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Research Article

Free Radical Scavenging Activity of Some Nigerian Medicinal Plant Extracts

M.O. Sofidiya, O.A. Odukoya , O.B. Familoni and S.I. Inya-Agha
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The present research evaluates the DPPH radical scavenging, total antioxidant activities, reducing power and total contents of phenolic compounds in methanolic leaf extracts of five Nigerian medicinal plants (Dalbergia saxatilis Hook.f. (Papilionacae), Ekebergia senegalensis A.Juss.(Meliaceae), Hymenocardia acida Tul. (Hymenocardiaceae), Icacina tricantha Oliv. (Icacinaceae) and Salacia pallescens Oliv.(Celastraceae). Total phenols were analysed according to the Folin-Ciocalteu method. Each sample under assay condition, showed a dose-dependent effect both on free radical scavenging 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) and also on Fe3+ reducing power. The antioxidant activity of the plant extracts with the DPPH radical scavenging and reducing power method, were in the order Hymenocardia> Ekebergia> Salacia> Icacina> Dalbergia. H. acida and E. senegalensis possess very high radical scavenging activity in both assays. Potency of H. acida extract was of the same magnitude as that of reference α-tocopherol. Total phenols in all the samples expressed as GAE (Gallic Acid Equivalent) varied from 1.83 to 15.47mg g-1 of dry plant material. Total antioxidant activities correlated with total phenols (R2 = 0.6640) an indication that 66% of the antioxidant capacity of these extracts results from contribution of phenolic compounds. A linear positive relationship existed between the reducing power and total phenolics of the tested plant extracts (R2 = 0.9564).

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  How to cite this article:

M.O. Sofidiya, O.A. Odukoya , O.B. Familoni and S.I. Inya-Agha , 2006. Free Radical Scavenging Activity of Some Nigerian Medicinal Plant Extracts. Pakistan Journal of Biological Sciences, 9: 1438-1441.

DOI: 10.3923/pjbs.2006.1438.1441



Free radicals are compounds generated from normal body processes and also from environmental pollutants. They tend to attack the cells of our body causing them to deteriorate. Antioxidants quench free radicals so that they will not attack living cells. Antioxidant properties have been extensively studied and are among the first links between chemical reactions and biological activity (Trouillas et al., 2003). It has been shown that antioxidants and free radical scavengers are relevant in the prevention of pathologies such as arteriosclerosis, heart diseases, cancer and arthritis, in which reactive oxygen species or free radicals are, implicated (Middleton, 2000).

Several types of plant materials such as vegetables, fruits leaves, oilseeds, cereals crops, barks and roots, spices and herbs and crude plant drugs are potential sources of antioxidant compounds. Most of the isolated constituents with antioxidant activity are phenolic compounds (Ramarathnam et al., 1997; Odukoya et al., 2005; Olukemi et al., 2005).

Phenolic natural products such as flavonoids are of particular interest because of their antioxidant activity through scavenging oxygen radicals. The antioxidant activity of these phenolic compounds is mainly due to their redox properties, which allow them to act as reducing agents, hydrogen donators and singlet oxygen quencher. In addition, they have metal chelating potentials (Rice-Evans et al., 1995).

A systematic search for useful bioactivities from medicinal plants is now considered to be a rational approach in nutraceutical and drug research. There is a worldwide trend toward the use of natural antioxidants. For this reason, an extensive search for different types of antioxidants in various kinds of plants has been undertaken. According to a conservative estimation, 300,000-400,000 plants species grow on earth; however, only a small percentage have had their phytochemistry and biological function investigated (Kitani et al., 2001). In this regard, while some individual plant species such as Ginkgo biloba and Panax ginseng have been investigated in some detail, relatively little information is available concerning the antioxidant potential of plant species in general and Nigerian plant species in particular. The objective of the present research is to carry out a systematic survey of the relative levels of antioxidant activity in selected Nigerian medicinal plant species used in traditional medical practice. D. saxatilis is used in the treatment of sores, sharp stomach pains and gonorrheoa; E. senegalensis and I. tricantha for dysentery; H. acida as a febrifuge and also in the treatment of gonorheoa and S. pallescens as an antidiabetic and anti obesity.


Plant collection and extraction: The plants were collected between November 2004 and January 2005, from various locations in Oyo State, Nigeria and were identified at the herbarium of the Forestry Research Institute of Nigeria [FRIN], Ibadan. Voucher specimens were deposited at the Institute and also in the Department of Pharmacognosy, University of Lagos. The plants were air dried at room temperature.

