Antioxidant and free radical scavenging activity of Spondias pinnata
© Hazra et al; licensee BioMed Central Ltd. 2008
Received: 30 June 2008
Accepted: 09 December 2008
Published: 09 December 2008
Many diseases are associated with oxidative stress caused by free radicals. Current research is directed towards finding naturally-occurring antioxidants of plant origin. The aim of the present study was to evaluate the in vitro antioxidant activities of Spondias pinnata stem bark extract.
A 70% methanol extract of Spondias pinnata stem bark was studied in vitro for total antioxidant activity, for scavenging of hydroxyl radicals, superoxide anions, nitric oxide, hydrogen peroxide, peroxynitrite, singlet oxygen and hypochlorous acid, and for iron chelating capacity, reducing power, and phenolic and flavonoid contents.
The extract showed total antioxidant activity with a trolox equivalent antioxidant concentration (TEAC) value of 0.78 ± 0.02. The IC50 values for scavenging of free radicals were 112.18 ± 3.27 μg/ml, 13.46 ± 0.66 μg/ml and 24.48 ± 2.31 μg/ml for hydroxyl, superoxide and nitric oxide, respectively. The IC50 for hydrogen peroxide scavenging was 44.74 ± 25.61 mg/ml. For the peroxynitrite, singlet oxygen and hypochlorous acid scavenging activities the IC50 values were 716.32 ± 32.25 μg/ml, 58.07 ± 5.36 μg/ml and 127.99 ± 6.26 μg/ml, respectively. The extract was found to be a potent iron chelator with IC50 = 66.54 ± 0.84 μg/ml. The reducing power was increased with increasing amounts of extract. The plant extract (100 mg) yielded 91.47 ± 0.004 mg/ml gallic acid-equivalent phenolic content and 350.5 ± 0.004 mg/ml quercetin-equivalent flavonoid content.
The present study provides evidence that a 70% methanol extract of Spondias pinnata stem bark is a potential source of natural antioxidants.
It is increasingly being realized that many of today's diseases are due to the "oxidative stress" that results from an imbalance between formation and neutralization of pro-oxidants. Oxidative stress is initiated by free radicals, which seek stability through electron pairing with biological macromolecules such as proteins, lipids and DNA in healthy human cells and cause protein and DNA damage along with lipid peroxidation. These changes contribute to cancer, atherosclerosis, cardiovascular diseases, ageing and inflammatory diseases [1, 2]. All human cells protect themselves against free radical damage by enzymes such as superoxide dismutase (SOD) and catalase, or compounds such as ascorbic acid, tocopherol and glutathione . Sometimes these protective mechanisms are disrupted by various pathological processes, and antioxidant supplements are vital to combat oxidative damage. Recently, much attention has been directed towards the development of ethnomedicines with strong antioxidant properties but low cytotoxicities.
Spondias pinnata (Linn. f.) Kurz (Family – Anacardiaceae) is a deciduous tree distributed in India, Sri Lanka and South-East Asian countries. In India it is commonly seen in the deciduous to semi-evergreen forests of the Western Ghats. The genus Spondias includes 17 described species, 7 of which are native to the neotropics and about 10 are native to tropical Asia. The phytochemistry of this plant has been studied . The gum exudate of the species has been found to contain acidic polysaccharides . A crude extract of S. pinnata has been reported to show antibacterial activity . In ethnomedicine, equal quantities of bark juice of S. pinnata and Syzygium cumuni are prescribed as a remedy for dysentery . An aqueous extract of this plant inhibits the citrus canker of lime . However, there has been no report on the antioxidant properties of this species.
The objective of the present study was to evaluate the antioxidant potential and free radical scavenging activity of a 70% methanol extract of S. pinnata. The extract was examined for different reactive oxygen species (ROS) scavenging activities including hydroxyl, superoxide, nitric oxide, hydrogen peroxide, peroxynitrite, singlet oxygen and hypochlorous acid, and for phenol and flavonoid contents, iron chelating capacity and total antioxidant activity.
