Correlation of antioxidant activity and volatile oil chemical components from Schizonepeta tenuifolia herbs by chemometric methods

ABSTRACT

In this paper, 15 batches of volatile oil from Schizonepeta tenuifolia herbs (STVO) originating from different areas of China were used to identify and quantify eight main components by GC-MS and to evaluate the antioxidant activity with 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity by the HPLC method. The results indicated that with respect to the antioxidant activity and the contents of the main compounds, there were notable differences among the STVOs. IC₅₀ values of the STVOs ranged from 0.016 to 1.518 mg/mL. To determine the correlation of the chemical components and the antioxidant activities, multiple linear regression analysis (MLRA) and partial least square analysis (PLSA) were performed. As a result, (+)-pulegone and (-)-menthone showed much closer relationships than the other components in each analysis. Hence, it was concluded that (+)-pulegone and (-)-menthone were the characteristic components that may exhibit potent antioxidant activities in STVO.

KEYWORDS:

• Antioxidant activity
• Chemometric method
• Correlation analysis
• GC-MS
• Schizonepeta tenuifolia
• Volatile oil

Introduction

Schizonepeta tenuifolia (ST), belonging to the Lamiaceae family, is classified as a Traditional Chinese Medicine (TCM) to release the exterior, disperse wind, promote eruption, and relieve sores. As an aromatic medicinal plant, the herb is widely distributed in China, Korea, and Japan. The dried aerial part of ST, called Jingjie in China, is recorded in the Chinese Pharmacopoeia for the treatment of common cold, headache, measles, rubella, early onset of sore, and ulcer.^[1] Modern pharmacological studies have shown that ST herbs have various biological functions, such as anti-inflammation,^[2–4] immunomodulation,^[2,5] relieving itching,^[6] and antioxidation.^[3,7] As we know, with anti-inflammatory,^[4,8,9] antitumor,^[10] and fumigant^[11] activities, the volatile oil of ST herbs (STVO) was used as a raw material in many TCM prescriptions recorded in Chinese Pharmacopoeia . Thus, STVO was considered responsible for the efficacies of ST herbs.

STVO contains many monoterpenoids, which mainly include (−)-limonene, (-)-menthone, (+)-menthofuran, and (+)-pulegone. These components have different bioactive or toxic activities. (+)-Pulegone is a natural monoterpene obtained from the volatile oils of a variety of plants. It was employed as flavouring agents and in perfumery. (+)-Pulegone decreased myocardial contractility and markedly reduced both the intracellular Ca²⁺ transient and L-type Ca²⁺ current. Additionally, (+)-pulegone caused changes in the action potential waveform and reductions of the outward and inward potassium current.^[12] As an important oxygenic terpenoid, (-)-menthone has been widely used for fragrance and flavour in the cosmetic, perfume, drug, and food industries. The component can also freely cross the blood–brain barrier and have been used as a cooling agent, a counterirritant for pain relief, and an antidepressant-like substance.^[13] (−)-Limonene is one of the most common monoterpenes in nature and has shown anxiolytic effect, regulatory effect on neurotransmitters, and antinociceptive effect.^[14] However, (+)-menthofuran is a toxic furan terpenoid that was oxidized by mammalian cytochrome P450 to hepatotoxic metabolites.^[15] In addition, 1-octen-3-ol, 3-octanone, β-myrcene, and β-caryophyllene are also the main constituents of STVO. Both 1-octen-3-ol and 3-octanone are typical “mouldy” odorants.^[16] β-myrcene has shown potent hepatoprotective, potential antimicrobial, antioxidant, and anti-inflammatory activities.^[17,18] Thus, it is obvious that all these eight compounds are the key ones to STVO due to their bioactives.

However, to our knowledge, the antioxidant activity of STVO has not been documented while there have been several literatures about the bioactivity of other essential oils extracted from herbs.^[19,20] At present, 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay,^[21,22] 2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS) assay,^[23,24] and β-carotene bleaching assay^[25,26] are the common in vitro methods of antioxidant activity evaluation. Among these methods, DPPH assay by HPLC was considered as the rapid, stable, and sensitive one.^[27–29]

In this study, a DPPH assay method was designed and validated to evaluate the antioxidant activities of different STVOs. Furthermore, we also aimed to investigate the correlation of chemical components and antioxidation of STVO and identify the characteristic component(s) responsible for the bioactivity by some chemometric methods.

