The antioxidant properties of bufadienolides, analogous to the orbicusides of Cotyledon orbiculata L. var orbiculata (Haw.) DC
The use of traditional and natural medicines in primary healthcare or alternative therapy is on the increase. However, the safety and efficacy of these medicines have not yet been confirmed. Pharmacognosy, the study of the properties of drugs, potential drugs or drug substances of natural origin and the search for new drugs from natural resources, is therefore of extreme importance in today’s healthcare environment. Cotyledon orbiculata L. var. orbiculata (Haw.) DC., a succulent shrub that is widely distributed over the whole of southern Africa, is an example of a plant used in traditional medicine for its antiepileptic effects. Oxidative stress can either be the cause of, or be secondary to epilepsy pathogenesis. Lipid peroxidation causes the disruption of cell membranes which leads to cell destruction and, in the case of neurological disorders, neurodegeneration. Reactive species have also been found to influence neurotransmission by affecting neurotransmitter metabolism and functions. Reactive species can therefore be responsible for the development of convulsions. Conventional anti-epileptics have shown to exert neuroprotective effects but information or research regarding their ability to prevent epilepsy from becoming chronic does either not exist or is not promising. Antioxidants have potential in the treatment of epileptic seizures as well as the prevention of chronic epilepsy by preventing the effects that oxidative stress has on neurotransmitter metabolism and functions that cause alterations in neuronal excitability and seizure threshold, ultimately leading to epileptic foci. The aim of this study was to evaluate the potential of the bufadienolide orbicusides of C. orbiculata and analogues as anti-epileptic treatment through antioxidant activity. Initially the isolation of novel antioxidants from C. orbiculata leaf juice was attempted. The antioxidant activity of the concentrated juice and fractions resulting thereof were evaluated with two assays. The thiobarbituric acid (TBA) assay was used to measure the extent of lipid peroxidation and nitroblue tetrazolium (NBT) assay was used to measure superoxide scavenging activity in rat brain homogenate. The low concentrations of orbicusides prompted the determination of the activity of two commercial bufadienolides (bufalin and cinobufotalin) and two bufadienolide analogues, synthesised by the esterification of trans-androsterone and androstanolone, respectively, using coumalic acid, producing Compound 1 and Compound 2. The toxicity of the commercial bufadienolides and synthesised analogues were evaluated by using the MTT assay (a cell viability assay). C. orbiculata juice showed significant pro-oxidant activity in both assays. Bufalin showed significant pro-oxidant activity in the TBA assay. Cinobufotalin showed no significant activity. Compound 1 showed pro-oxidant activity in the TBA assay and Compound 2 showed slight antioxidant activity in the NBT assay. The commercial bufadienolides showed low cell viability, indicating significant toxicity. The synthesised analogues showed a significant reduction in toxicity (despite Compound 2 being moderately toxic) when compared to the toxicity of the commercial bufadienolides. The low concentrations of orbicusides in the plant material and the antioxidant assay results of the two commercial bufadienolides suggested that the orbicusides may not be involved in the antioxidant properties of C. orbiculata. However, the antioxidant activity of Compound 2 showed that altering the pyrone moiety of bufadienolides could possibly improve antioxidant activity. The reduced toxicity and slight antioxidant activity of the synthesised bufadienolide analogues motivates further investigation.
- Health Sciences