|
|
|
|
Research Article
|
|
Phenolic Content, Antioxidant Capacity and Toxicity of 3 Varieties of Living Stone Potato (Rizga)
|
|
C.O. Eleazu,
K.C. Eleazu
and
A.I. Ikpeama
|
|
|
ABSTRACT
|
The phenolic content, antioxidant activity and toxicity of three varieties of extracts of rizga flour on white albino male rats was evaluated. The antioxidant activities of the extracts as determined by the quantities of phenols present and scavenging activities on 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical indicated that all three varieties of rizga flour contained significant quantities of antioxidants and phenols. However, the total antioxidant capacity and polyphenolic contents of langaat was lower than that of beebot and riyom (p<0.05) with an IC50 of 90.16 μg mL-1 for langaat, 30.28 μg mL-1 for beebot and 60.27 μg mL-1 for riyom as compared with the standard (Quercetin) that had an IC50 of 23.17 μg mL-1. The phytochemical screening of the flours was determined using standard qualitative methods indicated the presence of cyanogenic glucosides, tannins and flavonoids in all the three varieties studied but beebot and riyom contained alkaloids in addition. The extracts of the 3 varieties of rizga had a low toxic effect on the rats studied in addition with an LD50 of 630.1 mg kg-1 for Riym, 398.1 mg kg-1 for Langaat and 446.7 mg kg-1 for Beebot. Correlation between the total antioxidant capacity and polyphenolic composition of the 3 varieties of rizga was found to be significant (R2 = 0.917). These findings suggest rizga to be a natural source of antioxidants and thus could be used in the treatment of ailments implicating free radicals. In addition, it is safe for consumption.
|
|
|
|
|
Received: December 20, 2011;
Accepted: January 28, 2012;
Published: June 11, 2012
|
|
INTRODUCTION
Living stone potato (Plectranthus esculentus N.E.Br) which is known
locally as rizga is one of the widely cultivated minor root crops in the middle
belt regions especially Kaduna and Plateau States of Nigeria for the finger-like
edible tubers (Schippers, 2000; Olojede
et al., 2004). It is grown in Nigeria as it is rich in carbohydrates
like most other tuber crops. In terms of protein content, when compared with
yam, cassava, sweet potato and cocoyam, living stone potato ranks highest relative
to major food crops grown in Nigeria. Despite its nutritive potential, it is
classified among the lesser known and under exploited species of food crops
in Africa (Schippers, 2000).
The medicinal value of plants have assumed a more important dimension in the
past decades owing largely to the discovery that their extracts contain not
only minerals but also a diverse array of secondary metabolites with antioxidant
potentials (Akinmoladun et al., 2007). These
antioxidants have been implicated in the therapeutic effects of several plants
and vegetables that are used in traditional medicine (Ames
et al., 1993; Kumar et al., 2005).
As, these compounds are predominantly found in fruit tissues, it would be worthwhile
investigating the nature of phenols that are present in them. In addition, phytochemicals
also act as potent antioxidants in both fat soluble and water soluble body fluids
and cellular components (Marthur and Marthur, 2001)
and also possess biological characteristics like anti-carcinogenicity, anti-mutagencity,
anti-aging and anti-cholesterol activity.
Since, free radicals have been associated with some of these disorders and
being that the phytochemicals present in plants are known to possess either
anti-oxidative or free radical scavenging activity, the antioxidant and phytochemical
composition of this plant ought to be investigated. In addition, the biosafety
of its consumption need to be ascertained. This leads to the basis of this research
which is designed to understudy the total antioxidant capacity, phenolic composition
and toxicity of three varieties of living stone potato (rizga) to white albino
male rats.
