The in vitro and in vivo anti-oxidative and anti-diabetic effects of some African medicinal plants and the identification of the bioactive compounds.
This thesis examined the in vitro and in vivo anti-oxidative and anti-diabetic activities of five African medicinal plants which are traditionally used for the treatment of diabetes mellitus viz; Ziziphus mucronata, Cassia singueana, Parkia biglobosa, Khaya senegalensis and Vitex doniana. Ethanol, ethyl acetate and aqueous crude extracts of the stem bark, root and leaf samples of each of the plants (a total of 45 crude extracts) were investigated for detailed anti-oxidative activity and the most active crude extract from each plant was selected for further fractionation with solvents of increasing polarity. Subsequently, the solvent fractions derived from these crude extracts (a total of 21 fractions) were also subjected to the anti-oxidative assays as well as α-glucosidase and α-amylase inhibitory activities assays. Results from these assays revealed that the butanol fractions from crude extracts of Z. mucronata, P. biglobosa, K. senegalensis and V. doniana and the acetone fraction from the crude extract of C. singueana were the most bioactive. Kinetic delineation of the types of enzyme inhibitions exerted by these most active fractions as well as measurement of relevant kinetic parameters (KM, Vmax and Kᵢ) were done using Lineweaver-Burke’s plot. Furthermore, the most active fractions were also subjected to GC-MS analysis and in vivo intervention trial in a type 2 diabetes (T2D) model of rats (except fraction from V. doniana). The in vivo studies revealed that all the fractions possessed potent in vivo anti-T2D activity (to varying extent) and the possible mechanisms of actions were proposed. Furthermore, most of the fractions were able to ameliorate the T2D-associated complications. Analysis of in vivo oxidative stress markers such as glutathione, thiobarbituric acid reactive substances, superoxide dismutase and catalase in the serum, liver, kidney, heart and pancreas of the animals also gave a clue in to the possible mechanism of action. Bioassay guided isolation was used to track the bioactive anti-diabetic compounds from these fractions via column chromatography. The isolated compounds were characterized by ¹H NMR, ¹³C NMR, 2D NMR (in two cases) and mass spectroscopy (in one case). From this study, 2,7-dihydroxy-4H-1-benzopyran-4-one, 3β-O-acetyl betulinic acid, lupeol and bicyclo[2.2.0]hexane-2,3,5triol were identified as the possible bioactive compounds from Z. mucronata, C. singueana, P. biglobosa, K. senegalensis solvent fractions respectively. The findings of this work are important for the relevant government agencies, pharmaceutical industries, scientific community and poor diabetic patients because it might open an avenue for the development of viable and cost effective anti-diabetic herbal products and/or novel plant-derived anti-diabetic drugs.