Inhibitory potential and antidiabetic activity of leaf extracts of Vitex doniana

Pancreatic alpha (α)-amylase and intestinal α-glucosidase involved in the enzymatic breakdown of dietary carbohydrates are implicated in postprandial hyperglycemia. The inhibition of these two enzymes has been exploited in the design of antidiabetic drugs. This study was therefore designed to investigate the ability of the leaf extracts of Vitex doniana to inhibit α-amylase and α-glucosidase as well as the in-vivo antidiabetic potential of the ethanol leaf extract. The inhibitory potentials of the extracts on α-amylase were assayed through the method of starch-iodine colour change while α-glucosidase inhibition was assessed by measuring the release of 4-nitrophenol by p-nitrophenyl-α-D-glucopyranoside (pNPG). Acarbose was used as the standard. The extracts displayed a dose-dependent significant inhibition of the enzymes. The aqueous leaf extract of V. doniana was the strongest inhibitor of α-amylase with half-maximal inhibitory concentration (IC50) of 3.09 ± 0.22 mg/ml while the ethanol leaf extract was the strongest inhibitor of α-glucosidase with IC50 of 17.12 ±1.42 mg/ml. The result also showed that both extracts contain mixed inhibitors of the two enzymes. The result of the in-vivo antidiabetic study conducted for 14 days showed a significant (p<0.05) reduction in the fasting blood glucose level of the extract treated groups of animals compared to the untreated diabetic group.  These results strongly suggest that V. doniana can be exploited in the treatment of diabetes mellitus.   Key words: Vitex doniana, antidiabetic, inhibitory potential, Lineweaver-Burk, α-amylase, α-glucosidase.