Chemical Composition, Antioxidative and Anticancer Activities of the Essential Oil: Curcumae Rhizoma–Sparganii Rhizoma, a Traditional Herb Pair

As a classical herb pair in clinics of traditional Chinese medicine, Curcumae Rhizoma–Sparganii Rhizoma (HP CR–SR) is used for activating blood circulation to remove blood stasis. The essential components in HP CR–SR and its single herbs were comparatively analyzed using gas chromatography-mass spectrometry data. 66, 22, and 54 components in volatile oils of Curcumae Rhizoma, Sparganii Rhizoma, and HP CR–SR were identified, and total contents accounted for 75.416%, 91.857%, and 79.553% respectively. The thirty-eight components were found in HP CR–SR, and not detected in single herbs Curcumae Rhizoma and Sparganii Rhizoma. The highest radical trapping action was seen by an essential oil of HP CR–SR (IC50 = 0.59 ± 0.04 mg/mL). Furthermore, the HP CR–SR essential oil showed more remarkable cytotoxicity on tumor cell lines than that of the single herbs Curcumae Rhizoma and Sparganii Rhizoma in a dose-dependent manner: IC50 values showing 32.32 ± 5.31 μg/mL (HeLa), 34.76 ± 1.82 μg/mL (BGC823), 74.84 ± 1.66 μg/mL (MCF-7), 66.12 ± 11.23 μg/mL (SKOV3), and 708.24 ± 943.91 μg/mL (A549), respectively. In summary, the essential oil of HP CR–SR is different from any one of Curcumae Rhizoma and Sparganii Rhizoma, nor simply their superposition, and HP CR–SR oil presented more remarkable anticancer and antioxidant activities compared with Curcumae Rhizoma and Sparganii Rhizoma oils.


Results and Discussion
Zedoary turmeric oil, the essential oil extracted from CR, is documented in the Chinese Pharmacopoeia (2010) that it has exhibited many potential medical characteristics in the aspects of anticancer and antivirus [26,27]. In records of Chinese medical formula books or traditional application, CR is always combined with SR [7,8]. The difference between HP oil and CR oil has been analyzed by GC-MS.

The Volatile Components of CR, SR and HP CR-SR
The essential oils of CR, SR, and HP CR-SR were extracted by hydrodistillation, and the yield was 6.0 mL/kg, 0.1 mL/kg, and 2.8 mL/kg, respectively. By means of the overall volume integration method, quantitative results of each component in pure chromatographic profiles of HP CR-SR, single herbs CR and SR can be obtained, as shown in Table 1. In total, 66, 22, and 54 volatile components in CR, SR, and HP CR-SR were determined quantitatively, accounting for 75.416%, 91.857%, and 79.553%, respectively. The volatile chemical components in HP CR-SR are notably different from those of single herb CR or SR.
It is noteworthy that curcumenol has broad-spectrum anticancer activity [21] and its content in HP oil was approximately two times higher than that of CR oil. Clinically, germacrone, as the official quality control marker for Zedoary turmeric oil, exhibited many pharmacological activities including anti-tumor activity, antiviral activity, and so on [28,29]. Comparing with CR oil, the content of germacrone was increased in HP oil. Three common components in volatile oils of HP and SR were detected, including 4-hydroxy-4-methyl-2-pentanone (5.691%) and two other trace amount ingredients. The conclusion from the analysis above is that the volatile chemical components in HP CR-SR were not completely the same as those of single herb CR or SR.

