**3. Prooxidant/antioxidant ratio (proantidex) as a better index of net free radical scavenging potential**

Antioxidants are substances that protect other chemicals of the body from damaging oxidation reactions by reacting with free radicals and other reactive oxygen species within the body, hence hindering the process of oxidation (Halliwell & Gutteridge, 1995). Plants contain active components namely phenolics and polyphenolics that are known to act as antioxidants (Cai et al., 2003).

Every antioxidant is in fact a redox agent and might become a pro-oxidant to accelerate lipopolysaccarides and induce DNA damage under special conditions and concentrations. Studies have revealed pro-oxidant effects of antioxidant vitamins and several classes of plant-derived polyphenols such as flavonoids (Rahman et al., 1990) and tannins (Singh et al., 2001). As reported earlier, resveratol (Lastra & Villegas, 2007), phloroglucinols from *Garcinia subelliptica* (Wu et al., 2008) and curcumin (Ahsan & Hadi, 1998) can exhibit pro-oxidant properties, leading to oxidative breakage of cellular DNA in the presence of transition metal ions such as copper. Therefore, it is essential to discover natural compounds that good antioxidant activity but low pro-oxidant capabilities.

Pro-oxidant and antioxidant effect of plant extracts are due to the balance of two activities, free radical-scavenging activity and reducing power on iron ions, which may drive the Fenton reaction via reduction of iron ions. In a Fenton reaction, Fe2+ reacts with H2O2, resulting in the production of hydroxyl radical, which is considered to be the most harmful radical to biomolecules. Fe2+ is oxidized to Fe3+ in the Fenton reaction initially. By the action of many reductants, such as ascorbic acid, the oxidized forms of iron ion can be reduced to reduced forms (Fe2+) later, which can enhance the generation of hydroxyl radicals. A predominant reducing power (on iron ions) over the free radical-scavenging activity in a mixture of compounds results in the pro-oxidant effect (Tian & Hua, 2005). In this study, the pro-oxidant capacity of the extracts were compared to the IC50 (mg/mL) of the antioxidant scavenging activity of DPPH radical. This ratio of pro-oxidant/antioxidant activity enabled us to evaluate the net antioxidant capacity of the extracts as this index will include not only the effective free radical-scavenging ability, taking into account pro-oxidant effect of the extracts as shown in equation (1).

$$\text{ProAntiidex} = \frac{\text{Proxidant capacity at the absorbance set at arbitrary 1.0 (mg/mL)} }{\text{IC}\_{50} (\text{mg/mL) from DPPH saving away}} \quad \text{(1)}$$

conclude that although a broaden use of these plants are in aqueous form, its commercial preparation can be achieved using ethanol since a high total phenolic content and antioxidant activity was seen in this preparation. It is desirable that these extracts be further purified to gain a better understanding of the active compounds contributing to its

Similar results were observed in the lipid peroxidation inhibition studies (Palanisamy et al., 2008). There was a strong correlation between antioxidant activity and the total phenolic content of the extracts. The high antioxidant extracts had below the permissible value of heavy metal content for nutraceutical application. Most of the extracts were also not cytotoxic to 3T3 and 4T1 cells at concentration as high as 100μg/mL (Ling et al., 2010a).

Antioxidants are substances that protect other chemicals of the body from damaging oxidation reactions by reacting with free radicals and other reactive oxygen species within the body, hence hindering the process of oxidation (Halliwell & Gutteridge, 1995). Plants contain active components namely phenolics and polyphenolics that are known to act as

Every antioxidant is in fact a redox agent and might become a pro-oxidant to accelerate lipopolysaccarides and induce DNA damage under special conditions and concentrations. Studies have revealed pro-oxidant effects of antioxidant vitamins and several classes of plant-derived polyphenols such as flavonoids (Rahman et al., 1990) and tannins (Singh et al., 2001). As reported earlier, resveratol (Lastra & Villegas, 2007), phloroglucinols from *Garcinia subelliptica* (Wu et al., 2008) and curcumin (Ahsan & Hadi, 1998) can exhibit pro-oxidant properties, leading to oxidative breakage of cellular DNA in the presence of transition metal ions such as copper. Therefore, it is essential to discover natural compounds that good

