Isolation and identification of the primary toxin in the smoke of the Namibian milk bush, Euphorbia damarana

Euphorbia damarana L.C.Leach, also known as Damara milk bush, grows only in the north-western desert areas of Namibia and southern Angola. It is often the only vegetation in these desert regions and the local inhabitants know very well that it is extremely poisonous to humans and animals. During the 1960s, twenty-seven migrating mineworkers used its dead branches as firewood for their barbecue with deadly consequences. They all passed away after consuming meat that was infused with toxic volatiles in the smoke. We report in this study, on the isolation of the toxic triterpenoid, euphol that is present in high concentrations in the stems and smoke of E. damarana. Small concentrations of compounds with phorbol ester skeletons were also detected in this species. The cytotoxicity of euphol in the smoke and stem extracts of E. damarana was determined by the sulfo-rhodamine-B stain (SRB) assay on eight human cell lines (A549, PC-3, HeLa, HepG2, MCF-7, MCF-12A, MRC-5 and HaCaT). Significant cytotoxic activity was observed from the purified euphol, stem and smoke extracts with IC50 values ranging from 1.99 to 3.99 μg/mL, 5.00 to 20.00 μg/mL and from 11.75 to 40.00 μg/mL respectively, on all the tested human cell lines. Since euphol is the primary compound (concentration 10.3 mg/g) in the smoke extract and toxic to all cell lines tested at a significantly low IC50 level, it can be considered as the main toxin in E. damarana and responsible for the deaths of the miners.