African Journal of Biomedical Research Ibadan Biomedical Communications Group ISSN: 1119-5096 Vol. 11, Num. 1, 2008, pp. 55-63

African Journal of Biomedical Research, Vol. 11, No. 1, Jan, 2008, pp. 55-63

Full Length Research Article

Free Radical Scavenging Activity of Crude Extracts and 4´-O-Methylepigallocatechin Isolated From Roots of Cassine Transvaalensis Burtt-Davy from Botswana 

^aMotlhanka D.M.T,   Habtemariam^c S, & ^bHoughtonP.,   

^a Medicinal Plant Research Laboratory, Basic Sciences Department, Botswana College of Agriculture, Bag 0027, , Gaborone, Botswana.
^b  Pharmacognosy Research Laboratories, Pharmaceutical Sciences Research Division, King’s College University of London, 150 Stamford Street, SE1 9NH, London, UK.
 ^cPharmacognosy & Phytotherapy Research  Laboratory, School of Chemical and Life Sciences, The University of Greenwich at Medway, Central Avenue, Chatham Maritime, Kent, ME4 4TB, KENT, UK.
*Address for Correspondence: dmotlhan@bca.bw Tel: 002673650221 

Received: June 2007
Accepted (Revised): November  2007
Published: January 2008

Code Number: md08008

ABSTRACT

Water and ethanol extracts of roots from Cassine transvaalensis Burtt-Davy (celastraceae) were assessed for in vitro antioxidant activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay. The ethanolic extract exhibited higher free radical scavenging effect than the water extract at all tested concentrations. Above 100µg/ml, the ethanolic extract showed 80% scavenging activity, similar to control antioxidant compounds quercetin, rutin and L-ascorbic acid. The water extract reached a similar level of activity (80%) at 200µg/ml. Between 20-50µg/ml, 4'-O-methyl-epigallocatechin isolated by bioassay directed fractionation exhibited scavenging activity greater than that of either the ethanolic or aqueous crude extract. However, at concentrations above 50µg/ml, the scavenging activity of the ethanolic extract exceeded that of 4'-O-methyl-epigallocatechin.The results suggest that extracts from the roots of Cassine transvaalensis have strong antioxidant activity. These findings support the ethnomedical use of this plant to promote good health.

Key Words;Cassine transvaalensis; antioxidant; DPPH radicals; 4’-O-methyl-epigallocatechin; epicatechin 

Abbreviations:

NMR: Nuclear Magnetic Resonance
HSQC: Heteronuclear Single Quantum Correlation
HMBC:Heteronuclear Multiple Bond Correlation 

INTRODUCTION

Cassine transvaalensis Burtt-Davy (vernacular name -monamane) is a small medium-sized tree much branched with rigid, arching stems. It occurs in bushveld, occasionally on termitaria. It is widely distributed along the eastern to South eastern Botswana. The roots are used extensively in traditional medicine to treat backache. In Botswana, a decoction of the root is believed to treat arthritis (Source: traditional healer). The Zulu people in South Africa drink large quantities of the root infusion as a general stomach conditioner (Source: traditional healer). It is also used to prepare an enema to relieve stomach aches and fevers (Coates-Palgrave, 1977). These ethnomedical uses prompted us to investigate the roots extracts for antioxidant activity using the DPPH free radical scavenging assay and to further isolate the principles responsible for the antioxidant activity.

Reactive oxygen species (ROS), including free radicals such as superoxide anion radicals(O^-₂), hydroxyl radical species (OH^.), singlet oxygen (¹O₂), and hydrogen peroxide, are active oxygen species that are often generated by biological oxidation reactions of exogenous factors (Crutti, 1991; Halliwell and Gutteridge, 1990). These ROS are known to cause severe damage to biological molecules (Aruma, 1994; Kehrer, 1993). Several phenolic compounds from plants can trap the free radicals directly or scavenge them through a series of coupled reactions with antioxidant enzymes (Lewis, 1993; Rao, Paliyath & Ormrod, 1996).