Pro-oxidant and antioxidant effect of plant extracts are due to the balance of two activities, free radical-scavenging activity and reducing power on iron ions, which may drive the Fenton reaction via reduction of iron ions. In a Fenton reaction, Fe2+ reacts with H2O2, resulting in the production of hydroxyl radical, which is considered to be the most harmful radical to biomolecules. Fe2+ is oxidized to Fe3+ in the Fenton reaction initially. By the action of many reductants, such as ascorbic acid, the oxidized forms of iron ion can be reduced to reduced forms (Fe2+) later, which can enhance the generation of hydroxyl radicals. A predominant reducing power (on iron ions) over the free radical-scavenging activity in a mixture of compounds results in the pro-oxidant effect (Tian & Hua, 2005). In this study, the pro-oxidant capacity of the extracts were compared to the IC50 (mg/mL) of the antioxidant scavenging activity of DPPH radical. This ratio of pro-oxidant/antioxidant activity enabled us to evaluate the net antioxidant capacity of the extracts as this index will include not only the effective free radical-scavenging ability, taking into account pro-oxidant effect of the

<sup>50</sup>

Prooxidant capacity at the absorbance set at arbitrary 1.0 mg /mL ProAntidex=

IC mg /mL from DPPH scavenging assay (1)

**3. Prooxidant/antioxidant ratio (proantidex) as a better index of net free** 

antioxidant activity.

**radical scavenging potential** 

antioxidants (Cai et al., 2003).

extracts as shown in equation (1).

antioxidant activity but low pro-oxidant capabilities.


Table 1. DPPH scavenging activity, Pro-oxidant activity and ProAntidex in ethanolic extracts of selected Malaysian plants and standard. ProAntidex was devised using the ratio of prooxidant activities to the IC50 DPPH scavenging activity. All values represent means ± SD, *n*=3.


Table 2. DPPH scavenging activity, pro-oxidant and ProAntidex in aqueous extracts of selected Malaysian plants and standards. ProAntidex was devised using the ratio of prooxidant activities to the IC50 DPPH scavenging activity. All values represent means ± SD, *n*=3. \*\*Designates a significance difference from EmblicaTM, *p*<0.01.

Review: Potential Antioxidants from Tropical Plants 69

*Mangifera indica,* commonly known as the mango plant has been the focus of many researchers for the next source of potent anti-oxidants. Previously, a standardised aqueous extract from the bark of *Mangifera indica* was reported to contain anti-inflammatory activity immunomodulatory and antioxidant activities (Garrido et al., 2004). The extract, is composed of a variety of phenolic acids, phenolic esters, flavanols and the xanthone mangiferin (Janet et al., 2006). When fed orally to mice that have been induced to have ear oedema by arachidonic acid and phorbol myristate acetate injection, the ear edema was observed to reduce markedly. *In vitro* studies showed that the extract also inhibited the induction of prostaglandin E (PGE)

Fig. 3. HPLC chromatogram of *M. indica* extracts and standard, mangiferin. The mobile phase consisted of solvent A: 3% acetic acid in water and solvent B : acetonitrile; starting from 90%B for 5 minutes, 80%B for 15minutes and finally 100% B for 10minutes for washing and recondition the column (a) Mangiferin (b) ethanolic *M indica* extract and (c) aqueous *M* 

In our laboratory, the standardised ethanolic and aqueous extracts of Mangifera indica leaf was analyzed for its free radical scavenging activity using a variety of other assays. Its IC50 values using the DPPH assay was 0.17mg/mL 0.02 and 0.49mg/mL 0.4 respectively. Mangiferin, the main active compound in M.indica plant has been established to contribute to its biological activities (Ling et al., 2009). Standardised ethanolic extracts of the Mangifera indica leaf was found to have a mangiferin concentration of 71mg/g extract, free radical scavenging activity (IC50) of 0.17mg/mL 0.02 and total phenolic content of 590mg/g 48.08 of extract. The protection seen by Mangifera indica extracts against lipid peroxidation was observed to be far better than butlylated hydroxytoluene (BHT; a commercial anti oxidant used to prevent rancidity of oils) and commercial grape seed extract. The Mangifera indica extracts at higher concentrations did not exhibit pro-oxidant activities when compared to Vitamin C is yet another interesting feature of this extract. We also found that

*indica* extract.

**4. Standardised** *Mangifera indica* **leaf extract as an ideal antioxidant** 

and leukotriene-B4 (LTB4) release by macrophages (Garrido et al., 2004).

