Antioxidative and antidiabetic effects of some African medicinal plants.
Three (3) medicinal plants [Aframomum melegueta K. Schum., Xylopia aethiopica (Dunal.) A. Rich. and Capsicum annuum L.] were selected based on their traditional uses in the treatment of diabetes in Africa. Various crude extracts and fractions from different parts of the plants were screened using several anti-oxidative and anti-diabetic tests in vitro. Most active fractions from each plant were used to examine in vivo anti-diabetic activity in type 2 diabetes (T2D) rat model. Additionally, possible bioactive compounds from most active extracts and fractions were analyzed by using GC-MS, TLC and NMR spectroscopy. The results showed that ethanolic extracts derived from the fruits of the plants demonstrated excellent anti-oxidative and anti-diabetic activities in vitro compared to other extracts from the same or different parts of these plants. After fractionation, ethyl acetate fraction from A. melegueta and acetone fractions from X. aethiopica and C. annuum exhibited strong radical scavenging (IC₅₀: 1-120 μg/mL) activity, inhibition of hemoglobin glycation (IC₅₀: 100-150 μg/mL), α-amylase (IC₅₀: 50-170 μg/mL) and α-glucosidase (IC₅₀: 40-87 μg/mL) activities hence were used for the in vivo study. The GC-MS analysis of the three (3) most active fractions revealed the presence of mostly phenolic compounds of 4-hydroxy-3-methoxyphenyl derivatives. Furthermore, the data of the in vivo study showed that oral intervention of the fractions (150 and 300 mg/kg bw) for 4 weeks demonstrated potent anti-diabetic actions via improving body weight gain, reducing feed and fluid intake and hyperglycemia, improving glucose tolerance ability, insulin sensitivity, amelioration of pancreatic β-cell histology and β-cell functions, improving dyslipidemia in a T2D rat model. Additionally, the pancreatic histopathological damages and other oxidative damages caused by the induction of diabetes were attenuated to near normal in the liver, kidney, heart and pancreas of the treated animals. The bioassay-guided fractionations lead to the isolation of 3 arylalkanes (6-paradol (1), 6-shagaol (2), and 6- gingerol (3)) and oleanolic acid (4) from A. melegueta fruits, when oleanolic acid (4) was the first to be isolated from A. melegueta. Moreover, 6-gingerol (3) and oleanolic acid (4) were similarly isolated for the first time from X. aethiopica fruits as well. These compounds have exhibited significant inhibitions against the α-amylase and α-glucosidase actions and thus are possible anti-diabetic agents and the anti-diabetic action of A. melegueta and X. aethiopica fruits is attributed to the presence of these compounds. This study also confirmed the use of these plants in African anti-diabetic traditional medicines by traditional healers. However, further clinical study is required to confirm these effects in human subjects.