MATERIALS AND METHODS Chemicals: Quercetin (3,3,4,5,7-pentahydroxyflavone), Folin-ciocalteau reagent, DPPH and Chlorogenic acid used were products of Sigma Chemical Company (UK). Other chemicals used were purchased from John-J-Scientific Company, Jos, Nigeria and were of analytical grade. Plant materials: Living stone potato used was freshly harvested from the experimental farm of National Root Crops Research Institute, Potato Programme, Jos. It was thoroughly washed, peeled and dried in an oven for 24 h at a temperature of 50°C. Preparation of plant materials for analysis: The peeled portion of the Living stone potato (rizga) was ground into flour using a food processor and the flour was then used for analysis.
Phytochemical screening of rizga flour: The phytochemical screening
of the plant flour using organic solvent and hot water extracts was carried
out using standard qualitative procedures (Trease and Evans,
1984; Sofowara, 1984).
Assay of DPPH radical scavenging activity: The free radical scavenging
activity of the extracts of the rizga varieties was determined using the modified
method of Blois (1985). One milliliter of different
concentrations (500, 250, 125, 62.5 and 31.25 μg mL-1) of the
methanolic extracts and standard quercetin was added to 1 mL of 0.3 mM DPPH
in a test tube methanol to bring the final concentration to 250, 125, 62.5,
31.25 and 15.62 μg mL-1. The mixture was vortexed and incubated
in a dark chamber for 30 min and the absorbance was read at 517 nm using a uv
spectrophotometer against a DPPH control which contained 1 mL of methanol. The
percentage inhibition was calculated using the equation:
IC50 values denote the concentration of the samples which is required to scavenge 50% of DPPH radical. Higher IC50 values denote lower antioxidant activities.
Total phenol assay: Ten grams each of the fresh samples was soaked in
100mls of ethanol overnight. The mixture was filtered and centrifuged at 3000
x g for 10mins to obtain a clear supernatant. The total phenol assay was measured
using the modified Folin-Ciocalteau method (Singleton et
al., 1999). The hydrophilic extract (0.5 mL) was diluted with distilled
water to 5 and 0.5 mL of Folin-Ciocalteau reagent was added and allowed to react
at room temperature for 3 min. One milliliter of 1N sodium carbonate was added
and allowed to react at room temperature for 1 h. The absorbance was measured
at 725 nm using a UV spectrophotometer with distilled water as blank. Chlorogenic
acid and gallic acid were used as standards. Total phenol content was reported
as mg of chlorogenic acid equivalents per gram fresh weigh sample (mg CAE g-1
fw) which could be converted to mg of gallic acid by multiplying by a factor
of 0.445.
Toxicity assay
Selection of animals: Fourty two matured male albino rats weighing
between 119.86±48.2 g were used for the toxicity tests. Animals were
acclimatized for a period of 7 days to the laboratory conditions prior to the
experiment. Rats were housed in colony cages with 2 rats per cage at room temperature
with 12 h light and dark cycle and they had access to drinking water and their
food.
Experimental procedure: The rats were divided into 3 groups with 14
animals in each group. The animals received varying doses of extracts of langaat,
beebot and riyom flour corresponding to 330, 300, 280, 250, 220 and 120 mg kg-1
body weight. The dosage that killed 50% of the experimental animals after 24
h of oral administration of the extracts of the different varieties of rizga
flour was recorded and the LD50 of each of the rizga varieties computed
using the methods of Karber (1931) and Litchfield
and Wilcoxon (1949) and results were expressed in mg kg-1 body
weight. The percentage dead for 0 and 100% mortality were corrected before the
determination of probits as follows: For 0% mortality, corrected = 100 (0.25/n)
and for 100% mortality, corrected = 100 (n-0.25/n). The probit values were plotted
against log-doses and the dose corresponding to probit 5 was noted and its antilog
taken to be the LD50.
Statistical analysis: Statistical analysis was conducted using the Mean±SD of 3 experiments. Results were considered significant at p<0.05. Correlation analysis was carried out using Pearsons model and results were considered significant at p<0.05. RESULTS AND DISCUSSION Phytochemical screening of the rizga flours: The phytochemical screening of the 3 varieties of rizga showed that they contained some quantities of flavonoids, tannins and cyanogenic glucosides but beebot and riyom contained alkaloids in addition (Table 1).
