**6.1. Non-enzymatic plant antioxidants and their mode of action**

Though plants have enzymatic antioxidants, it is usually difficult to isolate these enzymes for therapeutic uses in humans. Also, they are usually denatured during food processing, preparation and not sufficiently present in diets such as fruits and vegetables. On the contrary, non-enzymatic antioxidants are readily present in plants leaves, fruits and food in sufficient amounts and can easily be extracted from plants. For these reasons, this section will focus on the non-enzymatic plant antioxidants.

**Bioflavonoids:** This is a group of natural benzo-γ-pyran derivatives which are widely distributed in fruits and vegetables. They are the most abundant polyphenols found to possess strong antioxidant activities in scavenging free radicals. They have generally been reported to protect against hydroxyl radical induced DNA damage [62]. Also, bioflavonoids are capable of chelating metal ions, such as copper or iron thereby preventing the generation of ROS [63]. These bioflavonoids include flavonol, flavones, flavonolols, flavan-3-ols, flavonone, anthocyanidin, isoflavone, etc.

Free Radicals and the Role of Plant Phytochemicals as Antioxidants Against Oxidative Stress-Related Diseases

**Flavonoids:** In plants, most flavonoids are attached to sugars (glycosides), although they are occasionally found as aglycones. Most flavonoids are not completely absorbed and reach the circulatory system except for some flavan-3-ols and proanthocyanidins. **Quercetin** is a flavonol, known to protect DNA from oxidative damage resulting from the attack of •OH, H2

In general, flavonoids are oxidized by radicals, resulting in a more stable, less-reactive radical. In this reaction, flavonoids stabilize the ROS by reacting with them to become a flavonoids radical. This is achieved due to high reactive hydroxyl group of the flavonoids as shown below.

Flavonoid(OH) + R• → flavonoid(O•) + RH (15)

As reviewed from Nijveldt et al. [65], certain flavonoids can directly scavenge superoxides as well as peroxynitrite. Other flavonoids may act as antioxidants by inhibiting the activity of free radical generating enzymes such as xanthine oxidase and nitric-oxide synthase. Quercetin, rutin and silibin have shown to inhibit xanthine oxidase activity while silibin has been reported to inhibit nitric` oxide dose dependently. By scavenging radicals, flavonoids can inhibit LDL oxidation in vitro. This action protects the LDL particles and, theoretically,

**Carotenoids:** Carotenoids are among the common lipid soluble phytonutrients synthesized from phytoene. They include Xanthophyll (zeaxanthine, lutein) and Carotines (lycopene, b-carotene), the latter been the most abundant. Carotenoids are generally known to scavenge peroxyl radicals which are generated during the process of lipid peroxidation of cell membrane. As such, scavenging of peroxyl radicals prevents cellular lipids and membrane damage. Carotenoids are highly lipophilic and are known to play an important role in the protection of cellular membranes and lipoproteins against ROS due to their peroxyl radical scavenging activity [66]. **Lycopene** is the most potent antioxidant naturally present in many fruits and vegetables. The high number of conjugated double bonds in lycopene endows it with singlet oxygen quenching ability. Lycopene demonstrate the strongest singlet oxygen quenching ability as compared to α-tocopherol or β-carotene [67]. **β-carotene** is a naturally occurring orange-colored carotenoid, abundantly found in yellow orange fruits and in darkgreen leafy vegetables [68]. Just like lycopene, β-carotene is well-known to quench singlet

inhibition of lipid oxidation due to their metal ion-chelating activity.

where R• is a free radical and O• is an oxygen free radical.

flavonoids may have preventive action against atherosclerosis.

• on DNA oligonucleotides. However, at high concentrations of cupric ion, quercetin is reported to be a carcinogenic agent by enhancing DNA damage via ROS [64]. 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. **Anthocyanidin** is a class of flavonoids with antioxidant potentials. They are effective in the

and O2

O2 , 61

http://dx.doi.org/10.5772/intechopen.76719

**Glutathione:** Glutathione is a low-molecular-weight, tripeptide of glutamic acid-cysteineglycine containing a thiol. It exist as GSH in its reduced form and 2 GSH molecules can be joined via oxidation at their SH groups of the cysteine residue into a disulfide bridge to form GSSG which is the oxidized form.

GSH generally acts as a cofactor for glutathione peroxidase, thus serving as an indirect antioxidant by donating the necessary electrons for the decomposition of H2 O2 . GSH can directly scavenge ROS such as ROO•, OH• and RO• radicals as well as •O2 and HCLO•. Upon reacting with ROS, GSH becomes a glutathione radical, which can be reconverted to its reduced form [54]. Glutathione also has other cellular functions such metabolism of ascorbic acid [55]. Also, glutathione prevents the oxidation of SH protein groups and acts as a chelating agent for copper preventing its participation in the Haber-Weiss reaction [54].

