USING HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY

Background: Cyclocarya paliurus is an endangered plant and endemic to China. The leaves of C. paliurus have been used in drug formulations and as ingredients in functional foods in China. The aim of this study was to develop an effective method to extract most of the compounds and to establish a simplified HPLC analytical method to determine the contents of major bioactive compounds simultaneously. Materials and methods: High-performance liquid chromatography (HPLC) coupled with a photodiode array detector (PDA) was used for the simultaneous determination of the major flavonoids and triterpenoids in C. paliurus leaves. Results: Ultrasonic extraction in 100% methanol for 30 min was adopted as the optimal extraction method for C. paliurus leaves. The separation conditions were optimized using a Phenomenex C18 ODS column (250 mm × 4.6 mm, 5 µm) with a mobile phase of acetonitrile and 0.02% formic acid and a detection wavelength of 205 nm. The validation data indicated that this new HPLC analytical method successfully quantified the provenance and seasonal variations of seven major compounds (three flavonoids and four triterpenoids) in C. paliurus leaves. Conclusion: The study provided a novel and simplified approach to simultaneously determine the quantity of major flavonoids and triterpenoids in C. paliurus leaves. The results could promote the optimization of silvicultural systems for quality control of C. paliurus .


Introduction
Cyclocarya paliurus (Batal.)Iljinskaja is an endangered plant endemic to China and a monotypic genus of the Juglandaceae family.C. paliurus is commonly called "sweet tea tree" because of the sweet taste of its leaves (Fang et al., 2006).Sweet tea tree is naturally distributed in many provinces of Southern China, including Anhui, Fujian, Guangxi, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Sichuan, Guizhou and Zhejiang (Fang and Fu, 2007).The leaves of C. paliurus have been used as herbal tea for hundreds of years (Zhao et al., 2013;He et al., 2013), and were used for drug formulations in traditional Chinese medicine (TCM) or as ingredient of functional foods in China (Xie et al., 2010).The main chemical composition of C. paliurus leaves include flavonoids, triterpenoids, and polysaccharides (He et al., 2012).The extracts of C. paliurus were reported to have multiple beneficial effects, such as antihypertensive (Zhang et al., 2010), antihyperlipidemic (Wang et al., 2013;Kurihara et al., 2003), antihyperglycemic (Wang et al.，2013；Li et al.，2011), antioxidant activity (Wang et al.，2013), and inhibitors of PTP1B (protein tyrosine phosphatase 1B) (Zhang et al., 2010).
Up to now, the raw materials of C. paliurus were harvested in the wild for herbal tea or nutraceutical products.As the market demand increased, the natural resources were seriously destroyed.More leaves must be produced within the plantation.Recently, a number of such plantations of C. 126 paliurus have been developed in Zhejiang and Hunan province, China.However, the chemical profile of herbal plants is readily affected by many factors such as origin, climate, harvesting time and other agricultural practices (Zhao et al., 2013).The differences in chemical composition may lead to potential variations in health related effects, effective dosage, stability, shelf life, flavor, functional properties, as well as safety (Xie et al., 2013).However, one or even a few active constituents in a traditional Chinese medicine do not reflect its overall efficacy (Liu et al., 2010).To ensure the efficacy and safety of TCMs, qualitative and quantitative analysis of more major bioactive components is very important (Peng et al., 2013).The previous studies of C. paliurus have focused only on the quantification of total flavonoids (Mi et al., 2009;Chen et al., 2009;Jin et al., 2007) or a few selected flavonoids (kaempferol, quercetin and isoquercitrin) (Fang et al., 2011;Yang et al.,2009;Xie et al., 2003) because many marker compounds are not commercially available, especially some characteristic compounds.The purposes of this study were to develop an effective method to extract the overall compounds and develop a simple HPLC analytical method to quantify seven major bioactive components (isoquercitrin, kaempferol-3-O-α-L-rhamnopyranoside, kaempferol-3-O-β-glucuronide, arjunolic acid, cyclocaric acid B, pterocaryoside B and pterocaryoside A) simultaneously.Based on the established analytical method, the provenance and temporal variation of C. paliurus were investigated.
The results could provide useful information for quality evaluation of C. paliurus and for its further development in establishing a good agriculture practice (GAP) standard in China.HPLC grade acetonitrile and water were obtained from Mallinckrodt (Phillipsburg, NJ, USA).Formic acid was purchased from Sigma-Aldrich (St.

Reference
Louis, MO, USA).Other reagents were all analytical grade.

Plant Material
Seeds of C. paliurus from 11 provenances were collected from October to November, 2006 in China.The geographical information of the 11 provenances and seedling cultural techniques were described in detail previously (Fang et al., 2011).The one-year-old seedlings were planted in Zhenjiang Nursery, Jiangsu Province at around 32°16′ N latitude and 119°32′ E longitude in Mid-March of 2008.For seasonal variation analysis, leaves of ZAJ provenance were collected in July, October, and December of 2011, respectively.For provenance variation analysis, the leaves were collected in October 2011.A voucher specimen was deposited in Silviculture Lab of Nanjing Forestry University (Voucher code: 2011ZJ).

