Kaempferol Rhamnosides from Geranium sibiricum as Aldose Reductase Inhibitors and Their Content by HPLC Analysis

: The study aimed to assess the aldose reductase (AR) inhibition of selected Geranium species and determine the bioactive ﬂavonoid constituents. Flavonoids are known to be good AR inhibitors. Among the species examined, G. sibiricum exhibited potent inhibition of AR (IC 50 value, 2.4 µ g / mL). Further examination of G. sibiricum , after solvent extraction and fractionation, revealed that the ethyl acetate fraction (IC 50 value, 0.41 µ g / mL) had a potent AR inhibitory e ﬀ ect. Kaempferol rhamnosides were the active compounds from this fraction. Moreover, G. sibiricum showed the highest content of kaempferol-7- O -rhamnoside and kaempferol-3,7- O -dirhamnoside among the samples examined with a concentration in the extracts of 28.1 and 2.2 mg / g, respectively. This study shows that G. sibiricum exhibits promising AR inhibitory activity, which can be explored further as a natural therapy for treating and managing complications associated with diabetes.


Introduction
The polyol pathway is involved in glucose to fructose conversion via the formation of sorbitol as an intermediate [1]. It is one of the major cellular pathways linked to many complications associated with diabetic hyperglycemia, such as cataract formation, nephropathy, neuropathy, and other microvascular diseases [2]. Investigations in animal models have shown that diabetic complications can be prevented and delayed by inhibiting aldose reductase (AR) [3][4][5]. Currently, studies have aimed to identify potent agents that can effectively inhibit AR activity without causing adverse side effects, such as those observed when using synthetic aldose reductase inhibitors (ARIs) [6]. Inhibition of AR activity prevents and alleviates these complications. Thus, research on ARIs has been geared toward the use of novel agents from natural sources, as they may be a safer and more effective alternative. Among the different types of natural products, polyphenolic compounds, such as flavonoids, are the most widely reported to possess notable inhibitory activity against AR [7][8][9][10]. Furthermore, other compounds, such as capsaicin [11], pyrogallol [12], and bromophenol [13], have also shown good AR inhibition.
The genus Geranium is composed of herbaceous plants taxonomically classified under the Geraniaceae family, comprising 420 species distributed all over the world and mainly present in Southern Africa [14]. They have simple, glandular, and deeply lobed leaves and flowers that are composed of five carpels, sepals, petals, five to fifteen stamens, and yield five fruits. A distinct morphological feature shared by the Geraniaceae family is the beak-like shape of the fruits [15]; hence the etymology of the word Geranium, from "geranos" meaning crane [16]. Studies on the phytochemical composition of different Geranium species have frequently reported on the abundance of polyphenolic compounds, such as flavonoids, tannins, catechins, and phenolic acids. Flavonoids and other polyphenolic compounds are classes of compounds that have been shown to exert AR inhibitory activities. As far as we know, there are no studies yet regarding the inhibitory effects of Geranium species against AR. Therefore, the study aimed to assess the AR inhibitory activities of several species of Geranium and to isolate and characterize the flavonoids from G. sibiricum that showed the highest IC 50 values. Moreover, quantitative analysis was performed using high-performance liquid chromatography (HPLC).

Plant Materials
Six species of the genus Geranium (G. eriostemon var. reinii, G. koreanum, G. kunthii, G. nepalense subsp. thunbergii, G. sibiricum, and G. wilfordii) were tested for their in vitro activity against crude rat lens AR. The methanol (MeOH) extracts of the selected species were purchased from Korea Plant Extract Bank, KRIBB, South Korea. Aerial parts of G. sibiricum were collected from Korea National Arboretum, South Korea on 13 July 2016. A voucher specimen (K. Choi and H.J. Kim, 2016001) was deposited in Korea National Herbarium.

