Antioxidants from Nigerian Medicinal Plants: What Are the Evidence?

1. Introduction

Since the discovery of enzyme superoxide dismutase (SOD) and the evidence that emerged in support of the role of free radicals in biological systems, human understanding of free radical biochemistry in health and disease continue to advance [1]. This provided the basis for continuous search on natural antioxidants from foods and phytomedicines. Overwhelming reports on Google search engine has indicated 92,800 hits for “antioxidant activity” of medicinal plants in the last 10 years (2008–2018). This is due to growing interest on the antioxidant properties of medicinal plants. Several chemical and biochemical protocols have been used in the evaluation of antioxidant activity including the oxygen radical absorbance capacity (ORAC), total radical-trapping antioxidant potential (TRAP), total oxidant scavenging capacity (TOSC), chemiluminescence (CL), croton bleaching, low density lipoprotein (LDL) oxidation, ferric reducing antioxidant power (FRAP), copper reduction assay (CUPRAC), 2,2′ azinobis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) assay, 2,2-diphenyl-1-picrylhydrazyl (DPPH), nitric oxide (NO), hydroxyl radical (OH) hydrogen peroxide (H₂O₂) and total phenolic assay among others [2]. Biochemical protocols are based on animal models for in vivo evaluations of oxidative stress biomarkers. However, this study is focused on in vitro evaluations of antioxidants from plants based on hydrogen atom transfer or single electron transfer mechanisms [2]. The strength of antioxidant activity measured from a combination of different methodologies was used to evaluating antioxidant effectiveness [3]. This review provides fundamental background on free radical and ROS in human health and disease with a view to understand the roles of natural antioxidants. We reviewed 250 Nigerian medicinal plants evaluated for antioxidant activity in search of evidence for effective antioxidants.

2. Reactive oxygen species (ROS) in human health and disease

Human system uses oxidation for normal metabolic activities in the transformation of nutrients into energy. During oxidation, reactive oxygen species (ROS) are also produced at low levels in normal physiological conditions, which are necessary for maintaining normal cell functions such as signaling immunity and homeostasis [4]. These activities are maintained by endogenous antioxidant (enzymatic) defense systems produced by the body for protection against harmful effects. These include superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), glutathione reductase (GSH-Rx) and catalase [5]. Excessive production of ROS beyond the body defense mechanisms can be extremely harmful to cellular functions by damaging nucleic acids, oxidizing proteins, and causing lipid peroxidation [6]. The resultant cell damage by free radicals and ROS appeared as major contributor to aging and degenerative diseases of aging such as cancer, cardiovascular disease, immune system decline, liver diseases, diabetes mellitus, inflammation and brain dysfunction among others [7, 8]. These ROS and reactive nitrogen species (RNS) including superoxide anion O₂^−*, hydroxide ion OH^−*, hydroxyl radical OH^*, peroxyl radical ROO^* and nitric oxide NO^* as well as H₂O₂, lipid peroxides ROOH, and singlet O₂ are very reactive and can initiates free radical reactions or lipid peroxidation in living cells.

ROS can be produced either by external sources (e.g., tobacco smoke, stress, etc.), as by-products during the mitochondrial electron transport of aerobic respiration or by oxidoreductase enzymes and metal catalyzed oxidation [9]. But the biological effects of ROS depend on the types of cell or tissue in relation to enzyme production, signal transduction and DNA repair [10]. ROS are harmful when excessive productions are not balanced by body antioxidant mechanism. This imbalance between ROS production and enzymatic antioxidant defense systems is called oxidative stress [11]. Antioxidants counteract oxidative stress by neutralizing free radicals because they are reducing agents that react with and buffer ROS as a form of defense against oxidative stress [12].

