- About this Journal
- Abstracting and Indexing
- Aims and Scope
- Annual Issues
- Article Processing Charges
- Articles in Press
- Author Guidelines
- Bibliographic Information
- Citations to this Journal
- Contact Information
- Editorial Board
- Editorial Workflow
- Free eTOC Alerts
- Publication Ethics
- Reviewers Acknowledgment
- Submit a Manuscript
- Subscription Information
- Table of Contents
Evidence-Based Complementary and Alternative Medicine
Volume 2012 (2012), Article ID 252758, 7 pages
Antimicrobial Constituents of Artemisia afra Jacq. ex Willd. against Periodontal Pathogens
1Department of Plant Science, University of Pretoria, Pretoria 0002, South Africa
2Chemitsry Deparment, Universirty of Western Cape, Private Bag X17, Bellvile 7535, South Africa
Received 23 January 2012; Accepted 6 March 2012
Academic Editor: Victor Kuete
Copyright © 2012 Garland More et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The phytochemical investigation of an ethanol extract of Artemisia afra led to the isolation of six known compounds, acacetin (1), 12α,4α-dihydroxybishopsolicepolide (2), scopoletin (3), α-amyrin (4), phytol (5), and a pentacyclic triterpenoid betulinic acid (6). The compounds were evaluated for antimicrobial activity against Gram positive (Actinomyces naeslundii, Actinomyces israelii, and Streptococcus mutans), Gram negative bacteria (Prevotella intermedia, Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans previously known as Actinobacillus actinomycetemcomitans), and Candida albicans. The crude extract of A. afra inhibited the growth of all tested microbial species at concentration range of 1.6 mg/mL to 25 mg/mL. The compounds 1–6 also showed activity range at 1.0 mg/mL to 0.25 mg/mL. Three best compounds (scopoletin, betulinic acid, and acacetin) which showed good antimicrobial activity were selected for further studies. Cytotoxicity of extract and compounds was determined using the XTT cell proliferation kit. The antioxidant activity of the extract and compounds was done using the DPPH scavenging method. The extract showed good antioxidant activity with an IC50 value of 22.2 μg/mL. Scopoletin had a strong transformation of the DPPH radical into its reduced form, with an IC50 value of 1.24 μg/mL which was significant to that of vitamin C (1.22 μg/mL). Acacetin and betulinic acid exhibited a decreased scavenging activity with the IC50 of 2.39 and 2.42 μg/mL, respectively. The extract and compounds showed moderate toxicity on McCoy fibroblast cell line and scopoletin was relatively nontoxic with an IC50 value of 132.5 μg/mL. Acacetin and betulinic acid also showed a smooth trend of non-toxic effects with IC50 values of 35.44 and 30.96 μg/mL. The obtained results in this study confirm the use of A. afra in the treatment of microbial infections.
Periodontal disease is a chronic, multifactorial disease of the tissue supporting teeth . It is characterised by local infection and inflammation in teeth supporting tissue leading to connective tissue destruction and alveolar bone loss . The etiology of periodontitis is the oral bacteria [3, 4]. If left untreated, periodontitis can have medical consequences such as weight loss, chronic pain, sore or loss teeth, swollen gums, tooth decay, breakage of the maxillary or mandibular bones, and renal, coronary, and hepatic diseases [5–7]. The population of periodontal bacteria begins to increase as an anaerobic environment is produced due to oxygen scavenging activity of the early subgingival colonizers . These bacterial populations lead to biofilm formation which consists of microcolonies, extracellular layers, fluid channels, and communication systems . Biofilms may consist of more than 700 different microbial species which leave symbiotically and incubate each other . The biofilm formation and associated disease can be prevented by daily tooth brushing and chemotherapeutic agents such as chlorhexidine (CHX), fluorides biguanide antiseptics, quaternary ammonium-antiseptics and phenol derivatives [10, 11]. Although these chemotherapeutic agents are effective they can cause side effects, such as gastrointestinal irritation, tooth staining, and gum irritation .