The plant extracts having a high antioxidant activity were simultaneously analyzed for its pro-oxidant capability. Interestingly, in our laboratory, we established a Prooxidant/Antioxidant ratio (ProAntidex) which represents an index of the net free radical scavenging ability of whole plant extracts. The ethanolic extracts, *Nephelium lappaceum* peel, *Fragaria x ananassa* leaf, *Lawsonia inermis* leaf, *Syzygium aqueum* leaf and grape seed had lower Pro-Antidex than the commercially available Emblica™ extract which is a commercially available extract from *Phyllantus emblica* claims to have high antioxidant but low pro-oxidant activity (Table 1) (Ling et al., 2010b).

Among the aqueous extracts on the other hand; *Lawsonia inermis* leaf, *Nephelium mutobile* leaf and grape seed had low pro-oxidant activity (Table 2). In this study, Emblica™, green tea, vitamin C and grape seed were used as the positive controls in comparison to other plant extracts as shown in Figure 2. Among these extracts, the aqueous extract of *Nephelium mutobile* leaf had a very low ProAntidex of 0.05 compared to 0.69 for Emblica™. Most of the extracts had a far lower ProAntidex value compared to vitamin C. This index enables us to identify extracts with high net free radical scavenging activity potential. The ProAntidex is therefore beneficial as a screening parameter that can be used in food and healthcare industries.

Fig. 2. Prooxidant activity of the standards used in the study. The prooxidant assay was carried out by measuring reducing power on Fe3+ in the Fenton reaction. Grape seed, green tea, Emblica TM and vitamin C were used as positive controls. All the values represent means ± SD, *n*=3.

Pro-Antidex is a useful indicator in free radical research. The ratio of pro-oxidant to the antioxidant activity capacity gives a better picture of the real antioxidant capacity of the plant extracts. The pro-oxidant assay will enable the nutritionists and chemists to formulate antioxidant mixtures that balance between the two activities, which is higher antioxidant activity with lower pro-oxidant capacity. In other words, the net ProAntidex should be low to reflect that the particular plant extract has good overriding antioxidant property.

The plant extracts having a high antioxidant activity were simultaneously analyzed for its pro-oxidant capability. Interestingly, in our laboratory, we established a Prooxidant/Antioxidant ratio (ProAntidex) which represents an index of the net free radical scavenging ability of whole plant extracts. The ethanolic extracts, *Nephelium lappaceum* peel, *Fragaria x ananassa* leaf, *Lawsonia inermis* leaf, *Syzygium aqueum* leaf and grape seed had lower Pro-Antidex than the commercially available Emblica™ extract which is a commercially available extract from *Phyllantus emblica* claims to have high antioxidant but

Among the aqueous extracts on the other hand; *Lawsonia inermis* leaf, *Nephelium mutobile* leaf and grape seed had low pro-oxidant activity (Table 2). In this study, Emblica™, green tea, vitamin C and grape seed were used as the positive controls in comparison to other plant extracts as shown in Figure 2. Among these extracts, the aqueous extract of *Nephelium mutobile* leaf had a very low ProAntidex of 0.05 compared to 0.69 for Emblica™. Most of the extracts had a far lower ProAntidex value compared to vitamin C. This index enables us to identify extracts with high net free radical scavenging activity potential. The ProAntidex is therefore

beneficial as a screening parameter that can be used in food and healthcare industries.

Fig. 2. Prooxidant activity of the standards used in the study. The prooxidant assay was carried out by measuring reducing power on Fe3+ in the Fenton reaction. Grape seed, green tea, Emblica TM and vitamin C were used as positive controls. All the values represent means

Pro-Antidex is a useful indicator in free radical research. The ratio of pro-oxidant to the antioxidant activity capacity gives a better picture of the real antioxidant capacity of the plant extracts. The pro-oxidant assay will enable the nutritionists and chemists to formulate antioxidant mixtures that balance between the two activities, which is higher antioxidant activity with lower pro-oxidant capacity. In other words, the net ProAntidex should be low

to reflect that the particular plant extract has good overriding antioxidant property.

low pro-oxidant activity (Table 1) (Ling et al., 2010b).

± SD, *n*=3.

