INTRODUCTION:

Medicinal plants play a significant role in treating and preventing many diseases. Many chemical compounds have been isolated from plant extracts and their biological activities have been proven¹. In recent years, many studies have demonstrated the effectiveness of plants as antioxidants in reducing oxidative strees-induced tissue injury². The family Polygonaceae consists of approximately 1200 species distributed within 50 genera³. The genus Rheum belongs to this family and contains about 60 species, and it is mainly spread in mountains and desert regions in Asia and Europe⁴. Rhubarb has many biological activities such as purgative effect⁵, anti-bacterial⁶,anti-oxidant^7,8 and anti-cancer activity⁹. Many compounds have been isolated from Rheum genus¹⁰.

Antraquinones are the most important secondary metabolites found in all species. Most of these compounds are derived from the basic structure 9, 10-anthracendione¹¹. Free antraquinones such as aloe-emodin, emodin, rhein, chrysophanol and physcion. In addition, the antraquinones glycosides such as aloe-emodin-8- O-β-D-glucoside, rhein-8-O-β-D-glucoside, chrysophanol-8-O- β-D-glucoside, emodin-8-O-β-D-glucoside and physcion-8-O- β-D-glucoside¹². Twenty six anthrones were isolated from some species like sennosides⁵. Stilbenes such as resveratrol and rhaponticin are also found in the genus and have shown significant efficacy as an anti-inflammatory, anti-fungal and anti-oxidant⁵. Catechin, epigallocatechin-3-O-gallate (EGCg) and kaempherol are the most flavonoids isolated from species⁵.

Rheum palaestinum Feinbr is also known as the desert rhubarb (Figure 1). It is distributed in the Syrian Desert and eastern desert of Jordan and extended to the southern part of Palestin⁴. It has a thick vertical root, the leaves are rounded, huge, 20-50cm in diameter, with wrinkled surface¹³. Popularly it was used as a laxative, anti-platelet and anti-spasmodic. Emodin and chrysophanol were isolated from the roots of the plant. In addition, two compounds of stilbenes (trans-resveratrol-3-O-β-d-glucopyranosid and Rhapontigenin 3-O-β-d-glucopyronoside) were isolated from aerial parts¹⁴.

The aim of this study was to determine the antioxidant activity and the total content of phenols, flavonoids and anthraquinones of different extracts of Rheum Palaestinum roots and aerial parts.

A

Figure 1: Rheum palaestinum Feinbr.

A.Aerial parts of Rheum palaestinum B. Roots of Rheum palaestinum

MATERIALS AND METHODS:

Materials

Gallic acid was purchased from AVONCHEM, United Kingdom, Quercetin from Sigma-Aldrich, Germany, Rhein from MedChemExpress, Germany, 2,2-Diphenyl-1-picrylhydrazyl (DPPH^•) from Tokyo Chemical Industry, Japan, Folin-ciocalteu from Merck KGaA Darmstadt, Germany, magnesium acetate from Merck, Germany, sodium carbonate from AVONCHEM, United Kingdom, Aluminium chloride from Riedel-de Haen, Germany, sodium acetate from Merck, Germany, absolute ethanol and methanol from Sigma-Aldrich, Germany, distilled water.

Plant materials:

The plant was collected from the countryside of AL-SUWAYDA during the months of March and April 2022. It was identified by Dr. Jourjet Baboujian, Department of plant Biology, Faculty of Science, Damascus University. Plant samples were cleaned, then the aerial parts were separated from the roots and dried in the shade, at room temperature for 15 days and were powdered using an electrical grinder to prepare the extracts.

Preparation of plant extracts:

The powdered plant parts (20g of each part) were extracted by Ultrasound device using 200ml of solvent (Ethanol 95% and water) at 50^ᵒC for half an hour ¹⁵. The extracts were filtered using filter papers and then the extraction was repeated three times using the same procedure. The combined extracts were evaporated using a rotatory evaporator under low pressure to remove the solvent. The crude extracts were kept in sealed packages and stored at 4^ᵒc for further usage. The extraction yield was calculated using the following equation:

Yeild% = (Weight of dry extract/ Weight of dry plant material) × 100

Phytochemical screening:

Four studied extracts were assessed for the existence of flavonoids, tannins, coumarins, saponins, anthraquinones and alkaloids using standard protocols as per the description in the literature^{16,17,18,19,20,21,22,23}.

