Crude extracts of, and purified compounds from, Pterocarpus angolensis, and the essential oil of Lippia javanica: their in-vitro cytotoxicities and activities against selected bacteria and Entamoeba histolytica

Abstract

In a recent study, various extracts of Pterocarpus angolensis were prepared and tested against bacteria. The acetone extract was found to be the most active against all the bacteria investigated, with minimum inhibitory concentrations varying from 0.0156 mg/ml against Staphylococcus aureus to 2 mg/ml against Enterobacter cloacae. Seven pure compounds were subsequently isolated from the ethanol extract of P. angolensis. Using several chromatographic and spectroscopic methods, the structures of five of these compounds — phthalate and four derivatives of epicatechin [(-)-epicatechin, epicatechin-3-O-galate, epicatechin (4β–8)-epicatechin (B2), and a hexamer of epicatechin] — were successfully determined. The seven purified compounds were then further tested, in vitro, against Staphylococcus aureus and Entamoeba histolytica, and for their in-vitro cytotoxic activity. Although all seven were active against S. aureus, just one of the purified compounds from P. angolensis and piperitenone, a pure compound isolated from Lippia javanica essential oil, were found to have marked activity against Entamoeba histolytica, with median inhibitory concentrations (IC₅₀) of 25 and 100 μg/ml, respectively. The other P. angolensis compounds were either weakly active or showed no activity against the amoebae when tested at concentrations up to 400 μg/ml. All seven compounds isolated from P. angolensis showed less toxicity against cultures of human (HCT-8) cells than piperitenone, with IC₅₀ of 175–375 μg/ml. The presence of epichatechin and derivatives (with strong antibacterial activities but generally weak activities against Entamoeba histolytica) in the stem bark of P. angolensis has thus been demonstrated. Further investigation of the activities of these compounds and their potential use in the treatment of bacterial diseases appears justified.