Characterization of Phenolic Compounds of Ulva rigida ( Chlorophycae ) and Its Antioxidant Activity

Ulva rigida is a worldwide distributed green alga and is commenly used for human nutrition. Extracts of this seaweed were shown anti-hypercholestierinemic, antioxidant and antihyperglycemic activities. The antioxidant effect was often ascribed to the presence of a huge amount of polyphenols. The aim of this study was to characterise by high-performance liquid chromatography-electrospray ionisation mass spectrometry (HPLC-ESI-MS) the phenolic molecules present in extracts obtained from U. rigida. The antioxidant activities of different extracts were evaluated in vitro by DPPH assay and on HeLa cells culture. Original Research Article Mezghani et al; EJMP, 12(1): 1-9, 2016; Article no.EJMP.22935 2


INTRODUCTION
Ulva rigida is a very common green marine seaweed distributed worldwide that is commonly used as a popular food ingredient in Asian countries as well as in North and South America [1,2].The high protein, lipid, mineral and vitamin content of marine U. rigida have encouraged its extensive use as a dietary supplement for humans and animals and as an organic fertilizer [3,4].Moreover, U. rigida has shown biological activities that are related to the presence of polyphenols [3,5,6], polysaccharides [3], terpenoids [3], fatty acids [3,5] and vitamins [3].Several studies have demonstrated its antihyperglycemic [7,8], anti-hypercholestierinemic [7,8], anti-bacterial [5], anti-genotoxic [5], antioxidant [9] and immunostimulating activities [10,11].In particular, U. rigida has received much attention as novel sources of natural antioxidants.Previous investigation on the phytochemistry suggested that U. rigida extracts produced large amounts of phenolic compounds [5,6].However, until now there is no report focused in the chemical identification of those molecules.The aim of this study was to identify by High-performance liquid chromatographyelectrospray ionisation mass spectrometry (HPLC-ESI-MS) the main phenolic molecules present in the different extracts obtained from U. rigida in order to evaluate the correspondant phenolic profile.The evaluation of the free radical-scavenging properties, cytotoxicity and cytoprotective action of U. rigida extracts are also investigated.These data will offer a strong framework for new discoveries, particularly the pharmaceutical, cosmetic and agri-food processing industries.

Algal material
Ulva rigida was collected from a shore of Ras-Djebel region in Tunisia.The algae were successively washed with water.The sample was botanically identified and a voucher specimen was deposited in the Laboratory of Functional Neurophysiology and Pathology (LFNP.URA 02).

Algal Extract Preparation
A fresh sample of U. rigida (1 kg) was mixed with 4 L of distilled water and sonicated.The mixture was then ground with an electric mortar and pressed.The resultant liquids (4.8 L) were pooled and subjected to chloroform and ethyl acetate extractions.The obtained extracts (chloroform, ethyl acetate, water) were concentrated with a rotary evaporator, freeze dried and stored until use.Approximately 15 ml of water extract was hydrolysed by H 2 SO 4 (2 M, 8 h, 100°C) to allow glycoside separation.The obtained hydrolysate was neutralised, filtered and concentrated under vacuum.The methanol extract was obtained by using air-dried powdered U. rigida (5 g).The obtained solution was filtered through Whatman No. 1 paper and then evaporated at reduced pressure by rotary evaporator.

Total Phenolic Determination
The total phenolic contents in different extracts (ethyl acetate, methanol, water, hydrolysis water) were determined by the Folin-Ciocalteu method [12].The total phenolic content was expressed as gallic acid equivalents (GAE) (mg/g of dry weight).

DPPH Free-radical Scavenging Activity
The effect of the extracts on DPPH (2,2-diphenyl-1-picrylhydrazyl) radicals scavenging was estimated using a spectrometric method [13].The test compound concentration providing 50% inhibition (IC 50 expressed in µg.mL -1 ) was calculated from the graph of the inhibition percentage plotted against the extract concentration.Butylated hydroxytoluene (BHT) was used as the positive control.

Cell Culture and Treatment
HeLa cells were maintained in RPMI medium containing 2 mM L-glutamine, 10% Foetal Bovine Serum (FBS) and 100 U/ml of antibiotic solution in a humidified 5% CO 2 incubator at 37°C.The cells were grown in 24-well microplates until 70-80 % confluence.Different concentrations (250, 350, 400, 500 and 1000 µg/ml) of the water extract of U. rigida and/or H 2 O 2 (10, 100, 1000 and 10 000 µM) were added to the cells and incubated for 4 h.