2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was obtained from Roche Diagnostics, Mannheim, Germany. 6-hydroxy-2,5,7,8-tetramethychroman-2-carboxylic acid (Trolox) was obtained from Fluka, Buchs, Switzerland. Potassium persulfate (K2S2O8), ethylenediamine tetraacetic acid (EDTA), ascorbic acid, 2-deoxy-2-ribose, trichloroacetic acid (TCA), mannitol, nitro blue tetrazolium (NBT), reduced nicotinamide adenine dinucleotide (NADH), phenazine methosulfate (PMS), sodium nitroprusside (SNP), sulfanilamide, naphthylethylenediamine dihydrochloride (NED), L-histidine, lipoic acid, sodium pyruvate, quercetin and ferrozine were obtained from Sisco Research Laboratories Pvt. Ltd, Mumbai, India. Hydrogen peroxide, potassium hexacyanoferrate, Folin-Ciocalteu reagent, sodium carbonate (Na2CO3), butylated hydroxytoluene (BHT), sodium hypochlorite (NaOCl), aluminium chloride (AlCl3), ammonium iron (II) sulfate hexahydrate ((NH4)2Fe(SO4)26H2O), potassium nitrite (KNO2), N, N-dimethyl-4-nitrosoaniline and xylenol orange were obtained from Merck, Mumbai, India. Gallic acid and curcumin were obtained from MP Biomedicals, France. Ferrous sulfate and catalase were obtained from HiMedia Laboratories Pvt. Ltd, Mumbai, India. Evans Blue was purchased from BDH, England. Manganese dioxide was obtained from SD Fine Chemicals, Mumbai, India. Diethylene-triamine-pentaacetic acid (DTPA) was obtained from Spectrochem Pvt. Ltd, Mumbai, India. Thiobarbituric acid (TBA) was obtained from Loba Chemie, Mumbai, India. Sodium nitrite was obtained from Qualigens Fine Chemicals, Mumbai, India.
The bark of the S. pinnata plant was collected from the Bankura district of West Bengal, India and authenticated through the Central Research Institute of Ayurveda, Kolkata, India.
The stem bark of S. pinnata was dried at room temperature for 7 days, finely powdered and used for extraction. The powder (100 g) was mixed with 500 ml methanol:water (7:3) using a magnetic stirrer for 15 hours, then the mixture was centrifuged at 2850 × g and the supernatant was decanted. The pellet was mixed again with 500 ml methanol-water and the entire extraction process was repeated. The supernatants collected from the two phases were mixed in a round bottom flask and concentrated under reduced pressure in a rotary evaporator. The concentrated extract was then lyophilized. The residue was kept at -20°C for future use.
Total antioxidant activity
The ability of the test sample to scavenge ABTS.+ radical cation was compared to trolox standard . The ABTS.+ radical cation was pregenerated by mixing 7 mM ABTS stock solution with 2.45 mM potassium persulfate (final concentration) and incubating for 12–16 h in the dark at room temperature until the reaction was complete and the absorbance was stable. The absorbance of the ABTS.+ solution was equilibrated to 0.70 (± 0.02) by diluting with water at room temperature, then 1 ml was mixed with 10 μl of the test sample (0.05–10 mg/ml) and the absorbance was measured at 734 nm after 6 min. All experiments were repeated six times. The percentage inhibition of absorbance was calculated and plotted as a function of the concentration of standard and sample to determine the trolox equivalent antioxidant concentration (TEAC). To calculate the TEAC, the gradient of the plot for the sample was divided by the gradient of the plot for trolox.
Hydroxyl radical scavenging
This was assayed as described by Elizabeth and Rao  with a slight modification. The assay is based on quantification of the degradation product of 2-deoxyribose by condensation with TBA. Hydroxyl radical was generated by the Fe3+-ascorbate-EDTA-H2O2 system (the Fenton reaction). The reaction mixture contained, in a final volume of 1 ml, 2-deoxy-2-ribose (2.8 mM); KH2PO4-KOH buffer (20 mM, pH 7.4); FeCl3 (100 μM); EDTA (100 μM); H2O2 (1.0 mM); ascorbic acid (100 μM) and various concentrations (0–200 μg/ml) of the test sample or reference compound. After incubation for 1 h at 37°C, 0.5 ml of the reaction mixture was added to 1 ml 2.8% TCA, then 1 ml 1% aqueous TBA was added and the mixture was incubated at 90°C for 15 min to develop the color. After cooling, the absorbance was measured at 532 nm against an appropriate blank solution. All tests were performed six times. Mannitol, a classical OH. scavenger, was used as a positive control. Percentage inhibition was evaluated by comparing the test and blank solutions.
