α-Amylase inhibitory activity of some Malaysian plants used to treat diabetes; with particular reference to Phyllanthus amarus

Extracts of six selected Malaysian plants with a reputation of usefulness in treating diabetes were examined for α-amylase inhibition using an in vitro model. Inhibitory activity studied by two different protocols (with and without pre-incubation) showed that Phyllanthus amarus hexane extract had α-amylase inhibitory properties. Hexane and dichloromethane extracts of Anacardium occidentale, Lagerstroemia speciosa, Averrhoa bilimbi Pithecellobium jiringa and Parkia speciosa were not active when tested without pre-incubation. Extraction and fractionation of Phyllanthus amarus hexane extract led to the isolation of dotriacontanyl docosanoate, triacontanol and a mixture of oleanolic acid and ursolic acid. Dotriacontanyl docosanoate and the mixture of oleanolic acid and ursolic acid are reported from this plant species for the first time. All compounds were tested in the α-amylase inhibition assay and the results revealed that the oleanolic acid and ursolic acid (2:1) mixture was a potent α-amylase inhibitor with IC₅₀ = 2.01 μg/ml (4.41 μM) and that it contributes significantly to the α-amylase inhibition activity of the extract. Three pure pentacyclic triterpenoids, oleanolic acid, ursolic acid and lupeol were shown to inhibit α-amylase.