Angiotensin-Converting enzyme inhibitory activity of some Vietnamese medicinal plants

Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering 77june 2020 • Volume 62 number 2 Angiotensin-converting enzyme inhibitory activity of some Vietnamese medicinal plants Xoan Thi Le1*, Xuyen Thi Phi1, Duong Thi Thuy Nguyen1, Hang Thi Nguyet Pham1, Tai Nguyen Van1, Olapoju Samuel O.1, Kinzo Matsumoto2 1National Institute of Medicinal Materials, Vietnam 2Department of Kampo Pharmacology, Daiichi University of Pharmacy, Japan Received 25 March 2020; accepted 29 May 2020 *Corresponding author: Email: xoanle@gmail.com Abstract: This study investigated the angiotensin 1-converting enzyme (ACE) inhibitory activity of certain medicinal plants that have been traditionally used as an anti- hypertensive, diuretic, or related diseases treatment in Vietnam. Ten different medicinal plants were selected and screened for in vitro ACE inhibitory activity. A spectrophotometric assay was developed for the determination of ACE activity in the presence of ACE inhibitors using hippuryl-L-histidyl-L-leucine as the ACE-specific substrate. When the in vitro activities of ACE were assessed in the presence and absence of aqueous or ethanol extracts from each medicinal plant, 2 out of the 22 medicinal plant extracts inhibited the ACE activity by more than 90% at a concentration of 50 µg/ml. The estimated IC50 value of the L. rubra and U. sessilifructus were 1.31±0.44 and 12.86 µg/ml, respectively. These results suggest that the L. rubra rhizome and U. sessilifructus extract are potential anti- hypertensive drug-like candidates with ACE inhibitory activity. Keywords: angiotensin 1-converting enzyme, antihypertensive, Leea rubra, Uncaria sessilifructus. Classification number: 3.3 Introduction Hypertension is a leading cause of death worldwide. About 7.6 million mortalities are annually recorded due to hypertension and this number accounts for approximately 13.5% of total mortality [1]. Currently, many types of synthetic anti-hypertension drugs have been used for the treatment of patients with hypertension. Unfortunately, the proportion of treated and managed cases with such antihypertensive drugs is relatively low in developing countries, probably because of high medical care costs combined with low annual incomes [2]. Therefore, the development of alternative therapeutics for hypertension remains a critical issue in those countries [3]. Various medicinal plants have been investigated to explore natural resources with therapeutic potentials for the patients with hypertension by focusing on calcium channels, β-receptors, diuretic activity, and the renin-angiotensin system (RAS). Particularly, the angiotensin-converting enzyme (ACE) in the RAS has been targeted to develop anti-hypertensive drugs since ACE plays a vital role in blood pressure regulation. This enzyme catalyses the conversion of angiotensin I to angiotensin II and thereby causes vasoconstriction. At the moment, ACE inhibitors such as captopril and lisinopril are clinically used as first-line drugs for the treatment of patients with hypertension. However, the use of these drugs is, in some cases, limited because of their various side effects such as cough, numbness, and mild skin itching, etc. Therefore, the natural source-based development of ACE inhibitors with fewer side effects has been attracting much interest. DoI: 10.31276/VJSTE.62(2).77-82 Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering78 june 2020 • Volume 62 number 2 These backgrounds have prompted us to investigate the ACE inhibitory of certain medicinal plants that are traditionally used as anti-hypertensives, diuretics, and as treatments of related diseases in Vietnam by using an in vitro screening system of ACE inhibitors. Numerous methods for the measurement of ACE activity have been developed and reported, including ultraviolet spectrophotometric (USP), visible spectrophotometric (VSP), fluorometric, radiochemical, high-performance