Four New Furostanol Saponins from the Rhizomes and Roots of Smilax scobinicaulis and Their Cytotoxicity

Four new furostanol saponins 1–4, along with two known furostanol saponins 5 and 6 and one known spirostanol saponin 7 were isolated from the rhizomes and roots of Smilax scobinicaulis. The structures of the new saponins were elucidated as 26-O-β-d-glucopyranoside-3β,26-dihydroxy-(25R)-5α-furostan-22-methoxyl-6-one-3-O-α-l-arabinopyranosyl-(1→6)-β-d-glucopyranoside (1), 26-O-β-d-glucopyranoside-3β,26-dihydroxy-(25R)-5α-furostan-22-methoxyl-6-one (2), 26-O-β-d-glucopyranoside-3β,26-dihydroxy-(25R)-5α-furostan-20(22)-en-6-one (3), 26-O-β-d-glucopyranoside-3β,23,26-trihydroxy-(23R, 25R)-5α-furostan-20(22)-en-6-one (4) on the basis of spectroscopic analysis. The isolated saponins were evaluated for cytotoxic activity against two human cancer cell lines including Hela (cervical carcinoma) and SMMC-7221 (hepatocellular carcinoma). Compounds 1 and 7 demonstrated cytotoxicity against the tested cell lines.


Introduction
The genus Smilax (family Liliaceae) is mainly distributed in the tropical, subtropical and temperate areas of the world [1].It is well known that steroidal saponins are abundant in the genus Smilax [2].Many species of this genus have a long history of use as herbal remedies.Smilax scobinicaulis (C.H.) Wright, commonly known as Hei Ci Ba Qia in Chinese is one of them, which is distributed in Shaanxi, Gansu, Sichuan, Yunnan and other regions of China [3].The rhizomes and roots of this plant, known as "Jin Gang Teng" and "Wei Ling Xian" in North China, have long been used in folk medicine for the treatment of rheumatic arthritis, lumbago, gout, tumor and inflammatory diseases [1].Previous phytochemical investigations of this plant led to the isolation of steroidal saponins [4], flavonoids [5] and phenylpropanoids [6].Bioactivity investigations showed that some of the isolated steroidal saponins had antimicrobial and cytotoxic activity [4,7].
In a continuing search for new steroidal saponins from this plant, a series of steroidal saponins have now been obtained, including four new furostanol saponins 1-4, two known furostanol saponins 5 and 6 and one known spirostanol saponin 7 (Figure 1), which are reported here as S. scobinicaulis compounds for the first time.All the compounds have been tested for cytotoxicity against Hela and SMMC-7221 human cancer cell lines.In this paper, we report the isolation and cytotoxic activity of these compounds and the structural elucidation of the new compounds.

Results and Discussion
The rhizomes and roots of S. scobinicaulis were extracted with ethanol (EtOH).The extract was suspended in water (H2O) and partitioned successively with petroleum ether (PE), ethyl acetate (EtOAc) and n-butanol (n-BuOH).The n-BuOH soluble fraction was subjected to silica gel CC (column chromatography), Sephadex LH-20, reversed phase silica gel CC and semi-preparative High Performance Liquid Chromatography (HPLC)