**Vitamin E (α-tocopherol):** Vitamin E is a lipid soluble antioxidant that functions as an efficient 'chain breaker' during lipid peroxidation in cell membranes and various lipid particles including low-density lipoprotein (LDL). Its role is to scavenge lipid peroxyl radicals (LOO•) and to terminate the lipid peroxidation chain reactions [56].

$$\text{LCO}^\* + a \text{ -tocopherol-OH} \rightarrow \text{LCOOH} + a \text{ -tocopherol-O}^\*. \tag{14}$$

Also, α-tocopherol can scavenge other ROS, such as •O2 to become tocopherolquinone and subsequently tocopherylquinone. However, it is not an efficient scavenger of OH• and alkoxyl (•OR) radicals in vivo [57]. The resultant tocopheroxyl radical in these reactions can be recycled to its active form but this radical is relatively stable in normal circumstances and insufficiently reactive to initiate lipid peroxidation itself, which makes it a good antioxidant [58].

**Ascorbic acid (Vitamin C):** Ascorbic acid is a water-soluble antioxidant. It also functions as a chain breaker to terminate the lipid peroxidation chain reaction. In this reaction, it donates an electron to the lipid radical (LOO•) to become ascorbate radical. Two molecules of ascorbate radicals can react rapidly to produce a molecule of ascorbate and a molecule of dehydroascorbate which do not have any scavenging activity. Dehydroascorbate can be reconverted to ascorbate by the addition of two electrons catalyzed by oxidoreductase. More so, ascorbate can react with GSH to regenerate vitamin E in cell membranes [59].

**Vitamin A:** Though not fully understood, vitamin A is considered as a vital antioxidant that prevents humans LDL against copper stimulated oxidation [60]. The antioxidant potential of vitamin A was first revealed by Monaghan and Schmitt who showed that vitamin A can protect lipids against rancidity [61].

**Bioflavonoids:** This is a group of natural benzo-γ-pyran derivatives which are widely distributed in fruits and vegetables. They are the most abundant polyphenols found to possess strong antioxidant activities in scavenging free radicals. They have generally been reported to protect against hydroxyl radical induced DNA damage [62]. Also, bioflavonoids are capable of chelating metal ions, such as copper or iron thereby preventing the generation of ROS [63]. These bioflavonoids include flavonol, flavones, flavonolols, flavan-3-ols, flavonone, anthocyanidin, isoflavone, etc.

**Flavonoids:** In plants, most flavonoids are attached to sugars (glycosides), although they are occasionally found as aglycones. Most flavonoids are not completely absorbed and reach the circulatory system except for some flavan-3-ols and proanthocyanidins. **Quercetin** is a flavonol, known to protect DNA from oxidative damage resulting from the attack of •OH, H2 O2 , and O2 • on DNA oligonucleotides. However, at high concentrations of cupric ion, quercetin is reported to be a carcinogenic agent by enhancing DNA damage via ROS [64]. 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. **Anthocyanidin** is a class of flavonoids with antioxidant potentials. They are effective in the inhibition of lipid oxidation due to their metal ion-chelating activity.

In general, flavonoids are oxidized by radicals, resulting in a more stable, less-reactive radical. In this reaction, flavonoids stabilize the ROS by reacting with them to become a flavonoids radical. This is achieved due to high reactive hydroxyl group of the flavonoids as shown below.

$$\text{Flavonoid(OH)} + \text{R}^\* \rightarrow \text{ flavonoid(O}^\*) + \text{RH} \tag{15}$$

where R• is a free radical and O• is an oxygen free radical.

**6.1. Non-enzymatic plant antioxidants and their mode of action**

60 Phytochemicals - Source of Antioxidants and Role in Disease Prevention

the non-enzymatic plant antioxidants.

GSSG which is the oxidized form.