Flavonoids, alkaloids and tannins are polyphenolic compounds with antioxidant
properties. Phenolics have been associated with antioxidant properties of food
(Robbins, 2003). Kirakosyan et
al. (2003) reported that phenolic compounds in plants possess antioxidant
activity and may help protect cells against the oxidative damage caused by free
radicals.
Cyanogenic glucosides are the compounds in cassava that make them toxic. This
is due to the production of cyanide which activates the enzyme cytochrome oxidase
in the mitochondria of cells by binding to the Fe2+/Fe3+
contained in the enzyme. This leads to a decrease in the utilization of oxygen
in the tissues, increase in blood glucose, lactic acid levels, decrease in ATP/ADP
ratio, drop in blood pressure and death in severe cases.
Table 1: |
Phytochemical screening of 3 varieties of rizga (langaat,
beebot and riyom) |
 |
+ve: Present, -ve: Absent, CG: Cyanogenic glucoside |
However, we could not determine the actual concentration of these glucosides
in the 3 varieties of rizga studied.
The present study shows that, rizga flour contains considerable amount of phenolics and this implies that they may be useful in relation to diseases involving free radical reactions.
DPPH radical scavenging activity: The effect of antioxidants on DPPH
is thought to be due to their hydrogen donating ability (Liu
et al., 2008). The DPPH stable free radical method is an easy, rapid
and sensitive way to survey the antioxidant activity of a specific compound
or plant extracts (Devasagayam et al., 2004).
The inhibitory activities of the extracts and quercetin decreased in the following
order: Langaat<Riyom<Beebot<quercetin: The inhibitory activities of
the extracts on DPPH free radical indicates that the extracts have a proton-donating
ability and could serve as free radical inhibitors or scavengers, acting possibly
as primary antioxidants. The inhibitory actions of the three varieties of rizga
is attributed to the phenolic content and presence of other phytochemicals in
them. This suggests that rizga might be a useful therapeutic agent for treating
radical-related pathological damage. The lower antioxidant activity of langaat
when compared with beebot and riyom is thought to have arisen from the absence
of alkaloids in it which are polyphenols with high antioxidant capacity (Table
2). Kirakosyan et al. (2003) have reported
that polyphenols present in plants are responsible for their antioxidant activities.
The generation of reactive oxygen species beyond what the bodys ability
can cope with leads to oxidative stress and can lead to tissue damages and necrosis
in many instances. Furthermore, oxidative stress plays an important role in
malaria (Kremser et al., 2000; Kulkarni
et al., 2003). The immune system of the body is stimulated by infection
including malaria thus causing the release of reactive oxygen species.
Table 2: |
DPPH radical scavenging activity of three rizga varieties |
 |
Linear equation: (a) y = 33.81x-16.10 (b) y = 25.25x +12.60
(c) y = 29.51x-2.532 (d) y = 34.28x+3.209 |
Table 3: |
Total phenolic contents of 3 varieties of rizga (langaat,
beebot and riyom) |
 |
Each value in the table is the average of triplicate Experiments±SD
at (p<0.05). CAE: Chlorogenic Acid Equivalent, FW: Fresh weight |
Table 4: |
Lethal dose (LD50) (oral) of extracts of 3 varieties
of rizga on white albino rats |
 |
LD50: Lethal dose |
In addition, the malaria parasite also stimulates certain cells to produce
reactive oxygen species resulting in hemoglobin degradation (Das
and Nanada, 1999). Results indicate that these rizga varieties could play
significant roles in malaria therapy.