Antioxidants are of great importance in terms of reducing oxidative stress that is thought to cause damage to biological molecules (Bektas et al, 2005). Several studies have described the antioxidant properties of medicinal plants rich in phenolic compounds (Tsao, 2004; Nijveldt et al, 2001). Natural antioxidants such as a-tocopherol and L-ascorbic acid are widely used because they are seen as being safe and causing few adverse effects, but their antioxidants effects are however, lower than those of synthetic antioxidants such as butylated hydroxytoluene (BHT) (Seung Hwa baek et al, 2004). Hence, the need exists for safe and economic antioxidants with high activity from natural sources to replace these synthetic chemicals. Medicinal plants represent a constant interest as sources of new antioxidant substances. The large majority of substances isolated from plants with antioxidant activities are flavonoids (Nijveldt et al, 2001). Some of the best described flavonoids, are qurcetin, from appleskin (Tagliaferro et al, 2002); myricetin, from red wine (Tagliaferro et al 2002); (-)epicatechin, from tea (Lopez et al, 2001); rutin, from red wine (Hara et al, 1995); luteolin, from red pepper (Hertog et al, 1992). These compounds are able to scavenge free radicals and thus inhibit oxidative mechanisms that lead to oxidative stress. Oxidative stress can lead to inactivation of cellular components and can have serious effects on the cells, probably leading to ageing as well as several diseases (Kerry et al., 2003).

The root bark of Cassine transvaalensis contains a pentogynoid, (+)-11,11-dimethyl-1,3,8,10tetrahydroxy-9-methoxypeltogynan and pentacyclic triterpenes; canaphylol, canophyllal, 6-beta-hydroxylup-20(30-en-3-one (Drewes  et al.,  1991).

MATERIALS AND METHODS 

Plant  materials and extraction

The plant was collected from Goo-Mosweu, in Tswapong North, Botswana. The botanical identification of the collected plant was done by Dr Bruce Hegreaves; (PhD plant taxonomy) and Queen Turner of the National Herbarium and Gallery- Gaborone, Botswana where voucher specimen (DMT 61) has been deposited. 

Powdered root material was extracted in water or ethanol (2.5l) using soxhlet apparatus for 48hours. The resultant crude ethanol extract was evaporated to dryness using rota vapour whilst the aqueous extract was concentrated using a freeze drier. The dried crude extracts were then stored in the fridge until ready for use.

Chemicals

DPPH (Sigma), positive controls: [Quercetin (Fluka), Rutin (Sigma), Chrysin (Aldrich), (-)epicatechin (Fluka), L-ascorbic acid]; Cassine transvaalensis water and ethanol extracts, 4'-O-methyl-epigallocatechin and ethanol (AR).

Plant material 

The plant was authenticated by comparison with herbarium sample at the Botswana National Herbarium and Gallery, Gaborone, Botswana, where voucher specimen has been deposited.

Thin layer chromatography (TLC)

The TLC plates used were analytical TLC (Merck Si-gel 60 F_254nm 0.25mm thick). A TLC plate spotted with 80-100µg of sample (1mg/ml) in methanol, was developed using a mixture of Petroleum spirit/ethylacetate/methanol/water (30:70:30:10)       (fig. 1a,b-2). After drying, the plate was sprayed with a 0.2% solution of DPPH (Sigma) in methanol and heated at 100ºC for 5 minutes. Active compounds appeared as yellow spots against a purple background. Plant extracts that showed activity were further subjected to fractionation to identify the compounds responsible for the antioxidant activity.

Scavenging activity on DPPH radical   (Quantitative method) 

The free radical scavenging activity of aqueous and ethanol extracts, were quantitatively assessed using the DPPH radical method adopted for spectrophotometry. Extracts that exhibited strong antioxidant activity were further pursued to quantify their antioxidant capacity using the DPPH method proposed by Brand-Williams, Cuvelier, and Berset (1995). Briefly, a 0.1mM solution of DPPH in ethanol was prepared and 1.0ml of this solution was added to 0.5ml of samples in different concentrations. After 20minutes, the absorbance was measured at 525nm. The DPPH radical-scavenging activity was calculated according to the following equation: 

DPPH^· scavenging activity (%) = [(A₀-A₁)/A₀] x 100

Where A₀ was the absorbance of the blank i.e no sample, DPPH solution only) and A₁ was the absorbance in the presence of the test compound.

Structural elucidation of  the antioxidant principle from Cassine transvaalensis 

¹³C-NMR, ¹H-NMR, HSQC, and MS were used to elucidate the structure of the compound responsible for the antioxidant activity. The observed chemical shifts are as shown on table ( 1-1). From the spectroscopic data and also from the comparison with published data (Monache et al., 1992) compound 1 was identified as4'-O-methyl-epigallocatechin.  