Extraction and Isolation of Flavonoids from G. sibiricum
Air-dried aerial parts of G. sibiricum (684 g) were extracted with MeOH under a reflux at 80 • C for 3 h. The resulting extract after 6 times of extraction was filtered with filter paper (185 mm) and evaporation was completed using a rotary evaporator to obtain a concentrated MeOH extract (82 g). The MeOH extract was subjected to water suspension and subsequently partitioned with solvents of varying polarity, namely n-hexane, CHCl 3 , EtOAc, and n-BuOH. The partitioned layer of organic solvent was then evaporated to afford the n-hexane, CHCl 3 , EtOAc, and n-BuOH fractions. Compounds from the EtOAc fraction were isolated using open column chromatography (CHCl 3 : MeOH, 1:0 to 0:1). In the EtOAc fraction, 35 sub-fractions were obtained. Compounds 1 and 2 were obtained from the recrystallization of Sub-fractions 10 and 15, respectively. Both compounds were prepared by the MeOH recrystallization method. NMR assignment of compounds 1 and 2 was in Table 1.

Preparation of AR from Rat Lenses
AR was prepared from the lenses of healthy Sprague Dawley rats (Koatech Co., Pyeongtaek, Korea) based on a previous literature [17]. Lenses were resected from the rats and 0.1 M sodium phosphate buffer was used for homogenization (pH 6.2) followed by centrifugation at 10,000 rpm (4 • C for 20 min). The supernatant collected was used as an enzyme source.

Measurement of AR Inhibitory Activity
The AR inhibitory activities of G. sibiricum were measured by the decrease in the absorbance of β-NADPH at 340 nm for a period of 4 min using dl-glyceraldehyde as the substrate. The assay mixture was composed of the crude rat lens AR, 25 mM dl-glyceraldehyde, 1.6 mM NADPH, 100 mM sodium phosphate buffer, 100 mM potassium phosphate buffer (pH 7.0), and the sample dissolved in DMSO (1 mL). The AR inhibition of the samples was expressed as a percentage (Inhibition = (absorbance of control-absorbance of sample/absorbance of control) × 100). The IC 50 values were obtained from the least-squares regression line of the log of concentration plotted against residual activity.

Sample Preparation and HPLC Conditions
The MeOH extract of the Geranium species were dissolved in 20 mg/mL MeOH. Standard solutions of compounds 1 and 2 were prepared by 1 mg/mL MeOH. All samples were filtered through a 0.45 µm membrane-filter before use. Chromatographic separation (Figure 1) of the G. sibiricum extract was performed on a C 18 reversed-phase INNO column (4.6 × 250 mm, 5 µm). The mobile stationary was a mixture of water (Solvent A) and acetonitrile (Solvent B), and a gradient elution was followed (90% A to 60% A for 30 min). The flow rate and injection volume were set at 1 mL/min and 10 µL, respectively. The column temperature was maintained at room temperature and UV absorbance was monitored at 270 nm. All injections were done in triplicate. In Figure 1, the arrows of compounds 1 and 2 in MeOH extracts were checked by spike tests. As cab be observed in Figure 1, the retention time values of compounds 1 and 2 were similar to the retention time values of the spikes found in the chromatograms of the samples.
Processes 2020, 8, x FOR PEER REVIEW 4 of 10 followed (90% A to 60% A for 30 min). The flow rate and injection volume were set at 1 mL/min and 10 µL, respectively. The column temperature was maintained at room temperature and UV absorbance was monitored at 270 nm. All injections were done in triplicate. In Figure 1, the arrows of compounds 1 and 2 in MeOH extracts were checked by spike tests. As cab be observed in Figure  1, the retention time values of compounds 1 and 2 were similar to the retention time values of the spikes found in the chromatograms of the samples.

Limit of Detection and Quantification (LOD and LOQ)
LOD is the lowest amount of analyte having a signal that is three times greater than the noise level and LOQ is the lowest amount of analyte that can be quantitated with a signal-to-noise ratio of 10. The equations used to determine LOD and LOQ are as follows: LOD = 3.3 (σ/S) and LOQ = 10 (σ/S) where σ is the intercept and S is the slope ( Table 2).