Total phenolic contents of 3 varieties of rizga flour: In recent years,
phenolics have attracted the interest of researchers because they show promise
of being powerful antioxidants that can protect the human body from free radicals,
the formation of which is associated with the normal metabolism of aerobic cells
(Mohammadpour and Rocha, 2007; Eleazu
et al., 2011a, b). That the phenolic compounds
present in plants possess antioxidant activity and may help protect cells against
the oxidative damage caused by free radicals has been reported by Kirakosyan
et al. (2003). All three varieties investigated contained significant
quantities of phenols (Table 3). However, the phenolic composition
of langaat was lower than that of the other 2 varieties analyzed and this is
also attributed to the absence of alkaloids in it. Results obtained indicate
that rizga contains significant quantities of phenolics.
Toxicity assay: The extracts of the 3 varieties of rizga flour had low
toxic effects on the rats studied with an LD50 of 630.1 mg kg-1
for riyom, 398.1 mg kg-1 for langaat and 446.7 mg kg-1
for beebot (Table 4). LD50 denotes the dosage of
the extracts that can kill 50% of the experimental animals in 24 h. High LD50
values indicate low toxic levels of the extracts and vice versa (Table
4).
| Fig. 1: |
Correlation of total phenolic composition versus antioxidant
activity of 3 rizga varieties |
The implication of this is that all three varieties of rizga flour have a very
low toxic level and are thus safe for consumption.
Correlation analysis carried out revealed that the antioxidant activities of
the rizga varieties correlated positively with their total phenolic contents
(Fig. 1) suggesting that the phenols present in all the varieties
rizga evaluated could be responsible for their antioxidant activities. This
is consistent with the findings of many researchers who reported such correlation
between total phenolic content and IC50 antioxidant activity (Llobera
and Canellas, 2007; Lecumberri et al., 2007).
CONCLUSION The results of the research analysis carried out reveals that Living stone potato (rizga) is a potential natural source of antioxidants with free radical scavenging activity. Since the lethal dosage of its consumption in the rats studied was found to be high, its use could be recommended especially in ailments implicating free radicals and oxidative stress. However, since it was found to contain cyanogenic glucosides and being that these compounds could be toxic in high concentrations, more efforts need to be geared to determine the actual status of these glucosides in Living stone potato and thus confirm the safety of its consumption.
|
REFERENCES |
1: Akinmoladun, A.C., E.O. Ibukun, E. Afor, B.L. Akinrinlola and T.R. Onibon et al., 2007. Chemical constituents and antioxidant activity of Alstonia boonei. Afr. J. Biotechnol., 6: 1197-1201. Direct Link |
2: Ames, B.N., M.K. Shigenaga and T.M. Hagen, 1993. Oxidants, antioxidants anddegenerative diseases of aging. Proc. Natl. Acad. Sci. USA., 90: 7915-7922. Direct Link |
3: Blois, M.S., 1985. Antioxidant determination by use of stable free radicals. Nature, 29: 1199-1200.
4: Das, B.S. and N.K. Nanda, 1999. Evidence for erythrocyte lipid peroxidation in acute falciparum malaria. Trans. Royal Soc. Trop. Med. Hyg., 93: 58-62. CrossRef | Direct Link |
5: Devasagayam, T.P.A., J.C. Tilak, K.K. Boloor, K.S. Sane, S.S. Ghaskadbi and R.D. Lele, 2004. Free radicals and antioxidants in human health: Current status and future prospects. J. Assoc. Phys. India, 52: 794-804. PubMed | Direct Link |
6: Karber, G., 1931. Contribution to the collective treatment of pharmacological serial experiments. Arch. Exp. Path. Pharmacol., 162: 480-483.
7: Kirakosyan, A., E. Seymour, O.B. Kaufman, S. Warber, S. Bolling and S.C. Chang, 2003. Antioxidant capacity of polyphenolic extracts from leaves of Crataegus laevigata and Crataegus monogyna (Hawthorn) subjected to drought and cold stress. J. Agric. Food Chem., 51: 3973-3976. Direct Link |
8: Kremser, P.G., B. Greve, B. Lell, D. Luckner and D. Schmidt, 2000. Malaria anemia in African children associated with high oxygen radical production. Lancet, 355: 40-41.