Statistical analysis

All experiments were performed at least three times with each assay in triplicate with means for each assay recorded.

RESULTS AND DISCUSSION

DPPH radical scavenging activity of the extracts (Qualiatative)

The reduction of DPPH has been used to detect extracts with antioxidant activity, including those considered as free radical scavengers (Gamez et al.,1998). 

Scavenging activity on DPPH radical  (quantitative)

Fig. 1a, 1b, 2, 3, 4, 5, 5a and 6 shows the DPPH radical-scavenging activity of various solvent extracts of Cassine transvaalensis. As positive controls, epicatechin and L-ascorbic acid were also examined for DPPH radical scavenging activity. Chrysin, rutin and quercetin were run to explore the effect of presence or absence of –OH group on C or B ring of the flavonoid nucleus free radical scavenging capacity.

The scavenging capacity of flavonoids is determined by the substitution pattern (Cos et al., 1998) on the flavonoid nucleus (Fig.5a).

4'-O-methyl-epigallocatechin was isolated from the crude water extract of Cassine transvaalensis (Fig. 1, b) using bio-assay guided fractionation. The structure of the isolated compound was confirmed by proton NMR and ¹³C-NMR spectroscopy, and mass spectroscopy (Table 1 and Fig. 4). The structure was further confirmed by comparing the data with published work by Monache et al., 1992 (Table 1). 

Table 1: DPPH radical scavenging activity by plant crude extracts

Key: (++++)- strong anti-oxidant activity
This grading of activity was based on visual observation of the intensity of bleaching of DPPH solution.

Table 1:¹³C-NMR  and ¹H-NMR 500MHz chemical shifts of the isolated compound 1 (methanol-d₄ ) and published work on 4'-O-methyl-(-)epigallocatechin (acetone-d₆ ) (Monache et al:1992)

^{a, b} Values in the same column may be interchanged

Mass spec data: EI+2.58e6
m/z (100%) is found at 139.2 (C7H7O3 requires 139.03948) is consistent with the loss of ring B.

[M]^+• m/z = 320.2 is consistent with C₁₆ H₁₆ O₇ :

(C₁₆ H₁₆ O₇ requires 320.08956)

[M-H₂ O]+ at 302.2 (C₁₆ H₁₄ O₆ requires 302.0790) consistent with loss of H₂ 0

Figure 6, shows the DPPH radical-scavenging activity of ethanolic, water extracts of Cassine transvaalensis and 4'-O-methyl-epigallocatechin isolated from Cassine transvaalensis water extract. As positive controls, (-)epicatechin and L- ascorbic acid were also examined for DPPH radical scavenging activity. In order to investigate the effect of –OH groups of different flavonoids on DPPH radical scavenging, flavonoids that differed in hydroxyl substitution on the B or C- flavane ring were tested for their DPPH free radical scavenging capacity. Substitution pattern for each tested flavonoid is as depicted in brackets and the structures are as shown in Figure 5b.

4'-O-methyl-epigallocatechin ((3,5,7,3',5'-OH,4'-OMe), quercetin, a flavonol (3,5,7,3',4'-OH), (-)epicatechin, a flavanol (3,5,7,3',4'-OH), rutin, a flavone (5,7,3',4'-OH, 3-rutinose), chrysin, a flavone (5,7-OH (lacking the 3, 3', 4'-OH)) were examined for their DPPH free radical scavenging capacity at concentrations (25-200µg/ml).

L-ascorbic acid showed the best results (~88%) through all the tested concentrations. Epicatechin, quercetin and rutin exhibited good scavenging activity (~80%). However, at 25µg/ml, rutin had a relatively lower scavenging activity (~78%) relative to either quercetin or (-)epicatechin. At all tested concentrations, chrysin, had the lowest DPPH radical scavenging activity (~26%).

The isolated compound had good scavenging activity (~70%) at 100µg/ml. As can be seen from Fig. 1a, 1b, 2, 3, 4, 5, 5a and 6, this activity was lower than that of ascorbic acid, quercetin, rutin and epicatechin. At concentrations between 25-100µg/ml, significant differences were observed between compound 1(70%) and water extract (65%). However, as the concentration of the compound in the assay system increased, the differences in scavenging activities between compound and water extract became less significant.