Materials and methods

Chemicals and samples

Chemicals and reagents we used in the experiment are listed in Table 1. Fifteen batches of the ST herbs were collected from different locations in China, and were identified by Prof. Qi-Nan Wu in Nanjing University of Chinese Medicine. The voucher specimen was stored at the herbarium of Nanjing University of Chinese Medicine, China. Table 2 lists the sample’s information of different batches of ST herbs.

Table 1. Chemicals and reagents.

Chemicals and reagents	Supplier
3-octanone (C8H16O, 98.2% purity)	Sigma-Aldrich (Vienna, Austria)
β-myrcene (C10H16, 98.9% purity)	Sigma-Aldrich (Vienna, Austria)
(+)-menthofuran (C10H14O, 98.7% purity)	Sigma-Aldrich (Vienna, Austria)
(-)-limonene (C10H16, 96.0% purity)	National Institutes for Food and Drug Control (Beijing, China)
(-)-menthone (C10H18, 99.8% purity)	National Institutes for Food and Drug Control (Beijing, China)
(+)-pulegone (C10H16O, 99.8% purity)	National Institutes for Food and Drug Control (Beijing, China)
1-octen-3-ol (C8H16O, 99.8% purity)	Nanjing JinYibai Biological Technology Co., Ltd. (Nanjing, China).
β-caryophyllene (C15H24, 95.8% purity)	Nanjing JinYibai Biological Technology Co., Ltd. (Nanjing, China).
Naphthalene (C10H8,AR)	Sinopharm Chemical Reagent (Shanghai, China)
Sodium sulfate anhydrous (AR)	Sinopharm Chemical Reagent (Shanghai, China)
Ultrapure water	Milli-Q system (Millipore, Bedford, MA, USA)
Methanol (HPLC grade)	Merck (Darmsdadt, Germany)
Formic acid (HPLC grade)	Merck (Darmsdadt, Germany)
Ethyl acetate	Xilong Chemical Industry Co., Ltd. (Guangzhou, China)
DPPH	TCI Chemical Industry Co., Ltd. (Shanghai, China)

Table 2. Sample information of different bathes of ST herbs.

NO.	Location (China)	Lot.	NO.	Location (China)	Lot.	NO.	Location (China)	Lot.
1	Nanjing Jiangsu	151024	6	Suzhou Jiangsu	141022	11	Pingnan Guangxi	150819
2	Xuzhou Jiangsu	150712	7	Anguo Hebei	150820	12	Guilin Guangxi	151215
3	Chengdu Sichuan	140304	8	Bozhou Anhui	150912	13	Liuzhou Guangxi	150619
4	Chuxiong Yunnan	150202	9	Baoding Hebei	150724	14	Gaoyao, Guangdong	150725
5	Tongling Anhui	150939	10	Zhaoqing Guangdong	150408	15	Maoming Guangdong	160309

Herbal extraction and sample preparation

According to the Chinese Pharmacopoeia,^[1] STVOs were separately extracted with the hydro-distillation method using a set of standard instruments. Anhydrous sodium sulphate was added into the volatile oil at the ratio of 5 g:100 mL to remove the trace water. After standing overnight, all the STVOs were filtered and then stored in dark glass bottles at 4°C until use.

Gas chromatography–mass spectrometry analysis

Each STVO was dissolved and diluted with ethyl acetate to the concentration of about 80 μg/mL. Qualitative and quantitative analyses for 15 batches of the STVO were carried out using an Agilent 7890B-7000C GC-MS with an Agilent HP-5MS capillary column (30 m × 0.25 mm ID, 0.25 µm film thickness). Helium was the carrier gas at the flow rate of 1.0 mL/min. The oven temperature was set at 50°C for 2 min, increased to 90°C at a rate of 10°C/min and held for 3 min, and then increased to 150°C at a rate of 8°C/min and held for 3 min. The volume injected was 1 μL with a split ratio of 5:1. The inlet temperature was 220°C and the aux temperature was 200°C. Electron ionization (EI) worked as the ion source at 230°C with 70 ev and mass quadrupole was at 150°C. Scan mode was accepted from m/z 50 to 550.