Dental treatment usually is expensive and not so easily accessible, especially in developing countries; therefore humans have turned to the use of natural traditional remedies to prevent oral ailments [12–15]. Artemisia afra (African wormwood, family Asteraceae) is widely distributed along the eastern parts of Africa. It grows in thick, bushy areas, usually with tall stems up to 2 m high but sometimes as low as 0.6 m. A. afra is a common species in South Africa with a wide distribution from the Cederberg Mountains in the Cape, northwards to tropical East Africa and stretching as far north as Ethiopia [16, 17]. Worldwide there are about 500 species of Artemisia, mainly from the northern hemisphere. Many of the other Artemisia species are aromatic perennials and are used medicinally . In southern Africa it is used to treat coughs, colds, diabetes, malaria, sore throat, asthma, headache, dental care, gout and intestinal worms . In vitro studies done have revealed that A. afra is a potential antidepressant, cardiovascular, spasmolytic effects, antioxidant, and antimycobacteria [18, 20, 21]. Furthermore, the extracts of this plant species have shown activity against Trypanosoma brucei brucei .
The rationale of this study was to determine the antimicrobial, antioxidant, and cytotoxicity of Artemisia afra and isolated compounds against oral microorganisms which are responsible for dental caries, gingivitis, and periodontitis.
2. Materials and Methods
2.1. Plant Material
Artemisia afra was collected at the South African National Botanical Institute (SANBI), Pretoria. Voucher specimen was prepared and identified at the H. G. W. J. Schwelcherdt Herbarium (PRU), University of Pretoria.
2.2. Preparation of Extracts
Fresh plant material was soaked in 96% ethanol and homogenized into fine mesh. The extract was then filtered through the Whatman No.1 filter paper. The filtrates were evaporated to dryness in a (BUCHI) Rotavapor under reduced pressure of 40°C.
2.3. Antimicrobial Assay
2.3.1. Microbial Strains
The microorganisms used in this study include Actinomyces naeslundii (ATCC 19039), Actinomyces israelii (ATCC 10049), Aggregatibacter actinomycetemcomitans (ATCC 33384), Candida albicans (Med I), Porphyromonas gingivalis (ATCC 33277), Prevotella intermedia (ATCC 25611), and Streptococcus mutans (ATCC 25175). Bacteria were grown in the Casein-peptone Soy Agar medium (CASO) (Merck SA (Pty) Ltd.) under anaerobic conditions in a jar with anaerocult A (Merck SA (Pty) Ltd.), at 37°C for 48 hours. Sabouraud Dextrose Agar medium (SDA) (Merck SA (Pty) Ltd.) was used for the culturing of Candida albicans and incubated at 37°C for 24 hours under aerobic conditions. Subculturing was done once weekly.
2.4. Determination of Minimum Inhibitory Concentration (MIC) and Minimum Microbicidal Concentration (MMC)
The microdilution technique using 96-well microplates  was used to obtain the MIC and MMC values of the crude extract against microorganisms under study. The extract was serially diluted in the 96-well plate with 48 hours old microorganisms (5 × 106 CFU/mL) grown at 37°C and the final concentration of extract and positive control (CHX) ranged from 25.0 mg/mL to 0.8 mg/mL. Microbial growth was indicated by adding 40 μL of (0.2 mg/mL) p-iodonitrotetrazolium violet (INT) (Sigma-Aldrich, South Africa) to microplate wells and incubated at 37°C for 48 hours. MIC was defined as the lowest concentration that inhibited the colour change of INT. The MMC was determined by adding 50 μL of the suspensions from the wells, which did not show any growth after incubation during MIC assays, to 150 μL of fresh broth. These suspensions were reincubated at 37°C for 48 hours. The MMC was determined as the lowest concentration of extract which inhibited 100% growth of microorganisms .
2.5. Antioxidant Activity
The free radical scavenging activity was measured using 1, 1 diphenyl-2-picryl-hydraxyl (DPPH) assay  with slight modifications. The ethanol extract of A. afra and Vitamin C (positive control) 1000 μg/mL (20 μL) was added in the first three wells of a 96-well plate containing 200 μL of distilled water to make up final concentration of 100 μg/mL and the remaining wells were filled with 110 μL of distilled water. The first raws containing the extract/compounds were serially diluted to wells which contain 110 μL of distilled water, and later, 90 μL of methanolic solution of DPPH (90 mM) was added to all the wells. The final concentrations of the extract/compounds ranged from 100 to 0.8 μg/mL. The plates were incubated at 37°C for 30 min and the absorbance was measured at 517 nm using the ELISA plate reader. The percent radical scavenging activity by A. afra was determined by comparison with ethanol (blank). The inhibition ratio was calculated as follows: % DPPH radical-scavenging = (AC-AS)/AC × 100, where AC is absorbance of the control solution (containing only DPPH solution), and AS is the absorbance of the sample in DPPH solution. The percentage of DPPH radical-scavenging was plotted against the plant extract/compounds concentrations (μg/mL) to determine the concentration of extract/compound required to scavenge DPPH by 50% (EC50).