#### **4. Standardised** *Mangifera indica* **leaf extract as an ideal antioxidant**

*Mangifera indica,* commonly known as the mango plant has been the focus of many researchers for the next source of potent anti-oxidants. Previously, a standardised aqueous extract from the bark of *Mangifera indica* was reported to contain anti-inflammatory activity immunomodulatory and antioxidant activities (Garrido et al., 2004). The extract, is composed of a variety of phenolic acids, phenolic esters, flavanols and the xanthone mangiferin (Janet et al., 2006). When fed orally to mice that have been induced to have ear oedema by arachidonic acid and phorbol myristate acetate injection, the ear edema was observed to reduce markedly. *In vitro* studies showed that the extract also inhibited the induction of prostaglandin E (PGE) and leukotriene-B4 (LTB4) release by macrophages (Garrido et al., 2004).

Fig. 3. HPLC chromatogram of *M. indica* extracts and standard, mangiferin. The mobile phase consisted of solvent A: 3% acetic acid in water and solvent B : acetonitrile; starting from 90%B for 5 minutes, 80%B for 15minutes and finally 100% B for 10minutes for washing and recondition the column (a) Mangiferin (b) ethanolic *M indica* extract and (c) aqueous *M indica* extract.

In our laboratory, the standardised ethanolic and aqueous extracts of Mangifera indica leaf was analyzed for its free radical scavenging activity using a variety of other assays. Its IC50 values using the DPPH assay was 0.17mg/mL 0.02 and 0.49mg/mL 0.4 respectively. Mangiferin, the main active compound in M.indica plant has been established to contribute to its biological activities (Ling et al., 2009). Standardised ethanolic extracts of the Mangifera indica leaf was found to have a mangiferin concentration of 71mg/g extract, free radical scavenging activity (IC50) of 0.17mg/mL 0.02 and total phenolic content of 590mg/g 48.08 of extract. The protection seen by Mangifera indica extracts against lipid peroxidation was observed to be far better than butlylated hydroxytoluene (BHT; a commercial anti oxidant used to prevent rancidity of oils) and commercial grape seed extract. The Mangifera indica extracts at higher concentrations did not exhibit pro-oxidant activities when compared to Vitamin C is yet another interesting feature of this extract. We also found that

Review: Potential Antioxidants from Tropical Plants 71

HO

O

O HO HO

OH

O

HO

OH

sample

O OH

OH

OH

HO OH HO OH

O O <sup>O</sup> <sup>O</sup> O

OH

H O O O O

**10 20 30 40**

**Minutes** 

Fig. 4. Purification on Prep-HPLC showing geraniin as the major compound in the ethanolic *Nephelium lappaceum* rind extract. The solvent gradient consisted of 0-10% acetonitrile for 3 minutes, 10-40% for 12 minutes and finally 100% acetonitrile for 5 minutes to recondition the

13minutes. 1 was detected at 210nm and 2 at 275nm. Insert shows the chemical structure of

Sample/Fraction Extraction Method Yield (%) Geraniin (%) in

The rind of *N.lappaceum* extract was standardized to 13% of geraniin, the active hydrolysable ellagitannins responsible for over 50% of the antioxidant potential of the ethanolic extract of *Nephelium lappaceum* rind. In a single dose acute toxicity studies, oral LD50 of the rind extract in ICR mice was found to be greater than 5 g/kg body weight. In a subsequent study, Sprague Dawley rats were given via oral gavage 0 (control), 1000 mg/kg body weight/day of the extract for 28 days to evaluate the subacute toxicity of the extract to animals. Animals in a satellite group scheduled for follow-up observations were kept for 14 days without treatment to detect for any delayed effects. At the end of the experiment, kidney, liver, brain and testis were collected and followed by histopathological studies. No behavioural or organs to body weight changes were found in all the groups. Furthermore, no obvious

abnormal changes were observed histologically in all the groups (unpublished data).

column. at a flow rate of 18mL/min. Geraniin was obtained at the retention time of

*N.lappaceum* rind Ethanol extraction 30.58 3.79 Ethanolic extract LiChroprep RP-18 60.00 12.68 F1 Preparative HPLC 21.15 21.13

Table 3. Quantification of geraniin in the rapid purification method

**1**

**Geraniin**

**1500**

**2000**

**mVolts**

**1000**

**1200**

**500**

**250**

**0**

geraniin (Palanisamy, Uma D. et al., 2011).

**750**

**2**

the aqueous and ethanolic extracts of Mangifera indica leaf protects the mouse fibroblasts cells, NIH/3T3, from oxidant-induced cell death by about 84% and it is also non-toxic to cultured splenocytes .