Cytotoxicity Assay
5-6-chloromethyl 2',7'-dichlorodihydrofluorescein diacetate (CM-H 2 DCFDA) was used as an intracellular esterase substrate to indicate cell integrity [14].The CM-H 2 DCFDA assay solution was freshly made by adding 15 µl of CM-H 2 DCFDA stock solution (5 mg/ml DMSO, -20°C) to 1700 µl of RPMI.The culture medium was removed from the microplate wells, and the cells were incubated with CM-H 2 DCFDA solution for 8 min at 37°C in the dark.After the incubation period, the solution was aspirated, and the cells were rinsed with PBS (phosphate-buffered saline) at 37°C; then, the cells were lysed in lysis buffer (Tris-HCl).Fluorescence was measured using microplate reader (λ excitation = 480 nm, λ emission = 528 nm) and was normalised to control cells levels, which were set at 100% fluorescence.H 2 O 2 was used as a positive control for cyotoxicity.

LC-MS Analysis
The LC-MS/MS experiments were carried out with an Agilent 1100 LC system.For the chromatographic separation a Zorbax 300Å Extend-C-18 Column (2.1 x 150 mm) was used.The column was held at 95% solvent A (0.1% formic acid in water) and 5% solvent B (0.1% formic acid in ACN) for 1 min, followed by an 11 min step gradient from 5% B to 100% B, then kept for 4 min with 100% B. Finally, elution was achieved with a linear gradient from 100% B to 5% B in 2 min.For MS experiments, the capillary voltage was set to 3.5 kV for electrospray ionisation with positive ion polarity.

Statistical Analysis
The data are presented as the mean ± S.D (standard deviation) and were evaluated using Student's t-test.

Total Phenol Contents
Several studies of the Ulva species revealed that they are good dietary sources of antioxidants [15].Thus, we evaluated the levels of total phenolic compounds in extracts of U. rigida.As shown in Table 1, the ethyl acetate and hydrolised water extracts showed the highest total phenolic contents (582.93±0.8 and 457.12±4.8mg g -1 , respectively).These data are consistent with previous studies indicated that U. rigida extracts produced large amounts of phenolic compounds [5,6,16].Overall, the results showed that phenolic content of all the extracts was quite high.

LC-MS/MS Analysis
LC-MS/MS analyses were performed to characterise the major phenolic compounds contained in U. rigida extracts.Several phlorotannins and phenolic acids (peak marked with number in Fig. 1) were tentatively identified using mass spectrometry and compared with literature data.The MS study of the ions allowed the detection of compound 1 with protonated molecular ion ([M+H] + ) at (m/z 127) (Table 2).This compound corresponds to phloroglucinol with a fragment at m/z 108, which is due to the loss of one molecule of water (-18).The compound 3 with ([M+H] + ) at m/z 499, composed by four phloroglucinol units, was also observed, and is tentatively identified as fucodiphloroethol (Table 2, Fig. 1).This tetramers showed a fragmentation pattern with losses of one molecule of water (-18, m/z 481), three methyl (-42, m/z 439) and six methyl (-84, m/z 355), successively.Such phlorotannins characterization were demonstrated in Fucales extracts [17,18].In a similar manner, compound 5 with ([M+H] + ) at m/z 747 can be a fucophloroethols derivatives composed of six units of phloroglucinol.The fragmentation pattern showed losses of one molecule of water (-18, m/z 729), two molecules of water (-36, m/z 711), phloroglucinol and two molecules of water (m/z 585, -126 -36) and one molecule of phloroglucinol, two molecules of water and methyl (m/z 571, -126 -36 -14).A low-intensity product ion was detected (m/z 220) which is likely a result of the cleavage of benzene ring structure (Table 2).These results are similar with those obtained in brown seaweeds [19].The compound 7, ([M+H] + at m/z 743) was identified as dieckol molecules composed of six units of phloroglucinol, with ion peaks observed at m/z 329, 311 and 227.This might be due to the loss of three phloroglucinol, two water molecules, and one molecule water and six methyl, respectively (Table 2).The compound 8 with [M+H] + at m/z 375 and product ion peaks observed at m/z 357, 339, 249 and 235 led us to assume that the compound correspond to fucophlorethol composed by three    [18,20].The compound 10 at m/z 497 can be a phloroeckol molecule and showed a fragmentation pattern with losses of one water (m/z 479, -18), one water and two methyl (m/z 451, -18 -28) and other fragments at m/z 386, 368 and 258 (Table 2).The compound 13 at m/z 371 correspond to dioxinodehydroeckol.The product ions are m/z 329 corresponding to a loss of three methyl (-42), 311 (-18) and six methyl (-84, m/z 227), succesively.The compound 14 at m/z 373 is suggested to be a polyphenolic compound composed by three phloroglucinol units, possibly a eckol.The fragmentation pattern is m/z 331 (-42, three methyl), 313 (-18, one water) and 295 (-18, one water), successively.The compound 15 at m/z 513 is identified as diphloroethohydroxycarmalol, the product ions are successively, m/z 495 (-18 one water), 327 (-168, twelve methyl) and other low-intensity product ions (m/z 301 and 257 ions) which were probably a result of the cleavage of benzene ring structure.Moreover, the chromatogram of U. rigida extracts showed the presence of quinic acid and phenolic acids such as dihydroxybenzoic, quinic, siringic, vanillic acids, phenylethanol and feruloyl-hexose which are the main phenolic acids found in U. rigida (Table 2, Fig. 1).The present results indicated for the first time the identification of phlorotannins in Chlorophyta species.These compounds have been reported to occur in brown marine algae and in several families of Angiosperms [21].