2.6. Determination of Cytotoxicity
2.6.1. Preparation of Extract and Compounds
Extract and compounds were dissolved in dimethyl sulfoxide (DMSO) and stored at −20°C. All tested compounds were diluted to the final concentration with RPMI 1640 and control cultures were diluted with 0.1% DMSO.
2.6.2. Cell Culture
McCoy cells were maintained in monolayer culture at 37°C and 5% CO2 with 10% PBS Medium, 10 μg/mL of penicillin, 10 μg/mL, streptomycin, 40 μg/mL gentamycin, and 0.25 μg/mL fungizone.
2.6.3. Cell Proliferation Assay
A microtiter plate with McCoy cells was used for testing all the ethanol extracts for cytotoxicity following the method of . Cytotoxicity was measured by the XTT (sodium 3′-[1-(phenyl amino-carbonyl)-3,4-tetrazolium]-bis-[4-methoxy-6-nitro] benzene sulfonic acid hydrate) method using a cell proliferation kit II (Roche Diagnostics GmbH). Hundred microlitres of McCoy cells (1 × 105 mL) was seeded onto a microtiter plate and incubated for 24 h to allow the cells to attach to the bottom of the plate. Dilution series were made of the extract and compound and the various concentrations (400 to 3.1 μg/mL) were added to the microtitre plate and incubated for 48 h. The XTT reagents were added to a final concentration of 0.3 mg/mL and the cells were incubated for 1-2 hours. The positive drug control (Zearalenone) at concentrations range of (10 μg/mL to 0.6 μg/mL) was included in the assay. After incubation the absorbance of the colour was spectrophotometrically quantified using an ELISA plate reader, which measured the optical density at 490 nm with a reference wavelength of 690 nm. The assay was carried out in triplicate.
2.6.4. Statistical Analysis
Statistical analysis was conveyed as means ± SD using GraphPad Prism 4.0 with a significant difference of ().
3. Isolation and Identification of Compounds
The antibacterial compounds present in A. afra extract were determined by the direct bioautography method of chromatograms using S. mutans . The extract was spotted onto a TLC plate and developed using hexane: ethyl acetate at different ratios (1 : 1, 3 : 7, and 7 : 3). The plates were thoroughly dried and then the chromatograms were sprayed with a dense culture of S. mutans, incubated overnight at 37°C. The plates were further sprayed with 0.2 mg/mL of p-iodonitrotetrazolium (INT) (Sigma). Clear zones of inhibition indicated compounds which inhibited bacterial growth. The isolation of compounds was performed using column chromatography. Fractionation was preceded by using silica gel 60 (70–230 mesh) and Sephadex LH 20. Thin layer chromatography (TLC) was performed on aluminum sheets coated with silica gel 60 F254 (Merck) and UV light was used to detect compounds. TLC plates were further sprayed with vanillin/sulphuric acid reagent. H1-NMR and 13C-NMR spectra were obtained by using Nuclear magnetic resonance (NMR) Germin 200 AT 199, 50 Hz, respectively.
The Isolation of A. afra was started with 200 g of ethanol extract on to a 100 mm diameter column. The column was filled with 2 kg silica gel, eluted with a mixture of hexane: ethyl acetate of increasing polarity (100 : 0 to 0 : 100). Forty fractions were obtained and combined to make up 12 (I-XII) main fractions according to similarities of compounds as determined by TLC plate. A Sephadex column was conducted on the fraction X using 100% MeOH and it yielded 25 subfractions which were combined in to three subfractions (L, M, N). Fraction N was fractionated using MeOH and yielded a pure compound 1. Fraction M was isolated using a gradient of solvents DCM: Hex, 9 : 1 increasing polarity to 3% to obtain a florescent blue compound 3.
