21 Antioxidant Activity of African
Medicinal Plants
Mikhail Olugbemiro Nafiua, Musa Oyewole Salawua
and Mutiu Idowu Kazeemb
a

Department of Biochemistry, University of Ilorin, Ilorin, Nigeria,
Department of Biochemistry, Lagos State University, Ojo, Nigeria

b

21.1

Introduction

Antioxidants are powerful free radical scavengers in the body, while free radicals
are highly reactive chemical substances such as superoxide, hydroxyl radical, or
singlet oxygen [1] that travel around in the body and cause damage to body cells.
Free radical damage is one of the most prominent causes of devastating diseases
that are responsible for killing many people in the world, such as cardiovascular
disease, which can manifest as heart attacks, and cancer [2]. The aging process has
been linked by some researchers to free radical damage in the body [3]. Free radicals naturally occur in the body as a result of chemical reactions during normal cellular processes. They can also be formed in response to environmental factors such
as excess pollution, excessive UV rays, and exposure to cigarette smoke, automobile exhaust, and pesticides. Inadequate rest or sleep, inability to manage stress
responses, and unhealthy eating habits can also cause free radical damage [3À5].
In chronic infections and inflammation, as well as in other disorders, release of
leukocytes and other phagocytic cells readily defends the organism from further
injury. The cells do this by releasing free oxidant radicals, and these by-products
are generally reactive oxygen species (ROS) such as superoxide anion, hydroxyl
radical, nitric oxide, and hydrogen peroxide, which result from cellular redox processes [6,7]. At low or moderate concentrations, ROS exert beneficial effects on
cellular responses and immune function [7,8]. At high levels, however, free radicals
and oxidants generate oxidative stress, a deleterious process that can damage cell

Organization (WHO) has estimated that 80% of earth’s inhabitants rely on
traditional medicine for their primary health care needs, and most of this therapy
involves the use of plant extracts and their active components [18]. These natural
plant antioxidants can therefore serve as a type of preventive medicine. Recent
reports indicate that there is an inverse relationship between the dietary intake of
antioxidant-rich foods and the incidence of human disease [19]. However, synthetic
antioxidants, such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), have been widely used as antioxidants in the food industry and may be
responsible for liver damage and carcinogenesis [20,21]. For this reason, interest in
the use of natural antioxidants has increased. Plants have been the basis of
traditional medicines throughout the world for thousands of years and continue to
provide new remedies to humankind; a great deal of effort has therefore focused on
using available experimental techniques to identify natural antioxidants from
plants.

21.3

African Medicinal Plants with Antioxidant Potential

21.3.1 Diospyros abyssinica Hiern
Diospyros abyssinica (also known as kˆoforonto and baforonto) is a species of trees
in the Ebenaceae family that grows in the southern part of Africa, particularly in
Mali. It can also be found in Angola, Guinea, Eritrea, and Ethiopia [22].


Antioxidant Activity of African Medicinal Plants

789

The trees have been used in many traditional medical systems around the world,
including traditional Ayurvedic, African, and Chinese medicine. Nearly every part


O

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OH O

4

OH

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O

OH O

HO

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5

OH

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H H

HOH

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COOCH3

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8

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Morocco, and Egypt. P. lentiscus is important because of its medicinal value. The
aerial parts have traditionally been used as a stimulant, for their diuretic properties,
and to treat hypertension, coughs, sore throats, eczema, stomach aches, kidney
stones, and jaundice [25,26]. The reducing power and radical scavenging activity
of the extracts from the leaves of P. lentiscus in solvents such as ethanol, ethyl acetate, aqueous/ethyl acetate, hexane, aqueous/hexane, chloroform, and aqueous/chloroform have been studied in vitro [28].
Using the diphenylpicryhydrazyl (DPPH) scavenging activity assay, it was found
that all of the P. lentiscus extracts, except for the chloroform extract, have a high
radical scavenging activity (90%), equivalent to that of the standard, BHA (89%).
The ethanolic and aqueous fractions from the ethyl acetate extract have high scavenging activities, with values of 78% and 90.29%, respectively. Overall, P. lentiscus exhibited outstanding reducing power, good radical scavenging activity against
DPPH and H2O2, slow inhibition of lipid peroxidation, and richness in tannins;
however, it also showed a lack of flavonoids [28].
In vitro antioxidant and antimutagenic activities of two polyphenols isolated
from the fruits of P. lentiscus have been investigated. These polyphenols are gallic
acid (2) and 1,2,3,4,6-pentagalloyl glucose (3) [27].