Determination of total phenolic contents (TPC):

This calibration depends on using Folin-ciocalteau reagent. 20µl of each extract was mixed with 1.58 ml distilled water and 100µl Folin-ciocalteau reagent. After waiting for 5 minutes, 300µl of 20% sodium bicarbonate was added²⁴. The samples were mixed and then put in dark at room temperature for two hours, the absorbance was measured at 765nm using the UV-VIS spectrophotometer. The blank was contained 2ml of distilled water. The samples were prepared in triplicate and the mean value of three absorbance was obtained. The total phenols were determined by the standard curve of gallic acid in distilled water within the concentration range 0-500µg/ml. The concentration was expressed as gallic acid equivalents in mg/g of dry extract.

Determination of total flavonoids contents (TFC):

The total contents of flavonoids was determine using the Aluminum chloride (AlCl₃) reagent. 2ml of each extract was mixed with 0.1ml Aluminum chloride 2% and 0.1 ml sodium acetate and 2.8ml distilled water²⁵. The samples were mixed and then left in dark at room temperature for half an hour, the absorbance was measured at 430nm using the UV-VIS spectrophotometer. The blank was contained 2ml of methanol. The samples were prepared in triplicate and the mean value of three absorbance was obtained. The total flavonoids were determined by the standard curve of quercetin in methanol within the concentration range 0-50µg/ml. The concentration was expressed as quercetin equivalents in mg/g of dry extract.

Determination of total Anthraquinones content:

The extracts were analyzed for total content of anthraquinones by UV spectrophotometer at 522nm²⁶. 50mg of each extract was accurately weighed and 10ml of distilled water was added. The mixture was well mixed, weighed and refluxed on a water bath for 15 minutes. The flask was cooled, weighed and adjusted to the original weight by water. The mixture was filtered and then 20ml of 10.5% ferric chloride solution was added and it was refluxed on a water bath for 20 minutes. 1ml of 2M Hydrochloric acid was added and also it was refluxed on a boiling water bath for 20 minutes. The mixture was extracted with chloroform (25×3). The water layer was discarded. The chloroform layer was combined and washed with 15ml distilled water twice. The combined chloroform was transferred to a 100ml volumetric flask and adjusted to the volume. 25ml of the solution was evaporated to dryness and the residue was dissolved with 10ml of 0.5% magnesium acetate in methanol yielding a red solution. The UV absorbance was measured at 522nm. The samples were prepared in triplicate and the mean value of three absorbance was obtained. The total anthraquinones were determined by the standard curve of Rhein within the concentration range 0.96-8.64µg/ml. The concentration was expressed as rhein equivalents in mg/g of dry extract.

Determination of DPPH^• scavenging activity:

The DPPH^• radical scavenging activity was evaluated according to Kilic et al. method with some modification ²⁷. 1.5ml of 4.5mg\100ml DPPH^• in ethanol was mixed with 300 µl of each extract at different concentration (25-125µg/ml). The samples were mixed and then left in dark at room temperature for half an hour, the absorbance was measured at 517nm using the UV-VIS spectrophotometer. Results were presented by preparing a series of gallic acid as a reference compound (0.5-10 µg/ml). The percentage of scavenging activity was determined by the following equation:

DPPH^• radical scavenging activity%

= (A₀ – A₁)/ A₀ × 100

Where:

A₀: The absorbance of control (Ethanol + DPPH^•) A₁: The absorbance of sample (Extract + DPPH)

Ferric reducing capacity:

The reducing power for the studied extracts was determined as Khatoon et al²⁸. 2.5ml of Different concentrations of extracts and ascorbic acid as standard (25-200µg/ml) were mixed with 2.5ml of phosphate buffer (0.2 M, pH 6.6) and 2.5ml of 1% potassium ferricyanide. The mixture was incubated at 50ᵒC for 20 minutes. Then 2.5ml of 10% trichlorocacetic acid was added and the mixture was centrifuged at 3000 rpm for 10minutes. 1.8ml of the upper layer was mixed with 1.8 of distilled water and 0.5ml of 0.1% ferric chloride solution and the absorbance was measured at 700nm. The samples were prepared in triplicate and the mean value of three absorbance was obtained.

Total antioxidant capacity using phosphomolybdate assay:

The total anti-oxidation capacity was determined using phosphomolybdate method according to the protocol used by Khatoon et al²⁸. 0.3ml of each extracts (0.25µg/ml) and different concentrations of ascorbic acid (25-200µg/ml) was mixed with 3 ml of reagent solution (0.6M sulfuric acid, 28mM sodium phosphate and 4mM ammonium molybdate). The tubes were incubated at 95ᵒC for 90 minutes. After cooling the tubes to room temperature, the absorbance was measured at 695nm. The samples were prepared in triplicate and the values are expressed as equivalent of ascorbic acid in mg per g of extract.