Fraction VII was chromatographed using DCM: MeOH (95 : 5), and 20 fractions were obtained and combined to 3 subfractions. Fraction G was fractionated using DCM: MeOH, increasing polarity on a Sephadex column and two fractions were obtained of which one was a pure compound 2, the second fraction was again fractionated using DCM: MeOH (95 : 5) and compound 6 was obtained. Compound 5 was isolated using Hex: EtoAc, at a ratio of 9 : 1, using silica gel column. A succession of a blue-coloured compound was observed on a TLC plate after application of vanillin/sulphuric acid. Subfraction H from fraction VII was chromatographed using a sephadex column with ethanol as its mobile phase and it yielded a white precipitate of a pure compound 4.
4. Results and Discussion
The TLC profile gave a clear antibacterial activity of the extract and guide to isolate ideal compounds. In all solvent systems tested on TLC, both polar and nonpolar bends demonstrated activity by inhibiting the growth of S. mutans. Isolation from A. afra extract yielded six compounds (Figure 1), one known flavone Acacetin (1), sequiterpene12α,4α-dihydroxybishopsolicepolide (2), a diterpene Phytol (5) [28, 29], and two pentacyclic triterpenes (4). A thorough revision of literature indicated that the data for pentacyclic triterpene matched with those of α-Amyrin, a widely spread triterpene in nature [30, 31]. The presence of the signal at 3.0 of H-17 confirms the structure of Betulinic acid (6). We have compound (3) of which the forgoing data is identical with the known compound Scopoletin. All these compounds were characterized by their 1H-NMR and 13C-NMR spectra.
The strong antimicrobial activity demonstrated by the ethanol extract of A. afra has provided us with more evidence that needed further chemical investigation of bioactive compounds present. The extract of A. afra showed good inhibitory effects against all Gram positive bacteria. It can be noted that the MIC and MMC values varied from 1.6 to 25.0 mg/mL among Gram positive bacteria. C. albicans which is a thick grower fungus was inhibited at a concentration of 6.3 mg/mL. However, the MMC value insignificant as compared to the results of the MIC. The A. actinomycetemcomitans was the resistant Gram negative bacteria as compared to all other tested bacteria (Table 1). The lowest MIC and MMC value of isolated compounds was recorded at 0.25 mg/mL of compounds (6), (3), and (1) against A. israelii and A. naeslundii. Of all microorganisms, A. actinomycetemcomitans and C. albicans showed to be resistant against all compounds tested (Table 1).
The antioxidant activity of three selected compounds (scopoletin, acacetin and betulinic acid) revealed that they are effective antioxidant agents (Figure 2). Scopoletin had a strong transformation of the DPPH radical into its reduced form, with an IC50 value of 1.24 μg/mL which was in close range to that of vitamin C (1.22 μg/mL). Acacetin and Betulinic acid exhibited a slightly low DPPH scavenging activity with the IC50 of 2.39 and 2.42 μg/mL, respectively.
The cytotoxicity effects of ethanol crude extract of A. afra and three selected compound on the growth of Fibroblast cells are shown in Figure 3 and Table 2. The extract as a mixture of different components showed to be nontoxic on lower concentrations of 6.12 and 3.06 μg/mL, with the cell viability of 120%. However, toxic effects were apparent at higher concentration range of 12.50 to 400 μg/mL, with the cell viability of 60 to 20%. Acacetin and betulinic acid also showed a smooth trend of nontoxic effects at lower concentrations and toxic at higher concentrations with IC50 values of 35.44 and 30.96 μg/mL respectively. The effect of acacetin on lung cancer (A549) cell proliferation was observed to have a dose-dependent manner with an IC50 value of 9.46 μM . Apoptotic activity of betulinic acid against murine melanoma B16 cell line was reported and it was discovered that betulinic acid induces apoptotic effects with an IC50 of 22.5 μg/mL . Reports postulates that triterpenes with a carboxyl group at c-28 shows more cytotoxic activity against cancer cell lines [34–36] and induce apoptosis . Unexpectedly, one out of three compounds tested, scopoletin was relatively nontoxic with an IC50 value of 132.5 μg/mL.
This work supported by the National Research Foundation.
- W. J. Loesche and N. S. Grossman, “Periodontal disease as a specific, albeit chronic, infection: diagnosis and treatment,” Clinical Microbiology Reviews, vol. 14, no. 4, pp. 727–752, 2001.
- L. P. Samaranayake, in Essential Microbiology for Dentistry with a Contribution by BM. Jones; foreword by Crispian Scully, Churchill Livingstone, New York, NY, USA, 2nd edition, 2000.
