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Phytochemical investigation and biological studies of some South African plant species (Asteraceae and Hyacinthaceae)

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Oyetunde-Joshua, Funsho Mary.

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Plants used in conventional medicine for the treatment of different ailments have been the bedrock for modern therapeutics. Scilla nervosa from the Hyacinthaceae family and Helichrysum panduratum and Helichrysum actutaum from the Asteraceae family are plants used in traditional medicine in South Africa. However, limited information exists on the biological potential and safety of these plants. This research aimed to phytochemically investigate these plants by isolating and characterizing their secondary metabolites and testing their biological activities. The phytochemical investigation of S. nervosa yielded eleven compounds, including two novel homoisoflavonoids, two novel lanostane-type triterpenes, five known homoisoflavonoids, one stilbene and one sterol glucoside. The cytotoxicity of the homoisoflavonoids was good against the Caco-2 tumor cell line but moderate against the HepG2 cell line and the methanol extract of the leaves showed promising activity against Caco-2 and HepG2 cell lines, giving IC50 values of 7.79 and 9.29 μg/mL, respectively. For homoisoflavonoids, polarity influenced activity, with the least polar compounds being more active. Likewise, saturation between the benzopyrone ring and ring C contributed to activity. The homoisoflavonoids with methoxy substituents displayed better antibacterial activity than those with hydroxy substituents but these were still lower than flavonoids. Molecular docking using MraY phospho-MurNAc-pentapeptide translocase was conducted on the bioactive homoisoflavonoids to rationalize their antibacterial activity. The results showed isolates to bind in the same active site of the substrate, with a slight difference due to the presence of the hydroxy group. The phytochemical investigation of H. panduratum led to the isolation of a sterol, a sterol glucoside, three triterpenes, a phenolic glucoside, and one homoisoflavonoid, which is the first report of this group of flavonoids from Helichrysum genus. The plant was shown to have moderate antibacterial activity. Screening of the quorum sensing-controlled phenotype of bioluminescence in Vibrio harveyi BB120 was conducted. The methanol extract of the leaves could inhibit Gram-negative N-acyl homoserine lactone-based and global crosstalk autoinducer-2-based quorum sensing. The cytotoxicity assay showed reduced activity towards Caco-2, HepG2 and the regular cell line Hek 293, making it safe for human use. H. acutatum yielded three compounds, a sterol, a sterol glucoside and a cinnamic acid derivative. The plant showed no antibacterial activity and no cytotoxicity towards the normal cell line, Hek 293, making it safe for human consumption. The ethyl acetate extract of the root demonstrated good antioxidant activity, which could be attributed to the cinnamic acid derivative. The leaves of H. panduratum are rich in arbutin, a natural hydroquinone form, making it valuable in cosmetology. The antibacterial potential of the isolated homoisoflavonoids could be enhanced by synthetic manipulations of the molecular framework. These modifications could also improve selectivity towards tumor cell lines. The findings from this study provide scientific evidence for the use of the plants in traditional medicine, especially H. acutatum, for which there are no reports on its use or biological activity.


Doctoral Degree. University of KwaZulu-Natal, Durban.