Potential antidiabetic effects by alpha-amylase inhibition and free radical scavenging activity of extracts from five medicinal plants used in Nigeria

The cost of managing diabetics and side effects from available synthetic drugs has led to the exploration of the floral diversity for the discovery of potent drug molecules. Therefore, this study aimed to assess the potential antidiabetic effects of alpha-amylase inhibition and free radical scavenging properties of Lagenaria breviflora, Macaranga barteri, Solanum erianthum, Spigelia anthelmia, and Uvaria afzelii growing in southwestern Nigeria. Total phenolic content (TPC), 2,2-Diphenyl-1-Picrylhydrazyl (DPPH), and in vitro alpha-amylase inhibitory activities of the five medicinal plant methanol extracts were evaluated. The methanol extract of Macaranga barteri leaves (MBL) had the highest TPC (107.37 mg GAE/g), and free radical scavenging activity (IC50 = 6.58 μg/mL) as compared to ascorbic acid (IC50 = 8.39 ± 0.64 µg/mL). It also had the highest α-amylase inhibitory activity (IC50 = 0.54 µg/mL) when compared to acarbose (IC50 = 0.68 ± 0.21 µg/mL). From the resulting partitioned fractions, ethylacetate fraction had the highest TPC (176.81 mg GAE/g) followed by dichloromethane (DCM) fraction (93.23 mg GAE/g). The DCM fraction had the highest free radical scavenging and α-amylase inhibitory activities with IC50 = 5.17 µg/mL and IC50 = 0.29 µg/mL respectively. Among the subfractions from the DCM fraction using vacuum liquid chromatography, subfraction C showed the highest α-amylase inhibitory activity (IC50 = 0.35 µg/mL). The major molecules including 3,5-dicaffeoylquinic acid, acteoside, kampferol-7-O-glucoside and bastadin 11, were identified from the DCM fraction using high-performance liquid chromatography-ultraviolet–visible detector. These activities of M. barteri leaves revealed that it could be a potent source of novel anti-diabetic agent. Also, the Nigeria flora could be a veritable source of bioactive leads for drug development.