Though plants have enzymatic antioxidants, it is usually difficult to isolate these enzymes for therapeutic uses in humans. Also, they are usually denatured during food processing, preparation and not sufficiently present in diets such as fruits and vegetables. On the contrary, non-enzymatic antioxidants are readily present in plants leaves, fruits and food in sufficient amounts and can easily be extracted from plants. For these reasons, this section will focus on

**Glutathione:** Glutathione is a low-molecular-weight, tripeptide of glutamic acid-cysteineglycine containing a thiol. It exist as GSH in its reduced form and 2 GSH molecules can be joined via oxidation at their SH groups of the cysteine residue into a disulfide bridge to form

GSH generally acts as a cofactor for glutathione peroxidase, thus serving as an indirect anti-

with ROS, GSH becomes a glutathione radical, which can be reconverted to its reduced form [54]. Glutathione also has other cellular functions such metabolism of ascorbic acid [55]. Also, glutathione prevents the oxidation of SH protein groups and acts as a chelating agent for cop-

**Vitamin E (α-tocopherol):** Vitamin E is a lipid soluble antioxidant that functions as an efficient 'chain breaker' during lipid peroxidation in cell membranes and various lipid particles including low-density lipoprotein (LDL). Its role is to scavenge lipid peroxyl radicals (LOO•)

LOO• + α − tocopherol–OH → LOOH + α − tocopherol– O•. (14)

subsequently tocopherylquinone. However, it is not an efficient scavenger of OH• and alkoxyl (•OR) radicals in vivo [57]. The resultant tocopheroxyl radical in these reactions can be recycled to its active form but this radical is relatively stable in normal circumstances and insufficiently reactive to initiate lipid peroxidation itself, which makes it a good antioxidant [58].

**Ascorbic acid (Vitamin C):** Ascorbic acid is a water-soluble antioxidant. It also functions as a chain breaker to terminate the lipid peroxidation chain reaction. In this reaction, it donates an electron to the lipid radical (LOO•) to become ascorbate radical. Two molecules of ascorbate radicals can react rapidly to produce a molecule of ascorbate and a molecule of dehydroascorbate which do not have any scavenging activity. Dehydroascorbate can be reconverted to ascorbate by the addition of two electrons catalyzed by oxidoreductase. More so, ascorbate

**Vitamin A:** Though not fully understood, vitamin A is considered as a vital antioxidant that prevents humans LDL against copper stimulated oxidation [60]. The antioxidant potential of vitamin A was first revealed by Monaghan and Schmitt who showed that vitamin A can

O2

to become tocopherolquinone and

. GSH can directly

and HCLO•. Upon reacting

oxidant by donating the necessary electrons for the decomposition of H2

scavenge ROS such as ROO•, OH• and RO• radicals as well as •O2

per preventing its participation in the Haber-Weiss reaction [54].

and to terminate the lipid peroxidation chain reactions [56].

Also, α-tocopherol can scavenge other ROS, such as •O2

can react with GSH to regenerate vitamin E in cell membranes [59].

protect lipids against rancidity [61].

As reviewed from Nijveldt et al. [65], certain flavonoids can directly scavenge superoxides as well as peroxynitrite. Other flavonoids may act as antioxidants by inhibiting the activity of free radical generating enzymes such as xanthine oxidase and nitric-oxide synthase. Quercetin, rutin and silibin have shown to inhibit xanthine oxidase activity while silibin has been reported to inhibit nitric` oxide dose dependently. By scavenging radicals, flavonoids can inhibit LDL oxidation in vitro. This action protects the LDL particles and, theoretically, flavonoids may have preventive action against atherosclerosis.

**Carotenoids:** Carotenoids are among the common lipid soluble phytonutrients synthesized from phytoene. They include Xanthophyll (zeaxanthine, lutein) and Carotines (lycopene, b-carotene), the latter been the most abundant. Carotenoids are generally known to scavenge peroxyl radicals which are generated during the process of lipid peroxidation of cell membrane. As such, scavenging of peroxyl radicals prevents cellular lipids and membrane damage. Carotenoids are highly lipophilic and are known to play an important role in the protection of cellular membranes and lipoproteins against ROS due to their peroxyl radical scavenging activity [66]. **Lycopene** is the most potent antioxidant naturally present in many fruits and vegetables. The high number of conjugated double bonds in lycopene endows it with singlet oxygen quenching ability. Lycopene demonstrate the strongest singlet oxygen quenching ability as compared to α-tocopherol or β-carotene [67]. **β-carotene** is a naturally occurring orange-colored carotenoid, abundantly found in yellow orange fruits and in darkgreen leafy vegetables [68]. Just like lycopene, β-carotene is well-known to quench singlet