Superoxide radical scavenging
This activity was measured by the reduction of NBT according to a previously reported method . The non-enzymatic phenazine methosulfate-nicotinamide adenine dinucleotide (PMS/NADH) system generates superoxide radicals, which reduce nitro blue tetrazolium (NBT) to a purple formazan. The 1 ml reaction mixture contained phosphate buffer (20 mM, pH 7.4), NADH (73 μM), NBT (50 μM), PMS (15 μM) and various concentrations (0–20 μg/ml) of sample solution. After incubation for 5 min at ambient temperature, the absorbance at 562 nm was measured against an appropriate blank to determine the quantity of formazan generated. All tests were performed six times. Quercetin was used as positive control.
Nitric oxide radical scavenging
At physiological pH, nitric oxide generated from aqueous sodium nitroprusside (SNP) solution interacts with oxygen to produce nitrite ions, which may be quantified by the Griess Illosvoy reaction . The reaction mixture contained 10 mM SNP, phosphate buffered saline (pH 7.4) and various doses (0–70 μg/ml) of the test solution in a final volume of 3 ml. After incubation for 150 min at 25°C, 1 ml sulfanilamide (0.33% in 20% glacial acetic acid) was added to 0.5 ml of the incubated solution and allowed to stand for 5 min. Then 1 ml of napthylethylenediamine dihydrochloride (NED) (0.1% w/v) was added and the mixture was incubated for 30 min at 25°C. The pink chromophore generated during diazotization of nitrite ions with sulphanilamide and subsequent coupling with NED was measured spectrophotometrically at 540 nm against a blank sample. All tests were performed six times. Curcumin was used as a standard.
Hydrogen peroxide scavenging
This activity was determined according to a previously described method  with minor changes. An aliquot of 50 mM H2O2 and various concentrations (0–2 mg/ml) of samples were mixed (1:1 v/v) and incubated for 30 min at room temperature. After incubation, 90 μl of the H2O2-sample solution was mixed with 10 μl HPLC-grade methanol and 0.9 ml FOX reagent was added (prepared in advance by mixing 9 volumes of 4.4 mM BHT in HPLC-grade methanol with 1 volume of 1 mM xylenol orange and 2.56 mM ammonium ferrous sulfate in 0.25 M H2SO4). The reaction mixture was then vortexed and incubated at room temperature for 30 min. The absorbance of the ferric-xylenol orange complex was measured at 560 nm. All tests were carried out six times and sodium pyruvate was used as the reference compound .
Peroxynitrite (ONOO-) was synthesized by the method described by Beckman et al. . An acidic solution (0.6 M HCl) of 5 ml H2O2 (0.7 M) was mixed with 5 ml 0.6 M KNO2 on an ice bath for 1 s and 5 ml of ice-cold 1.2 M NaOH was added. Excess H2O2 was removed by treatment with granular MnO2 prewashed with 1.2 M NaOH and the reaction mixture was left overnight at -20°C. Peroxynitrite solution was collected from the top of the frozen mixture and the concentration was measured spectrophotometrically at 302 nm (ε = 1670 M-1 cm-1).
An Evans Blue bleaching assay was used to measure peroxynitrite scavenging activity. The assay was performed by a standard method  with a slight modification. The reaction mixture contained 50 mM phosphate buffer (pH 7.4), 0.1 mM DTPA, 90 mM NaCl, 5 mM KCl, 12.5 μM Evans Blue, various doses of plant extract (0–200 μg/ml) and 1 mM peroxynitrite in a final volume of 1 ml. After incubation at 25°C for 30 min the absorbance was measured at 611 nm. The percentage scavenging of ONOO- was calculated by comparing the results of the test and blank samples. All tests were performed six times. Gallic acid was used as the reference compound.