Antioxidant activity assay

DPPH and samples preparation: Fresh DPPH stock solution was prepared by dissolving 80.32 mg DPPH in 100 mL methanol, which was stored at 4°C, and then diluted to the appropriate concentrations before use. Fifty milligrams of STVO were accurately weighed and transferred into a 10 mL volumetric flask, and then diluted to different concentrations for analysis (0.1, 0.25, 0.5, 1, 2, and 3 mg/mL).

HPLC analysis: The antioxidant activity was determined by the HPLC method as reported previously.^[27] Initially, an aliquot of 0.5 mL STVO solution was added to 0.5 mL DPPH solution (50 μg/mL). The mixture was vortexed for 30 s and reacted in the darkness for 60 min at room temperature. Then the mixture was filtered through a 0.45 μm microfiltration membrane and 10 μL of the sample was injected for HPLC analysis. A waters 2695 HPLC coupled with a photodiode array detector (Waters 2998) was used. The blank control was prepared by adding 0.5 mL methanol to 0.5 mL DPPH solution. The chromatographic separation was performed on a Hedera ODS-2 column (150 mm×4.6 mm×5 μm). Methanol-0.2% formic acid (80:20, v/v) was used as the mobile phase at the flow rate of 1.0 mL/min. The DPPH peak was monitored at 517 nm. The difference of DPPH peak areas (PAs) between the blank and the sample was used to calculate the radical scavenging activity according to the following equation: Radical scavenging rate (%) = (PA_blank—PA_sample)×100%/PA_blank.

Optimization of reaction condition between DPPH and STVO: As is known, reaction time and DPPH concentration are the key factors to the final result. With radical scavenging rate as the evaluation index, the reaction time and DPPH concentration were optimized in the range of 0~120 min and 50~800 μg/mL, respectively.

Antioxidant capacity for samples using IC₅₀: In this paper, IC₅₀ value was named as the concentration of STVO scavenging 50% DPPH free radicals. STVO solutions of different concentrations (0.1, 0.25, 0.5, 1, 2, and 3 mg/mL) and their corresponding radical scavenging rates were used to establish the probit interpretation equation with IBM SPASS Statistics 19.0 software. IC₅₀ value of each STVO could be calculated with the equation of this STVO.

Correlation analysis

Multiple linear regression analysis (MLRA)^[25,30] was used to model the relationship between two or more variables with SPSS statistics software (IBM SPASS Statistics19.0). Partical least square analysis (PLSA)^[31,32] was a wide class of methods for modelling relations between sets of observed variables by means of latent variables with SIMCA-P + 11.0 (Umetrics, Umea, Sweden). The general purposes of the two models were to learn about the relationship between the contents of the eight compounds and the antioxidant activity (IC₅₀) of STVOs. p-Values less than 0.05 and 0.01 were considered statistically significant and very significant, respectively.

Results and discussion

Chemical composition analysis of STVO by GC-MS

The GC-MS method was used to analyse the chemical compositions of 15 batches of hydro-distilled STVOs. Eight compounds (S1, 1-octen-3-ol; S2, 3-octanone; S3, β-myrcene; S4, (-)-limonene; S5, (-)-menthone; S6, (+)-menthofuran; S7, (+)-pulegone; and S8, β-caryophyllene) were identified with the data bank mass spectra (NIST libraries) and with the individual standard in each batch of STVO. The total ion chromatograms (TICs) and extract ion chromatograms (EICs) of STVO (sample 2) and the reference substances are shown in Fig. 1. The EI mass spectra of the eight compounds with their chemical structures are illustrated in Fig. 2. These figures showed good separation for the eight components and their accurate data spectra.

Figure 1. Total ion chromatograms of STVO (sample 2, A1) and reference substances (B1); extract ion chromatograms of STVO (sample 2, A1) and reference substances (B2).

