Antimicrobial lupenol triterpenes and a polyphenol from Elaeodendron transvaalense, a popular southern African medicinal bark

• Published in: South African journal of botany 2019-05, Vol.122, p.518-521
• Main Authors: Khumalo, G.P., Sadgrove, N.J., Van Vuuren, S.F., Van Wyk, B.-E.
• Format: Article
• Language: English
• Subjects: Cassine transvaalensis; Gastrointestinal infections; Lupane triterpene; MIC; Skin infections; Traditional medicine
• ISSN: 0254-6299; 1727-9321
• DOI: 10.1016/j.sajb.2018.07.020

Elaeodendron transvaalense stem bark is widely used in southern African traditional medicine, mainly for gastrointestinal tract disorders and skin ailments. The aim of the study was to investigate the possible scientific rationale for the popularity and ethnobotanical uses of E.transvaalense. To achieve this aim, target pathogens related to infections were tested with crude bark extracts and also the main active antimicrobial compounds. Stem bark was purchased from traditional medicine (muthi) markets, powdered and extracted with either methanol or dichloromethane. For the antimicrobial testing, the micro-titre plate broth two-fold serial dilution assay in 96 well plates was undertaken. Bioassay guided fractionation and polar stationary phase column chromatography was used to isolate the active compounds. Crude bark extracts demonstrated moderate activity against Gram-negative(Escherichia coliATCC 8739, Pseudomonas aeruginosaATCC 27853, Salmonella typhimuriumATCC 9290, Shigella sonneiATCC 14028) and Gram-positive(Staphylococcus aureusATCC 25923, Staphylococcus epidermidisATCC 12228) organisms. A dichloromethane extract yielded three major triterpenes; lup-20(30)-ene-3α,29-diol(1), 6β-hydroxylup-20(29)-ene-3-one(2) and 30-hydroxylup-20(29)-ene-3-one(3) and one major polyphenol, 4′-O-methylepigallocatechin (4). Compounds 1, 3 and 4 were previously reported from the stem bark of E.transvaalense, but this is the first report of 2 in the stem bark of this species, although it was previously reported from the root bark. The presence of 1–3 was confirmed to be also in the aqueous extract but a much higher relative abundance of 4′-O-methyl epigallocatechin (4) was found. The four compounds demonstrated moderate antimicrobial activity with the lowest MIC value of 0.093 mg/mL against E.coli and S.typhimurium from 4, the polyphenol. The antimicrobial activity of E.transvaalense stem bark is therefore at least partly due to the presence of lupenol triterpenes and 4′-O-methyl epigallocatechin. The presence of all four compounds in the dichloromethane and aqueous extracts was confirmed, but the overwhelmingly major component in the aqueous extract was 4. The results support the traditional uses of the bark against gastrointestinal tract and skin infections. •Elaeodendron transvaalense is one of the most popular sources of medicinal bark.•Three lupeol triterpenes and a major polyphenol were identified in the stem bark.•Crude extracts and isolated compounds showed moderat