Singlet oxygen scavenging
The production of singlet oxygen (1O2) was determined by monitoring N, N-dimethyl-4-nitrosoaniline (RNO) bleaching, using a previously reported spectrophotometric method [17, 18]. Singlet oxygen was generated by a reaction between NaOCl and H2O2, and the bleaching of RNO was monitored at 440 nm. The reaction mixture contained 45 mM phosphate buffer (pH 7.1), 50 mM NaOCl, 50 mM H2O2, 50 mM histidine, 10 μM RNO and various concentrations (0–200 μg/ml) of sample in a final volume of 2 ml. It was incubated at 30°C for 40 min and the decrease in RNO absorbance was measured at 440 nm. The scavenging activity of sample was compared with that of lipoic acid, used as a reference compound. All tests were performed six times.
Hypochlorous acid scavenging
Hypochlorous acid (HOCl) was prepared immediately before the experiment by adjusting the pH of a 10% (v/v) solution of NaOCl to 6.2 with 0.6 M H2SO4, and the concentration of HOCl was determined by measuring the absorbance at 235 nm using the molar extinction coefficient of 100 M-1 cm-1. The assay was carried out as described by Aruoma and Halliwell  with minor changes. The scavenging activity was evaluated by measuring the decrease in absorbance of catalase at 404 nm. The reaction mixture contained, in a final volume of 1 ml, 50 mM phosphate buffer (pH 6.8), catalase (7.2 μM), HOCl (8.4 mM) and increasing concentrations (0–100 μg/ml) of plant extract. The assay mixture was incubated at 25°C for 20 min and the absorbance was measured against an appropriate blank. All tests were performed six times. Ascorbic acid, a potent HOCl scavenger, was used as a reference .
The ferrous ion chelating activity was evaluated by a standard method  with minor changes. The reaction was carried out in HEPES buffer (20 mM, pH 7.2). Briefly, various concentrations (0–120 μg/ml) of plant extract were added to 12.5 μM ferrous sulfate solution and the reaction was initiated by the addition of ferrozine (75 μM). The mixture was shaken vigorously and incubated for 20 min at room temperature, then the absorbance was measured at 562 nm. All tests were performed six times. EDTA was used as a positive control.
The Fe3+-reducing power of the extract was determined by the method of Oyaizu  with a slight modification. Different concentrations (0.0–0.4 mg/ml) of the extract (0.5 ml) were mixed with 0.5 ml phosphate buffer (0.2 M, pH 6.6) and 0.5 ml potassium hexacyanoferrate (0.1%), followed by incubation at 50°C in a water bath for 20 min. After incubation, 0.5 ml of TCA (10%) was added to terminate the reaction. The upper portion of the solution (1 ml) was mixed with 1 ml distilled water, and 0.1 ml FeCl3 solution (0.01%) was added. The reaction mixture was left for 10 min at room temperature and the absorbance was measured at 700 nm against an appropriate blank solution. All tests were performed six times. A higher absorbance of the reaction mixture indicated greater reducing power. Butylated hydroxytoluene (BHT) was used as a positive control.
Determination of total phenolic content
Total phenolic content was determined using Folin-Ciocalteu (FC) reagent according to the method of Singleton and Rossi  with a slight modification. Briefly, the plant extract (0.1 ml) was mixed with 0.75 ml of FC reagent (previously diluted 1000-fold with distilled water) and incubated for 5 min at 22°C, then 0.06% Na2CO3 solution was added. After incubation at 22°C for 90 min, the absorbance was measured at 725 nm. All tests were performed six times. The phenolic content was evaluated from a gallic acid standard curve.
Determination of total flavonoid content
The total flavonoid content was determined with aluminium chloride (AlCl3) according to a known method  using quercetin as a standard. The plant extract (0.1 ml) was added to 0.3 ml distilled water followed by NaNO2 (0.03 ml, 5%). After 5 min at 25°C, AlCl3 (0.03 ml, 10%) was added. After a further 5 min, the reaction mixture was treated with 0.2 ml 1 mM NaOH. Finally, the reaction mixture was diluted to 1 ml with water and the absorbance was measured at 510 nm. All tests were performed six times. The flavonoid content was calculated from a quercetin standard curve.
All data are given as the mean ± SD of six measurements. Statistical analysis was performed using KyPlot version 2.0 beta 15 (32 bit). The IC50 values were calculated by the formula Y = 100*A1/(X + A1), where A1 = IC50, Y = response (Y = 100% when X = 0), X = inhibitory concentration. The IC50 values were compared by paired t tests. p < 0.05 was considered significant.
