Kinetic of inhibition of carbohydrate-hydrolysing enzymes, antioxidant activity and polyphenolic content of Phyllanthus amarus Schum. & Thonn. (Phyllanthaceae)

Phyllanthus amarus (PA) Schum. & Thonn. (Phyllanthaceae) is a herbal medicine traditionally used in the management of diabetes mellitus. The aqueous extracts (decoction and macerate) of the aerial parts of PA as used traditionally, and the organic extracts (dichloromethane and methanol) were evaluated for possible inhibitory effect against α-amylase and α-glucosidase activity and the type of inhibition was established by the Lineweaver–Burke plot in vitro. Antioxidant capacity of PA was evaluated using standard assays and the total phenolic content (TPC) was determined. Highest inhibitory activity was observed for the aqueous leaf extract (IC50 = 44.62 μg/mL) and stem methanol extract (IC50 = 35.39 μg/mL) against α-amylase and comparable to acarbose (IC50 = 58.62 μg/mL). Highest inhibitory activity against α-glucosidase was observed in the methanolic extract of the leaf (IC50 = 0.674 μg/mL), which was significantly lower (p < 0.01) than acarbose. A mixed competitive form of inhibition against both enzymes was recorded. Reducing power was more significant for the aqueous stem extract (2023 μM Fe(II)/g) and leaf decoction (1576 μM Fe(II)/g). Aqueous extracts showed significant free radicals scavenging activity and strongest metal ion chelation was recorded in the leaf aqueous extract (IC50 = 1.705 mg/mL) and stem decoction (IC50 = 1.598 mg/mL). A strong positive correlation (R2 ≥ 0.7) between TPC and anti-oxidant activity was recorded. In conclusion, the local variety of PA has the potential to inhibit α-amylase and α-glucosidase activity, which can be further investigated for the management of hyperglycemia. Additionally, the significant antioxidant activity observed suggests its possible application in the mitigation of hyperglycaemia-induced oxidative stress.