9: Kulkarni, A.G., A.N. Suryakar, A.S. Sardeshmukh and D.B. Rathi, 2003. Studies on Biochemical changes with special reference to oxidant and antioxidant in malaria patients. Indian J. Clin. Biochem., 18: 136-149. Direct Link |
10: Kumar, R.S., T. Sivakumar, R.S. Sunderam, M. Gupta and U.K. Mazumdar et al., 2005. Antioxidant and antimicrobial activities of Bauhinia racemosa L. stem bark. Braz. J. Med. Biol. Res., 38: 1015-1024. CrossRef | PubMed | Direct Link |
11: Lecumberri, E., R. Mateos, M. Izquierdo-Pulido, P. Ruperez, L. Goya and L. Bravo, 2007. Dietary fibre composition, antioxidant capacity and physico-chemical properties of a fibre-rich product from cocoa (Theobroma cacao L.). Food Chem., 104: 948-954. CrossRef | Direct Link |
12: Litchfield, Jr. J.T. and F. Wilcoxon, 1949. A simplified method of evaluating dose-effect experiments. J. Pharmacol. Exp. Ther., 96: 99-113. PubMed | Direct Link |
13: Liu, X., M. Zhao, J. Wang, B. Yang and Y. Jiang, 2008. Antioxidant activity of methanolic extract of emblica fruit (Phyllanthus emblica L.) from six regions in China. J. Food Comp. Anal., 21: 219-228. CrossRef | Direct Link |
14: Llobera, A. and J. Canellas, 2007. Dietary fibre content and antioxidant activity of manto negro red grape (Vitis vinifera): Pomace and stem. Food Chem., 101: 659-666. CrossRef |
15: Marthur, N.K. and V. Marthur, 2001. Antioxidants: Natural ingredients and additives for food. Beverage Food World, 5: 13-15.
16: Mohammadpour, G. and J.B.T. Rocha, 2007. Polyphenols in red pepper [Capsicum annuum var. aviculare (Tepin)] and their protective effect on some pro-oxidants induced lipid peroxidation in brain and liver Eur. Food Res. Technol., 225: 239-247. Direct Link |
17: Olojede, A.O., C.C. Nwokocha, O.N. Eke-Okoro and J.K.U. Emehute, 2004. Varietal response of living stone potato to NPK fertilizer application at Umudike. National Root Crops. Research Institute (NRCRI) Annual Report, pp: 86-87.
18: Robbins, R.J., 2003. Phenolic acids in foods: An overview of analytical methodology. J. Agric. Food Chem., 51: 2866-2887. CrossRef | PubMed | Direct Link |
19: Schippers, R.R., 2000. African Indegenous Vegetables: An Overview of the Cultivation Species. University Greenwwish, England, pp: 193-205.
20: Sofowara, R.A., 1984. Medicinal Plants and Transitional Medicine in Africa. John Wiley and Sons, Chichester, UK.
21: Trease, G.E. and W.C. Evans, 1984. Pharmacognosy. 13th Edn., Bailliere Tindall, London, UK., Pages: 332.
22: Eleazu, C.O., P.N. Okafor, J. Amajor, E. Awa, A. Ikpeama and K.C. Eleazu, 2011. Chemical composition, antioxidant activity, functional properties and inhibitory action of unripe plantain (M. Paradisiacae) flour. Afr. J. Biotechnol., 10: 16948-16952. CrossRef | Direct Link |
23: Eleazu, C.O., J.U. Amajor, A.I. Ikpeama and E. Awa, 2011. Studies on the nutrient composition, antioxidant activities, functional properties and microbial load of the flours of 10 elite cassava (Manihot esculenta) varieties. Asian J. Clin. Nutr., 3: 33-39. CrossRef | Direct Link |
24: Singleton, V.L., R. Orthofer and R.M. Lamuela-Raventos, 1999. Analysis of total phenols and other oxidation substrates and antioxidants by means of Folin-Ciocalteu reagent. Methods Enzymol., 299: 152-178. CrossRef | Direct Link |
|
|
|
 |