Phytochemical and biological activities of South African traditional medicinal plants

Abstract

Natural remedies have become integral to modern healthcare, with plant-based therapies regaining prominence due to their therapeutic efficacy and cultural significance. Approximately 60-80% of the global population, particularly in developing regions, relies on medicinal plants for primary healthcare, involving over 70,000 species. However, many of these plants remain scientifically unexplored, emphasising the urgent need for research to validate their therapeutic potential and ensure safety. South Africa's Cape Floristic Region, renowned for its biodiversity, hosts over 30,000 plant species, including Lessertia frutescens (cancer bush), Protea venus, and Leucadendron xanthoconus. While L. frutescens is well-documented for its anti-inflammatory, antioxidant, antiviral, anti-HIV and anticancer properties due to its diverse phytochemicals (e.g., amino acids, saponins, phenolics, and flavonoids), the phytochemical and pharmacological properties of P. venus and L. xanthoconus species remain largely unexplored. These three South African plants were selected for their economic importance, ethnobotanical significance and phytochemical potential. Phytochemical analysis of L. frutescens (syn. Sutherlandia frutescens) led to the isolation of 11 compounds (6.1-6.11), including two novel metabolites, lessertiosides A (6.1) and B (6.2), characterised using IR, UV, NMR, and HR-ESIMS techniques. Notably, flavonoids 8- methoxyvestitol (6.7) and mucronulatol (6.8) were reported for the first time in this plant. The remaining compounds included the amino acid proline (6.10), the sugar D-pinitol (6.11), cycloartane glycosides [sutherlandiosides B (6.6), D (6.5), K (6.3) and 7S,24S,25-trihydroxy 9,10R-seco-9,19-cyclolanost-2(3),9(11)-diene-25-O-β-D-glucopyranoside (6.4)], and the flavonoid sutherlandin C (6.9). Compounds 6.3-6.11 were evaluated for their neuroprotective properties in SH-SY5Y cells under MPP⁺-induced PD conditions. At concentrations of 2.5, 5, and 10 µM, the compounds were non-toxic and demonstrated neuroprotective effects by enhancing cell viability, maintaining ATP production, and reducing apoptotic cell death. Compounds 6.6, 6.8, 6.9, and 6.10 significantly restored ATP levels from 51% (under insult) to 73%, 75%, 74%, and 75%, respectively. Furthermore, compound 6.8 exhibited the most potent anti-apoptotic effect, reducing caspase 3/7 activity from 5-fold to 1.5-fold. Phytochemical analysis of P. venus, a hybrid of P. repens and P. aristata, led to the isolation of 11 distinct compounds (7.1-7.11), including a novel metabolite, p-coumaroylcalleryananin (7.1). Other compounds included calleryanin (7.2), protocatechuoylcalleryanin (7.3), p hydroxybenzoylcalleryanin (7.4), lacticolorin (7.5), kaempferol-3-O-rhamnoside (7.6), rutin (7.7), quercetin-3-O-rhamnoside (7.8), 3,4-dihydroxybenzoic acid (7.9), p-hydroxybenzoic acid (7.10) and glucose (7.11). The methanolic leaf extract showed a phenolic content of 199.05±1.64 mg GAE/g and exhibited strong antioxidant activity (IC50= 0.042±0.07 µg/mL) compared to the positive control vitamin E (IC50= 0.152±0.67). Among the tested isolated compounds (7.3-7.6 and 7.9), compounds 7.3, 7.5, 7.6 and 7.9 showed significant antioxidant activity with IC50 of 0.018±1.11, 0.033±1.27, 0.048±0.53 and 0.018±0.39 mg/mL. MTT assay results for the tested compounds indicated minimal cytotoxic effects on the H9c2, MCF-7, and C3A cell lines. Moreover, the crude extract and compound 7.6 demonstrated potential cardioprotective activity by significantly enhancing ATP levels in H9c2 cells from 53% (HG+Pal) to 70% and 73%, respectively, in DCM model following treatment. Phytochemical analysis of L. xanthoconus yielded seven compounds (6.5, 8.1-8.6), including novel metabolites dichotomain C (8.1) and syringetin-3-O-(6´´-caffeoyl)-β-D-glucopyranoside (8.2). Compound 8.5 (Syringetin-3-O-β-D-glucopyranoside) displayed good antioxidant activity (IC50 = 0.071±0.82) compared to vitamin E and exhibited weak cytotoxicity against H9c2 cells (IC₅₀ = 85.79±1.56 µM). Sutherlandioside D (6.5), leucodrin (8.3), leudrin (8.4) and the methanolic extract did not show cytotoxic activities against H9c2 cells. Additionally, none of them exhibited cardioprotective effects. In silico docking and computational ADME analysis identified compounds 6.7 and 6.8 as potential neuroprotective agents, demonstrating high GI absorption, BBB permeability, drug likeness, and lead-likeness properties. These two compounds demonstrated favourable binding to MAO-B (-9.3 and -8.9 kcal/mol, respectively) compared to Selegiline (-7.3 kcal/mol) and DJ-1 (-6.3 and -5.3 kcal/mol) compared to levodopa (-5.4 kcal/mol), supporting their neuroprotective potential. Compound 7.6 from P. venus demonstrated strong binding to the KEAP1-NRF2 and weak binding to β1-AR, suggesting that β1-AR signalling is unlikely to play a significant role in its activity. These findings indicate that modulation of the KEAP1-NRF2 pathway is its primary mode of action. Like curcumin, its interactions with KEAP1-NRF2 highlight its potential to enhance antioxidant responses and improve cellular stress resistance, thereby supporting its moderate cardioprotective effects. Overall, this research study highlights underexplored South African plants' phytochemical and therapeutic potential, providing a foundation for further investigations into their phytochemical profiles, pharmacological properties and toxicological safety. It also provides new insights into L. xanthoconus secondary metabolites while highlighting their limited bioactivities.