Between 25-50µg/ml, the isolated compound had a greater scavenging activity than the ethanolic extract of Cassine transvaalensis, but, as the concentration of the ethanolic extract in the assay system increased, the extract exhibited a greater scavenging activity than the isolated compound. At all concentrations the ethanolic extract exhibited a greater scavenging activity than the water extract. At 100µg/ml and above, the scavenging activity of the ethanolic extract was greater than that of quercetin, rutin and epicatechin, This may suggest the presence of other constituents with free radical scavenging activity in the ethanolic extract.

The scavenging activity of the water extract though lower than that of rutin, L-ascorbic acid, quercetin, epicatechin, was greator than that of chrysin at all tested concentrations.

Previous studies of structure-activity relationships have generated several consistent lines of evidence supporting the role of specific structural components as requisites for radical scavenging (Tagliaferro et al., 2002). Consistent with most polyphenolic antioxidants, both the configuration and total number of hydroxyl groups substantially influence several mechanisms of antioxidant activity (Tagliaferro et al., 2002). Antioxidant activity of flavonoids and their metabolites in vitro depends upon the arrangement of functional groups about the nuclear structure. Flavonoids with a 3-OH, and 3',4'-OH substitution are reported to be more potent than those lacking this substitution pattern (Tagliaferro et al., 2002).

Work done by Cos et al., (1998), on structure- activity relationship of flavonoids as superoxide scavengers has also shown that the presence of a hydroxyl group at C-3' in ring B and at C-3 is associated with a high superoxide scavenging activity.

Steric obstruction of 3',4'-catechol structure by 4'-O-methylation significantly compromises antioxidant activity. This explains why 4'-O-methyl-epigallocatechin has a lower scavenging activity than  (-)epicatechin, rutin or quercetin. The effect of 4'-O-methylation can also explain why 4'-O-methylation of quercetin to tamarixetin (3,5,7,3'-OH, 4'-OMe) decreases percentage inhibition of ferrous sulphate induced lipid  peroxidation from 98% to -2.6% (Dugas et al.,2000).

The relatively low scavenging activity of rutin at 25µg/ml, could be attributed to the O-rutinose substituted 3-position of the C-ring. Removal of a 3-OH severely compromises scavenging ability (Acker et al., 1996).

Free radical scavenging of flavonoids is highly dependent on the presence of a free 3-OH (Burda and Oleszek, 2001). Quercetin exhibits a better scavenging activity than luteolin, supporting the role of the 3-OH group in free radical scavenging (Hirano et al., 2001)

The findings of this experiment have shown that the radical scavenging activity of extracts or antioxidant constituents, might be mostly affected by the presence and position of the hydroxyl group.

Whilst the antioxidant activity of the water extract of Cassine transvaalensis can be attributed to 4'-O-methyl-epigallocatechin, the results of the ethanolic extract have shown possible contribution of additional components whose combined effect might confer a stronger antioxidant activity. The findings of this work support the use of Cassine transvaalensis in traditional medicine.

Acknowledgements:

The authors acknowledge the Botswana College of Agriculture for Funding of (D.M.T) PhD work.

