1. Introduction
As a common ingredient in the Asia traditional medicine,
Angelica dahurica Radix (AD) is the dried root of
Angelica dahurica (Fisch. ex Hoffm.) Benth. et Hook. f. or
A. dahurica (Fisch. ex Hoffm.) Benth. et Hook. f. var. Formvsana (Boiss) Shan et Yuan. Especially in China, AD is one of the common traditional Chinese medicines (TCM), which has been used for the treatment of wind-cold type of common cold, headache, rhinitis, and toothache and has been officially listed in the Chinese Pharmacopoeia [
1].
Coumarin is the main chemical components in AD [
2,
3,
4]. The reported studies have shown that a majority of coumarins in AD are photosensitive and can be used for the treatment of hypopigmentation disease because of their photosensitization [
5,
6]. For instance, the compound Angelica Tincture, which is widely used in the treatment of vitiligo clinically, is prepared by taking AD and Psoraleae Fructus as the principal ingredients and then dipping them in 70% ethanol after being powdered [
7].
Tyrosinase is the key rate-limiting enzyme in melanin biosynthesis pathway [
8,
9]. Interestingly, photosensitive furocoumarin extracted from AD can enhance tyrosinase activity, thus increasing the synthesis of melanin and achieving the treatment of vitiligo [
7]. However, clinical practice and research have shown that AD also has whitening effect. According to the traditional Chinese medical science theory, the whitening TCMs can be used to treat skin disease caused by hyperpigmentation. AD is one of the whitening TCMs and is selected to make up the famous classical prescription for whitening named “Seven-White Ointment”, which was historically recorded as “Tai Ping Sheng Hui Fang” in Song Dynasty, “Yu Yao Yuan Fang” in Yuan Dynasty, and medicine “Pu Ji Fang” in Ming Dynasty). All of the water extracts of the TCMs in this prescription (except the egg white) had the inhibitory effect on tyrosinase activity [
10].
The bidirectional regulation of AD on melanogenesis is closely related to the extract fractions and concentration [
11,
12,
13,
14,
15]. There may be the components in AD with inhibitory and activating effects on tyrosinase activity, respectively. In order to prepare the components with different influences on tyrosinase activity in AD clear, the method of spectrum-effect relationship which is considered to be a systematic approach to TCM research was adopted firstly [
16]. We conducted study on the spectrum-effect relationship of TCMs by establishing mathematical model to connect the characteristic peaks with the pharmacodynamic value, and to explore the correlation between them, so as to provide a reliable method for elucidating the material basis of TCMs. In this study, we prepared the sample by water decoction method, which is commonly used for TCM clinically, and then established the chromatographic fingerprint of AD and the evaluation model of effect on tyrosinase activity in vitro, analyzed the relationship between the characteristic peaks with the pharmacodynamic value with the established mathematical model, fitted the active components related to inhibition on tyrosinase, so as to provide reference for the pharmacodynamic material basis of AD. Secondly, we applied component knock-out method to find out active components of AD on tyrosinase activity for a more comprehensive research. Pharmacodynamic material basis identification model of TCM based on component knock-out suggest that the target component will be knocked out from the complete sample and the changes in the efficacy of the samples before and after the target component is knocked-out can reflect the contribution of the target component to the efficacy of the TCM. By this way, the interactions between the target component and other components can also be investigated. The continued study was guided by the result of spectrum effect relationship research. High performance liquid chromatography (HPLC) method was used to “knock out” and obtain the target components and negative samples. The effects of target components, negative samples and water extracts of AD on tyrosinase activity were obtained by the evaluation model of effect on tyrosinase in vitro and compared in parallel, so as to identify the components in AD related to the activity and their interactive effects on tyrosinase activity.
