Investigation of neurotoxic principles of the Tumbleweed, Trachyandra Laxa, responsible for livestock losses in Southern Africa

A thesis submitted in fulfillment of the requirements for the Degree of Master of Science (Chemistry)

Livestock losses due to poisonous plants remain a major concern for livestock agriculture in southern Africa. However, for a number of such toxic plants, including Trachyandra laxa (family Xanthorrhoeaceae, sub-family Asphodeloideae), the toxic components are still unknown. Trachyandra laxa primarily affects the central nervous system when ingested by livestock, causing paralysis. Traditional bioactivity-guided fractionation approaches used for the isolation and identification of toxins are time consuming and laborious. Therefore, in this study, a chemical screening approach involving gas chromatography - mass spectrometry (GCMS) was used. The chemical constituents were extracted from the leaves, inflorescence, flowers, roots and rhizomes of T. laxa, using a sequential extraction approach with solvents of different polarity, starting with hexane and ending with methanol. Extracts were screened for cytotoxicity using the MTT-assay, for neurotoxicity using acetylcholinesterase (AChE) inhibition. Results showed no cytotoxicity and the acetylcholinesterase inhibition assay displayed positive results for the dichloromethane (DCM) extract, indicating neurotoxic activity. The chemical screening approach was subsequently pursued on the DCM extracts only. In this approach, the DCM extracts were analysed by GC - MS and their major volatile constituents were tentatively identified from their mass spectra and retention indices. A literature search was conducted to determine which of the tentatively identified compounds are toxic. This led to the discovery that two of the constituents, the α,β-unsaturated lactones, dihydroactinidiolide (DHA) and its hydroxylated analogue, are known to have potent AChE inhibition properties. It is envisaged that this study will pave the way to further research on specific treatment of animals affected by T. laxa poisoning or the development of a preventative agent that can be used to avoid T. laxa poisoning.