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Exploration of Wendan Pomelo Peel from Four Different Growing Regions by LC-MS and Bioactivity Analysis

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11 December 2023

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12 December 2023

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Abstract
Wendan (a type of pomelo), as a popular fruit in China, is less known in other countries. Its peel is widely used in food and traditional medicine. Four different origins of Wendan pomelo were selected, and crude extracts were obtained by the Soxhlet extraction method. The composition of Wendan peel was analyzed by high-performance liquid chromatography-mass spectrometry (HPLC-MS) in positive ion mode. The compounds were identified by searching the Metabolite Link (METLIN), Spectral Database for Organic Compounds (SDBS) and referring to literature reports. A total of 20 compounds were identified among the samples from the four origins, of which 8 compounds were common. The majority of the compounds belonged to the flavonoid and coumarin classes, Meranzin hydrate was identified for the first time in pomelo peel. In vitro antioxidant activity experiments showed that samples from Taizhou, Zhejiang exhibited the highest antioxidant activity in the DPPH, ABTS, and FRAP assays, with values of 0.59 mg/mL, 97.06 μmol TE/g, and 60.62 μmol Fe2+/g, respectively. Samples from Zhangzhou, Fujian showed antioxidant activity second only to the samples from Taizhou, Zhejiang. The sample from Zhangzhou, Fujian Province, showed excellent inhibitory activity in the α-glucosidase inhibition assay (IC50=7.99 mg/mL).
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Subject: Biology and Life Sciences  -   Food Science and Technology

1. Introduction

Citrus maxima (Burm.) Merr, commonly known as pomelo, is an evergreen tree belonging to the Rutaceae family. Its fruit has a smooth surface with small oil glands and is typically round or pear-shaped. The flesh of the fruit can be either white or red, with a few variants exhibiting a creamy yellow color. Pomelo cultivation can be found in various Southeast Asian regions such as Malaysia, Thailand, and Vietnam [1,2]. In China, it is primarily cultivated in Guangdong, Guangxi, Fujian and Zhejiang [2].
Pomelo has been widely used in food and cosmetic industry due to its special aroma, nutritional value and pharmacological activity [3]. Pomelo, in China, is used traditionally as functional food with the potential to balance insulin and glucose levels, thereby contributing to the management of diabetes [4]. Furthermore, pomelo products could be processed into pectin, essential oils and dried pomelo peel. [5]. In recent years, the pomelo processing industrial products, such as beverages, canned foods and wines have been developing rapidly.
The pulp portion of pomelo is used as fresh food to supplement the nutrients such as vitamins. The peel, accounting for approximately 30% of the whole pomelo, is consumed directly in the form of sweets, tea and medicine by locals on the southeast coast of China [2].The bioactivities and phytochemistry of pomelo have been reviewed, including flavonoids, essential oils, coumarin classes, and triterpenes. The antioxidation, antibacterial, anticancer, and alleviation of depression have been reported [6,7,8,9]. However, there is relatively limited research on the classification of different species of pomelo.
The main pomelo species in China can be divided into three categories: Wendan pomelo, Shatian pomelo, and inter-specific pomelo [2]. Especially Wendan pomelo is well-known for its crisp and tender flesh, large size, and rich flavor. It was first cultivated in Zhangzhou, Fujian and later introduced to other regions. Currently, the most famous Wendan pomelo species in China are Yuhuan Wendan from Taizhou, Zhejiang, Duwei Wendan from Putian, Fujian, and Madou Wendan from Tainan, Taiwan. With the widespread cultivation of Wendan pomelo, there has been a continuous increase in its production. However, there is a lack of systematic research on Wendan pomelo peel, especially in terms of comparing the chemical composition and biological activities of Wendan pomelo peel from different cultivation regions.
Therefore, the present work was to investigate the composition and antioxidant activity of Wendan pomelo peel grown in China from four different origins.

2. Materials and Methods

2.1. Plant Materials

The Wendan (Citrus maxima cv. Wentan Buntan) peels were obtained from four different regions of China (Table 1, Figure 1), including Xuzhou (Jiangsu), Taizhou (Zhejiang), Zhangzhou (Fujian), Meizhou (Guangdong). These materials were identified by Professor Chun-lin Long (College of Life and Environmental Sciences, Minzu University of China) and the Flora of China, and then the fresh Wendan peels were dried, crushed and stored at cool place.
The extractions were carried out by the Soxhlet extraction method. The ground peel of Wendan pomelo (6.0g) was tightly wrapped in filter paper and placed into a Soxhlet extractor, which was then placed in a round-bottom flask (500ml) containing 70% ethanol solution to obtain the extract. The extract was subjected to vacuum filtration, with the addition of 25% ethanol solution for washing. The resulting filtrate was transferred to a rotary evaporator and distilled under reduced pressure at 60°C until no alcohol odor remained. Finally, the extract was dried in a fume hood.