### **5. Rind of rambutan,** *Nephelium lappaceum***, a potential source of natural antioxidant**

*Nephelium lappaceum* L. belongs to the same family (Sapindaceae) as the sub-tropical fruits lychee and longan and it is native to Southeast Asia. This fruit is an important commercial crop in Asia, where it is taken freshly or processed. In Southeast Asia, the dried fruit rind has been employed in traditional medicine for centuries. Additionally, the rind is used in cooking and the manufacture of soap. The roots, bark, and leaves have various uses in medicine and in the production of dyes. Previous studies have shown *N.lappaceum* rind extract to exhibit high antioxidant activity (Palanisamy et al., 2008), antibacterial activity (Thitilertdecha et al., 2010) and anti-Herpes Simplex virus type 1 (Nawawi A, 1999). Recently in our laboratory, *N.lappaceum* rind was also shown to have anti hyperglycemic potential (Palanisamy, Uma et al., 2011). The utilisation of *Nephelium lappaceum* rind to manage hyperglycemia is seen as an important finding not only in traditional medicine but also in aspects of valorisation of food waste.

The rind of *Nephelium lappaceum* (rambutan) was selected as the rind contains extremely high antioxidant activity when assessed using several free radical scavenging methods. Although having a yield of only 18%, the ethanolic rambutan rind extract has a total phenolic content of 762 ± 10 mg GAE/g extract, which is comparable to the commercial grape seed extract. The rambutan rind had lower pro-oxidant activities compared to vitamin C, α-tocopherol, grape seed and green tea in a dose response experiment. In addition, the rind extract at 100 µg/ml reduced oxidant-induced cell death (DPPH at 50 µM) by apoptosis to an extent similar to that of grape seed extract. The rind extracts were not cytotoxic to normal mouse fibroblast cells or splenocytes. Powderised rind had low heavy metal content far below the permissible levels for nutraceuticals. This study is the first to show a unique combination of high phenolic content, low pro-oxidant capacity and strong antioxidant activity of the rind extract of *Nephelium lappaceum*.

Whole extracts of the rind of *N.lappaceum* was standardized using a reverse phase column on analytical HPLC. Bioassay-guided fractionation of the extract was attempted to establish the most effective method to extract fractions with high antioxidant activity. Two fractionated extracts of the rind having DPPH activity of 0.01±0.001 and 0.01±0.003mg/ml and total phenolic content of 6662±240 and 1761±239 mg/g GAE respectively were established using preparative HPLC.

Bioassay guided fractionation was found to be time and labour consuming; therefore we investigated a rapid purification method to isolate and purify the bioactive compound from *N.lappaceum* rind extract. It was pertinent that we isolate and identify the active compound(s) in this extract that contribute(s) to the said biological activities. Structural characterization of purified compounds can lead to the formulation of the new therapeutic products. Geraniin was found to be the major phenolic compound in the *N.lappaceum* rind extract. A composition of 13% geraniin contributed to the high free radical scavenging activity in the extract. The compound exhibited radical scavenging activity of IC50; 3.8 μg/mL (DPPH radical test), 1.7 μg/mL(ABTS radical test) and 1.7 μg/mL (Galvinoxyl radical test). The compound also displayed very low pro-oxidant capabilities.

the aqueous and ethanolic extracts of Mangifera indica leaf protects the mouse fibroblasts cells, NIH/3T3, from oxidant-induced cell death by about 84% and it is also non-toxic to

*Nephelium lappaceum* L. belongs to the same family (Sapindaceae) as the sub-tropical fruits lychee and longan and it is native to Southeast Asia. This fruit is an important commercial crop in Asia, where it is taken freshly or processed. In Southeast Asia, the dried fruit rind has been employed in traditional medicine for centuries. Additionally, the rind is used in cooking and the manufacture of soap. The roots, bark, and leaves have various uses in medicine and in the production of dyes. Previous studies have shown *N.lappaceum* rind extract to exhibit high antioxidant activity (Palanisamy et al., 2008), antibacterial activity (Thitilertdecha et al., 2010) and anti-Herpes Simplex virus type 1 (Nawawi A, 1999). Recently in our laboratory, *N.lappaceum* rind was also shown to have anti hyperglycemic potential (Palanisamy, Uma et al., 2011). The utilisation of *Nephelium lappaceum* rind to manage hyperglycemia is seen as an important finding not only in traditional medicine but