Calibration Curves
Different concentrations of compounds 1 and 2 were prepared by dilution of the standard solutions (0.1-1.0 mg/mL). The calibration curve (Table 2) for each standard was constructed using concentration (X, μg/mL) and peak area (Y). The assessment of linearity was based on the correlation coefficient (r 2 ). The contents of the analytes in the samples were estimated from the constructed calibration curves. a Y = peak area, X = concentration of standard compound (µg/mL); b r 2 = correlation coefficient for 6 data points in the calibration curve; LOD and LOQ were determined from the calibration curve of each standard compound using the values of the standard deviation of σ and S.

Results
To our knowledge, there are no studies regarding the investigation of the AR inhibitory activity of Geranium species. The study aimed to detect the inhibitory effects of selected Geranium species on AR and to characterize their bioactive constituents. The results revealed that among the Geranium species examined, the extracts of G. sibiricum exhibited potent AR inhibitory efficacy (IC50 value, 2.4 µg/mL) ( Table 3).

Limit of Detection and Quantification (LOD and LOQ)
LOD is the lowest amount of analyte having a signal that is three times greater than the noise level and LOQ is the lowest amount of analyte that can be quantitated with a signal-to-noise ratio of 10. The equations used to determine LOD and LOQ are as follows: LOD = 3.3 (σ/S) and LOQ = 10 (σ/S) where σ is the intercept and S is the slope ( Table 2). a Y = peak area, X = concentration of standard compound (µg/mL); b r 2 = correlation coefficient for 6 data points in the calibration curve; LOD and LOQ were determined from the calibration curve of each standard compound using the values of the standard deviation of σ and S.

Calibration Curves
Different concentrations of compounds 1 and 2 were prepared by dilution of the standard solutions (0.1-1.0 mg/mL). The calibration curve (Table 2) for each standard was constructed using concentration (X, µg/mL) and peak area (Y). The assessment of linearity was based on the correlation coefficient (r 2 ). The contents of the analytes in the samples were estimated from the constructed calibration curves.

Results
To our knowledge, there are no studies regarding the investigation of the AR inhibitory activity of Geranium species. The study aimed to detect the inhibitory effects of selected Geranium species on AR and to characterize their bioactive constituents. The results revealed that among the Geranium species examined, the extracts of G. sibiricum exhibited potent AR inhibitory efficacy (IC 50 value, 2.4 µg/mL) ( Table 3). The MeOH extract and solvent fractions of G. sibiricum were tested for AR inhibition. The EtOAc fraction potently inhibited the enzyme (IC 50 value, 0.41 µg/mL) ( Table 4). This means that the compounds from the EtOAc fraction were responsible for the inhibitory activity. Further examination of the bioactive components of the EtOAc fraction of G. sibiricum by open column chromatography yielded compounds 1 and 2 (Figure 1). The chemical structures of the compounds were elucidated by spectroscopic analysis, including 1 H-and 13 C-NMR and fast atom bombardment (FAB)-MS spectroscopy. The 1 H-NMR data for compounds 1 and 2 revealed signals typical of flavonoids ( Table 1). The FAB-MS analysis of compounds 1 and 2 showed a molecular ion peak at m/z 433 [M + 1] + and m/z 579 [M + 1] + , respectively. Compounds 1 and 2 were identified as kaempferol-7-O-rhamnoside (KR) and kaempferol-3, 7-O-dirhamnoside (KD), respectively [18,19] (Figure 2). The IC50 values of KR and KD on AR were 1.23 and 0.55 µM, respectively (Table 5). KD was more potent than KR, with IC50 value comparable to that of TMG (0.74 µM). Quantitative analysis of KR and KD in six Geranium species was carried out using HPLC. Good separation was observed in the samples analyzed, as seen in Figure 1. The calibration curves for each reference compound showed high linearity (Table 2), and the results of the analysis are summarized in Table 6. KR and KD were not detected in G. kunthii and G. wilfordii, whereas both compounds were present in the four remaining species analyzed. G. sibiricum contained a high concentration of KR in the methanol extract (28.10 mg/g).