The New Chemical Components of HP Volatile Oil
There are 38 terpene alcohol-type components that were first found in HP, such as epi-cedrol (1.009%), 10-epi-γ-eudesmol (1.194%), selin-11-en-4-α-ol (1.675%), epi-catalponol (1.348%), vetivenic acid (1.287%), and so on. Those new component contents in HP reach up to 19.380%. The reports about their pharmacological activities are rather rare. The following compounds were worth mentioning: epi-cedrol, as a sesquiterpene alcohol, was also detected in Artemisia annua L, which could increase production of artemisinin by genetically-engineered methods [30]. Selin-11-en-4-α-ol was the principle component in the oil of Ononis Sicula Guss., exhibited moderate antioxidant activity [31]. It is the purportedly unique sesquiterpene compound, 10-epi-γ-eudesmol, that has been suggested as a marker for distinguishing the various cultivars and hybrids of Pelargonium graveolens [32]. In addition, vetivenic acid was isolated by column chromatography and identified by NMR and CG-MS from Vetiveria zizanioides root, with promising antimicrobial effects [33]. Interestingly, 2-ethylhexyl-salicyate, a significant sunscreen, was widely used in cosmetics and the medicine industry [34]. Mayurone was first reported from Mayur pankhi and Thujopsis dolabrata, respectively, in 1965, and widely existed in many kinds of plants such as Panax ginseng and Franco Thujaorientalis [35]. Considering the relatively high content and potential activity of those new components, it is inferred that they may impact on the activity and efficacy of HP oil.
Moreover, some volatile chemical compounds of CR, such as α-eudesmol (3.310%, 8-cedren-13-ol accetate (2.781%), (Z)-β-santalol (2.550%), nootratin (1.775%), and cryptomerione (1.730%), have not been detected in HP. In addition, hexadecanoic acid (38.634%), linoleic acid (37.981%), and n-docosene (3.196%), as the main chemical ingredients of SR volatile oil, have not been seen in HP. Possible reasons could be listed as follows: for one thing, some organic acids of SR, like succinic acid or sanleng acid, could play a role of solubilization effect, which have improved the constituents of CR dissolving out during the process of decocting the two single herbs, CR and SR [36]. For another, it has a chance to easily interact among many active organic compounds, such as aldehydes, ketones, and alcohols, during the process of decocting CR and SR [3,18,[37][38][39][40]. In addition, the total contents of essential oils in CR and SR were reported to be quite uneven (2% and 0.04%, respectively) [36], thus, it probably means that volatile oil of SR were covered by the volatile oil of CR when the compatibility ratio of two herbs is one to one.

DPPH Radical Scavenging Activity
Free radicals and oxidants could affect the metabolism of the body [41]. Antioxidants have become important auxiliary products for health. DPPH is a useful reagent for investigating the free radical scavenging ability of compounds. The dose-response curve of DPPH radical scavenging activities of the essential oils from CR, SR, and HP CR-SR was distinct in Figure 1. The essential oil from HP CR-SR possessed the best activity with a rate of 90.6% under 5.0 mg/mL, and the essential oil from SR showed little DPPH radical scavenging activity.
Among total essential oils of the four samples, HP CR-SR possessed the highest antioxidant activity (IC50 = 0.59 ± 0.04 mg/mL). Compared with HP CR-SR oil, the volatile oil of CR showed moderate DPPH scavenging ability (IC50 = 1.39 ± 0.06 mg/mL), and the other two samples, SR and CR + SR, their IC50 were 8.39 ± 1.20 mg/mL and 2.20 ± 0.09 mg/mL, respectively. In addition, as a positive control, IC50 of vitamin C was 0.07 mg/mL in this condition.
In daily life, the increased dietary intake of natural antioxidants could reduce cancer mortality [42][43][44]. Moreover, antioxidants not only selectively inhibit the proliferation of cancer cells but also do not harm normal cells [45]. This means that the positive correlation exists between the antioxidant and antitumor activities.