- C. W. Cutler, J. R. Kalmar, and C. A. Genco, “Pathogenic strategies of the oral anaerobe, Porphyromonas gingivalis,” Trends in Microbiology, vol. 3, no. 2, pp. 45–51, 1995.
- R. J. Lamont and H. F. Jenkinson, “Life below the gum line: pathogenic mechanisms of Porphyromonas gingivalis,” Microbiology and Molecular Biology Reviews, vol. 62, no. 4, pp. 1244–1263, 1998.
- L. J. DeBowes, D. Mosier, E. Logan, C. E. Harvey, S. Lowry, and D. C. Richardson, “Association of periodontal disease and histologic lesions in multiple organs from 45 dogs,” Journal of Veterinary Dentistry, vol. 13, no. 2, pp. 57–60, 1996.
- K. Okuda, T. Kato, and K. Ishihara, “Involvement of periodontopathic biofilm in vascular diseases,” Oral Diseases, vol. 10, no. 1, pp. 5–12, 2004.
- F. A. Scannapieco, R. B. Bush, and S. Paju, “Associations between periodontal disease and risk for atherosclerosis, cardiovascular disease, and stroke. A systematic review.,” Annals of Periodontology, vol. 8, no. 1, pp. 38–53, 2003.
- H. K. Kuramitsu, X. He, R. Lux, M. H. Anderson, and W. Shi, “Interspecies interactions within oral microbial communities,” Microbiology and Molecular Biology Reviews, vol. 71, no. 4, pp. 653–670, 2007.
- J. M. ten Cate, “Biofilms, a new approach to the microbiology of dental plaque,” Odontology, vol. 94, no. 1, pp. 1–9, 2006.
- M. Addy, “Chlorhexidine compared with other locally delivered antimicrobials. A short review,” Journal of Clinical Periodontology, vol. 13, no. 10, pp. 957–964, 1986.
- S. Ciancio, “Improving oral health: current considerations,” Journal of Clinical Periodontology, vol. 30, no. 5, pp. 4–6, 2003.
- E. S. Akpata and E. O. Akinrimisi, “Antibacterial activity of extracts from some African chewing sticks,” Oral Surgery Oral Medicine and Oral Pathology, vol. 44, no. 5, pp. 717–722, 1977.
- K. A. Homer, F. Manji, and D. Beighton, “Inhibition of protease activities of periodontopathic bacteria by extracts of plants used in Kenya as chewing sticks (mswaki),” Archives of Oral Biology, vol. 35, no. 6, pp. 421–424, 1990.
- A. Hutchings, S. Haxton, S. G. Lewis, and A. Cunningham, Zulu Medicinal Plants. An Inventory, University of Natal Press, Pietermaritzburg, South Africa, 1996.
- H. Tapsoba and J. P. Deschamps, “Use of medicinal plants for the treatment of oral diseases in Burkina Faso,” Journal of Ethnopharmacology, vol. 104, no. 1-2, pp. 68–78, 2006.
- B. van Wyk and P. van Wyk, Field Guide to Trees of Southern Africa, McKenzie Street, Cape Town, South Africa, 1997.
- B. E. van Wyk, “A broad review of commercially important southern African medicinal plants,” Journal of Ethnopharmacology, vol. 119, no. 3, pp. 342–355, 2008.
- N. Q. Liu, F. Van der Kooy, and R. Verpoorte, “Artemisia afra: a potential flagship for African medicinal plants?” South African Journal of Botany, vol. 75, no. 2, pp. 185–195, 2009.
- B.-E. Van Wyk and N. Gerick, Peoples Plants, chapter 12, Dental Care, 2000.
- S. P. N. Mativandlela, J. J. M. Meyer, A. A. Hussein, P. J. Houghton, C. J. Hamilton, and N. Lall, “Activity against Mycobacterium smegmatis and M. tuberculosis by extract of South African medicinal plants,” Phytotherapy Research, vol. 22, no. 6, pp. 841–845, 2008.
- B. M. Lawrence, Antimicrobial/Biological Activity of Essential Oils, Allured, Illinois, Ill, USA, 2005.
- E. Nibret and M. Wink, “Volatile components of four Ethiopian Artemisia species extracts and their in vitro antitrypanosomal and cytotoxic activities,” Phytomedicine, vol. 17, no. 5, pp. 369–374, 2010.