Discussion
Studies on the phytochemistry of Geranium species have revealed that they are abundant in many phenolic constituents, including catechins, phenolic acids, flavonoids, and tannins [20]. The potent antioxidant and other bioactive properties of Geranium species have been attributed to the presence of these polyphenolic compounds [21][22][23]. Moreover, polyphenolic compounds, such as flavonoids, potently inhibit AR [24,25]. Among the Geranium species examined, G. sibiricum exhibited The IC 50 values of KR and KD on AR were 1.23 and 0.55 µM, respectively (Table 5). KD was more potent than KR, with IC 50 value comparable to that of TMG (0.74 µM). Quantitative analysis of KR and KD in six Geranium species was carried out using HPLC. Good separation was observed in the samples analyzed, as seen in Figure 1. The calibration curves for each reference compound showed high linearity (Table 2), and the results of the analysis are summarized in Table 6. KR and KD were not detected in G. kunthii and G. wilfordii, whereas both compounds were present in the four remaining species analyzed. G. sibiricum contained a high concentration of KR in the methanol extract (28.10 mg/g).

Discussion
Studies on the phytochemistry of Geranium species have revealed that they are abundant in many phenolic constituents, including catechins, phenolic acids, flavonoids, and tannins [20]. The potent antioxidant and other bioactive properties of Geranium species have been attributed to the presence of these polyphenolic compounds [21][22][23]. Moreover, polyphenolic compounds, such as flavonoids, potently inhibit AR [24,25]. Among the Geranium species examined, G. sibiricum exhibited potent AR inhibitory efficacy (IC 50 value, 2.4 µg/mL). G. sibiricum is an indigenous perennial herb in Europe and Asia, commonly referred to as the Siberian cranesbill.
It has been used in herbal preparations in traditional medicine for diarrhea treatment, eruptive skin diseases, wounds, and intestinal inflammation. Recent studies have reported that the beneficial effect of G. sibiricum on human health may be attributed to its potent anti-oxidant activity and to its high content of polyphenolic compounds [26][27][28]. Its extract has been shown to exert hair-promoting, xanthine oxidase inhibitory, anti-inflammatory, and anti-proliferative effects [26][27][28][29]. G. nepalense subsp. thunbergii is also used in ethnomedicinal applications for treating dysentery, influenza, and diabetes [30]. However, the ability of G. sibiricum to inhibit AR has never been examined to date, and as far as we know, this study is the first investigation regarding the inhibition of AR.
The tested EtOAc fraction for inhibitory efficacy against rat lens AR potently inhibited the enzyme (IC 50 value, 0.41 µg/mL). The EtOAc fraction of G. sibiricum has been previously reported to possess powerful anti-oxidant activity and xanthine oxidase inhibitory efficacy owing to its high phenolic concentration [28]. Similarly, in this study of AR inhibition, the EtOAc fraction was the bioactive fraction of G. sibiricum that showed the highest IC 50 value. KR and KD from the EtOAc fraction of G. sibiricum were tested for their inhibitory efficacy against rat lens AR. There have been no previous studies pertaining to the inhibition of the polyol pathway by KR and KD, and this study is the first to report on their AR inhibitory efficacies. KR and KD are flavonoid compounds. There are other AR inhibitory compounds, such as capsaicin [11], pyrogallol [12], and bromophenol [13] present in G. sibiricum. The potent AR inhibition showed by the G. sibiricum species can be attributed to its high concentrations of KR and KD, since these two compounds were the major components of the EtOAc fraction that showed the highest IC 50 value. This species also exhibited the best AR inhibition among all the species examined in this study. These results may have application in the development of novel therapeutic approaches to target complications associated with diabetic hyperglycemia.