1-octen-3-ol (S1), 3-octanone (S2), β-myrcene (S3), (-)-limonene (S4), (-)-menthone (S5).(+)-menthofuran (S6), (+)-pulegone (S7), β-caryophyllene (S8), and naphthalene (IS).

Figure 2. EI mass spectra and chemical structures of eight compounds in STVO.

1-octen-3-ol (S1), 3-octanone (S2), β-myrcene (S3), (-)-limonene (S4), (-)-menthone (S5), (+)-menthofuran (S6), (+)-pulegone (S7), β-caryophyllene (S8).

With naphthalene as the internal standard, the contents of the eight components were determined by our previously established method^[29,33] (Table 3). As far as the components were concerned, each sample exhibited varying contents with other samples. The results showed that the intrinsic qualities of these samples were different to some extent. Some differences in the contents or even in the qualities of the STVOs might be due to some environmental factors and planting habits. (+)-Pulegone and (-)-menthone were determined as the predominant components of STVO, accounting for 85%~95%.

Table 3. Contents of eight components from 15 batches of STVO (means±SD‚n = 3).

	Content (%)± SD (%)
No.	1-octen-3-ol	3-octanone	β-myrcene	(-)-limonene	(-)-menthone	(+)-menthofuran	(+)-pulegone	β-caryophyllene
1	0.67 ± 0.0091	0.14 ± 0.0012	0.64 ± 0.0062	4.44 ± 0.028	18.74 ± 0.093	0.32 ± 0.0089	67.36 ± 0.136	0.74 ± 0.0053
2	0.53 ± 0.0062	0.17 ± 0.0013	0.68 ± 0.0056	5.06 ± 0.048	19.35 ± 0.085	0.29 ± 0.0022	71.20 ± 0.134	0.80 ± 0.0064
3	0.46 ± 0.0054	0.13 ± 0.0019	0.62 ± 0.0052	3.59 ± 0.036	20.79 ± 0.102	0.40 ± 0.0011	71.97 ± 0.165	0.66 ± 0.0052
4	0.62 ± 0.0082	0.13 ± 0.0016	0.63 ± 0.0064	5.39 ± 0.054	19.80 ± 0.142	0.28 ± 0.0045	70.08 ± 0.243	0.86 ± 0.0062
5	0.63 ± 0.0063	0.15 ± 0.0017	0.59 ± 0.0056	4.28 ± 0.043	20.35 ± 0.149	0.21 ± 0.0021	65.05 ± 0.306	0.83 ± 0.0060
6	0.64 ± 0.0052	0.17 ± 0.0012	0.68 ± 0.0063	3.85 ± 0.040	15.68 ± 0.102	0.43 ± 0.0056	74.27 ± 0.218	0.73 ± 0.0070
7	0.69 ± 0.0074	0.25 ± 0.0020	0.68 ± 0.0076	6.05 ± 0.068	19.56 ± 0.084	0.21 ± 0.0038	63.60 ± 0.148	0.85 ± 0.0073
8	0.67 ± 0.0082	0.14 ± 0.0015	0.70 ± 0.0074	6.00 ± 0.060	19.04 ± 0.093	0.53 ± 0.0035	60.52 ± 0.241	0.94 ± 0.0084
9	0.63 ± 0.0073	0.12 ± 0.0013	0.68 ± 0.0067	6.70 ± 0.058	18.58 ± 0.106	0.22 ± 0.0036	65.28 ± 0.142	0.81 ± 0.0083
10	0.61 ± 0.0064	0.15 ± 0.0018	0.59 ± 0.0061	4.45 ± 0.047	16.37 ± 0.121	0.51 ± 0.0039	63.52 ± 0.284	0.87 ± 0.0074
11	0.59 ± 0.0051	0.17 ± 0.0019	0.58 ± 0.0062	6.95 ± 0.055	18.95 ± 0.162	0.29 ± 0.0013	62.71 ± 0.352	0.65 ± 0.0057
12	0.62 ± 0.0078	0.11 ± 0.0023	0.63 ± 0.0068	4.25 ± 0.048	20.56 ± 0.125	0.21 ± 0.0013	68.34 ± 0,265	0.69 ± 0.0072
13	0.58 ± 0.0064	0.32 ± 0.0016	0.72 ± 0.0073	4.09 ± 0.052	19.60 ± 0.116	0.38 ± 0.0027	63.64 ± 0,232	0.73 ± 0.0062
14	0.62 ± 0.0073	0.31 ± 0,0018	0.76 ± 0.0086	3.99 ± 0.038	19.96 ± 0.133	0.38 ± 0.0021	63.12 ± 0.148	0.81 ± 0.0070
15	0.59 ± 0.0065	0.12 ± 0.0013	0.61 ± 0.0070	4.21 ± 0.057	20.44 ± 0.164	0.32 ± 0.0042	66.19 ± 0.157	0.93 ± 0.0083