Sample Preparation
The dried leaves were pulverized to powder and passed through a 60-mesh sieve.The accurately weighed powder (0.5 g) was suspended in 6 mL of methanol.After the weight of the whole flask was recorded, the sample was ultrasonically extracted for 30 min at 60℃ and then cooled at room temperature.The original solvent weight was restored.After centrifugation at 4000 rpm for 10 min, 2 mL supernatant was subjected to C18 column, and filtered through a 0.45 μm syringe filter (Nalgene, NY, USA) prior to HPLC.For each sample, the complete assay procedure was conducted in triplicate and the standard deviation was calculated.

HPLC Apparatus and Chromatographic Conditions
An HPLC (e2695) form Waters (Milford, MA) connected to a UV/vis Photodiode Array (PDA) 2996 detector was used for all analyses.

Preparation of Standard solution
Seven stock solutions of standards (1000 μg/mL for three flavonoids and 2000 μg/mL for four triterpenoids) were prepared by dissolving them in methanol.Calibration curves were made by diluting the stock solutions with methanol.All solutions were filtered through a 0.45 μm syringe filter and stored in the refrigerator at 4 ℃ before analysis.

Linearity, Limits of Detection, and Quantification
Calibration curves were fit to the equation y = ax + b using a linear regression in which y and x were HPLC analyte peak areas of each compounds and each compound concentration.The lowest concentration of standard solution was diluted with methanol to yield a series of appropriate concentrations, and the LOD and LOQ under the chromatographic conditions were separately determined at signal-to-noise ratio (S/N) of about 3 and 10, respectively.

Precision and Accuracy
The measurement of intra-and inter-day variability was utilized to determine the precision of this newly developed method.For intra-day precision test, the standard solution of assayed components was injected for five times within a day.The inter-day precision was determined with the same standard solution on five consecutive days.The precision and accuracy of the method were assessed by analyzing control samples at three concentrations.Precision was expressed as the coefficient of variation (CV, %), calculated as the ratio of the standard deviation to the measure mean standard concentration.Accuracy was calculated by dividing the measured mean standard concentration by the theoretical mean

Recovery
A recovery test was used to evaluate the effectiveness of this method.Accurate standards were spiked with 80, 100, 120% of the amounts found in the JXS extract into the sample, then extracted and analyzed as described in the Sample Preparation section.The average percentage recoveries were evaluated by calculating the ratio of detected amount versus the added amount.

Optimization of Extraction Methods
To achieve a complete extraction of the components, four extraction solvents (55%, 70%, 85% and 100% methanol) and four extraction times (15, 30, 45 and 60min) were investigated.Four solvents were compared with regard to marker compound extraction efficiency using ultrasonication for 30 min.The results indicated that 55% and 70% methanol could not extract any triterpenoids, and the concentrations of three flavonoids were very low (Figure 2).100% Methanol could effectively extract the highest level of triterpenoids (compounds 4-7) and higher level of flavonoids (compounds 1-3).When samples were extracted with 100% methanol for 15 min, the extraction efficiency was low.However, the extraction efficiencies of 45 min and 60 min were similar to that of 30 min.Therefore, ultrasonically extracting in 100% methanol for 30 min was adapted as the optimal extraction method for C. paliurus in the present study.

Optimization of Chromatographic Conditions
To develop an accurate, valid, and optimal chromatographic fingerprint, different HPLC parameters including mobile phases (methanol-water, methanol-water-formic acid, acetonitrile-water or acetonitrile-water-formic acid) and types of column (Alltech Prevail C18 5 μm 4.6 × 250 mm, Waters Symmetry C18 5 μm 4.6 × 250 mm and Phenomenex C18 5 μm 4.6 × 250 mm) were examined and compared.The results showed that 0.02% v/v formic acid in the solvent system could give more symmetrical peaks and better separation than non-acid systems.A Phenomenex C18 ODS column provided a better separation of components in the plant than other brands of C18 columns.Finally, an optimized HPLC condition was developed by comparing the resolution, baseline, elution time and number of characteristic peaks in each chromatogram of different parameters comprehensively.Figure 3(a) showed the HPLC chromatogram of seven compounds obtained by optimized gradient elution with a mobile phase consisting of acetonitrile-water containing 0.02% formic acid.In the optimal HPLC condition, seven marker compounds showed retention times of 49.13 min, 67.32 min, 71.91 min, 127.03 min, 134.16 min, 137.08 min and 143.08 min, respectively, with good resolution and satisfactory peak shape.As shown in Figure 3