The rind of *Nephelium lappaceum* (rambutan) was selected as the rind contains extremely high antioxidant activity when assessed using several free radical scavenging methods. Although having a yield of only 18%, the ethanolic rambutan rind extract has a total phenolic content of 762 ± 10 mg GAE/g extract, which is comparable to the commercial grape seed extract. The rambutan rind had lower pro-oxidant activities compared to vitamin C, α-tocopherol, grape seed and green tea in a dose response experiment. In addition, the rind extract at 100 µg/ml reduced oxidant-induced cell death (DPPH at 50 µM) by apoptosis to an extent similar to that of grape seed extract. The rind extracts were not cytotoxic to normal mouse fibroblast cells or splenocytes. Powderised rind had low heavy metal content far below the permissible levels for nutraceuticals. This study is the first to show a unique combination of high phenolic content, low pro-oxidant capacity and strong antioxidant

Whole extracts of the rind of *N.lappaceum* was standardized using a reverse phase column on analytical HPLC. Bioassay-guided fractionation of the extract was attempted to establish the most effective method to extract fractions with high antioxidant activity. Two fractionated extracts of the rind having DPPH activity of 0.01±0.001 and 0.01±0.003mg/ml and total phenolic content of 6662±240 and 1761±239 mg/g GAE respectively were

Bioassay guided fractionation was found to be time and labour consuming; therefore we investigated a rapid purification method to isolate and purify the bioactive compound from *N.lappaceum* rind extract. It was pertinent that we isolate and identify the active compound(s) in this extract that contribute(s) to the said biological activities. Structural characterization of purified compounds can lead to the formulation of the new therapeutic products. Geraniin was found to be the major phenolic compound in the *N.lappaceum* rind extract. A composition of 13% geraniin contributed to the high free radical scavenging activity in the extract. The compound exhibited radical scavenging activity of IC50; 3.8 μg/mL (DPPH radical test), 1.7 μg/mL(ABTS radical test) and 1.7 μg/mL (Galvinoxyl

radical test). The compound also displayed very low pro-oxidant capabilities.

**5. Rind of rambutan,** *Nephelium lappaceum***, a potential source of natural** 

cultured splenocytes .

also in aspects of valorisation of food waste.

activity of the rind extract of *Nephelium lappaceum*.

established using preparative HPLC.

**antioxidant** 

Fig. 4. Purification on Prep-HPLC showing geraniin as the major compound in the ethanolic *Nephelium lappaceum* rind extract. The solvent gradient consisted of 0-10% acetonitrile for 3 minutes, 10-40% for 12 minutes and finally 100% acetonitrile for 5 minutes to recondition the column. at a flow rate of 18mL/min. Geraniin was obtained at the retention time of 13minutes. 1 was detected at 210nm and 2 at 275nm. Insert shows the chemical structure of geraniin (Palanisamy, Uma D. et al., 2011).


Table 3. Quantification of geraniin in the rapid purification method

The rind of *N.lappaceum* extract was standardized to 13% of geraniin, the active hydrolysable ellagitannins responsible for over 50% of the antioxidant potential of the ethanolic extract of *Nephelium lappaceum* rind. In a single dose acute toxicity studies, oral LD50 of the rind extract in ICR mice was found to be greater than 5 g/kg body weight. In a subsequent study, Sprague Dawley rats were given via oral gavage 0 (control), 1000 mg/kg body weight/day of the extract for 28 days to evaluate the subacute toxicity of the extract to animals. Animals in a satellite group scheduled for follow-up observations were kept for 14 days without treatment to detect for any delayed effects. At the end of the experiment, kidney, liver, brain and testis were collected and followed by histopathological studies. No behavioural or organs to body weight changes were found in all the groups. Furthermore, no obvious abnormal changes were observed histologically in all the groups (unpublished data).

Review: Potential Antioxidants from Tropical Plants 73

Moongkarndi, P.; Kosem, N.; Kaslungka, S.; Luanratana, O.; Pongpan, N.. & Neungton, N.

Rockenbach, I. I.; Gonzaga, L. V.; Rizelio, V. M.; Goncalves, A. E. d. S. S.; Genovese, M. I.. &

Chan, E.; Lim, Y.. & Wong, S. (2011). Antioxidant Properties of Ginger Leaves: An

Silva, S.; Gomes, L.; Leitao, F.; Coelho, A. V.. & Boas, L. V. (2006). Phenolic Compounds and

Agati, G.; Stefano, G.; Biricolti, S.. & Tattini, M. (2009). Mesophyll Distribution of

Tadhani, M. B.; Patel, V. H.. & Subhash, R. (2007). *In Vitro* Antioxidant Activities of *Stevia* 

Zhang, S.-J.; Lin, Y.-M.; Zhou, H.-C.; Wei, S.-D.; Lin, G.-H.. & Ye, G.-F. (2010). Antioxidant

Chang, S. T.; Wu, J. H.; Wang, S. Y.; Kang, P. L.; Yang, N. S.. & Shyur, L. F. (2001).