Statistical analysis:

All tests were repeated three times and expressed as the mean±standard deviation (SD).

RESULT:

Yield of extraction:

The extraction yield is shown in Table1. Roots showed a higher yield than the aerial parts. The yield ranged from (32.75%) in the aqueous roots extract to (18.7%) in 95% ethanolic aerial parts extract.

Table 1: Extraction Yield

Solvent	Rheum palaestinum feinbr. part	Yield %
Ethanol 95%	Aerial parts	18.7
Ethanol 95%	Roots	20.1
Water	Aerial parts	29.2
Water	Roots	32.75

Phytochemical screening:

The results of the identification tests are shown in Table 2. The Phytochemical screening showed the presence of phenolic compounds in the extracts, such as flavonoids and anthraquinones, so the total content of phenols, flavonoids and anthraquinones were determined.

Table 2: Phytochemical screening of extracts of R. palaestinm

		Ethanolic roots extract	Aqueous roots extract	Ethanolic aerial parts extract	Aqueous aerial parts extract
Flavonoids	Aluminum chloride	+	+	+	+
Flavonoids	Shinoda test	+	+	+	+
Tannins	Ferric chloride test	+	+	+	+
Tannins	Gelatin test	+	+	-	-
Coumarins	Fluorescence	+	+	+	+
Saponins	Foam test	+	+	-	-
Anthraquinones	Borntrager test	+	+	+	+
Anthraquinones	Modified Borntrager	+	+	+	+
Alkaloids	Dragendroff	-	-	-	-
	Mayer	-	-	-	-
	Hager	-	-	-	-
	Wagner	-	-	-	-

(+): present, (-): absent

Determination of DPPH^• scavenging activity:

The calibration curve of gallic acid and the regression equation are shown in Figure 4. The results showed that the value of the inhibitory concentration of 50% of DPPH^• ranged from 28.34µg\ml in the ethanolic extract of roots to 82.15µg\ml in the aqueous extract of aerial parts while gallic acid (the standard compound) had IC₅₀ of 1.24µg\ml Table 4.

Figure 4: Calibration curves

A.Gallic acid calibration curve, B. Calibration curve of ascorbic acid

Table 4: IC₅₀ values of extracts and standard

Solvent	*Rheum palaestinum* feinbr. part	IC₅₀ (µg/ml)
Ethanol 95%	Aerial parts	IC₅₀=58.33 ± 0.001
Ethanol 95%	Roots	IC₅₀=28.34 ± 0.001
Water	Aerial parts	IC₅₀=82.15 ± 0.002
Water	Roots	IC₅₀=53.27 ± 0.001
Gallic acid		IC₅₀= 1.24 ± 0.00

Ferric reducing capacity:

The following figure (Figure 5) shows the change in the reducing power of extracts and ascorbic acid at different concentrations (25-200mg\l).

Figure 5: Reducing capacity of extracts and ascorbic acid

Total antioxidant capacity using phosphomolybdate assay:

This calibration depends on measuring the ability of the studied compounds to reduce the Mo(VI) to Mo(V) and to form a green phosphate/Mo(V) complex. Total antioxidant capacity of the extracts expressed as the number of equivalent of ascorbic acid was obtained from the calibration curve as shown in Figure 4.

The following table (Table 5) shows the results of the total antioxidant capacity of the extracts.

Table 5: Total antioxidant capacity of studied extracts

Extract	Absorbance ± SD	Total antioxidant capacity (mg ascorbic acid equivalents/ g dry extract)
Aqueous aerial parts	0.16 ± 0.001	20.744 ± 2.85
Ethanolic aerial parts	0.232 ± 0.003	88 ± 5.31
Aqueous roots	0.406 ± 0.002	249.6 ± 1.16
Ethanolic roots	0.512 ± 0.001	348.184 ± 4.55

DISCUSSION:

Yields of aqueous extracts were higher than ethanolic extracts and this can be explained by the difference in polarity of the extracted compounds and thus the difference of solubility in the solvents, or most compounds were in glycosides form more than free aglycones ²⁹. Also, the higher yields of the roots cab be explained by the fact that they contain a large amount of antraquinones compounds and thus increase the yields ³⁰.

The highest concentration of phenols and flavonoids in this study was observed in the ethanolic roots extract (512.15mg GAE/g dry extract and 48mg QE/g dry extract), followed by aqueous extract of roots (402.62 mg GAE/g dry extract and 29.2mg QE/g dry extract), and the lowest was in aqueous extract of aerial parts (322.85mg GAE/g dry extract and 21.08mg QE/g dry extract), respectively.