2.2. HPLC-Q-TOF-MS/MS analysis

The analysis [10] was conducted using the Agilent 1260 Infinity system (Agilent, USA) and the 1200 HPLC 6520 Q-TOF-MS (Agilent, USA). The samples were separated on a Hypersill Gold C18 column (250 mm × 4.6 mm, 5 μm, SN10609405, Thermo Scientific, USA). The injection sample volume and flow rate were set at 2 µL and 1 mL/min, respectively. The gradient conditions were as follows: solvent A (H2O-CH2O2, 1%) and solvent B (acetonitrile, C2H3N 99%). The initial setting was 0-5 min with a linear gradient of 10% B, followed by 5-35 min with 30% B, 35-45 min with 100% B, and then held at 10% B for 5 min to allow column equilibration. The detection range was set from 200 nm to 600 nm using nitrogen gas as the carrier. Mass spectrometry conditions included electrospray ionization (ESI) source, with an ion source temperature of 300°C, positive ion detection, a scanning range of m/z 80-2000 Da, and a cone voltage of 30V.

2.3. Nuclear Magnetic Resonance

1H and 13C NMR spectra (one-dimensional) were obtained on a Bruker Avance DRX-600(Bruker, Germany) spectrometer with a 5 mm TCI cryoprobe and a 14.1 T magnetic field. The chemical shifts of 1H and 13C were referenced according to the peak of the DMSO (C2H6OS) solvent used to solubilize the samples.

2.4. DPPH (2, 2-diphenyl-1-picrylhydrazyl)-Assay

The DPPH free radical scavenging activity was assessed in accordance with a modified version of a known protocol [11]. The extract solutions were diluted with methanol to obtain a series of concentrations ranging from 10 to 500μg/mL. The 0.06 mM DPPH solution was prepared by dissolving DPPH in methanol. The 100 μL aliquot of the sample was mixed with 100 μL of the DPPH solution and kept in the dark at room temperature. After 30 minutes, the absorbance of the solution was measured at 517 nm. BHT was used as a positive control, and methanol served as the blank control, following the same experimental procedure. The radical scavenging activity was calculated using the equation:
SC50 (%) = [1 − (A1 − A2) / A0] × 100%
Where A0 represents the absorbance of the blank control (methanol and DPPH solution), A1 is the absorbance of the sample with DPPH solution, and A2 is the absorbance of the sample solution with methanol. The results were expressed as SC50 (mg/mL), and each batch of samples was tested in triplicate.

2.5. ABTS (2, 2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) assay

The ABTS free radical scavenging activity was determined with slight modifications based on a previously described method [12]. The ABTS solution was prepared by mixing equal volumes of potassium persulfate (2.45 mM) and ABTS stock solution (7 mM), and allowing the mixture to react in the dark at room temperature for 12-16 hours. The ABTS radical solution was then diluted with methanol to achieve an absorbance of 0.70 ± 0.02 at 734 nm. The Trolox (C14H18O4) standard solution (10 mM) was prepared using methanol and further diluted with methanol to obtain a series of concentrations ranging from 0.1 to 0.7 mM. For the assay, 20 μL of Trolox was added to 180 μL of the ABTS solution and incubated in the dark at room temperature. After 10 minutes, the absorbance of the solution was measured at 734 nm. The same procedure was followed for the sample analysis. The standard curve was constructed using the Trolox standard solution, with the absorbance as the ordinate and the Trolox concentration as the abscissa. The absorbance value obtained from the sample was then substituted into the standard curve to calculate the ABTS antioxidant activity. Each batch of samples was tested in triplicate, and the results were expressed as Trolox equivalent (μmol TE/g).

2.6. Ferric Reducing Antioxidant Power (FRAP) assay

The FRAP assay was conducted with slight modifications based on a previously described method [13]. The working solution was prepared by combining 300 mM/L acetate buffer (pH 3.6), 10 mM/L TPTZ (tripyridyltriazine) solution in 40 mM/L HCl, and 20 mM/L FeCl3•6H2O solution in a volume ratio of 10:1:1. Prior to analysis, the FRAP solution was incubated in a 37 °C water bath. The samples were diluted with methanol, and 20 μL of the diluted sample solution was added to react with 180 μL of the FRAP solution at 37 °C in the dark. After 30 minutes, the absorbance at 593 nm was measured. A standard curve was constructed using the Fe2+ concentration as the abscissa and the absorbance as the ordinate to calculate the FRAP antioxidant activity. Each batch of samples was tested in triplicate, and the results were expressed as Fe2+ equivalent (μmol Fe2+/g).