Antioxidant activity assay

In this paper, the antioxidant activity of STVO in vitro was expressed as DPPH free radical scavenging activity. Before the antioxidant activity assay, the conditions of reaction between STVO (sample 2) and DPPH were optimized as follows: DPPH concentration for 50 μg/mL and the reaction time for 60 min (Fig. 3). According to Fig. 3A, along with time increasing,the radical scavenging rate increased, and then reached the summit at 60 min, and would be a balanced condition. As seen in Fig. 3B, the radical scavenging rate has been nearly 60% at 50 μg/mL. Under the optimum conditions, the scavenging percentage kept stable at a medium level. For each STVO sample, the interpretation equation of STVO concentration (X) and scavenging percentage (Y) was used to calculate the IC₅₀ value, at which STVO could scavenge half of the DPPH free radicals. A lower IC₅₀ value represented better DPPH radicals scavenging activity. The IC₅₀ values of the STVOs ranged from 0.016 to 1.518 mg/mL, which are listed in Table 4 with the interpretation equations. The different IC₅₀ values of STVOs demonstrated that different antioxidant effects may be related to the different contents of eight constituents in STVOs and even the qualities of STVOs. The phenomenon might be caused by some environmental factors, planting habits, and harvesting time.

Table 4. Interpretation equation of scavenging DPPH free radical of STVOs (means ± SD).

No.	Probit	R^2	IC50 (mg/ml)	NO.	Probit	R^2	IC50 (mg/ml)
1	p = 0.160x-0.048	0.988	0.297 ± 0.022	9	p = 0.140x-0.088	1.000	0.630 ± 0.028
2	p = 0.175x-0.032	0.996	0.185 ± 0.013	10	p = 0.127x-0.193	0.974	1.518 ± 0.094
3	p = 0.209x-0.003	0.951	0.016 ± 0.001	11	p = 0.124x-0.145	0.978	1.178 ± 0.091
4	p = 0.176x-0.052	0.993	0.293 ± 0.021	12	p = 0.133x-0.030	0.999	1.172 ± 0.087
5	p = 0.165x-0.125	0.997	0.756 ± 0.031	13	p = 0.139x-0.097	0.996	0.695 ± 0.021
6	p = 0.169x-0.008	0.939	0.046 ± 0.004	14	p = 0.148x-0.113	0.996	0.762 ± 0.033
7	p = 0.226x-0.169	0.999	0.573 ± 0.021	15	p = 0.168x-0.129	0.998	0.765 ± 0.041
8	p = 0.143x-0.187	0.980	1.310 ± 0.014

Figure 3. Change curves of DPPH radical scavenging with reaction time (A) and DPPH concentration (B).

Multi-linear regression analysis

The relationship between the independent variable X and the dependent variable Y was analysed with MLRA.^[25,30] X1~X8 represented the contents of the eight components in one STVO while Y represented IC₅₀ of this STVO. As a result, the regression equation was as follows:

Y = 22.887 + 0.971×X1 + 0.629×X2 + 1.373×X3 − 0.029×X4 − 0.079×X5 − 0.085×X6 − 0.315×X7-0.344×X8.

In this equation, r² was 0.993 and F was 260.525, which showed the established MLRA model was available. The partial correlation coefficients and significant differences of the contents and IC₅₀ are listed in Table 5.

Table 5. Partical correlations of content and antioxidant activity.