REFERENCES

• Acker, van S.A.,  De Groot, M.J.,  van den Berg, D.J., Tromp, M.N., den Kelder, G.D., van der Vijgh, W.J.F and Bast, A (1996) A quantum chemical explanation of the antioxidant activity of flavonoid. Chemical  Research. Toxicology   9: 1305-1312.
• Aruma, O.I (1994) Nutrition and Health aspects of free radicals and antioxidant. Food and chemical toxicology. 62: 671-683.
• Bektas, T.,  Sokmen, M., Akpulat, H.A and  Sokmen, A  (2005) Food Chemistry  92: (1) 89-92.
• Brand-Williams, W., Cuvelier, M.E. and Berset, C. (1995).  Use of a free radical method to evaluate antioxidant activity. Lebensittel Wissenscaft and Technologie.  28:  25-30.
• Burda, S. and  Oleszek, W (2001) Antioxidant and antiradical activities of flavonoids. J. Agric. Food Chemisty    49: 2774-2779.
• Coates-Palgrave, K (1977): Trees of Southern Africa, p153, C. Strüik, Johannesburg.
• Cos, P., Ying, L., Calomme, M., Hu, J.P., Cimanga, K., Van Poel, B., Pieters, L., Vlietinck, A.J and Vanden Berghe, D (1998) Structure- Activity Relationship and Classification of Flavonoids as Inhibitors of Xanthine oxidase and superoxide scavengers. Journal of  Natural Products   61:  71-76.
• Crutti, P.P (1990) Oxidant stress and carcinogenesis. European Journal of clinical investigation. 21: 1-11.
• Drewers, S.E., Mashimbye, J.M.,  Field, J.S and  Ramesar, N (1991) 11,11-Dimethyl-1,3,8,10-Tetrahydroxy-9-methoxypeltogynan and three pentacyclic triterpenes from Cassine transvaalensis. Phytochemistry   30: (10) 3490-3493.
• Dugas, (Jr)A.J.  Castaneda-Acosta, J., Bonin, J.C.,  Price, K.L., Fischer, N.H and  Wisnston, G.W (2000) Evaluation of the total peroxyl radical-scavenging capacity of flavonoids; Structure-activity relationships.  Journal of  Natural  Products  63: 327-331.
• Gamez, E.J.C., Luyengi, L., Lee, S., Zhu, L., Zhou, B., Fong, H., Pezzuto, J., Kinghorn, A.D (1998) Antioxidant flavonoid glycosides from Daphniphyllum calcynum. Journal of  Natural  Products   61: 706-708.
• Halliwell, B and Gutteridge, J.M(1990). Role of free radicals and catalytic metal ions in human disease: An overview. Methods in enzymology. 186: 1-85
• Hara, Y., Luo, S.J., Wickremasinghe, R.L and Yamanishi, T (1995) Special issue on tea. Food Reviews  International    11: 371-542.
• Hertog, M.G.L., Hollman, P.C.H and Katan, M.B (1992) Content of potentially anticarcinogenic flavonoids of 28 vegetables and 9 fruits commonly consumed in the Netherlands. Journal of  Agricultural Food Chemistry   40: 2379-2383.
• Hirano, R., Sasamoto, W., Matsumoto, A., Itakura, H., Igarashi, O and Kondo, K (2001) Antioxidant ability of various flavonoids against DPPH radicals and LDL oxidation. Journal of Natural  Sciences and  Vitaminology  (Tokyo) 47: 357-362.
• Kehrer, J.P (1993) Free radicals as mediators of tissue injury and disease. CRC Critical Review Toxicology  23: 21-48.
• Kérry, A., Balázs, A., Kursinszki, P., Apáti, E., Szöke, A., Blázovics., Hagymasi, K., Nagy, E (2003) Radical scavenger and antioxidant activities of selected medicinal plants. International Society for Horticultural Science   597
• Lewis, N.G (1993) Plant phenolics. In R.G. Alscher & J.L Hess(Eds). Antioxidants in higher plants. Boca Raton, FL: CRC Press.
• Lopez, M., Martinez, F., Del Valle, C., Orte, C. and Miro, M (2001) Analysis of phenolic constituents of biological interest in red wines by high- performance liquid chromatography. Journal of  Chromatography   922: 359-363.
• Monache, F.D., Leoncio D’ Albuquerque, I., De Andrade Chiappeta, A and Jose’ Francisco De Mello. (1992). A bianthraquinone and 4'-O-methyl-ent-gallocatechin from Cassia trachypus.   Phytochemistry   3: (1) 259-261.
• Nijveldt, R.J., van Nood, E., van Hoorn, D.E., Boolens, P.G., van Norren, K., van Leeuwen, P.A (2001) Flavonoids: a review of probable mechanisms of action and potential implications  Euro. J. of cancer prevention  32: 401.
• Rao, M.V., Paliyath, G and Ormrod, D.P (1996) Ultraviolet-band ozone-induced biochemical changes in antioxidant enzymes of Arabidopsis thaliana.  Plant Physiology   110: 125-136.
• Seung Hwa Baek., Seong Soo Han., Seog Cho Lo.,  Yong Wa Choi., Jin Ho Kim. (2004) Antioxidant Activity of crude Extract and Pure Compounds of Acer  ginnala Max.  Bull. Korean  Chemical  Society   25:  (3)  389-391.
• Tagliaferro, A.R., Heim, K.E., Bobilya, D.J (2002) Flavonoid antioxidants: Chemistry, metabolism and structure-activity relationships. Journal of Nutritional Biochemistry  13: (10) 572-584.
• Tsao Rong and Deng Zeyuan (2004) Separation procedures for naturally occurring antioxidant Phytochemicals. Journal of Chromatography  B.  Vol. 812: (1-2) 85-99.

Copyright 2008 - Ibadan Biomedical Communications Group

The following images related to this document are available:

Photo images