21.3.3 Urtica dioica L.
Stinging nettle or common nettle, U. dioica, is a herbaceous perennial flowering
plant native to Europe, Asia, northern Africa, and North America, and is the bestknown member of the nettle genus Urtica. U. dioica L. (Urticaceae) leaves have
been used in Libya in the form of a medicinal tea or decoction as diuretic and antidiabetic therapies and to treat stomach disorders [29].
The antioxidant capacity of this plant was evaluated using several in vitro methods such as superoxide anion scavenging (SAS) and 2,2-diphenylpicryhydrazyl
methods. The SAS method determined that the antioxidant activity of U. dioica at
an acidic pH was 0.013 μg/mL resorcinol equivalents (Re eq.); the DPPH method
in methanolic solution determined that the antioxidant activity was 419 μg/mL. The
total phenolic content was found to be 0.35 mg/l gallic acid equivalent (GAE) [64].
Concentrations of U. dioica L. extract of 50, 100, and 250 μg/mL showed 39%,
66%, and 98% inhibition, respectively, of the peroxidation of a linoleic acid emulsion. By contrast, α-tocopherol, the positive control, at 60 μg/mL, exhibited only
30% inhibition [30]. It can be concluded that U. dioica L. has powerful antioxidant
activities.




Mali, Guinea

Anti-inflammatory [22]

Stimulant, diuretic, hypertension [25],
cough, kidney stones, jaundice [26]
Diuretic, antidiabetic [29]

Algeria, Morocco,
Egypt
Libya

Betulinic acid [23],
lupeol [24]
Gallic acid, 1,2,3,4,6pentagalloyl glucose [27]
Not identified

Antioxidant [30]

Anti-inflammatory, antiseptic,
antidiabetic [24], anticancer [31,32]
Antidiabetic [35]

Nigeria

Centaurin, centaureidin [33]

Antihypertensive [34]


(Humiriaceae)
Mallotus oppositifolius
(Euphorbiaceae)

Antioxidant, antimutagenic [27,28]

Tunisia

Oleuropein, oleside,
ligstroside [47]

South Africa

Not identified

Antihypertensive, antioxidant, antiinflammatory, antinociceptive [40]
Antimicrobial, analgesic,
inflammation, antioxidant [44]
Antimicrobial [48,49], gastroprotective
[50], antioxidant [51,52],
antiatherosclerotic [53], antiviral
[54], antitumor [55,56]
Antioxidant [58]

Gambia, Angola,
Senegal
Nigeria

Bergenin [60]



21.3.5 Momordica charantia L.
The bitter gourd (Momordica charantia L.) belongs to the family Cucurbitaceae
and has long been used in foods and medicines [66]. The bitter gourd is known by
different names, such as balsam pear and karela, and it grows in tropical and subtropical regions of India, Malaysia, China, Africa, the Middle East, the United
States, and Thailand [66]. It is common in the North African countries such as
Morocco and Egypt. The bitter gourd can be used to treat diabetes mellitus and
appears to be a safe alternative to reduce blood glucose [35].
In the DPPH radical scavenging assay, the activity of the positive control, ascorbic acid, was the highest (200 mg/mL), followed by the leaf, the green fruit, the
stem, and the ripe fruit fractions of the bitter gourd. The IC50 values were lowest in
the leaf fraction (9.72 mg/mL), followed by the green fruit fraction (11.00 mg/mL),
the stem fraction (17.8 mg/mL), and the ripe fruit fraction (27.6 mg/mL). In the
hydroxyl radical scavenging assay, the activity of the leaf fraction was greater than
that of the other fractions but lower than that of ascorbic acid. The green fruit had
the highest IC50 value (119 mg/mL), followed by the leaf (167 mg/mL), the stem
(267 mg/mL), and the ripe fruit (173 mg/mL) [37].
Bio-guided fractionation of the methanol extract of M. charantia dried gourds led to
the isolation of 11 compounds. These include momordicoside (6) and p-methoxybenzoic acid (7) [36].

21.3.6 Dorstenia picta L.
D. picta (Moraceae) is an herbaceous plant used in southern Cameroon as an antidiabetic and antihypertensive drug. Other traditional uses of the genus Dorstenia are


Antioxidant Activity of African Medicinal Plants

793

against headaches and abdominal pain [38]. It has been reported that Dorstenia psilurus, Dorstenia cilianta, and Dorstenia barteri have antihypertensive, antioxidant,
anti-inflammatory, and antinociceptive activity, respectively [40]. Many antioxidant
compounds have been isolated from this plant. They include quercitrin, 6,8-diprenyleridictyol, bartericin A, and 6-prenylapigenin [39].


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Medicinal Plant Research in Africa

such as malaria [71]. According to Tunisian folk medicine, olive leaves are recommended in a wide range of ailments, including inflammatory disorders, bacterial
infections, hypertension, and diabetes, but modes of preparation and administration
vary: earache is cured by using olive leaves in hot olive oil with salt [46]. Olive
leaf juice, despite its irritation, is recommended for curing trachoma. When
chewed, this plant organ is used to relieve tooth pain and to treat lip irritation. A
decoction of the leaves, used as a liquid mouthwash, is used for treating aphthous,
gingivitis, and glossitis [72,73].
Previous studies have demonstrated that olive leaves are used for their antimicrobial [48,49], gastroprotective [50], antioxidant [51,52], hypotensive [74], hypoglycemic [75], antiatherosclerotic [53], antiviral [54], antitumor [55,56], and antiinflammatory properties [76].
The pharmacological properties of olive oil, the olive fruit, and its leaves have
been recognized as important components of medicine and a healthy diet because
of their phenolic content [77]. Phenolic compounds are found in all parts of the
olive plant, but their nature and concentration varies greatly among the various tissues. In O. europaea, oleuropein, demethyl-oleuropein, ligstroside, and oleoside (8)
represent the predominant phenolic oleosides [47], whereas verbascoside [78] is the
main hydroxycinnamic derivative of the olive fruit. [79] Oleuropein (9) is generally
the most prominent phenolic compound in olive cultivars.
Oleuropein possesses good antioxidant properties. It potently and dose dependently inhibits copper sulfate-induced oxidation of low-density lipoproteins (LDL)
[80,81]. Oleuropein has the ability both to scavenge nitric oxide and to cause an
increase in inducible nitric oxide synthase (iNOS) expression in the cell. A scavenging effect of oleuropein was also demonstrated with respect to hypochlorous
acid (HOCl) [80].