2.7. α-Glucosidase inhibitory assay

The α-glucosidase inhibitory assay was conducted with slight adjustments based on a previously described method [14]. The 1 U/mL α-glucosidase solution was prepared using phosphate buffer (0.1M, pH 6.8). For the assay, 10 μL of the sample diluted in DMSO at various concentrations and 30 μL of α-glucosidase solution were mixed in 80 μL of potassium phosphate buffer and incubated for 5 minutes at a 37 °C water bath. Then, 30 μL of 20 mM PNPG (p-Nitrophenyl-β-D-G-alactopyranoside) was added to initiate the reaction, which was further incubated in a 37 °C water bath for 30 minutes. The reaction was terminated by adding 40 μL of Na2CO3 solution (2M), and the absorbance was measured at 405 nm. Acarbose was used as a positive control. The inhibition ratio was calculated using the following equation:
Inhibition (%) = [1 − (A1 − A0) / (A3 − A2)] × 100%
Where A0 represents the absorbance of the sample blank (phosphate buffer instead of PNPG), A1 is the absorbance of the sample, A2 is the absorbance of the control blank (phosphate buffer instead of the sample and PNPG), and A3 is the absorbance of the control (phosphate buffer instead of the sample). The results were expressed as IC50 (mg/mL), and each batch of samples was tested in triplicate.

3. Results and Discussion

3.1. Identification Components of Wendan pomelo peel

The ion chromatograms in positive ion mode obtained by HPLC-MS/MS for the pericarp of four different origins of Wendan pomelo peels are shown in Figure 2. The chemical composition results are presented in Table 2. A total of 20 compounds (Figure 3) were detected in the samples from the four origins, with 8 compounds common to all four samples and 2 compounds were identified but their names remain unknown. The major compounds were coumarins and flavonoids. The compound meranzin hydrate F13 (CPF13), which belongs to the coumarin compounds, exhibited the highest peak area in the chromatogram. This compound was first discovered in the root of Prangos ferulacea in 1972 [15] and is commonly studied in the context of Chaihu-Shugan-San [16]. It was the first time that this compound was discovered in the Wendan pomelo peel. The flavonoid compounds with higher peak areas in the chromatogram were identified as rutin (CP5), narirutin (CP10), hesperidin (CP11), neohesperidin (CP12), and kaempferol (CP14). There were significant differences in peak areas among samples from different origins, with the proportions of coumarin compounds and flavonoid compounds in Zhangzhou samples being similar, while the coumarins were much more abundant than the flavonoids in the samples from Xuzhou and Meizhou.
The significant variations were observed in the chemical composition of samples from different origins. For instance, the sample from Zhejiang Taizhou (W3) exhibited the highest number of detected compounds with a total of 16 compounds. The sample from Guangdong Meizhou (WF4) had the lowest number of detected compounds, with 11 compounds identified only. For these compounds, CP2, CP3, CP6, and CP16 were exclusively found in the sample of Taizhou, while CP14 was detected only in the sample of Zhangzhou, and CP15 was present solely in the sample of Xuzhou, compound CP8 was not detected in the sample of Meizhou. Previous studies have suggested that variations in plant chemical composition may be attributed to ecological differences [17,18]. The chemical composition differences have been observed in samples of the same plant species collected from different regions or even different locations within the same region as described by Zhang et al. [19]
Table 2. The chemical constituents of Wendan pomelo peel from four areas.
Table 2. The chemical constituents of Wendan pomelo peel from four areas.
Number1 Rt (Min) Molecular formula Fragments (m/z) Compound W1 W2 W3 W4
1a 2 3
CP1 7.81 C10H8O3 177.2405 133.1216 121.1081 7-methoxycoumarin* + + + -
CP2 12.13 C15H16O9 341.2512 323.1103 305.0924 Unknown - + - -
CP3 12.52 C15H16O9 341.0807 179.0340 151.0388 Esculetin-6-O-glucoside c - + - -
CP4 19.73 C16H18O9 355.1029 163.0392 145.0287 Chlorogenic acid c + + + +
CP5 21.17 C27H30O16 611.1611 465.1029 303.0501 Rutin* + + + +
CP6 21.95 C27H30O14 579.1710 433.1132 271.0603 Apigenin-O-(deoxyhexosyl)hexoside c - + - -
CP7 22.31 C21H20O10 433.1135 415.1034 397.0923 Isovitexin c - - - +
CP8 24.49 C28H32O16 625.2113 607.1564 589.3071 Lucenin-2 4′-methylether d + + + -
CP9 25.61 C23H34O15 573.1811 551.2143 227.1936 Genipin-β-gentiobiosidee + + + +
CP10 26.68 C27H32O14 581.1881 419.1349 273.0763 Narirutin* + + + +
CP11 27.55 C28H34O15 611.1994 449.1457 303.0870 Hesperidin* + + + +
CP12 28.91 C28H34O15 611.1987 449.1447 303.0871 Neohesperidin* + + + +
CP13 29.87 C15H18O5 279.1243 261.1294 243.1027 Meranzin hydrate*,x + + + +
CP14 30.88 C15H10O6 287.0553 258.0522 213.0543 Kaempferol *,c - + + -
CP15 31.14 C11H6O3 187.0401 159.0444 143.0499 Angelicin c + - - -
CP16 31.36 C21H20O11 449.1078 303.0501 229.0495 Quercitrin c - - + -
CP17 32.08 C20H24O11 441.2495 352.3446 282.2769 Unknown + + - -
CP18 33.88 C21H20O12 465.4033 303.2861 274.3367 Isoquercitrin* + + + +
CP19 34.83 C28H32O15 609.1820 463.1244 301.1069 Diosmetin-7-O-rutinoside c - - + +
CP20 35.62 C15H10O7 303.0507 285.0409 257.0453 Quercetin c - + + +
1Compound numbers are denoted as CPX (X = 1-20), a represents the main fragment ion [M+H]+, * indicates confirmation through standard compounds; "c-e" represents references [20,21,22]; x indicates determination through nuclear magnetic resonance analysis. "+" indicates the presence of the compound; "-" indicates the absence of the compound.