liquid chromatography (HPLC), and capillary electrophoresis [4-6]. In this study, using the spectrophotometric method based on the detection of the production of Hippuric acid (HA) from Hip-His- Leu (HHL) by the action of ACE in the presence of ACE inhibitors, we determine the ACE inhibitory activities of 22 different extracts from 10 medicinal plants. This study suggests that the L. rubra rhizome and U. sessilifructus extract are potential anti-hypertensive drug-like candidates with ACE inhibitory activity. Materials and methods Plant materials Ten medicinal plants were used for the in vitro screening study: Eucommi aulmoides, Apium graveolens, Calliasia fragrans, Catharanthus roseus, Artocarpus altilis, Diospyros kaki, Annona muricata, Uncaria sessilifructus, Leea rubra, and Hibiscus sabdariffa. The plants were collected from several locations in Vietnam in 2019, identified by Dr. Pham Thanh Huyen [Department of Medicinal Plant Resources, National Institute of Medicinal Materials (NIMM), Vietnam] and deposited at NIMM with voucher specimen numbers as shown in Table 1. Extract preparation The plant materials were cleaned, dried at 60oC in a hot air oven, and then cut into small pieces. For the in vitro study, each plant material (20 g) was extracted with water or ethanol 50% (1:7 w/v) at reflux for 2 h and filtered using a filter paper (Whatnam® qualitative filter paper, Grade 93, 580×580 cm, Merck, Darmstadt, Germany). This step was repeated 3 times. The filtered samples were combined and then concentrated under reduced pressure at 60oC. The obtained extract was then dried in a vacuum oven at 60oC until the moisture content of the extract was below 5%. The yield of the extraction from each plant is described in Table 1. These plant extracts samples were stored at 4oC and deposited at NIMM. ACE inhibition assay The plant extracts were dissolved in DMSo to make a stock solution with a concentration of 100 mg/ml and stored at -20oC until use. For the in vitro ACE inhibition assay, the stock solutions were diluted by a borate buffer of pH 8.3. The concentration of each plant extract in the ACE enzyme reaction was 50 µg/ml. To estimate the IC50 values of the herbal extracts, a dilution of the stock solutions were made with the borate buffer to achieve test concentrations of 0.156, 0.3125, 0.625, 1.25, and 2.5 µg/ml. Captopril (Across organics, Belgium), an ACE inhibitor, was used as a positive control, which was diluted in water at a concentration of 1 mM, then the stock was diluted in the borate buffer to reach the test concentration range. The ACE inhibitory activity was assayed according to the previous method [5] with minor modifications. The composition and reaction procedure are as follows: 12.5 µl of the inhibitor samples (plant extracts or captopril), 27.5 µl borate buffer, and 10 µl ACE 0.02 U/ml were mixed. After incubation for 5 min at 37oC, the mixture was gently blended with 75 µl HHL solution (N-Hippuryl-His-Leu hydrate powder, Sigma, Germany) by using a vortex and subsequently incubated at 37oC for 30 min. The reaction was terminated by adding 100 µl HCl 1 M and mixing. The control sample was prepared by replacing the inhibitor solution with the borate buffer. The blank sample (of both control sample and tested samples) was prepared by replacing the ACE solution by the borate buffer. Each sample was mixed with 150 µl quinoline (Across organics, Belgium). Subsequently, 75 µl of benzene sulfonyl chloride (Sigma, Germany) was added into mixture. After 45 min incubation in dark at 25oC, 300 µl of ethanol was introduced into the mixture. The absorbance of 200 µl of the mixture was determined at 490 nm using an ELISA system 96 well - EL x808 Biotek. Each reaction was done in replicas of two. In parallel with each reaction, the samples and controls had a blank to draw a comparison. The controls were similar to the test tubes but the samples were replaced with buffer. ACE inhibitory activity was