The phytochemistry and biological activity of secondary metabolites from Kenyan Vernonia and Vepris species.

Abstract

This work is an account of the phytochemical analysis of two genera, Vernonia and Vepris which are used as remedies for illness by the Kalenjin community of Kenya. Species of Vernonia are known to yield sesquiterpene lactones, which typify the genus whereas Vepris is rich in alkaloids and limonoids which have a wide range of biological activities. The species studied in this work were Vernonia auriculifera, Vernonia urticifolia, Vepris glomerata and Vepris uguenensis. Phytochemical studies revealed a range of compounds being present in the four species. From Vernonia, triterpenoids, a sesquiterpene amine, a carotenoid and a polyene were isolated. This was the first account of a sesquiterpene amine from a plant species and the first account of the novel polyene. The triterpenoids showed moderate antibacterial activity, with b-amyrin acetate and oleanolic acid being effective at decreasing adhesion of selected gram-negative and gram-positive bacteria. Lutein and urticifolene showed good antibacterial activity against Enterococcus feacium and Pseudomonas aeruginosa. In Vepris, a range of compounds were isolated, belonging to the furoquinoline alkaloids, coumarins, flavonoids, cinnamic acid derivatives, lignins, cinnamaldehydes, triterpenoids and limonoids. Five new compounds; a cinnamaldehyde derivative (glomeral), two flavonoids (veprisinol, uguenenprenol) and two A, D-seco-limonoids (uguenensene and uguenensone) were amongst the compounds isolated. Antibacterial studies showed that glomeral inhibited the growth of Staphylococcus aureus and Shigella dysentrieae at low concentrations (MIC of 2 μg mLˉ¹ and 0.4 μg mLˉ¹ respectively). Antioxidant assays of several compounds revealed that, veprisinol, isohaplopine-3,3’-dimethylallyl ether, uguenenprenol and 7-O-methylaromadenrin are good antioxidant agents. The limonoids isolated from Vepris uguenensis also make up an interesting biogenetic relationship.

Structural elucidation was carried out by 1D and 2D NMR spectroscopy in conjuction with mass spectrometry, infrared, ultraviolet and circular dichroism analysis where applicable. Biological assays were carried out using standard methods at laboratories in the University of KwaZulu-Natal and Kenya Medical Research Institute (KEMRI-Nairobi).