Method Validation
The HPLC method in the present study was validated by assessing the linearity, limit of determination (LOD) and quantification (LOQ), intraand inter-day accuracy and precision, recovery and stability.As shown in Table 1, all of the analytes showed good linearity.The correlation coefficients (R 2 ) of the calibration curves for the seven marker compounds were greater than 0.999 in relatively wide concentration ranging from 10 to 2060 μg/mL for all the compounds.The LOD (the limit of determination, S/N = 3) and LOQ (the limit of quantifiation, S/N = 10 ) were less than 92.34 and 307.86 ng/mL, respectively.The intra-and inter-day accuracy and intra-and inter-day precision were assessed at three different concentrations of the seven standards.The accuracy and precision data are presented in Table 2.The intra-day accuracy was 95.95 to 104.95% and the inter-day accuracy was 96.75 to 105.44 %.The intra-day precision was 0.83 to3.92% and the inter-day precision was 0.91 to 3.69%.As shown in Table 3, the mean recoveries found to be in the range of 96.38 and 102.20%, and their RSD values were less than 1%.As shown in Figure 4, the analytes remained very stable and remained at more than 96.30 % in the methanol solution at 4 ℃ over the tested period and the RSD values of all the analytes were less than 2.84 %.Hence, this verified HPLC method was precise, accurate and sensitive for the quantitative evaluation of seven active compounds in the extracts of C. paliurus.

Analysis and evaluation of C. paliurus samples
Seven marker compounds in C. paliurus were positively identified by comparing the retention time, UV spectra and MS fragmentation with the corresponding data obtained from the analysis of reference compounds (Figure 3   The data revealed that there were great provenance differences in the chemical constituents and their contents among eleven samples (Table 4).
In general, levels of four triterpenoids in most of samples were higher than that of the three flavonoids.The contents of three flavonoids ranged from 0.20 to 2.29 mg/g, while the contents of four triterpenoids ranged from 0.92 to 14.12 mg/g.In samples from FZP, YKM, GZY, GLP, GJH and JXS provenances, all seven marker compounds were observed, but with a high degree of content variability.The higher content of each flavonoid and triterpenoid were found in JXS provenances, and its total contents of three flavonoids (isoquercitrin, ) reached its highest level, 4.00 mg/g and 25.31 mg/g, respectively.Though AQLF contained higher contents of total flavonoids, reached to 3.60 mg/g, its total triterpenoids was the lowest among eleven provenances, only 4.66 mg/g.In this study, the content of isoquercitrin was 0.23 mg/g in HHF provenance, and it was much lower than the report from Fang et al. (2011), likely caused by seedling ages and extraction methods.Therefore, JXS is selected to be the optimal provenance based on the quantification of seven bioactive compounds.
Seasonal variations in natural compounds are fairly common and can contribute to variations of the finished products (Chou et al., 2009).Black et al. (2011) reported that there were significant seasonal variations in fifteen constituents of Northern Labrador Tea (Rhododendron tomentosum ssp.subarcticum).The temporal variations of the three selected compounds (quercetin, kaempferol, isoquercitrin) in the leaves of C. paliurus were determined by Fang et al. (2011), who reported that the mean concentrations in most of the provenances reached the maxima in July and September.In this study, the leaves of C. paliurus collected in July and October also showed higher contents in three major flavonoids (Table 5).
The total content of four triterpenoids was the lowest in July; however, the total triterpenoids in October was similar to that in December.In summary, July and October are the optimal harvesting seasons for flavonoids while October and December are the best harvesting time for triterpenoids.paliurus leaves.This HPLC method was applied successfully to determine the provenance and temporal variation of isoquercitrin, kaempferol-3-O-α-L-rhamnopyranoside, kaempferol-3-O-β-glucuronide, arjunolic acid, cyclocaric acid B, pterocaryoside B and pterocaryoside A.
Obviously, the developed quantitative analytical method of these bioactive components could be helpful for the quality assessment of C. paliurus leaves and provide useful information toward establishing good agriculture practice standard in China.

D
Center for Standardization of Chinese Medicines (Shanghai, China).Arjunolic acid was purchased from BioBioPha Company limited (Yunnan Province, China).Kaempferol-3-O-α-L-rhamnopyranoside, cyclocaric acid B, pterocaryoside B and pterocaryoside A were isolated in the Laboratory of Translational Medicine, Jiangsu Province Academy of Traditional Chinese Medicine (Nanjing, China) and were identified by spectral data (UV, MS, 1 H NMR,13 C NMR).Their structures are shown in Figure1and the purities of all chemicals were greater than 95%.

Table 2 :
Intra-and inter-day precision and accuracy in the assay of the seven compounds.

Table 3 :
Recovery of the marker compounds through the addition of standards.

Table 4 :
Quantification of seven compounds in the samples of C. paliurus collected from different provenances.
ConclusionsA novel and simplified HPLC analytical method was developed for the simultaneous quantification of seven bioactive components in C.