Recent studies showed that a significant portion of small-molecule drugs act on enzymes. Because enzymes are such important drug targets, it is not a surprise that constantly updated technologies and ideas have been performed with enzymes. High throughput screening is one of the most frequently used methods of enzyme analysis that can be defined as the implementation of assays in the wells of microplates in combination with liquid handling robotics [
17]. Our study drew on the idea of high throughput screening but made with adjustments. It was performed with the spectrum effect relationship research reoriented and the component knock-out method for verification, to achieve the high-throughput enzyme analysis of different compounds in AD with more purposiveness.
3. Materials and Methods
3.1. Materials
Acetonitrile was chromatographic grade. Glacial acetic acid was analytical grade. The pure water was purchased from Hangzhou Wahaha Baili Food Co., Ltd., (Hangzhou, Zhejiang, China). (l-3-(3,4-Dihydroxyphenyl) alanine was obtained from Alfa Aesar (Shanghai, China); Tyrosinase was from Worthington Biochemical Corporation (Lakewood, NJ, USA).
3.2. Plant Materials
Angelicae dahuricae Radix, identified by Professor Changqin Li of College of Pharmacy, Henan University (Kaifeng, Henan), were purchased in October 2014 in Yuzhou, Henan Province.
3.3. Classical Constant Temperature Method
AD was uniformly packed and weighted, then placed in 4 constant temperature drying box. The heating temperature and time were set (
Table 7). For each heating time point, 3 parallels were used. When heating process ended, the samples were cooled to room temperature and weighed.
3.4. Extraction
Fragments of each sample (about 2 g) was put in a test tube after being weighted precisely, 10 times amount (w:v) of distilled water was added in it. After being soaked for 30 min, the solution was heated to boiling and kept faint boiling for 30 min by water bath, then 4 layers of gauze was used to remove filter residue. 6 times the amount of distilled water (w:v) was added into residue and process was repeated. The filtrates from two times filtration were combined, dried, and then prepared to solution, which was equivalent to the amount of raw medicinal herbs at the concentration of 1 g·mL−1 by 50% methanol solution.
3.5. HPLC Analysis
A LC-20AT HPLC system was obtained from Shimadzu (Kyoto, Japan), and equipped with a degasser, a quaternary gradient low pressure pump, the CTO-20A column oven, a SPD-M20A UV-detector and a SIL-20A automatic sampler. All of the solutions were filtered through the 0.22 μm microporous membrane before they were injected into HPLC system.
Chromatography was performed with an InertSustain RP-C18 column (4.6 mm × 150 mm, 5 μm) at a column temperature of 30 °C The mobile phase was a mixture of acetonitrile (A)-0.1% glacial acetic acid-water (B). The gradient elution steps were set as shown in
Table 8 and the flow rate was set at 1.0 mL·min
−1. The UV detection wavelength was set at 312 nm with the sample volume of 30 μL.
3.6. Tyrosinase Inhibition Assay In Vitro
Water extracts of AD were dissolved in 50% methanol solution, and stored at 4 °C in refrigerator.
Tyrosinase inhibition assay was performed in a 96-well microplate format using Multiskan MK3 microplate reader (Thermo Electron) according to the method reported by Zhang [
29]. The compounds were screened for the inhibitory effects on tyrosinase activity using levodopa (LOP) as substrate. 45 μL of K-phosphate buffer (pH 6.8), and 25 μL of mushroom tyrosinase (TYR, 0.2 U·mL
−1) were incubated with 5 μL of sample at 30 °C for 10 min in water-jacket thermostatic incubator (Sumsung GRP-9270). Then LOP (0.5 mmol·L
−1) was added to the reaction mixture and incubated at 30 °C for 5 min. The enzymatic reaction was monitored by measuring the change in absorbanceat 492 nm (A
492) (at 30 °C) due to the formation of the dihydroxyphenylalanine (DOPA) chrome for 5 min. The percentage of inhibition of the enzymatic activity was calculated as follows: tyrosinase activation activity was expressed as activation rate under a certain concentration. The inhibition rates (%) were calculated according to the formula as follows:
Inhibition rate (%) = [(ASample + LOP + TYR − ASample + LOP)/(A50%Me + LOP + TYR − A50%Me + LOP) − 1] × 100%
3.7. Partial Least Squares Analysis
The software < Chinese traditional medicine chromatographic fingerprint similarity evaluation system 2004, 1.0 A Edition > that Chinese Pharmacopoeia Commission recommended was used to correct the retention times of each peak, and the peak area was processed by equalization. Then the quantitative data were obtained. The partial least squares regression equation was established with the analysis software DPS 7.05, and the peak area was set as the independent variable (X), tyrosinase inhibition rate was taken as the dependent variable (Y). Chromatographic peaks, which were significantly correlated with inhibitory effects on tyrosinase ability, were determined, respectively.