determined using the formula: where oD control is the absorbance value (optical density) Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering 79june 2020 • Volume 62 number 2 of the control tubes, oD test is the absorbance (optical density) value of test tubes, and the IC50 value is the concentration of herbal extract or captopril that inhibited 50% of ACE activity under experimental conditions using regression analysis. Total phenolic content of aqueous L. rubra rhizome extract Total phenolic content of the L. rubra rhizome extract was performed using the Folin-Ciocalteau method as previous described [7, 8]. Briefly, 20 µl of the sample (5 mg/ml) and 930 µl of 2% Na2Co3 were seeded in a tube to which 50 µl of Folin-Ciocalteau reagent was added. The reaction mixture was incubated at 40º C for 60 min and the absorption of the mixtures was read at 760 nm. The sample was tested at a final concentration of 100 µg/ml in triplicate and a calibration graph with four data points for gallic acid was obtained. The total phenolic content of the sample was expressed as mg of gallic acid equivalents (GAE) per gram of extract. on the basis of this analysis, the total phenolic content of the aqueous L. rubra rhizome extract amounts to 65.1 mg GAE/g. Results Ten medicinal plants were obtained from different locations in Vietnam. Aqueous and ethanolic extract from these medicinal materials were screened for their in vitro activities to inhibit ACE. As shown in Table 1, the plant extracts showed more than 70% ACE inhibition at a concentration of 50 µg/ml, including the L. rubra rhizome (aqueous extract) and U. sessilifructus (ethanol extracts), C. fragran (aqueous and ethanolic extract), C. roseus (ethanolic extract), A. graveolens (aqueous extract), and A. muricata (aqueous extract). Especially, the L. rubra rhizome aqueous and U. sessilifructus ethanol extracts showed more than 90% inhibition at a concentration of 50 µg/ml. The ACE inhibition effects of the aqueous L. rubra rhizome and U. sessilifructus ethanol extracts were further analysed. As shown in Fig. 1, the inhibitory effect of the L. rubra rhizome aqueous extract and U. sessilifructus ethanol extracts on the in vitro ACE activity were concentration dependent. The estimated IC50 values of the L. rubra rhizome aqueous extract and U. sessilifructus ethanol extract on the in vitro ACE activity were 1.3±0.5 µg/ml and 12.86±3.08 µg/ml, respectively. on the other hand, captopril, an ACE inhibitor used as a positive control had an IC50 value of 18.0±2.0 nM. Plants Family name Collection data Plant parts Extraction solvent Yield of extraction (%) % ACE inhibition (50 µg/ml)Date Site Voucher no. Callisia fragran (Lindl.) Woodson Commelinaceae 3/2019 Thai Nguyen province DL-260219 Leaves Water 29.2 82.8±1.7 Ethanol 36.2 71.9±6.4 Leea rubra Blume Leeaceae 4/2019 Yen Bai province DL-230219 Rhizome Water 20.9 100±0.0 Ethanol 23.9 63.5±9.8 Aerial part Water 8.2 50.0±7.1 Ethanol 9.8 12.0±6.8 Eucommia ulmoides oliv. Eucommiaceae 6/2018 Lao Cai province DL-240219 Stem bark Water 17.7 67.5±14.2 Ethanol 18.3 16.0±14.2 Catharanthus roseus G. Don Apocynaeae 4/2018 Thai Binh province DL-220219 Aerialpart Water 31.1 58.6±12.2 Ethanol 29.9 79.7±4.2 Apium graveolens Linn. Apiaceae 4/2018 Thai Binh province DL-210219 Aerialpart Water 34.2 79.5±7.6 Ethanol 35.1 37.5±2.1 Artocarpus altilis Fosberg Moraceae 4/2018 Ho Chi Minh city DL-200219 Leaves Water 15.0 48.6±6.6 Ethanol 14.7 67.8±10.7 Diospyros kaki Thunb. Ebenaceae 8/2018 Lang Son province DL-250219 Leaves Water 15.9 22.5±7.5 Ethanol 16.90 25.6±9.38 Annona muricata Linn. Annonaceae 3/2019 Thai Nguyen province DL-270219 Leaves Water 30.7 73.1±18.8 Ethanol 32.7 19.4±4.3 Uncaria sessilifructus Roxb. Rubiaceae 3/2018 Bac Can province DL-190219 Stem with hook Water 18.3 67.0±7.0 Ethanol 12.1 93.5±4.0 Hibiscus sabdariffa Linn. Malvaeae 4/2019 Phu Yen province DL-280219 Calyxes Water 35.1 65.8±4.5 Ethanol 45.1 39.1±7.4 Table 1. Plant materials used for ACE inhibition in vitro assay, part of plant, extraction solvent and the percentage of ACE inhibition. Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering80 june 2020 • Volume 62 number 2 Discussion In this study, we used the VSP method to screen 22 extracts from 10 traditional herbs for their in vitro ACE inhibitory potentials using captoptil as the positive control. The IC50 values of captopril determined in this study using this VSP method (18.0 nM) are in agreement with the range of 5-23 nM reported previously [4, 5, 9]. USP/VSP and HPLC are commonly used in the assay for determining ACE activity and inhibition in vitro. The USP method was developed by Cushman and Cheung in 1970 based on the hydrolysis of hippuryl -L-histidy where Hip-His-Leu converts into Hippuric acid (HA) and His-Leu (HL) by ACE. HA then was extracted by ethyl acetate and measured by USP at a wavelength 228 nm. However, this method has weak points such as a complicated process, wasted time, and instability during separation of HHL from HA because both absorb wavelengths of 228 nm. Therefore, HA may be overestimated [5]. The VSP method was developed by modifying the USP method by using spectrophotometry in the visible light wavelength range. For the modified VSP methods, the ethyl acetate extraction step was replaced by adding a colorimetric agent that binds with HA such as benzene sulfonyl chloride in the presence of quinoline. The VSP method process has the advantages of simplicity, high sensitivity, and cost effectiveness because of the absence of the procedure that separates HA from the reaction mixture [5, 6]. The HPLC method was employed to assay ACE activity with reaction principles similar to the USP method. Although the HPLC method exhibits higher precision and reproducibility than VSP method, the use of HPLC requires costly instrumentation requiring proper preventative Fig 1. Inhibition activity of L. rubra rhizome extract (A), U. sessilifructus ethanol extract (B) and captopril (C) on ACE in vitro. Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering 81june 2020 • Volume 62 number 2 maintenance, the highest grade of purity solvents, and a complex and time consuming procedure. In contrast, the VSP provides a simple and rapid means for chemical analysis [5] that is suitable for screening proposes with a large number of samples. The present study demonstrated that the L. rubra rhizome aqueous extract exhibited the highest in vitro ACE inhibitory potency of all the plant materials screened with an IC50 of 1.31±0.44 µg/ml. L. rubra belongs to the family Vitaceae. L. rubra, which is widely distributed in Vietnam, Thailand, and Cambodia. L. rubra is an herb with dusty red fruits that ripens to a black, purple red stem with an average height of about 2.5 m. Traditionally, this plant is used for the treatment of inflammation and pain [10]. Surprisingly, there is another species of Leea genus that is often used in traditional remedies for hypertension [11]. According to the previous study demonstrating the in vitro ACE inhibitory effect of some Brazilian medicinal herbs, the ethanolic extract of aerial part of L. rubra inhibited the activity of ACE by 57.0±12.5% at a concentration of 100 µg/ml [11]. Consistent with Braga’s study, our in vitro results show that aqueous and ethanolic extracts of L. rubra’s aerial part inhibited ACE activity by 50.0 and 12.0%, respectively, at a concentration 50 µg/ml. Moreover, we found that aqueous and ethanolic L. rubra rhizome extracts inhibited the ACE activity by 100 and 63.5%, respectively. This finding demonstrates that the L. rubra rhizome aqueous extract exhibits a higher ACE inhibitory activity than the aerial part. Moreover, with an IC50 as low as 1.31±0.44 µg/ml, the L. rubra rhizome extract is considered as a crude plant extract that possesses potent ACE inhibition. In our dedication to discover effective anti-hypertension therapies from natural products, the effect of this plant extract