Anti-Tumor Activity
The essential oils (CR, SR, and HP) were tested in vitro for their potential human tumor cell growth inhibitory effect on five human tumor cell lines (BGC823, A549, MCF-7, HeLa, and SKOV3), using MTT assay. The five human tumor cell lines are the most conventional tumor cell lines for the screening of antitumor activity, which are deemed to be high in veracity and reliability.
The concentration that inhibited cell vitality by 50% (IC50) is presented in Table 2. Apparently, HP essential oil was the most effective in all tested cell lines, and presented IC50 values ranging from 32.32-74.84 μg/mL, except for human lung cancer cell lines (A549). The CR oil had less activity compared with HP oil, IC50 in the range between 31.57 and 162.87 μg/mL. The oil of SR had the lowest antitumor activity, and it showed nearly no efficacy in inhibiting, instead promoting five human tumors cell lines. On the other hand, the HeLa cell line was the most susceptible to the oils of CR and HP, while, among the tumor cell lines, the A549 was the most resistant one (Figure 2).
In particular, in order to further compare the activity differences between HP and two single herbs (CR and SR), group (CR + SR) is also set in the test. Different from group HP, the group (CR + SR) was made of the oils of CR and SR with 1:1 input quantity. Seeing from Table 2, IC50 values of group HP are all lower than that of group (CR + SR) on human tumor cell growth inhibitory effect. Based on these findings, it can be speculated that the HP oil was remarkably better than the oil of any single one on anticancer activity.  In the inhibitory effect assays on five tested human tumor cell lines, the essential oil of HP, in a dose-dependent manner, was the most effective compared to CR and SR oils. In other words, with the SR being put into HP, more effective ingredients in CR were dissolved out. This is highly consistent with chemical research results, especially being related with new components in HP oil. In spite of having found new constituents in HP oil, their pharmacological activities were less reported. Their activities should be focused on in the further studies.

Materials
Single herb CR and SR, with vinegar processing, were purchased from Hebei Anguo Medical Materials Corporation (Anguo, China), and identified as Curcuma kwangsiensis S.G. Lee et C.F. Liang and Sparganium stoloniferum Buch.-Ham., respectively, by Yuan Zhang from the Beijing University of Chinese Medicine, China. n-alkane standard solutions of C7-C40 (No. 49452-U) was purchased from SUPELCO (Bellefonte, PA, USA).

Extraction of the Essential Oils
A total of 100 g of CR and 100 g of SR single herbs were mixed and soaked with 1000 mL deionized water in a standard extractor for extracting volatile oil at room temperature for 12 h. Then, the essential oil was prepared by using the standard steam distillation method, according to the Chinese Pharmacopeia [9]. The essential oil was collected and stored in a refrigerator at 4 °C before use. The essential oils of single herb CR or SR was extracted in the same method.

GC-MS Analysis
Volatile oils were analyzed by an Agilent Technologies 5975C gas chromatograph (GC) equipped with a HP-5 MS capillary column (5% phenyl methyl Siloxane, 30 m × 0.25 mm i.d., 0.25 μm film thickness) and an HP 5973 mass selective detector (Agilent Technologies, Palo Alto, CA, USA) in the electron impact ionization mode (70 eV) under the following operating conditions: split ratio, 1:40; injection volume, 1.0 µL (TBME solution); inlet temperature, 260 °C; detector temperature, 260 °C; The oven temperature was programmed to start at 50 °C, increased at 5 °C/min to 100 °C, 100 °C increased at 3 °C/min to 150 °C, 150 °C increased at 5 °C/min to 250 °C, then held at 250 °C for 30 min. The helium (99.999%) carrier gas was kept at a constant flow of 1.2 mL/min.

Identification of Compounds
The chemical ingredients of volatile oils were identified by the retention indices (RI), relative to a series of alkanes (C7-C40) at the same chromatographic conditions, consulting the Van Den Dool method [46]. The data were analyzed by the Xcalibur 1.1 software (Thermo Fisher Scientific, Waltham, MA, USA, version 1.1), compared with the NIST/EPA/NIH database (2005, version 2.0d, Scientific Instrument Services, Inc., Ringoes, NJ, USA) and several references [47][48][49][50]. The book "Identification of Essential Oils Components by Gas Chromatogrtaphy/Mass Spectrometry, 4th edition" [51] played an equally vital role in identifying the individual compounds.