Compounds	Correlation	t	p-Value	Compounds	Correlation	t	p-Value
1-octen-3-ol	0.038	0.482	0.104	(-)-menthone	−0.536	3.132	0.006
3-octanone	0.072	0.578	0.092	(+)-menthofuran	−0.116	1.543	0.072
β-myrcene	0.177	1.412	0.086	(+)-pulegone	−0.737	3.243	0.005
(-)-limonene	−0.274	2.003	0.053	β-caryophyllene	0.092	0.623	0.082

t > 2.145(df = 14), p < 0.05; t > 2.977, p < 0.01(df = 14).

As shown in Table 5, (+)-pulegone and (-)-menthone had significant negative correlations with IC₅₀ for −0.737 and −0.536, respectively, whose absolute values were much higher than those of the other components. Moreover, the absolute correlation value of (+)-pulegone was higher than that of (-)-menthone. The result indicated that these two compounds were most positively correlated with the antioxidant activity of STVO among the eight compounds and (+)-pulegone was the most prominent constituent to the antioxidant capacity.

Partial least square analysis

PLSA^[31,32] is a latent-variables technique to express the relation between X and Y to derive the principal components (PCs). In this section, X and Y represent the same as in the MLRA section above. As shown in Table 6, PC1 relied heavily on the abundance of (+)-pulegone (coefficient = —0.298, VIP = 1.0814) whereas (-)-menthone (coefficient = —0.2169, VIP = 1.0496) had a greater impact than the other six chemicals on PC2. It was concluded that the two compounds were the characteristic ones and were responsible for the antioxidant function. In addition, according to coefficient and VIP value, (+)-pulegone was more prominent than (-)-menthone in antioxidant capacity. The result was consistent with that of MLRA above. The values of R²X and R²Y were 0.918 and 0.896, respectively, indicating that the model was stable and suitable for prediction. The PLSA scores plot of the 15 STVOs is shown in Fig. 4, which showed the good distribution from the 15 batches of samples on PLSA scores plot by PC1 and PC2.

Table 6. Coefficients of eight components and their antioxidant activity in PLEA.

Compounds	Coefficient	VIP	Compounds	Coefficient	VIP
1-octen-3-ol	0.0427	0.4038	(-)-menthone	−0.2169	1.0496
3-octanone	0.0536	0.2054	(+)-menthofuran	0.1878	0.8802
β-myrcene	0.1219	0.6963	(+)-pulegone	−0.2985	1.0814
(-)-limonene	−0.0696	0.3602	β-caryophyllene	0.0569	0.2543

VIP (Variable Importance Plot).

Figure 4. PLSA scores plot of the 15 STVOs.

Conclusion

In this study, our result (Table 4) showed that STVO exhibited potential antioxidant activity and could be used as a promising source in food, cosmetic, and medicine due to the effect. Furthermore, it was also proved with two chemometric methods (MLRA and PLSA) that (+)-pulegone and (-)-menthone might be the two characteristic constituents responsible for the antioxidant effect of STVO. The study was reported for the first time to our knowledge and the findings were expected to be helpful in the researches of STVO and of antioxidant components from other phytomedicines.

Funding

This work was supported by Natural Science Foundation of China [81473313], University Science Research Project of Jiangsu Province [15KJB360010, 14KJD360001], Top-notch Academic Programs Project of Jiangsu Higher Education Institutions [TAPP-PPZY2015A070], Key Research Project of Jiangsu Collaborative Innovation Center of Chinese Medicinal Resources Industrialization [ZDXM-1-3], Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD-2014], and Postgraduate Innovation Project of Jiangsu Province [KYLX16-0001].

Additional information

Funding

This work was supported by Natural Science Foundation of China [81473313], University Science Research Project of Jiangsu Province [15KJB360010, 14KJD360001], Top-notch Academic Programs Project of Jiangsu Higher Education Institutions [TAPP-PPZY2015A070], Key Research Project of Jiangsu Collaborative Innovation Center of Chinese Medicinal Resources Industrialization [ZDXM-1-3], Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD-2014], and Postgraduate Innovation Project of Jiangsu Province [KYLX16-0001].