3.2. NMR

The compound CP13 was characterized based on the analysis of 1H NMR and 13C NMR spectra. In the 1H NMR spectrum (Figure 4), methyl signals at δ 1.13 (s, H-17), 1.14 (s, H-18), and 3.87 (s, H-13) were observed on three carbon atoms. Additionally, four olefinic signals were detected at δ 6.24 (t, H-2), 7.95(t, H-3), 7.53 (t, H-7), and 7.03 (t, H-8). Analysis of the 13C NMR spectrum (Figure 5) revealed a total of 15 carbon atoms in the compound, including three signals of CH3, one signal of CH2, five signals of CH, and six signals of C. The 1H NMR and 13C NMR spectra displayed characteristic signals for a coumarin nucleus substituted on 7 by a methoxyl group. The molecular formula of CP13 was determined as C15H18O5 based on the results obtained from MS, in conjunction with the analysis of the 1H NMR and 13C NMR spectra. The compound was identified as a meranzin hydrate through structural elucidation. The compound meranzin hydrate, identified as a major constituent of the traditional Chinese medicine Chaihu-Shugan-San, has been proven to possess anti-atherosclerotic and antidepressant properties [16]. In the four samples analyzed, this compound exhibited the highest proportion, indicating its significant research potential.

3.3. Antioxidant Activities

The DPPH assay, as an in vitro antioxidant activity assessment, exhibits a lower value indicating a more significant antioxidant activity. The concentration of BHT corresponding to a 50% DPPH radical scavenging rate (SC50) was found to be 0.11 mg/mL (Figure 6). The Sample of W2 and W3 demonstrated values of 0.59 mg/mL and 1.75 mg/mL respectively, indicating better antioxidant activity among samples. Additionally, sample of W3 exhibited slightly lower antioxidant activity compared to sample of W1. Both sample W1 and W4 exhibited relatively weaker antioxidant capabilities with SC50 values of 7.84 mg/mL and 5.96 mg/mL respectively.
The ABTS and FRAP assay results were expressed in terms of Trolox equivalents and Fe2+ concentrations respectively, where higher values indicate stronger antioxidant capacity. The samples of W2, according to Figure 7, exhibited the highest antioxidant capacity in both the ABTS and FRAP assays with values of 97.06 μmol TE/g and 60.62 μmol Fe2+/g respectively. The W3 showed a slightly lower antioxidant capacity than W2 in the ABTS and FRAP assays with values of 64.08 μmol TE/g and 45.29 μmol Fe2+/g respectively. However, samples of W1 and W4 demonstrated relatively weaker antioxidant capacities, especially the W1 displayed the weakest antioxidant activity in both tests.
The previous studies have demonstrated the favorable antioxidant activity of flavonoid compounds in the Citrus fruits [23]. Rutin (CP5), for instance, exerts its free radical scavenging effect through its ability to eliminate hydrogen peroxide and peroxides [24]. Naringin (CP10) exhibits strong antioxidant properties due to its ability to eliminate free radicals [25,26], while hesperidin (CP11) exerts antioxidant effects by attenuating oxidative stress reactions [27]. Neohesperidin (CP12) has also been confirmed to possess excellent free radical scavenging abilities in both superoxide and hydroxyl radical tests [28]. These four compounds were detected in all samples, providing a comprehensive explanation for the observed antioxidant activity. However, the compound kaempferol (CP14) has been shown to possess antioxidant activity [29] and considering the differences in compound composition among samples and the potential synergistic effects between compounds [30], this may explain why some samples exhibit stronger antioxidant capacity while others show relatively weaker activity.