in hypertensive rat models is under investigation in our laboratory. The main chemical compositions of the L. rubra rhizome extract that causes the angiotensin-converting enzyme inhibitory activity are unclear. As far as we know, no specific work on L. rubra rhizome isolate has been reported to date. However, based on a chemical analysis, we demonstrated that this crude extract consists of high phenolic content with the total phenolic content was estimated as 65.1 mg GAE/g. Moreover, Phuong, et al. (2015) [12] isolated and identified 5 phenolic compounds including gallic acid, proto-catechuic acid, 4-hydroxybenzoic acid, arctiin, and kaempferol-3-o-α-L-rhamnopyranosyl(1→2)-α-L- arabinofuranoside from L. rubra leaves. From the L. rubra leaves, this group also isolated and identified 4 flavonoids including myricitrin, 7-methoxymyricitrin, rhamnetin-3- o-a-L-rhamnopyranoside, and juglanin [13]. In addition, there was a study demonstrating the hypotensive effect of gallic acid suspected by acting on the renin-angiotensin system [14]. Thus, there is a possibility that gallic acid as well as flavonoids constituents of L. rubra may be the active ingredients contributing to the ACE inhibition effect of the L. rubra rhizome extract, however, further study is needed to demonstrate this possibility. Besides the L. rubra rhizome extract, we found that U. sessilifructus extracts exhibit significant ACE inhibition activity. As far as we know, the ACE inhibition effect of U. sessilifructus has not been reported previously. However, Hansen, et al. (1995) [15] reported that U. rhynchophylla is one of 7 species that inhibit ACE by more than 50% among the 31 species investigated. Notably, five Uncaria species, namely U. rhynchophylla, U. macrophylla, U. hirsuta, U. sinensis, and U. sessilifructus are documented in Chinese Pharmacopoeia as the raw materials of Uncariae Ramulus Cum Uncis. This Uncaria species has been traditionally used for the treatment of hypertension, headache, and fever. The indole alkaloid content of U. sessilifructus is considered as the antihypertensive active ingredient [16, 17]. Feng, et al. (2019) [18] reported that the Uncaria species including U. sessilifructus reduced the systolic blood pressure in spontaneously hypertensive rats (SHRs). Thus, these results allow us to speculate the ACE inhibition activity contributing to the anti-hypertensive action of this plant. A negative result of the ACE inhibition screening does not always mean that this plant species does not work as an anti-hypertensive drug as compounds may influence other hypotensive mechanisms. It also should be noted that the presence of potent ACE inhibition does not mean that the species are potent anti-hypertensive drugs. Therefore, further studies on pure compounds isolated from the active extracts are necessary. Also, the effect of a potential candidate should be evaluated using an animal model of hypertension. Conclusions In conclusion, our results showed that the L. rubra rhizome and U. sessilifructus extracts exhibit significant ACE inhibitory activity. This study suggests that the L. rubra rhizome and U. sessilifructus may be beneficial for the treatment of hypertension. Life ScienceS | Pharmacology Vietnam Journal of Science, Technology and Engineering82 june 2020 • Volume 62 number 2 ACKNOWLEDGEMENTS This work was in part supported by National Institute of Medicinal Materials, Vietnam. The authors declare that there is no conflict of interest regarding the publication of this article. REFERENCES [1] H. Arima, F. Barzi, J. Chalmers (2011), “Mortality patterns in hypertension”, J. Hypertens., 29, pp.S3-S7. [2] H.A. Hien, N.M. Tam, V. Tam, A. Derese, D. Devroey (2018), “Prevalence, awareness, treatment, and control of hypertension and its risk factors in (Central) Vietnam”, Int. J. Hypertens., 2018(29), pp.1-12, DoI: 10.1155/2018/6326984. [3] R.V. Picon, F.D. Fuchs, L.B. Moreira, S.C. 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