3. Phytochemicals as sources of natural antioxidants

Antioxidants are molecules that prevent oxidation or inactivates the reactive oxygen species and thus prevent oxidative damage to the cells and body tissues [13]. Antioxidants can also inhibit, quench or scavenge free radicals converting them into new and stable chemical compounds [14]. Broadly, antioxidants are classified as enzymatic and non-enzymatic with each class providing complementary role of protection against free radicals in human systems. Previous work has concisely discussed on antioxidants classification [3] as summarily reproduced in Figure 1. But our focus is the non-enzymatic antioxidant involving flavonoids, phenolic acids, vitamins, carotenoids, minerals and cofactors. They are exogenous sources of protection through diet. Plants foods contain a variety of nutrients and non-nutrients chemicals which are good antioxidant agents. These sources of natural antioxidants including Vitamin A (retinol) obtained from β-carotene, vitamin C (ascorbic acid), Vitamin E (α-tocopherol), lycopene and carotenoids occur naturally in fruits, vegetables, legumes and grains which are commonly consumed and play important role in the defense against free radicals [3, 15]. Medicinal plants are rich source of phenolic compounds such as flavonoids, phenolic acids and coumarins [16, 17]. Flavonoids are antioxidants compounds composed of anthocyanins, flavanones, flavonols, flavones, isoflavonoids and flavanones, while hydroxycinnamic and hydroxybenzoic acids such as gallic acid are components of phenolic acids widely distributed secondary metabolites in plants with antioxidant and antiradical properties [18]. They are important as chelators and free radical scavengers of hydroxyl and peroxyl radicals, superoxide anions and peroxynitrites [19]. Carotenoids natural pigments are important phytochemical antioxidants obtained from plants. They are structurally grouped into carotenes and xanthophyll based on the degree of oxygenation of carotenoid hydrocarbons and exert antioxidant effect by singlet oxygen quenching ability [3]. Several studies on the antioxidant activities of various herbal plants have indicated their enormous medicinal values as inhibitors of free radical and ROS [20].

4. Antioxidants from Nigerian medicinal plants

Nigeria is a west African country with an area of 923,769 km² having a population of 198 million with 250 ethnic groups [21]. The country shares border with republic of Cameroun to the east, Niger and Chad republics to the north, Benin republic to the west and Gulf of Guinea to the south. Nigeria has favorable climate conditions with enormous diversity of plant species, which are distributed across geographical contrast of the mangrove swamps in South-South (SS), to the tropical rain forests covering South-West (SW) and South-East (SE) and to the grassland vegetation of North-Central (NC) up to the Sahel savannah of semi-arid North-East (NE) (Figure 2). Many of these plants are used as medicines for treatment of illness or management of human and animal health among rural and urban dwellers.

The application of herbal recipes especially in the management of human metabolic diseases such as diabetes and cancer is common knowledge among Nigerians. This prompted research interest in academia on the potentials of phytomedicines as complimentary or alternative treatment agents, and consequent research efforts to validate their pharmacological properties. The number of Nigerian medicinal plants reported for antioxidants is enormous. However, 250 medicinal plants evaluated for antioxidant activity were studied in addition to the 28 compounds isolated from 44 plants. But antioxidant evaluations on crude extracts rather than on pure compounds largely dominated the literature. Thus, effective activity based on concentrations required to inhibit 50% free radicals (IC₅₀) for selected extracts are presented (Table 1) together with concentrations of various standard antioxidants used.

5. Antioxidant activities of crude extracts

The antioxidant efficacies of Nigerian plants were largely evaluated using protocols involving DPPH, ABTS, FRAP, TAC, NO, OH and or H₂O₂ targets. The DPPH radical scavenging assay is one of the commonly used techniques for quick evaluation of antioxidant capacity. Plant extracts tested for DPPH inhibition have demonstrated interesting efficacies for instance, crude extracts of P. reticulatum (40.10 μgmL⁻¹) and P. thoninngii (50.94 μgmL⁻¹) showed comparable activity with Ginkgo biloba (EC₅₀ 40.72 μgmL⁻¹) [22]. Nigerian plants evaluated for antioxidants activity between 2008 and 2012 were reported in 40 publications representing over 166 extracts from 119 plants. These studies showed 29 extracts with effective activity on various free radical targets. However, 15 extracts have comparable antioxidant efficacies to standard antioxidants, while 14 have higher percent (%) inhibition or lower IC₅₀ values than the standards used. These include stem methanol extract of C. kerstingii (IC₅₀ 26.27 μgmL⁻¹, ascorbic acid 33.59 μgmL⁻¹) [23] and leaf methanol extract of P. guajava (IC₅₀ 0.037 mgmL⁻¹, BHA 0.049 mgmL⁻¹) [24]. But DPPH inhibition studies on selected vegetable plants showed better effective activity for L. sativa (IC₅₀ 0.26 mgmL⁻¹), Z. officinale (IC₅₀ 0.29 mgmL⁻¹) and C. frutescens (IC₅₀ 0.67 mgmL⁻¹) respectively compared to BHA (IC₅₀ 0.96 mgmL⁻¹) and quercetin (IC₅₀ 0.83 mgmL⁻¹) [25]. The activity of L. inermis was most profound of the 36 medicinal plants surveyed in Southwestern Nigeria, with lower IC₅₀ of 3.80 μgmL⁻¹ than ascorbic acid (7.26 μgmL⁻¹) [26]. Similar evaluations of DPPH inhibition on 15 medicinal plants showed S. oleracea extract with lower IC₅₀ of 12.6 mgmL⁻¹. But S. macrocarpon extract was most effective with IC₅₀ 6.21 mgmL⁻¹ lower than α-tocopherol (13.20 mgmL⁻¹) [27].

