000 times more powerful and is nature's most powerful antioxidant that delays aging [12].
Astaxanthin is a special oxidized carotenoid. It not only has a long conjugated double bond in the molecule like other carotenoids, but also has a hydroxyl group at the 3 and 4 positions of its two violet rings. and unsaturated ketone groups. This adjacent hydroxyl group and ketone group can constitute α-hydroxyketone. These knots
The structure has a relatively active electronic effect, which can provide electrons to free radicals or attract unpaired electrons of free radicals, and can easily capture free radicals. Therefore, astaxanthin has stronger antioxidant properties than ordinary carotenoids [13].
There are 8 isomers of lutein, which are mainly found in dark green vegetables such as cabbage and spinach, and flowers such as marigolds and marigolds. Zeaxanthin is found primarily in foods such as goji berries, corn, spinach, and Asian persimmons. Lutein and zeaxanthin always exist together, and their functions are very similar.
In terms of antioxidant, it can reduce the damage of oxidative stress to the eyes, that is, it has the ability to resist the oxidation induced by light in the macula of the retina, and can prevent aging caused by the degradation of the visual spot [1 4]. In addition, it can prevent the oxidation of proteins and lipids in the lens, thereby reducing the occurrence of senile cataracts [15].

 

2 Research on the antioxidant mechanism of plant extracts
The properties and antioxidant mechanisms of different natural plant extracts are not the same. The antioxidant mechanisms reported in current research mainly include the following aspects:

 

2.1 Direct scavenging or inhibiting free radicals
Plant extracts can act as hydrogen proton or electron donors, directly quench or inhibit free radicals, terminate the chain reaction of free radicals, and exert antioxidant function.

 

2.1.1 Proton supply
Most antioxidant ingredients are oxygen free radical scavengers, such as polyphenols, sterols, VE, etc. One of the reasons is that they can release small and highly affinity hydrogen protons to capture highly active free radicals with high potential energy and convert them into inactive or relatively stable compounds. At the same time, they themselves are converted into substances that are more stable than the free radicals generated by the oxidation chain reaction, thereby interrupting or delaying the chain reaction [29].

2.1.2 Providing electrons

Another reason why plant extracts exert antioxidant effects is that they directly donate electrons through electron transfer to scavenging free radicals, such as polyphenols, plant polysaccharides, vitamins, etc. β-carotene has excellent antioxidant properties and can inhibit the generation of reactive oxygen species by providing electrons to achieve the purpose of scavenging free radicals. VC, on the other hand, transforms into semi-dehydroascorbic acid and dehydroascorbic acid by gradually providing electrons to achieve the purpose of scavenging reactive oxygen free radicals [30].

 

2.2 Acting on enzymes related to free radicals

Enzymes related to free radicals are divided into two categories: oxidases and antioxidant enzymes. The antioxidant effect of plant extracts is reflected in inhibiting the activity of related oxidases and enhancing the activity of antioxidant enzymes.

 

2.2.1 Inhibiting the activity of oxidases

Many oxidases in the body, such as xanthine oxidase (XOD), P-450 enzymes, myeloperoxidase (MPO), lipoxygenase and cyclooxygenase, are related to the generation of free radicals and can induce a large number of free radicals. In addition, the activity of inducible nitric oxide synthase (iNOS) increases during ischemia-reperfusion,producing a large amount of NO and causing oxidative damage. Studies have shown that many plant extracts have inhibitory effects on the above-mentioned oxidases, inhibiting the generation of free radicals from the source. Quercetin and curcumin in flavonoids can inhibit the activity of iNOS during ischemia-reperfusion injury, thereby playing an antioxidant role [31]. Gynostemma pentaphyllum saponins can reduce abnormally elevated XOD and MPO activities, improve oxidative stress in the kidneys of diabetic rats, and delay the progression of kidney damage [32].

 

2.2.2 Enhance the activity of antioxidant enzymes

 

The body has antioxidant enzymes that protect, remove and repair excessive free radical damage, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase (CAT) and peroxidase.

SOD is the main scavenger of superoxide anions in the body, catalyzing their decomposition into H2O2, but H2O2 also has oxidative damage, and CAT converts it into O2 and H2O. At the same time, H2O2 can also generate H2O through the catalysis of GSH-Px and the reaction with reduced glutathione (GSH), and at the same time generate oxidized glutathione.
Many studies have shown that plant-extracted antioxidant ingredients can not only protect the body's antioxidant enzymes, but also enhance the activity of antioxidant enzymes in the body. For example, quercetin in flavonoids can reduce the oxidative damage of pancreatic β cells, and at the same time restore the activity of SOD, GSH-Px and CAT in animals with Fe2+-induced renal cell damage [33]. Saponins have little effect on oxygen free radicals themselves, but most of them can increase the activity of antioxidant enzymes such as SOD and CAT in the body, thereby enhancing the function of the body's antioxidant system [34].
In addition, some natural substances can induce the expression of antioxidant enzymes such as SOD in the body at the gene and transcription levels, exerting their antioxidant effects [35].

2.3 Chelating and passivating transition metal ions

Transition metal ions (such as Fe2+, Cu2+, etc.) are essential in the process of oxygen free radical generation. For example, Fe2+ can mediate lipid peroxidation and is also a catalyst for the generation of free radicals such as ·OH. Flavonoids in plant extracts have a molecular structure of 4-keto and 5-hydroxyl, and the vicinal hydroxyl groups at the 3′ and 4′ positions of the B ring contain lone pairs of electrons [1], so they can chelate metal ions. Other antioxidant ingredients that can chelate and passivate pro-oxidant metal ions by coordinating electrons include tannins, polysaccharides, active peptides [3 6], phytic acid, citric acid, etc.

 

2.4 Complementarity and synergy between antioxidant components

The antioxidant components in plant extracts complement and coordinate each other. They jointly exert antioxidant effects in vivo through electron and/or proton transfer, acting on oxidases and antioxidant enzymes, chelating and passivating transition metal ions, and affecting gene expression. Studies have found that different concentrations of tea polyphenols and American ginseng have obvious synergistic effects, and the synergistic effect increases with the increase of concentration [37]. VE and VC have a significant synergistic effect on the reducing ability of chickpea antioxidant peptides, and the synergistic effect of VC with chickpea antioxidant peptides is stronger than that of VE. All synergistic effects increase with the increase of addition amount and action time [38].

 

3 Conclusion

A considerable part of the current research on natural antioxidant ingredients in my country is still on unpurified or partially purified extracts. In the research, we try to separate and collect a series of monomer compounds, study the relationship between their chemical structure and antioxidant activity and stability, antioxidant mechanism, multi-component synergy, etc., which has important application value and guiding significance for the development of new, efficient and safe antioxidants. It can also provide a theoretical basis for future systematic research on plant antioxidant activity, discovery of lead compounds, and structural modification and synthesis of natural substances.
In addition, data show that most of the current antioxidant research uses in vitro experiments, and there are few overall or in vivo experimental data, which makes it difficult to systematically and accurately reflect the full picture of the antioxidant effect of natural ingredients. Based on overall experiments, supplemented by in vitro experiments, and integrating multidisciplinary knowledge such as enzymology, immunology, and pharmacology to establish a comprehensive, objective, efficient and rapid animal experimental model to comprehensively evaluate the antioxidant activity of substances is a key issue that needs to be addressed in future research. With the promulgation and implementation of the "Food Safety Law", the application of safe plant extract antioxidants in food additives will also have a broader prospect.

 

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