Total antioxidant activity
Hydroxyl radical scavenging
Scavenging of reactive oxygen species and iron chelating activity (IC50 values) of Spondias pinnata and reference compounds
Hydroxyl radical (OH.) scavenging
112.18 ± 3.27 (6)
571.45 ± 20.12 (6)***
Superoxide anion (O2 .-) scavenging
13.46 ± 0.66 (6)
42.06 ± 1.35 (6)***
Nitric oxide radical (NO) scavenging
24.48 ± 2.31 (6)
90.82 ± 4.75 (6)***
Hydrogen peroxide (H2O2) scavenging
44.74 ± 25.61 (6)
3.24 ± 0.30 (6) *
Peroxynitrite (ONOO-) scavenging
716.32 ± 32.25 (6)
876.24 ± 56.96 (6)***
Singlet oxygen (1O2) scavenging
58.07 ± 5.36 (6)
46.15 ± 1.16 (6) **
Hypochlorous acid (HOCl) scavenging
127.99 ± 6.26 (6)
235.95 ± 5.75 (6)***
66.54 ± 0.84 (6)
1.27 ± 0.05 (6)***
Superoxide radical scavenging
Nitric oxide radical scavenging
Hydrogen peroxide scavenging
Singlet oxygen scavenging
Hypochlorous acid scavenging
Determination of total phenolic content
Phenolic compounds may contribute directly to antioxidative action. The total phenolic content was 91.47 ± 0.004 mg/ml gallic acid equivalent per 100 mg plant extract.
Determination of total flavonoid content
The total flavonoid content of the 70% methanolic extract of S. pinnata was 350.5 ± 0.004 mg/ml quercetin equivalent per 100 mg plant extract.
In living systems, free radicals are constantly generated and they can cause extensive damage to tissues and biomolecules leading to various disease conditions, especially degenerative diseases, and extensive lysis . Many synthetic drugs protect against oxidative damage but they have adverse side effects. An alternative solution to the problem is to consume natural antioxidants from food supplements and traditional medicines [26, 27]. Recently, many natural antioxidants have been isolated from different plant materials [28, 29]. The antioxidant and polyphenol contents of S. pinnata have also been determined by Chuyen et al. , using concentrated leaf extract. The antioxidant activity has been studied by the inhibition of ascorbic acid-induced lipid peroxidation and Catechin has been used as the standard for phenolic content, whereas in the present study, the antioxidant capacity of bark extract was measured by an improved ABTS radical cation decolorization assay and gallic acid was used as the standard for phenolic content measurement; both studies showed promising results. Therefore, it is clear that both the leaf and stem bark extracts of the plant have good antioxidant activities as well as high polyphenolic contents.
ABTS.+ is a blue chromophore produced by the reaction between ABTS and potassium persulfate. Addition of the plant extract to this pre-formed radical cation reduced it to ABTS in a concentration-dependent manner. The results were compared with those obtained using trolox and the TEAC value demonstrates that the extract is a potent antioxidant.
Hydroxyl radicals are the major active oxygen species causing lipid peroxidation and enormous biological damage . They were produced in this study by incubating ferric-EDTA with ascorbic acid and H2O2 at pH 7.4, and reacted with 2-deoxy-2-ribose to generate a malondialdehyde (MDA)-like product. This compound forms a pink chromogen upon heating with TBA at low pH . When S. pinnata extract was added to the reaction mixture, it removed the hydroxyl radicals from the sugar and prevented the reaction. The IC50 value indicates that the plant extract is a better hydroxyl radical scavenger than the standard mannitol.
Superoxide anion is also very harmful to cellular components . Robak and Glyglewski  reported that flavonoids are effective antioxidants mainly because they scavenge superoxide anions. As shown in figure 3, the superoxide radical scavenging activities of the plant extract and the reference compound are increased markedly with increasing concentrations. The results suggest that the plant extract is a more potent scavenger of superoxide radical than the standard quercetin.