Free-radical Scavenging Activity
The radical-scavenging activity of four Ulva rigida extracts were evaluated by DPPH assay based on their ability to quench radicals.The IC 50 values for radical scavenging showed that the ethyl acetate extract had the highest radical scavenging activity with an IC 50 value of 0.18 µg mL -1 followed by the hydrolysed water (0.21 µg mL -1 ), water (0.25 µg mL -1 ) and methanol (0.41 µg mL -1 ) extracts (Fig. 2).As shown in Fig. 2, the antioxidant activities of the ethyl acetate and the hydrolysed water extracts were comparable to that of butylated hydroxytoluene (BHT), which was used as positive reference.Furthermore, the total phenolic compounds showed a positive correlation with the radical-scavenging activity results suggesting that phenolic components constitute the major molecules acting as free radical terminators.These findings are consistent with previous reports that evaluated the antioxidant capacity of U. rigida alga [5,6,16].

Cytotoxicity and Cytoprotective Effects
To investigate the cytotoxic effect of U. rigida, HeLa cells were treated with concentrations of alga water extract (0 to 1000 µg mL -1 ) for 4 h or 24 h and then subjected to a cell viability assay.The results clearly indicate that no significant cell death occured in either algal dose, and the U. rigida extract was not toxic even after continuous exposure for a 24 h period (Fig. 3).Moreover, the cytoprotective action of the U. rigida water extract was tested in H 2 O 2induced cell death.Exposure to H 2 O 2 markedly reduced cell viability in a dose-dependent manner, and over 80% of the cell population was dead after 4 h treatment with 10 000 µM H 2 O 2 .However, the co-exposure of cells with H 2 O 2 and U. rigida water extract resulted in an increased percentage of viable cells.As shown in Fig. 3, U. rigida extract (400 µg/ml) prevented H 2 O 2induced damage, restoring cell viabilitiy to 74.12 (100 µM of H 2 O 2 ), 47.1 (1000 µM of H 2 O 2 ) and 20% (10 000 µM of H 2 O 2 ) versus 45.17, 15.2 and 4.14%.These results indicated that the U. rigida extract not toxic by it self and protects HeLa cells from cytotoxicity and the deleterious effect of H 2 O 2 -mediated oxidative damage.These data are in agreement with our previous study [16,22].The high polyphenol content found in U. rigida extracts could be partly responsible for the antioxidant power reported here.Phenolic acids and phlorotannins such as phloroglucinol, eckol, bieckol, dioxinodehydroeckol, and diphloroethohydroxycarmalol are reported to exhibit strong DPPH and ABTS (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) radical scavenging activities [19,23].The antioxidant activities of some phenolic acids and phlorotannins were also investigated in cellular models.In particular, phlorofucofuroeckol, dieckol, diphlorethohydroxycarmalol and dihydrobenzoic acids reduced the level of intracellular reactive oxygen species (ROS) [17][18][19][20]24,25].The phloroglucinol molecule was found to scavenge ROS and increased the catalase-antioxidant enzyme activity [26].Several reports have shown that phenolic acids (hydroxybenzoic, syringic, salicylic, etc.), phloroglucinol, phlorofucoeckol, eckol and bieckol markedly reduced lipid peroxidation, a hallmark of oxidative stress mediated through the free radicals produced in the cell [19,27].Finally, other studies showed that most phlorotannins possess a remarkable ability to protect cells from death trigged by oxidative stress and prevent damage to biomolecules like DNA [28,29].

CONCLUSION
This paper reports the characterization of the phenolic compounds of U. rigida extracts for the first time.On the other hand this preliminary work is the first report mentioned the identification of phlorotannins in green seaweed.In addition, the different extracts exhibited large phenolic contents and potent antioxidant activity.Therefore, U. rigida extracts enriched in phenolic molecules may provide a promising source of natural antioxidants, applicable in the pharmaceutical, food and cosmetic industries.However, more studies are needed involving NMR identification and investigation on Ulva species from several origins, in attempt to establish a chemical fingerprint of U. rigida phenols.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.

COMPETING INTERESTS
Authors have declared that no competing interests exist.

Fig. 1 .
Fig. 1.UV chromatograms of the U. rigida extracts recorded at 280 nmPeaks marked with numbers were identified as in Table2