Ten gram of each dried powdered sample was macerated with 100 mL of methanol for 24 h at room temperature. Each extract was filtered through Whatman No.1 filter paper and the residue re-extracted with the same solvent. All extracts were combined together and left on the bench to dry. The extracts were weighed and preserved for further use in the refrigerator.

DPPH Radical scavenging assay: Radical scavenging activity of plant extracts was measured by slightly modified method of Miliauskas et al. (2004). One mg mL–1 methanolic stock solution of extract was prepared and various concentrations of the extracts were obtained by serial dilution. To these was added 0.5 mL methanol solution of DPPH. (1 mM). The mixture was shaken and then left to stand at room temperature for 30 min. The absorbance of the resulting solution was read spectrophotometrically at 517 nm. α-Tocopherol (Sigma) was used as standard antioxidant while a blank of methanol (Analytical BDH) was ran with each assay. All determinations were carried out in triplicate. The same procedure was repeated using control sample (DPPH without the extracts). The inhibition of DPPH was calculated as a percentage according to the following equation:

Reducing power: The reducing power of the prepared extracts was determined using Ferricyanide Trichloroacetic acid method according to Yen and Chen (1995). Mean values from three independent samples were calculated for each extract.

Analysis of phenolic compounds: Plant extracts were screened for phenolics by the addition of few drops of 5% FeCl3 solution. Total phenol contents in plant methanol extract was determined with Folin-Ciocalteu reagent (1927) as recorded by Slinkard and Singleton (1977) using gallic acid as a standard. All determinations were performed in triplicate. The concentrations of total phenolic compounds in the extracts were determined as milligrams of Gallic Acid Equivalents (GAE) using the following linear equation based on the calibration curve:

A = 10.564X + 0.1036, R2 = 0.9998

A is the absorbance and X is gallic acid equivalents (mg).

Statistical analysis: Experimental results were the mean±SD of 3 measurements. Analysis of variance and regression coefficients were from Excel programme.


The DPPH assay measured hydrogen atom (or one electron) donating activity and hence provided an evaluation of antioxidant activity due to free radical scavenging. All extracts showed dose- dependent increase in activity (Table 1).

The reducing power of the crude methanol extracts of the samples was examined as a function of their concentration. In this assay, the yellow colour of the test solution changed to various shades of green and blue depending upon the reducing power of each extract. Reducing power was similar to antioxidant activity, H. acida showed a higher reducing power than the other plant extract and α-tocopherol (Table 2, 3).

Table 1: Plant yield to solvent

Table 2: Free radical scavenging effect % inhibition on DPPH at different concentrations of plant extract
Data are represented as means±SE (n = 3)

Table 3: Reducing power of extracts at different concentrations
Data are represented as means±SE (n = 3)

Table 4: Total phenol contents of the plant extracts
Data are represented as means±SE (n= 3)

However the absorbance value was lower than that of α-tocopherol at 0.4 mg mL–1. Using the DPPH radical scavenging method and reducing power method, the antioxidant activity of the plant extracts were in the order Hymenocardia> Ekebergia> Salacia> Icacina> Dalbergia. This result was consistent with the order of total phenolic content of each extract.

Several studies have reported the relationship between phenolic content and antioxidant activity. Velioglu et al. (1998) and Odukoya et al. (2005) reported a strong relationship between total phenolic content and antioxidant activity in selected fruits, vegetables and grain products, while Kahkonen et al. (1999) found no correlation between antioxidant activity and phenolic content of some plant extracts.

In this study, our findings showed a good linear correlation (R5 = 0.9564) between reducing power and the total phenol content in each extract. These results indicated that the reducing power of each extract might be mostly related to their concentration of hydroxyl hydrogen. Phenolic compounds could easily donate hydroxyl hydrogen due to resonance stabilization (Meir et al., 1995). Reducing power of a compound is related to electron transfer ability of the compound. Therefore, the reducing capacity of a compound may serve as a significant indicator of its potential antioxidant activity as indicated in our results.

However, free radical scavenging activity and total phenolic content of the plant extract had a correlation coefficient of R = 0.6640 (Y = 1.2281H-103.11) (Table 4). This suggests that about 66% of the antioxidant capacity of these extracts result from contribution of phenolic compounds. Thus confirming the radical scavenging and antioxidant activities of these extracts, as determined by reducing power and scavenging effect on the DPPH free radical.


The authors wish to place on record their heart felt thanks to Dr. Olayinka of Chemistry department, University of Lagos for providing the use of spectrophotometer used for this research.

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