It is well known that nitric oxide has an important role in various inflammatory processes. Sustained levels of production of this radical are directly toxic to tissues and contribute to the vascular collapse associated with septic shock, whereas chronic expression of nitric oxide radical is associated with various carcinomas and inflammatory conditions including juvenile diabetes, multiple sclerosis, arthritis and ulcerative colitis . The toxicity of NO increases greatly when it reacts with superoxide radical, forming the highly reactive peroxynitrite anion (ONOO-) . The nitric oxide generated from sodium nitroprusside reacts with oxygen to form nitrite. The extract inhibits nitrite formation by directly competing with oxygen in the reaction with nitric oxide. The present study proved that the extract studied has more potent nitric oxide scavenging activity than the standard curcumin.
Hydrogen peroxide is a weak oxidizing agent that inactivates a few enzymes directly, usually by oxidation of essential thiol (-SH) groups. It can cross cell membranes rapidly; once inside the cell, it can probably react with Fe2+ and possibly Cu2+ ions to form hydroxyl radicals and this may be the origin of many of its toxic effects . From the results, it appeared that the H2O2 scavenging activity of the plant extract is negligible compared to that of the standard sodium pyruvate.
Peroxynitrite (ONOO-) is relatively stable compared to other free radicals but once protonated it forms the highly reactive peroxynitrous acid (ONOOH) . Generation of excess ONOO- leads to oxidative damage and tissue injury . Peroxynitrite bleaches Evans Blue by oxidizing it. According to the present results, the plant extract inhibits Evans Blue bleaching by scavenging peroxynitrite and its activity is greater than that of the reference gallic acid.
Singlet oxygen is generated in the skin by ultraviolet radiation. It is a high energy form of oxygen and is known as one of the ROS. Singlet oxygen induces hyperoxidation and oxygen cytotxicity and decreases antioxidative activity . The present study indicates that the S. pinnata extract has good scavenging activity for singlet oxygen but is not as efficient as the standard lipoic acid.
At sites of inflammation, the oxidation of Cl- ions by the neutrophil enzyme myeloperoxidase results in the production of another harmful ROS, hypochlorous acid . HOCl has the ability to inactivate the antioxidant enzyme catalase through breakdown of the heme prosthetic group. Catalase inactivation is inhibited in the presence of the extract, signifying its HOCl scavenging activity. The results suggest that S. pinnata is a more efficient scavenger than the standard ascorbic acid.
Iron can stimulate lipid peroxidation by the Fenton reaction (H2O2 + Fe2+ = Fe3+ + OH- + OH.) and can also accelerate lipid peroxidation by decomposing lipid hydroperoxides into peroxyl and alkoxyl radicals that can perpetuate the chain reaction . Metal chelating capacity is significant since it reduces the concentration of the transition metal that catalyzes lipid peroxidation . According to the results, the plant extract is not as good as the standard EDTA; but the decrease in concentration-dependent color formation in the presence of the extract indicates that it has iron chelating activity.
The reducing capacity of a compound may serve as a significant indicator of its potential antioxidant activity. However, the activities of antioxidants have been attributed to various mechanisms such as prevention of chain initiation, decomposition of peroxides, reducing capacity and radical scavenging . As shown in figure 7, the reducing power of the plant extract was compared with the standard BHT and found to be superior.
The results indicate that S. pinnata plant extract contains significant amounts of flavonoids and phenolic compounds. Both these classes of compounds have good antioxidant potential and their effects on human nutrition and health are considerable. The mechanism of action of flavonoids is through scavenging or chelation . Phenolic compounds are also very important plant constituents because their hydroxyl groups confer scavenging ability .
On the basis of the results obtained in the present study, it is concluded that a 70% methanolic extract of Spondias pinnata bark, which contains large amounts of flavonoids and phenolic compounds, exhibits high antioxidant and free radical scavenging activities. It also chelates iron and has reducing power. These in vitro assays indicate that this plant extract is a significant source of natural antioxidant, which might be helpful in preventing the progress of various oxidative stresses. However, the components responsible for the antioxidative activity are currently unclear. Therefore, further investigation is needed to isolate and identify the antioxidant compounds present in the plant extract. Furthermore, the in vivo antioxidant activity of this extract needs to be assessed prior to clinical use.
Mr. B. Hazra is grateful to the Indian Council of Medical Research, Government of India for providing fellowship. The authors would like to thank Mr. Rhitajit Sarkar, Dr. Sourav Mandal and Mr. Ranjit Das for assistance.
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