3.8. Knock-Out Method
Under the optimized chromatography conditions described in section “HPLC analysis”, the water extract of AD was prepared as 1 g·mL−1 equivalent to raw medicinal herbs. Injection volume was 50 μL every time. The chromatogram of 312 nm was recorded. According to the peak retention time from the spectrum effect relationship analysis, the eluent solution containing the target component and the other eluent solution namely negative solution were collected, respectively. Each component was prepared and eluted in 10-fold series. The solution containing target component and negative solution were combined respectively, dissolved with 0.5 mL of 50% methanol water solution and filtered through the 0.22 μm microporous membrane, which was a sample containing target component (denoted as Sx+) and the corresponding negative sample (denoted as Sx−).
3.9. The High Performance Liquid Chromatography four Stage Rod-Electrostatic Field Orbit Trap High Resolution Mass Spectrometry
The compounds were detected using QExactive four stage rod-orbit trap LC-MS/MS system, containing Thermo Ultimate 3000 UHPLC system and QExactive (Thermo Fisher Scientific, Waltham, MA, USA). Separation was performed with a Waters BEH C18 column (2.1 mm × 50 mm, 1.7 μm; Waters, Milford, MA, USA). The mobile phase was a mixture of acetonitrile (C) and 0.1% formic acid-water (D), with an optimized linear gradient elution as follows: 0–2 min: 10–30% C; 2–10 min: 30–60% C; 10–18 min: 60–100% C; 18–25 min: 100% C; 25–26 min: 100–10% C; and 26–30 min: 10% C. The flow rate was 0.3 mL·min−1. The injection volume was 0.2 μL. The column temperature was set at 25 °C.
Compounds were analyzed with the full scan data in positive ion modes to provide complementary information for structural identification under the following mass spectrometry conditions: sheath gas flow rate, 35 arb; auxiliary gas flow rate, 10 arb; spray voltage, 3.5 kV; capillary temperature, 320 °C, a scan range, m/z 0–800 and a resolving power, 70,000. The automatic gain control (AGC) was set at 3e6 and the maximum injection time was set to 100 ms.
In addition to the full scan acquisition method, for confirmatory purpose, a targeted MS/MS analysis was also performed using the mass inclusion list and expected retention times of the target analytes, with a resolving power of 17,500. The AGC target was set to 1e5, with the maximum injection time of 50 ms. The isolation window was set at 4.0 m/z. Collision energy was optimized at 30 eV.
4. Conclusions
In this study, we investigated the components of AD that had inhibitory or activating effects on tyrosinase activity by using the spectrum-effect relationship and component knock-out method. The results showed that AD samples contained the activated and inhibitory components on tyrosinase activity simultaneously. The correlations of these components to inhibitory effects on tyrosinase activity were different, and there were either synergetic or antagonistic effects among these components. When the concentration of each sample was 1 g·mL−1 equal to the amount of raw medicinal herbs, oxypeucedanin hydrate, imperatorin, cnidilin, and isoimperatorin had inhibitory effects on tyrosinase activity whereas byakangelicin and bergapten had the activation effect on tyrosinase activity.