The analysis of antioxidant efficacies on medicinal plants reported from 2013 to 2017 in 55 publications, involving 211 extracts from 144 plants was carried out. We observed that 70 extracts from 50 plants have exhibited good antioxidant efficacies on various free radical targets with 51 extracts from 53 plants having comparable efficacies to standard antioxidants. However, lower IC₅₀ or higher percent (%) inhibitions compared to standards were observed with 20 extracts from 17 medicinal plants. The NO^● inhibition on root extract of L. africana (IC₅₀ 0.27 mgmL⁻¹) compared very well with rutin (IC₅₀ 0.28 mgmL⁻¹) [28]. The DPPH inhibition on P. guajava (IC₅₀ 1.564 μgmL⁻¹) extract also indicated effective activity compared to ascorbic acid (IC₅₀ 5.950 μgmL⁻¹) [29]. Other plant extracts including V. cinerea (IC₅₀ 6.50 μgmL⁻¹) compared to gallic acid (IC₅₀ 0.62 μgmL⁻¹) [30] and K. senegalensis stem bark (IC₅₀ 95.76 μgmL⁻¹) with ascorbic acid (223.35 μgmL⁻¹) indicated effective activity [31]. The inhibition of lipid peroxidation using thiobarbituric acid reactive substances (TBARS) assay on leaf extract of G. carpinifolia was very effective with IC₅₀ of 0.24 mgmL⁻¹ compared to ascorbic acid (IC₅₀ 0.38 mgmL⁻¹). Moreover, the ABTS assay indicated 100% inhibitions for both extracts and ascorbic acid [32]. The antioxidant evaluations on two of the most locally utilized vegetable plants such as V. amygdalina and O. gratissimum showed effective inhibitions compared to the standard ascorbic acid [33].

Furthermore, DPPH inhibitions on S. occidentalis (IC₅₀ 42.80 μgmL⁻¹) compared to gallic acid (48.77 μgmL⁻¹) was effective, but ABTS assay on F. gnaphalocarpa (44.63 μgmL⁻¹) was more effective than Trolox (72.92 μgmL⁻¹) [34]. Similarly, L. hastata (IC₅₀ 42.32 μgmL⁻¹) when compared to gallic acid (48.77 μgmL⁻¹) and ABTS on A. senegalensis (IC₅₀ 48.98 μgmL⁻¹) with Trolox (72.92 μgmL⁻¹) have interesting lower IC₅₀values [35]. But H. madagascariensis exhibited moderate activity [36] while A. precatorius [37] and B. micrantha [38] have demonstrated effective inhibitions (Table 1). The analyses of Nigerian plants in 2018 showed interesting activities with 15 plant extracts from 32 published reports. Plants with moderate DPPH inhibition include A. hispidum, A. laxiflora, C. christyanum, H. indicum and I. asarifolia [39]. However, effective inhibitions were observed on root extracts of D. tripetala (IC₅₀ 0.631 μgmL⁻¹) and M. excelsa (IC₅₀ 0.194 μgmL⁻¹) compared to 4.60 μgmL⁻¹ ascorbic acid [40]. Similarly, P. capitata (27.4 μgmL⁻¹) was effective than BHT (56.0 μgmL⁻¹) [41], and the evaluation of S. glauca stem bark on FRAP (4.70 μgmL⁻¹) and NO^● (11.90 μgmL⁻¹) were effective than 5.0 μgmL⁻¹ and 18.0 μgmL⁻¹ of BHT respectively [42]. Lastly, plant crude extracts have demonstrated varying but strong efficacies on different free radical targets which in many cases surpassed standard antioxidants. The report on DPPH inhibition of F. apodanthera root bark ethanol extract represents effective activity with IC₅₀ of 0.053 μgmL⁻¹ in comparison to vitamin C (0.048 μgmL⁻¹) standard [43].