Structure Elucidation
Compound 1 was isolated as a white amorphous powder and showed a positive reaction (red colour) to the Ehrlich reagent.The molecular formula, C45H74O19, was established from a positive molecular ion peak at m/z 941.4727 [M+Na] + (calcd.for C45H74NaO19: 941.4717) in the HR-ESI-MS spectrum and supported by the ESI-MS (m/z 941. 4 [M+Na] + and m/z 917.3 [M−H] − ) spectrum.Its IR spectrum displayed strong absorption bands for hydroxyl groups at 3408 cm −1 , for carbonyl group at 1707 cm −1 and absorption bands of alkyl groups at 2927 cm −1 .The 1 H-and 13 C-NMR assignments of 1 were based on the DEPT and 2D-NMR (COSY, HSQC, HMBC and NOESY) experiments and with the positive red colour reaction in Ehrlich's test suggested 1 was a furostanol saponin.To be specific, The 1 H-NMR spectrum of 1 showed four methyl proton signals including two tertiary methyl groups at δH 0.86 (3H, s, H-18) and 0.80 (3H, s, H-19) and two secondary methyl groups at δH 1.04 (3H, d, J = 6.0 Hz, H-21) and 0.98 (3H, d, J = 5.0 Hz, H-27), corresponding to C-atom signals at δC 15.5, 12.1, 14.7 and 15.9 in HSQC spectrum, typical steroid methyl signals.Besides, a signal for carbonyl at δC 212.0 was present in the 13 C-NMR.A correlation signal between H-7 at δH 2.16 and C-6 at δC 212.0 was observed in the HMBC spectrum (Figure 2), which suggested the carbonyl is located at C-6.Moreover, a methoxyl group at δH 3.17 (3H, s, OCH3) was also observed in the 1 H-NMR spectrum.In the HMBC spectrum, the correlations from OCH3 at δH 3.17 to C-22 at δC 112.6 to suggested the OCH3 was linked to the C-22 (Figure 2).The configurations of 1 were mainly determined by NOESY spectrum.A NOESY correlation signal between the H-5 proton at δH 2.40 and the H-9 proton at δH 1.40 was consistent with the 5α configuration.A NOESY cross-peak between H-5 (δH 2.40) and H-3 (δH 3.36) indicated that H-3 was α configuration.In addition, The NOE correlation between H-18 (δH 0.86, 3H, s) and H-20 (δH 2.21, 1H, m) suggested that the C-21 methyl group was α-configuration.Thus, the α-configurations of H-17 and the methoxy at C-22 position were unambiguously deduced based on the strong NOE correlations of H-21/H-17 and H-21/OCH3.The 14α configuration was further confirmed by the NOE correlations of H-14/H-16 and H-16/OCH3 [11].However, the C-25 configuration of 1 was assigned as 25R based on the observed difference (∆ab = δa − δb = 0.35) of the 1 H-NMR chemical shifts of the H2-26 geminal protons, which was in agreement with that of 25R furostane-type steroidal saponins (∆ab < 0.48 for 25R; ∆ab > 0.57 for 25S) [12].In addition, three anomeric proton signals at δH 4.42 (1H, d, J = 6.5 Hz, glc-1'), 4.34 (1H, d, J = 6.0 Hz, ara-1'') and 4.26 (1H, d, J = 7.0 Hz, glc-1''') existed in the 1 H-NMR spectrum and the HSQC showed correlations with anomeric carbon signals at δC 100.9, 103.8 and 103.2, respectively, indicating the presence of three sugar moieties.Combined the 1 H-NMR, 13 C-NMR (including DEPT) and 2D-NMR (HSQC, HMBC and COSY), two glucopyranosyls and one arabinopyranosyl were found.The relative configurations of the two glucopyranose moieties were all assigned as β-configurations based on their coupling constants (J = 6.5 Hz, Glc-1'; J = 7.0 Hz, Glc-1''') of the anomeric protons.The relative configuration of the arabinopyranose moiety was determined as α-confirguration by the coupling constants (J = 6.0 Hz, Ara-1'').The sugars were determined to be D-glucose and L-arabinose by acid hydrolysis of 1 and then trimethylsilylation and GC analysis on a chiral column in comparison with an authentic sample.Furthermore, the location of the glucopyranosyl was found to be C-26 of the aglycon on the basis of a glycosylation shift of C-26 at δC 74.6 and the HMBC correlation peak between H-1''' of glucopyranosyl at δH 4.26 and C-26 of the aglycone at δC 74.6, which is a structural feature in plant furostanol saponins [11].The attachments of sugar chain were deduced from the HMBC spectrum, in which the long range correlation between glc-H-1' at δH 4.42 with C-3 at δC 76.8 of the aglycone and ara-H-1'' at δH 4.34 with C-6' at δC 68.2 of the glucopyranosyl were observed (Figure 2), respectively.These results implied that the glucopyranosyl was linked to C-3 of the aglycone and the two sugars were the 1→6 linkage.Thus, the structure of 1 was elucidated as 26 Compound 2, obtained as a white amorphous powder, gave a positive red colour reaction in Ehrlich's test.The HR-ESI-MS spectrum of 2 showed a positive ion [M+Na] + at m/z 647.3762, consistent with the molecular formula C34H56O10 (calcd.for C34H56NaO10: 647.3766), which was supported by the ESI-MS (m/z 647. 