Biological activity of two related triterpenes isolated from Combretum nelsonii (Combretaceae) leaves

Abstract

Antifungal bioassay-guided fractionation of Combretum nelsonii leaf extracts afforded two closely related triterpenes, asiatic acid and arjunolic acid. Antifungal activities of the mixture of asiatic acid and arjunolic acid were determined against five fungal animal pathogens. The minimum inhibitory concentrations of the mixture to the different pathogens varied from 0.2 to 1.6 µg mL⁻¹; Candida albicans (0.9 µg mL⁻¹), Cryptococcus neoformans (0.4 µg mL⁻¹), Aspergillus fumigatus (1.6 µg mL⁻¹), Microsporum canis (0.2 µg mL⁻¹) and Sporothrix schenckii (0.2 µg mL⁻¹). Microsporum canis and S. schenckii were the most susceptible followed by C. neoformans. Aspergillus fumigatus was the most resistant. The R_f value of the mixture of asiatic acid and arjunolic acid was 0.27 in CEF (chloroform : ethylacetate : formic acid), 0.09 (BEA; benzene : ethanol : ammonium hydroxide) and 0.55 (EMW; ethylacetate : methanol : water) which was active against all pathogens. In vitro cytotoxicity of mixture gave an LC₅₀ of 10.58 µg mL⁻¹ towards Vero monkey kidney cells.

Keywords:

• Combretaceae
• antifungal
• Combretum nelsonii
• triterpenes
• asiatic acid
• arjunolic acid

Acknowledgement

The National Research Foundation (NRF) and National Department of Agriculture (South Africa) provided funding for the experiment. The authors thank Dr M. Adegroba and Mr P. Mokgotho for helping with the isolation, Dr L. McGaw for helping with toxicity studies and Rudi Kotze and Johan Hurter for allowing us to collect plant material from the Lowveld National Botanical Garden.