**Oxidative stress diseases**

Cardiovascular disease

Alzheimer's disease

Anti-obesity *Vaccinium floribundum*

*Aristotelia chilensis*

Cancer Polyphenols Ellagitannins and

*Crataegus pinnatifida* 

*fruit*

Green tea, grape seeds Polyphenols,

**Plant Phytochemical Mechanism of action References**

Free Radicals and the Role of Plant Phytochemicals as Antioxidants Against Oxidative Stress-Related Diseases

effects

reactivity

platelets

in vitro

cytokines

in rats

toxicity

skin cancers

signaling

in vitro

Crude extract Potential neuroprotective

anti-diabetic effect

protected vasodilator

inflammation activities

induced apoptosis of

Limits adipogenesis and inflammatory pathways

blocking proinflammatory

and reduction of glycemia

anti-oxidant activities

nitric oxide in vitro and protecting pancreatic β-cells against cytokine-induced

Protect the skin from the adverse effects of UV radiation preventing risk of

and potential ability as an anti-aging agent

healthy rats by reducing the damage of liver and kidney and improving ageassociated inflammation and oxidative stress through inhibiting NF-β

activity for preventing oxidative-related disorders

Anticarcinogenic properties [102]

[92]

63

http://dx.doi.org/10.5772/intechopen.76719

[93]

[94]

[95]

[96]

[97]

[98]

[99]

[100]

[101]

[103]

[104]

[105]

[106]

*Euterpe oleracea* Flavonoids *In vitro* atheroprotective

*Flos chrysanthemi* Flavonoids Vasodilating effects and

*Gnetum macrostachyum* Stilbenoids Antioxidant and anti-

Anthocyanins, proanthocyanidins

Diabetes *Ascophyllum nodosum* Phenolics Antioxidant activity and

Grape products Polyphenol Antioxidant action,

*Chrysobalanus icaco* Polyphenolics Strong antioxidant action


Polyphenol Butein Inhibit formation of

epicatechin

Aging *Elaeis guineensis leaves* Methanol extract High antioxidant activities

proanthocyanidins

*Epigallocatechin gallate* Crude extract Extended lifespan of

Polyphenols Crocin, carotenoid protected oxidative stress-

Free Radicals and the Role of Plant Phytochemicals as Antioxidants Against Oxidative Stress-Related Diseases http://dx.doi.org/10.5772/intechopen.76719 63


**Plant**

**In vitro antioxidant**

**RP**

✓

✓

✓

✓

✓

✓

−

− ✓

✓

✓

✓

−

−

−

−

−

*Torilis leptophylla*

*Clausena anisata*

*Peltophorum africanum*

Zanthoxylum capense

*Nypa fruticans* Wurmb

*Artemisia absinthium*

*Vitex doniana* *Mucuna pruriens*

*Schotia latifolia* Jacq

*Asphodeline Anatolica*

*Ziziphus mauritiana* Lam.

*Helichrysum longifolium* DC

*Strychnos henningsii* Gilg

Citrus sinensis

*Citrus anrantifolia*

*Citrus limonum* *Acalypha manniana*

*Chrysophyllum albidum*

*Murraya Koenigii*

Legend:

**Table 2.**

Some plants with *in vitro* and *in vivo* antioxidant activities.

✓

✓

✓

−

− −

−

−

− −

✓

✓ ✓ indicates present while − indicates not evaluated. RP: reducing power activity, HPS: hydrogen peroxide scavenging activity, NS: nitrogen oxide scavenging

activity; FS: FRAP scavenging activity, DS: DPPH radical scavenging activity, SS: superoxide anion scavenging activity, AS: ABTS radical scavenging activity, HS: hydroxyl

radical scavenging assay, TAC: total antioxidant capacity, TPC: total phenolic content, TFC: total flavonoid content, GSH: reduced glutathione, CA: catalase activity, SOD:

superoxide dismutase activity, GPx: glutathione peroxidase, GRx: glutathione reductase, MDA: malondialdehyde, PCC: protein carbonyl content, Ref: references.

