Identification of triterpenoids in Brachylaena discolor DC, a plant with antidiabetic activity.

Abstract

In recent times, metabolic diseases such as diabetes and hypertension have become a huge burden in populations around the world. Among these diseases, the number of diabetes mellitus cases have been increasing at an alarming rate. Problems such as poor access to medication and side effects of existing drugs are encountered and there is still a need for more therapeutic alternatives. Plants have been used extensively for the treatment of various diseases, among them diabetes. Brachylaena discolor DC is a South African medicinal plant that is used by people for the treatment of diabetes and it has been reported previously to have antidiabetic activity with little toxicity. This study aimed to investigate the triterpenoid content of B. discolor and further evaluate the plant for in vitro antidiabetic activity by inhibition of the enzyme α-glucosidase, a therapeutic target for treatment of diabetes. In literature, it has been reported that some triterpenoids are good inhibitors of α-glucosidase. Chromatographic techniques, including argentation chromatography on silica gel impregnated with silver nitrate, were used in the isolation of compounds. The compounds were identified by 1D and 2D NMR (i.e. 1H, 13C, COSY, DEPT, HSQC and HMBC), IR, GC-MS, and HR-MS. From the dichloromethane extract of B. discolor leaves, eight triterpenoids were isolated, α-amyrin acetate, β-amyrin acetate, ψ-taraxasterol acetate, taraxasterol acetate, lupeol acetate, α-amyrin palmitate, β-amyrin palmitate, and lupeol palmitate. All the compounds are reported for the first time from the plant, except lupeol acetate, which was previously identified in the plant. Hydrolysis of α-amyrin acetate, β-amyrin acetate, ψ-taraxasterol acetate, taraxasterol acetate, and lupeol acetate afforded the alcohol triterpenoids α-amyrin, β-amyrin, ψ-taraxasterol, taraxasterol, and lupeol. The DCM-MeOH (1:1) leaf extract of B. discolor was investigated in vitro against α-glucosidase and the extract was to inhibit the enzyme significantly with an IC50 value of 95.95 μg/mL when compared with the standard inhibitor acarbose with an IC50 value of 1149.07 μg/mL. The individual pure compounds were not assayed due to poor solubility in the assay medium. Molecular docking studies revealed that the triterpenoids, α-amyrin, β-amyrin, ψ-taraxasterol, taraxasterol, and lupeol all have binding affinity for the α-glucosidase active site, where α-amyrin and β-amyrin showed more pronounced results with binding energies of -8.90 and -8.00 kcal/mol, respectively. In the study the reported antidiabetic activity of B. discolor was corroborated.