4 [M+Na] + and m/z 623.4 [M-H] − ) spectrum.In the IR spectrum, the glycosidic nature of 2 was shown by strong hydroxyl groups absorptions at 3423 cm −1 , a carbonyl group at 1713 cm −1 and the alkyl groups at 2927, 2872 and 2853 cm −1 was also displayed in the IR spectrum.Compound 2 was suggested to be a furostanol glycoside with one sugar moiety by Ehrlich's test and the 1 H-and 13 C-NMR assignments of 2 based on the DEPT and 2D-NMR (COSY, HSQC, HMBC and NOESY) experiments.The aglycone of 2 was deduced as the same as that of 1 by comparison the 1 H-and 13 C-NMR spectra of 2 with those of 1.This was further confirmed by comparison Rf values of the aglycones obtained by acid hydrolysis of 1 and 2 on TLC.The only one sugar moiety was assigned to C-26 by a glycosylation shift of C-26 at δC 74.6 and the HMBC correlation between H-1 of the glucopyranosyl at δH 4.26 and C-26 at δC 74.6 (Figure 2).The relative configuration of the glucopyranose moiety was determined as β by the coupling constant (J = 8.0 Hz, Glc-1''') of the anomeric proton.The glucopyranose moiety was determined as D-glucose by acid hydrolysis of 2 and then trimethylsilylation and GC analysis on a chiral column in the same manner.The 25R configuration of 2 was deduced by the difference (∆ab = 0.35) of the chemical shifts of the H2-26 geminal protons.Thus, the structure of 2 was elucidated as 26-O-β-D-glucopyranoside-3β,26dihydroxy-(25R)-5α-furostan-22α-methoxyl-6-one.
Compound 3 was obtained as a white amorphous powder and positive ion HR-ESI-MS provided an [M+Na] + ion m/z 615.3510, corresponding to a molecular formula of C33H52O9 (calcd.for C33H52NaO9: 615.3504), which was supported by m/z 615.3 [M+H] + and m/z 591.3 [M-H] − in the ESI-MS spectrum.The IR spectrum showed the presence of hydroxyl groups at 3421 cm −1 , a carbonyl group at 1697 cm −1 and alkyl groups at 2943 cm −1 , 2915 cm −1 and 2865 cm −1 .It was apparent from the NMR spectroscopic data ( 1 H, 13 C, COSY, HSQC, HMBC and NOESY) of 3 that this compound differed from 2 only by the presence of the ∆ 20,22 double bond.The occurrence of a ∆ 20,22 double bond was confirmed from the HMBC spectrum which showed significant correlation peaks between the methyl proton signal of CH3-21 with the carbon resonances of C-20 (δC 103.6) and C-22 (δC 151.8) (Figure 2).The sugar moiety also was assigned as β-glucopyranose based on the coupling constant (J = 8.0 Hz, Glc-1''') of the anomeric proton.Its absolute configuration was also a D-as determined by acid hydrolysis of 3 and then trimethylsilylation and GC analysis on a chiral column.The α-configuration of H-17 was determined by the NOE correlations of H-14/H-17.The 25R configuration of 3 was assigned by the observed difference (∆ab = δa − δb = 0.32) of the 1 H-NMR chemical shifts of the H2-26 geminal protons (∆ab < 0.48 for 25R; ∆ab > 0.57 for 25S).Therefore, compound 3 was identified as 26-O-β-Dglucopyranoside-3β,26-dihydroxy-(25R)-5α-furostan-20( 22)-en-6-one.
Compound 4, a white amorphous powder, was assigned as C33H52O10 on the basis of its HR-ESI-MS data [M+Na] + m/z 631.3450 (calcd.for C33H52NaO10: 631.3453) and its ESI-MS data m/z 631.55 [M+Na] + , 609.34 [M+H] − and 1215.03[2M−H] − .The IR spectrum showed absorptions for hydroxyl groups (3422 cm −1 ), carbonyl group (1708 cm −1 ) and alkyl groups (2934 cm −1 and 2870 cm −1 ).The 1D and 2D NMR spectroscopic data of 4 were in good agreement with those of 3 except for noticeable differences in the δH-23 and δC-23 of the aglycone.The sugar moiety also was determined as β-D-glucopyranose by the coupling constant (J = 8.0 Hz, Glc-1''') of the anomeric proton and GC analysis of hydrolysis followed trimethylsilylation of 4 in comparison with an authentic sample.The configurations of H-17 and the C-25 of 4 was assigned as the same as those of 3 by the NOE correlation of H-14/H-17 and the ∆ab = 0.24 of the two geminal protons of H2-26.In the 1 H-NMR spectrum, the proton signal at δH 4.91 (1H, t-like, J = 7.0 Hz, H-23), corresponded C-atom signal at δC 63.6 in the HSQC spectrum showed the presence of the hydroxyl at the C-23.On the basis of this evidence, the structure of 4 was revised as 26-O-β-D-glucopyranoside-3β,23,26-trihydroxy-(25R)-5αfurostan-20(22)-en-6-one.Besides the four new furostanol saponins, three known steroid saponins were isolated from this plant.Their structures were identified by compared the NMR properties with the reported data.