6. Chemical composition and antioxidant activity

7. Antioxidant activities of isolated compounds

The antioxidant evaluations on isolated compounds from Nigerian medicinal plants are rarely reported. This is probably due to funding problems associated to plant chemistry research in Nigeria, coupled with dysfunctional analytical instruments such as the NMR spectrometer. Most of the published research on isolation and characterization of compounds were carried out abroad. Of the 250 plants analyzed for antioxidant evaluations, only 28 compounds were isolated from 44 plants together with full spectral characterization. The antioxidant activities of quercetin and quercetin-3-O-rutinoside from B. monandra were probably the first report on pure compounds [93]. Since then several isolated compounds were evaluated for antioxidant efficacies and in most cases compared with standard antioxidants. Thus, compounds’ efficacy only with IC₅₀ values of standards are presented in Table 2. The analysis of isolated compounds showed that flavonoids and flavonoids glycosides constitute major classes of antioxidants reported. Catechin isolated from A. senegalensis had effective DPPH inhibition (IC₅₀ 0.03 mgmL⁻¹) and Fe²⁺ chelating activity (1.29 mgmL⁻¹) when compared to ascorbic acid (0.01 mgmL⁻¹) and EDTA (IC₅₀ 0.05 mgmL⁻¹) respectively [94]. The evaluation on H₂O₂ inhibition by (−)-epicatechin isolated from A. floribunda showed effective activity with equal strength as standard ascorbic acid (IC₅₀ 8.0 μgmL⁻¹) [95]. Similarly, the ABTS inhibition by quercetin isolated from C. sieberiana has resulted to effective activity of equal strength to Tocopherol (0.81 mM) [96]. The DPPH inhibition by caffeic acid (IC₅₀ 3.03 μgmL⁻¹) from S. monostachys is another effective activity comparable to quercetin standard (IC₅₀ 2.32 μgmL⁻¹) [97]. However, the most outstanding DPPH inhibition was recorded on quercetin-3-O-rutinoside/rhamnoside isolated from A. occidentalis. The 1:1 mixture of flavonoid glycoside exhibited IC₅₀ 0.96 μgmL⁻¹ less than ascorbic acid (IC₅₀ 4.57 μgmL⁻¹). [74]. But generally, the antioxidant efficacies of isolated compounds from Nigerian plants are not interesting. Out of the 28 compounds isolated from 44 plants only 7 compounds from 6 plants exhibited the efficacies with strength of standard antioxidants.

8. Conclusion

Analysis of antioxidant efficacies of Nigerian medicinal plants reported from 1998 to 2018 was carried out. The aim was to provide evidence for effective antioxidants. Our findings have shown the enormous potentials of Nigerian plants as sources of natural antioxidants. We have observed various crude extracts obtained mainly from polar solvents with antioxidant efficacies better than standard compounds. Such preponderance of evidence indicated by broad spectrum of free radical and non-free radical inhibitions has defined the comparable strength of plant extracts to standard antioxidants. Nigerian plants have the capacity to protect or inhibit damage induced by free radical species. This study attempts to provide insights on the strength of antioxidant efficacies of plant extracts comparable to standard antioxidants. However, it is recommended that comprehensive approach to plant bioactive research must be adopted in search of antioxidants to avoid replication of studies especially on certain species. There is need for collaboration among Nigerian scientist working in related areas to enhance on the scope of research questions and improve on the quality of research output.

Acknowledgments

Authors acknowledged the plant taxonomist, Umar Shehu Gallah of National Research Institute for Chemical Technology (NARICT) Zaria, for providing the local (Hausa) names of plants. We are grateful to Ahmadu Bello University, Zaria for providing some of the facilities used during the study.