3.4. α-Glucosidase inhibitory activity

The α-glucosidase inhibition activity, according to Table 3, was evaluated as a measure of hypoglycemic with lower values indicating stronger blood glucose-lowering ability. All samples from the four regions exhibited weaker activity compared to the positive control acarbose. But the samples of W3 (I=7.99 mg/mL) demonstrated the highest inhibitory activity in all samples, followed by W1 with a value of 11.86 mg/mL. Sample of W2 showed moderate activity with a value of 12.38 mg/mL, while W4 exhibited the weakest inhibitory activity.
The studies have indicated that quercitrin exhibits significant inhibitory effects on α-amylase and α-glucosidase, making it an ideal compound for targeting diabetes management [31,32]. Based on the obtained chemical composition and without considering synergistic effects, it was observed that the compound quercitrin (CPF16) detected in sample of W3 was not found in other samples. Therefore, the presence of quercitrin in sample W3 may serve as a crucial indicator of its superior inhibitory activity compared to other samples.

4. Conclusions

This study compared the chemical composition and biological activities of Wendan peel samples collected from four different regions in China. The results revealed variations in the chemical composition of the samples from different origins. A total of 20 compounds, primarily flavonoids and coumarins, were detected among the four samples. The bioactivity assays demonstrated that the sample from Taizhou (W2) exhibited the highest antioxidant activity, followed by the sample from Zhangzhou (W3). These two samples could be considered as potential natural antioxidants. Additionally, the sample from Zhangzhou (W3) could be regarded as a promising source for the research and development of natural α-glucosidase inhibitors.

Author Contributions

Conceptualization, J.L.; methodology, J.L..; data curation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to acknowledge the technical guidance provided by the Institute of Microbiology, Chinese Academy of Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Jiangsu, Zhejiang, Fujian and Guangdong Province are represented by the colors green, pink, orange and purple respectively. Samples collection site are the red dot.
Figure 1. Jiangsu, Zhejiang, Fujian and Guangdong Province are represented by the colors green, pink, orange and purple respectively. Samples collection site are the red dot.
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Figure 2. The ion chromatogram of Wendan pomelo peel from different regions.
Figure 2. The ion chromatogram of Wendan pomelo peel from different regions.
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Figure 3. The structural formula of compounds.
Figure 3. The structural formula of compounds.
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Figure 4. The 1H NMR spectrum of meranzin hydrate.
Figure 4. The 1H NMR spectrum of meranzin hydrate.
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Figure 5. The 13C NMR spectrum of meranzin hydrate.
Figure 5. The 13C NMR spectrum of meranzin hydrate.
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Figure 6. DPPH scavenging ability of samples from four regions.
Figure 6. DPPH scavenging ability of samples from four regions.
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Figure 7. Antioxidant activities of samples from four regions in ABTS and FRAP assay.
Figure 7. Antioxidant activities of samples from four regions in ABTS and FRAP assay.
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Table 1. Characteristics and information of samples.
Table 1. Characteristics and information of samples.
Accessions collected Main geographical distribution Longitude and
Latitude		 Traditional Name Thickness of peels (mm)
W1 Xuzhou, Jiangsu N:34°12′36"
E:118°17′14″		 Suqianmiyou 1~2
W2 Taizhou, Zhejiang N:28°06’44"
E:121°14’25"		 Yuhuanwendan 2~3
W3 Zhangzhou, Fujian N:24°25′22″
E:117°28′16″		 Pingheyou 2~3
W4 Meizhou, Guangdong N:24°14′22"
E:116°31′10"		 Jiaxingmiyou 1~2
Table 3. The α-Glucosidase inhibitory activities of samples from four regions.
Table 3. The α-Glucosidase inhibitory activities of samples from four regions.
Sample I (mg/mL)
WF1 11.86±0.40
WF2 12.38±1.33
WF3 7.99±0.83
WF4 15.6±0.35
acarbose 0.96±0.07
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