21.3.9 Pelargonium reniforme Curt.
P. reniformeA, belonging to the family Geraniaceae, is native to the coastal regions
of South Africa [82]. The plant is notable for its narrow, deep-red flowers and its
large, heart-shaped leaves. Along with other closely related species, the root has
been used for centuries as a traditional herbal remedy in South Africa [83].

gonorrhea, and diarrhea, and are sometimes used to treat hypertension and diabetes
in various parts of Africa. In Senegal and Congo, a stem bark decoction is mixed
with other plants and added to bath water to treat ovarian troubles, vaginal infections, and children with fever. The stem bark of this plant is also used as a palm
wine additive, as it is claimed to prolong the shelf life of the wine, add potency,
reduce foaming, and impart a bitter taste [89,90]. It is also reported to have aphrodisiac properties.
The main active ingredient in the stem bark decoction of S. gabonensis is bergenin, an isocoumarin. The stem bark extract has been reported to have hepatoprotective properties and antilipid peroxidation activity in vivo in rats [91].
Bergenin has been reported to protect against 2,4-dinitrophenylhydrazine
(DNPH)-induced hepatotoxicity and toxicity to red blood cells in rats. The stem
bark extract of S. gabonensis and its main isolated compound bergenin have been
reported to have antioxidant properties [59]. However, more research is needed
to evaluate its potential as a lead drug. The effect of the bark extract on 2,4DNPH experimental lipid peroxidation and the side effects of 2,4-DNPH and ethanol on natural antioxidant enzymes and vitamins were studied. The bark extract,
like bergenin, exerted a protective action on brain tissue, though to a lesser
extent, as against oxidant 2,4-DNPH. The inhibitory effect is dependent on dose
and on activity per unit weight basis. The extract appears a more powerful inhibitor than vitamins E and C [60,92]. This suggests that the pharmacologic action
of the bark extract as an anticancer agent may possibly be due to the antioxidant
potentials of the extract and bergenin, which is believed to be the active substance in the S. gabonensis stem bark extract.


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Medicinal Plant Research in Africa

21.3.11 Mallotus oppositifolius Lour.
M. oppositifolius is a shrub of the family Euphorbiaceae that grows in many parts
of Africa. It is used in folk medicine and herbal preparations for the treatment of
dysentery, worms, and malaria. It is also used traditionally for the treatment of convulsion, epilepsy, parasitic eye and kidney infections, as a diuretic and painkiller,
and in treatment of paralysis, spasm, headache, and swelling. A decoction of the
root is used for anemia, pneumonia, and as an aphrodisiac, and the stick is chewed
for oral hygiene and teeth cleaning [61]. Anti-inflammatory [62] and antimicrobial
effects [63,93] of the plant have also been reported.


Antioxidant Activity of African Medicinal Plants

797

the plant are used in herbal medicine as vermifuge, astringent, and treatment for
dysentery, diarrhea, leprosy, cutaneous infections, abscesses, dental caries, gingivitis, hemorrhoids, and fever [96]. T. sanguinea extracts have also been reported to
possess antibacterial activity, including against multidrug-resistant strains [97].
The flowers of T. sanguinea are widely used in Ivory Coast in the treatment of
microbial diseases, mainly the salmonellae diseases [98]. In Africa, multidrugresistant nontyphoidal salmonellae (NTS) are one of the leading causes of morbidity and high mortality in children under 5 years of age [99]. T. sanguinea is also
known to possess antioxidant activity [100,101]. N’Guessan et al. [102] successfully isolated two phenolic antioxidant compounds from this plant. These are brevifolin carboxylic acid and gallic acid.

21.4

Conclusions

This review discussed medicinally significant plant species selected from Africa
with high antioxidant activities when compared to synthetic antioxidants. It focuses
on plants belonging to several different families from around Africa to understand
their therapeutic uses and their potential antioxidant activities. Different antioxidant
chemical compounds isolated from some of these plants were also discussed in
order to know the active constituents responsible for the antioxidant potential of
the plants.
However, it is worthy of note that most scientific studies on the antioxidant
potential of these medicinal plants were conducted in vitro using different methods
such as DPPH radical scavenging activity and SAS assay. The results obtained
from these in vitro assays may not necessarily imply that the same effect will be
produced when performed in the living organism. Therefore, there is need for
in vivo studies before these plants can be validated for use in medicine.


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