Cytotoxic Activity
The cytotoxicity of compounds 1-7 were tested against human cervical carcinoma cells (Hela) and human hepatocellular carcinoma cells (SMMC-7221) by the MTT method.The results revealed that compounds 2-6 were inactive (IC50 > 100 µM), while compounds 1 and 7 displayed cytotoxicity against Hela carcinoma cell lines with IC50 values of 18.79 ± 1.12 μM and 9.73 ± 1.64 μM, respectively and against SMMC-7221 cancer cell lines with IC50 values of 28.57± 1.57 μM and 21.54 ± 1.64 μM, respectively.Cisplatin used as the positive control had the IC50 values of 5.12 ± 0.26 μM (Hela) and 14.23 ± 0.44 μM (SMMC-7221).This investigation indicated that compound 7 which displays a spirostanol saponin structure showed stronger cytotoxic activity, whereas compound 1, with a furostanol saponin structure, displayed moderate cytotoxic activity.Thus, according to our results, the spirostanol saponin skeleton exhibits higher cytotoxic activity than furostanol saponins, which is consistent with the literature [13].

General
Optical rotations were measured with a Rudolph Research Analytical Autopol III automatic polarimeter (Rudolph Research Analytical, Hackettstown, NJ, USA) with MeOH as solvent.The IR spectra were conducted in KBr on a Bruker Tensor 27 spectrophotometer (Bruker, Billerica, MA, USA).The UV spectra were recorded on a Thermo Scientific Evolution 300 UV-visible spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA).The 1 H-(500 MHz), 13 C-(125 MHz) and 2D-NMR spectra were recorded on a Bruker DRX-500 instrument using TMS as internal standard.ESI-MS were collected on a Thermo Fisher LCQ Fleet mass spectrometer instrument (Thermo Fisher Scientific).HR-ESI-MS were carried out on a Bruker Bio-TOF-IIIQ mass spectrometer (Bruker).Absorbance value was measured by a Bio-Tek Epoch Absorbance Microplate Reader to calculate the inhibition rate (Bio-Tek, Winooski, VT, USA).

Plant Material
The rhizomes and roots of S. scobinicaulis were collected from Taibai Mountain of Shaanxi Province, China, in October 2010.The plant was identified by Zhenghai Wu of Northwest A&F University, Yangling, Shannxi, China.An authenticated voucher specimen (XB 00045) has been deposited at the Herbarium of Northwest A&F University, Yangling, Shannxi, China.

GC Analysis of the Sugar Moieties in 1-4
Compounds 1-4 (each 4 mg) in 1 M HCl (5 mL, dioxane-H2O 1:1, v/v) were heated at 95 °C for 6 h, respectively.The reaction mixtures were evaporated in vacuo.The residues were dissolved in water and then extracted with CHCl3 for three times, respectively.After evaporating the aqueous phases to dryness in vacuo, the residues were dissolved in pyridine (5 mL) and 1-(trimethylsilyl)-imidazole (0.5 mL) at room temperature for 30 min.The reaction mixtures were dried with a stream of N2.The residues were partitioned between CHCl3 and H2O.The organic layers were subjected to GC analysis using an L-Chirasil-Val column (0.32 mm × 25 m) [14].Temperature of the injector and detector was 200 °C.A temperature gradient system was used for the oven; the initial was temperature was maintained at 100 °C for 1 min and then increasing up to 180 °C at a rate of 5 °C/min.Peaks of the hydrolysate of 1 were detected at 8.92 and 9.80 min (L-arabinose) and 14.71 min (D-glucose).However, respectively detecting only one peak at 14.73, 14.72 and 14.72 min in the hydrolysate of 2, 3 and 4, suggested that all the sugar moieties of 2, 3 and 4 are D-glucose.Retention times for authentic samples of D-glucose and L-arabinose (Sigma Aldrich, St. Louis, MO, USA) after being treated in the same manner with 1-(trimethylsilyl)-imidazole in pyridine were detected at 8.90 and 9.78 min (L-arabinose), 14.71 min (D-glucose).

Cytotoxicity Assay
The cytotoxic activities of compounds 1-7 were performed against human cervical carcinoma cells (Hela) and human hepatocellular carcinoma cells (SMMC-7221) by MTT [3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide] assay [15], with cisplatin as positive control.Cell lines were maintained in Dulbecco's Modified Eagle Medium (DMEM) medium containing 10% fetal bovine serum and were cultivated in humidified incubator at 5% CO2 and 37 °C.The human cancer cells in the log phase of their growth cycle (2.5 × 10 4 cells/mL) were added to each well of the 96-well plates (100 µL/well) and were cultivated for 16 h.The test compounds were then added and the cells were further incubated for 48 h.After, 20 µL of MTT solution (5 mg/mL in PBS) were added to each well and the cells were incubated for addtitional 4 h.The supernatant was discarded and after washing with PBS, DMSO (100 µL/well) was added to dissolve the formazan crystals.The optical density (OD) was measured by enzyme immunoassay instrument at 570 nm.Each assay was done in triplicate.

Conclusions
Four new furostanol saponins 1-4 were isolated and identified from the rhizomes and roots of Smilax scobinicaulis.Besides, two known furostanol saponins 5-6 and one known spirostanol saponin 7 are described for the first time in this plant.Among the isolated saponins, compounds 1 and 7 displayed cytotoxicity against the Hela and SMMC-7221 cancer cell lines.The discovery of these new