Anti-Tumor Activity
Human gastric cancer cell lines (BGC823), human lung cancer cell lines (A549), human breast carcinoma cell lines (MCF-7), human cervix carcinoma cell lines (HeLa), and human ovary carcinoma cell lines (SKOV3) were purchased from Cell Resource Center, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China. These cell lines were maintained in RPMI 1640 culture medium plus 10% calf serum and 100 U/mL penicillin, 75 U/mL streptomycin, in a 37 °C incubator supplied with 95% room air and 5% CO2.
The growth inhibitory effect of essential oils of HP, CR, and SR were measured using the standard MTT assay [52,53]. Briefly, 100 μL samples of cell suspension (6 × 10 4 cells/well) were cultivated in 96-cell plates for 18 h, as described. The samples of HP, CR, and SR oil were well dissolved in DMSO, respectively, followed ultrasonic vibration. Then, cells were incubated for 24 h with varying concentrations of the oil. After adding 100 μL MTT (1 mg/mL) solution to each well, the cells were further incubated at 37 °C for15 min, in a humidified atmosphere of 95% air/5% CO2. For MTT assay, the supernatant was thrown away and 200 μL DMSO was mixed into, the 96 wells plate was oscillated on micro-vibrator for an additional 10 min, the absorbency of each well was determined at λ 560 nm by an enzyme-immunoassay instrument (Beijing Perlong New Technology Co., Ltd., Beijing, China). Anti-tumor activity was expressed as the concentration of the drug inhibiting cell growth by 50% (IC50 value).

DPPH Radical Scavenging Activity
The DPPH radical scavenging activity was determined according to the method reported by Sharififar and others with some modifications, and compared with the activity of vitamin C as a positive control [54]. An equal amount of (1 mL of 0.05 mg) DPPH solution in ethanol was mixed with 0.1 mL essential oil from CR, SR, and HP CR-SR in ethanol at various concentrations. After mixing, the solution was allowed to reach a steady state at room temperature for about 30 min. The DPPH radical scavenging activity was determined by the absorbance at λ 517 nm with the DNM9602A microplate reader (Beijing Perlong New Technology Co., Ltd., Beijing, China) and calculated by the following equation: where A0 is the absorbance of the control (blank, without extract) and A1 is the absorbance of the mixture with the essential oil.

Conclusions
In the present study, it is first reported that the essential oils of CR, SR, and HP CR-SR were comparatively explored from the aspects of the chemical compositions, antioxidant, and anticancer activities. The essential oil of HP is different from any, one of CR and SR, nor simply their superposition. Thirty-eight new components were detected in HP, and the relative contents of those new components reach up to 19.380%. Moreover, HP oil showed significant antioxidant and anticancer activities and the essential oils from four groups on antioxidant and anticancer activities were consistent, in order: SR < CR + SR < CR < HP. It is well known that the essential oil of CR reveals prominent activities in the clinic, and it has been documented in different versions of Chinese Pharmacopoeia since 1977. Compared with CR oil, HP oil possessed the stronger antioxidant and anticancer activities. Thus, the volatile oil of HP has the better medicinal application prospect and deserves more attention in the pharmaceutical industry.
HP CR-SR belongs to a mutual reinforcement relationship according to seven compatibilities in TCM theory. Comparison and analysis of the essential oils of CR, SR, and HP CR-SR from chemical compositions and pharmacological activities, we can infer that CR may play a predominant role in HP, namely sovereign drug according to TCM theory. In spite of the low concentration of SR oil, it also plays an important part in HP through improving the dissolution of effective constituents and helping interact to produce new ingredients. In order to further illustrate the role of SR in HP, more focus can be concentrated on the change of HP essential oils (produced from different proportion compatibilities of CR and SR) on the chemical composition and pharmacological activities. Furthermore, it can be concluded that the chemical composition in HP CR-SR is responsible for its potent anticancer and antioxidant activities. Further investigations are in progress in our laboratory to identity the exact active principle and their action mechanism involved in anticancer and antioxidant activity.
Recently, the studies about herb pairs mostly concentrated on the content change of some main components and traced the origin of chemical constituents [55][56][57][58]. Based on the integrity of TCM theory, it is suggested to be more focused on the content change of common components and the appearance of new constituents in herb pairs with different proportion compatibilities. Such experimental design and research approaches are encouraged to be applied in the further study on HP CR-SR decoction, in order to clarify the compatibility essence of HP.