No previous study were found about TPC and TFC of rheum palaestinum extracts. So, we compared these results with the results of studies conducted on other species of rheum genus.

The results for phenolic and flavonoids contents of R.palaestinum in this study exceed the ones of Ozturk et al. of R.ribes, methanolic extract of R.ribes where TPC and TFC were reported as (25.19±1.09µg Pyrocatechol/mg dry extract and 16.23±0.47µg QE/mg dry extract) respectively³¹. Also, in the study by Mohammed et al, the total phenolic contents of methanol extract of roots of R.ribes was (82.84±0.38 µg GAE/mg dry extract). However, the flavonoids content of R.palaestinum roots was lower than R.ribes roots, where TFC was reported as (278.19±5.76 µg QE/mg dry extract)³².

The total content of phenols varies according to the type of solvent used for extraction, its polarity, the method of analysis and the standard material used³³. The concentration of flavonoids is affected by several factors, including calibration method, reagents and its quantities, PH, temperature and calibration time^34,35.

The total content of anthraquinones in the ethanol extracts was higher than aqueous extracts. Also, the extracts of roots had a higher content than aerial parts extracts and this corresponds to the high content of phenolic compounds.

Anthraquinone compounds in R. palaestinm comprise of both anthraquinone aglycones and anthraquinone glycosides. Water could extract only the glycosides form while ethanol 95% could extract most of anthraquinone aglycones and some of anthraquinone glycosides. Therefore, extraction with ethanol 95% as a solvent gives a higher yield of total anthraquinones than water ³⁶.

Ethanolic roots extract had the highest free radical scavenging activity among the four studied extracts, this may be due to the high content of phenols³⁷. The anti-oxidation properties of phenols are directly related to their structure and can be affected by substitution of hydroxyl groups on phenolic aromatic ring. While the anti-oxidation efficacy increases with the number of hydroxyl groups, the association of a group such as methyl and methyoxyl reduces the activity³⁸.

In the study by Mohammed et al, methanolic extract of roots of R.ribes had IC₅₀ value of 96.7µg\ml which indicates that it has less efficacy in free radical scavenging compared to R.palaestinum. However, the ethanolic extract of roots of R.ribes which extracted by ultrasonic device had IC₅₀ of 14.03µg\ml¹⁵.

At the lowest concentration, the absorbance of aqueous aerial parts, ethanolic aerial parts, aqueous roots, ethanol roots and ascorbic acid was 0.122, 0.13, 0.133, 0.143, and 0.288. At the highest concentration, absorbance of the above extract was 0.194, 0.215, 0.192, 0.224, and 0.923. All extracts showed a good reducing power, increasing steadily with increasing concentration. Higher absorbance indicates a higher reducing power, therefore the results show that the ethanolic extract of the roots possesses the highest reducing capacity, while the aqueous extract of aerial parts has the lowest reducing power. Ascorbic acid has the highest reducing power in all concentrations³⁹.

Ethanolic extracts of the roots showed the highest total antioxidant activity followed by aqueous extract of the roots, ethanolic extract of the aerial parts and aqueous extract of aerial parts. The total antioxidant capacity of plant extracts may be due to their chemical composition and phenolic content²⁸.

CONCLUSION:

The results presented in this study are the first information about the content of phenols, flavonoids and anthraquinones of Rheum palaestinum and the efficacy as antioxidant. Ethanolic extract of the roots had a significant amount of phenolic compounds as well as the best efficacy as antioxidant. Certainly, these results will increase interest in research on using natural products as alternative treatments for many diseases causing by free radicals.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

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Received on 16.10.2023 Modified on 20.11.2023

Research J. Pharm. and Tech 2024; 17(5):2320-2326.

DOI: 10.52711/0974-360X.2024.00364

Solvent	*Rheum palaestinum* feinbr. part	Phenols (mg gallic acid equivalents/ g dry extract)	Flavonoids (mg quercetin equivalents/ g dry extract)	Anthraquinones (mg rhein equivalents/ g dry extract)
Ethanol 95%	Aerial parts	338.33 ± 1.185	25 ± 1.072	7.84 ± 0.05
Ethanol 95%	Roots	512.15 ± 0.692	48 ± 0.417	14.74 ± 0.08
Water	Aerial parts	322.85 ± 1.368	21.08 ± 0.585	2.04 ± 0.115
Water	Roots	402.62 ± 1.374	29.2 ± 1.183	10.41 ± 0.096