Diouf, P.; Stevanovic, T.. & Cloutier, A. (2009). Antioxidant Properties and Polyphenol

Dalton, D. A.; Joyner, S. L.; Becana, M.; Iturbe-Ormaetxe, I.. & Chatfield, J. M. (1998).

Hegde, K.. & Joshi, A. B. (2010). Hepatoprotective and Antioxidant Effect of *Carissa Spinarum*

Boruvka, L.; Kozák, J.. & Krištoufková, S. (1997). Heavy Metal Accumulation in Plants Grown in Heavily Polluted Soils. *Folia Microbiologica,* Vol. 42, No. 5, pp.524-526. Ang, H.; Lee, E.. & Matsumoto, K. (2003). Analysis of Lead Content in Herbal Preparations in Malaysia. *Human & Experimental Toxicology,* Vol. 22, No. 8, pp.445-51. Järup, L.; Berglund, M.; Elinder, C.; Nordberg, G.. & Vahter, M. (1998). Health Effects of

Yen, G.-C.; Chen, H.-Y.. & Peng, H.-H. (1997). Antioxidant and Pro-Oxidant Effects of Various Tea Extracts. *Journal of Agricultural and Food Chemistry,* Vol. 45, No. 1, pp.30-34. Halliwell, B. (1996). Commentary: Vitamin C: Antioxidant or Pro-Oxidant *in Vivo*? *Free* 

Ling, L. T.; Radhakrishnan, A. K.; Subramaniam, T.; Cheng, H. M.. & Palanisamy, U. D.

Winemaking. *Food Research International,* Vol. 44, No. 4, pp.897-901

Overview. *Free Radicals and Antioxidants,* Vol. 1, No. 1, pp.6-16.

Sunlight Irradiance. *Annals of Botany,* Vol. 104, No. 5, pp.853-861.

*of Agricultural and Food Chemistry,* Vol. 49, No. 7, pp.3420-3424.

*Bangladesh Journal of Pharmacology* Vol. 5, No. 1, pp.73-76.

*Journal of Work, Environment & Health,* Vol. 24, No. 1, pp.1-51.

Malaysian Plants. *Molecules,* Vol. 15, No. 4, pp.2139-2151.

*Ethnopharmacology,* Vol. 90, No. 1, pp.161-166.

*Technology International,* Vol. 12, No. 5, pp.385-395.

3-4, pp.323-329.

No. 8, pp.5658-5670.

No. 5, pp.457-470.

*Physiology,* Vol. 116, No. 1, pp.37-43.

*Radical Research,* Vol. 25, No. 5, pp.439-454.

(2004). Antiproliferation, Antioxidation and Induction of Apoptosis by *Garcinia Mangostana* (Mangosteen) on Skbr3 Human Breast Cancer Cell Line. *Journal of* 

Fett, R. (2011). Phenolic Compounds and Antioxidant Activity of Seed and Skin Extracts of Red Grape (*Vitis Vinifera* and *Vitis Labrusca*) Pomace from Brazilian

Antioxidant Activity of *Olea Europaea* L. Fruits and Leaves. *Food Science and* 

'Antioxidant' Flavonoid Glycosides in *Ligustrum Vulgare* Leaves under Contrasting

*Rebaudiana* Leaves and Callus. *Journal of Food Composition and Analysis,* Vol. 20, No.

Tannins from Stem Bark and Fine Root of *Casuarina Equisetifolia*. *Molecules,* Vol. 15,

Antioxidant Activity of Extracts from *Acacia Confusa* Bark and Heartwood. *Journal* 

Contents of Trembling Aspen Bark Extracts. *Wood Science and Technology,* Vol. 43,

Antioxidant Defenses in the Peripheral Cell Layers of Legume Root Nodules. *Plant* 

Root Extract against Ccl4 and Paracetamol-Induced Hepatic Damage in Rats.

Cadmium Exposure--a Review of the Literature and a Risk Estimate. *Scandinavian* 

(2010a). Assessment of Antioxidant Capacity and Cytotoxicity of Selected