−

✓

−

−

−

−

✓

−

− −

−

−

− −

✓

✓

−

✓

−

−

−

−

−

−

✓

− −

−

−

− −

✓

✓

−

−

−

−

−

−

−

−

−

✓

− −

−

−

− −

✓

✓

−

−

−

−

−

−

−

✓

− −

−

−

− −

✓

✓

−

−

−

−

−

−

−

−

✓

− −

−

−

− −

✓

✓

−

−

−

−

−

−

✓

✓

✓

✓

✓

−

✓

− ✓

✓

✓

✓

✓

✓

−

−

−

✓

✓

✓

✓

✓

−

✓

− ✓

✓

✓

−

−

−

−

−

−

 −

−

✓

− −

−

✓

− −

−

−

−

−

−

−

−

−

✓

−

✓

−

✓

−

−

✓

✓

−

−

−

−

−

−

−

−

−

✓

−

✓

−

✓

−

✓

− −

✓

✓

−

−

−

−

−

−

−

✓

− −

−

−

− −

−

−

−

✓

✓

−

−

−

−

−

−

✓

− −

−

−

− −

−

−

−

✓

✓

−

−

−

−

−

− −

−

−

− −

−

−

−

✓

✓

✓

✓

✓

✓

−

✓

−

✓

−

−

− −

✓

✓

−

−

−

−

−

−

−

−

✓

− −

−

−

✓ −

✓

✓

−

−

−

−

−

−

−

−

✓

− −

−

−

✓ −

✓

✓

−

−

−

−

−

−

−

−

✓

− −

−

−

✓ −

✓

✓

−

−

−

−

−

−

**HPS DS SS AS HS NS FS**

**TAC**

**TPC**

**TPC**

**GSH**

**CA**

**SOD**

**GRx GPx PCC**

**In vivo antioxidant activity**

**in vivo**

**Protective against Damage** 

**Ref**

**MDA**

✓ −

−

−

−

✓ ✓ ✓ −

−

−

−

✓

−

−

−

−

✓ ✓

[91]

[90]

[89]

[88]

[88]

[88]

[87]

[86]

[85]

[84]

[83]

[82]

[82]

[81]

[80]

62 Phytochemicals - Source of Antioxidants and Role in Disease Prevention

[79]

[79]

[79]

[78]


**7. Conclusion**

**Author details**

**References**

Godwill Azeh Engwa

2001;**54**:176-186

Gerontology. 1956;**11**:298-300

United States of America. 1981;**78**:7124-7128

Norton, Avon, England: Oxford University Press; 1999

American Journal of Physiology. 1986;**251**:F765-F776

pp. 1-43

Address all correspondence to: engwagodwill@gmail.com

Obvious deleterious effects of free radicals as regards man's heath cannot be over emphasized. Oxidative stress due to overwhelming levels of free radicals has promoted the progression of diseases such as diabetes, cancer, cardiovascular diseases, atherosclerosis etc. and even aging. Plants phytochemicals and some vitamins have shown to possess antioxidant properties capable of scavenging free radicals, preventing cellular damages and related diseases via several mechanisms. As such, plants phytochemicals are now being considered as the most sustainable alternative source of antioxidants to supplement the endogenous oxidative stress defense system in humans. Continuous efforts are needed to characterize plants phytochemicals for their antioxidant potentials and mode of action for various therapeutic uses against oxidative stress-related diseases while regular consumption of fruits and vegetables are encouraged for the prevention of these diseases.

Free Radicals and the Role of Plant Phytochemicals as Antioxidants Against Oxidative Stress-Related Diseases

http://dx.doi.org/10.5772/intechopen.76719

65

Biochemistry, Department of Chemical Sciences, Godfrey Okoye University, Enugu Nigeria

[1] Young IS, Woodside JV.Antioxidant in health and diseases. Journal of Clinical Pathology.

[2] Gilbert DL, editor. Perspective on the history of oxygen and life. In: Oxygen and the Living Process: An Inter-disciplinary Approach. New York: Springer Verlag; 1981.

[3] Harman D. Aging, a theory based on free radical and radiation chemistry. Journal of

[4] Harman D. The aging process. Proceedings of the National Academy of Sciences of the

[5] Halliwell B, Gutteridge JM. Free Radicals in Biology and Medicine. 3rd ed. Midsomer

[6] Beckman JS, Koppenol WH. Nitric oxide, superoxide, and peroxynitite: The good, the

[7] Baud I, Ardaillou R. Reactive oxygen species: Production and role in the kidney. The

[8] Sultan S. Reviewing the protective role of antioxidants in oxidative stress caused by free

bad and ugly. The American Journal of Physiology. 1996;**271**:C1424-C1437

radicals. Asian Pacific Journal of Health Sciences. 2014;**1**(4):401-406

**Table 3.** Some plants/phytochemicals with therapeutic effects on oxidative stress-related diseases and possible mechanism of action.

oxygen with higher efficiency as compared to the α-tocopherol. More so, β-carotene can be cleaved by β-carotene-15,150-dioxygenase into the two molecules of vitamin A, another antioxidant.

**Hydroxycinnamates:** Hydroxycinnamic acids which include ferulic acid, caffeic acid, p-coumaric acid, sinapic acid are another category of dietary antioxidants that are known to protect LDL from oxidation and can prevent coronary heart disease and atherosclerosis [69]. In vitro studies involving human LDL as the oxidizing substrate have shown hydroxycinnamic acids to have higher antioxidant activity than hydroxybenzoic acids [70].