- J. N. Eloff, “A sensitive and quick microplate method to determine the minimal inhibitory concentration of plant extracts for bacteria,” Planta Medica, vol. 64, no. 8, pp. 711–713, 1998.
- M. A. Cohen, M. D. Huband, S. L. Yoder, J. W. Gage, and G. E. Roland, “Bacterial eradication by clinafloxacin, CI-990, and ciprofloxacin employing MBC test, in-vitro time-kill and in-vivo time-kill studies,” Journal of Antimicrobial Chemotherapy, vol. 41, no. 6, pp. 605–614, 1998.
- J. W. K. Burrell, R. F. Garwood, L. M. Jackman, E. Oskay, and B. C. L. Weedon, “Carotenoids and related compounds. Part XIV. Stereochemistry and s ynthesis of geraniol, nerol, farnesol, and phytol,” Journal of the Chemical Society C, pp. 2144–2154, 1966.
- Y. T. Zheng, W. L. Chan, P. Chan, H. Huang, and S. C. Tam, “Enhancement of the anti-herpetic effect of trichosanthin by acyclovir and interferon,” FEBS Letters, vol. 496, no. 2-3, pp. 139–142, 2001.
- W. J. Begue and R. M. Kline, “The use of tetrazolium salts in bioauthographic procedures,” Journal of Chromatography A, vol. 64, no. 1, pp. 182–184, 1972.
- D. Arigoni, S. Sagner, C. Latzel, W. Eisenreich, A. Bacher, and M. H. Zenk, “Terpenoid biosynthesis from 1-deoxy-D-xylulose in higher plants by intramolecular skeletal rearrangement,” Proceedings of the National Academy of Sciences of the United States of America, vol. 94, no. 20, pp. 10600–10605, 1997.
- M. R. Seyed, N. Hossein, H. Rogayeh, et al., “Coumarins from the aerial parts of Prangos uloptera (Apiaceae),” Brazilian Journal of Pharmacognosy, vol. 18, no. 1, pp. 1–5, 2008.
- S. B. Mahato and A. P. Kundu, “13C NMR spectra of pentacyclic triterpenoids—a compilation and some salient features,” Phytochemistry, vol. 37, no. 6, pp. 1517–1575, 1994.
- M. Morita, M. Shibuya, T. Kushiro, K. Masuda, and Y. Ebizuka, “Molecular cloning and functional expression of triterpene synthases from pea (Pisum sativum): new α-amyrin-producing enzyme is a multifunctional triterpene synthase,” European Journal of Biochemistry, vol. 267, no. 12, pp. 3453–3460, 2000.
- Y. L. Hsu, P. L. Kuo, C. F. Liu, and C. C. Lin, “Acacetin-induced cell cycle arrest and apoptosis in human non-small cell lung cancer A549 cells,” Cancer Letters, vol. 212, no. 1, pp. 53–60, 2004.
- W. K. Liu, J. C. K. Ho, F. W. K. Cheung, B. P. L. Liu, W. C. Ye, and C. T. Che, “Apoptotic activity of betulinic acid derivatives on murine melanoma B16 cell line,” European Journal of Pharmacology, vol. 498, no. 1–3, pp. 71–78, 2004.
- C. I. Chang, C. C. Kuo, J. Y. Chang, and Y. H. Kuo, “Three new oleanane-type triterpenes from Ludwigia octovalvis with cytotoxic activity against two human cancer cell lines,” Journal of Natural Products, vol. 67, no. 1, pp. 91–93, 2004.
- I. Baglin, A. C. Mitaine-Offer, M. Nour, K. Tan, C. Cavé, and M. A. Lacaille-Dubois, “A review of natural and modified betulinic, ursolic and echinocystic acid derivatives as potential antitumor and anti-HIV agents.,” Mini reviews in medicinal chemistry, vol. 3, no. 6, pp. 525–539, 2003.
- K. Sakai, Y. Fukuda, S. Matsunaga, R. Tanaka, and T. Yamori, “New cytotoxic oleanane-type triterpenoids from the cones of Liquidamber styraciflua,” Journal of Natural Products, vol. 67, no. 7, pp. 1088–1093, 2004.
- K. Hata, K. Hori, and S. Takahashi, “Differentiation- and apoptosis-inducing activities by pentacyclic triterpenes on a mouse melanoma cell line,” Journal of Natural Products, vol. 65, no. 5, pp. 645–648, 2002.