(Alpha)-amylase and (alpha)-glucosidase inhibitory and antioxidant activities of three Sri Lankan plants

Abstract

Diabetes is a chronic metabolic disorder that has become a major health problem worldwide. Diabetes is associated with increased blood glucose levels and slowing down the metabolic conversion of carbohydrate to glucose is an effective strategy for managing the disease. α-Amylase and α-glucosidase enzyme inhibitors retard the conversion of carbohydrates to glucose. The search for such inhibitors in medicinal plants is attractive because of the anticipated low adverse effects and high efficacy of natural products present in medicinal plants.

𝘊𝘢𝘯𝘢𝘳𝘪𝘶𝘮 𝘻𝘦𝘺𝘭𝘢𝘯𝘪𝘤𝘶𝘮(Retz.) Blume (Burseraceae) and 𝘖𝘴𝘣𝘦𝘬𝘪𝘢 𝘰𝘤𝘵𝘢𝘯𝘥𝘳𝘢 (L.) DC. (Melastomataceae) are plants endemic to Sri Lanka and are used in the treatment of diabetes in the indigenous system of medicine. 𝘗𝘪𝘱𝘦𝘳 𝘣𝘦𝘵𝘭𝘦 L. (Piperaceae) is also known to have antidiabetic properties. In this study α-amylase- and α-glucosidase-inhibitory activities of the extracts of these three plants were investigated. Oxidative stress is implicated in the pathogenesis of diabetes and antioxidants may provide therapeutic benefits. Hence, the antioxidant activity of the extracts was also investigated using the 2,2-diphenyl-1picrylhydrazyl (DPPH) assay. Powders of shade-dried 𝘊. 𝘻𝘦𝘺𝘭𝘢𝘯𝘪𝘤𝘶𝘮 bark and 𝘖. 𝘰𝘤𝘵𝘢𝘯𝘥𝘳𝘢 leaves and freeze-dried 𝘗. 𝘣𝘦𝘵𝘭𝘦 leaves were separately extracted into hot water and a mixture (1:1) of dichloromethane and methanol. The concentrated extracts dissolved in aqueous dimethylsulfoxide (1%) were used in the enzyme-inhibitory bioassays. For the DPPH assay, the concentrated extracts were dissolved in ethanol. The average concentration of each extract for 50% inhibition of activity (IC50) in the bioassays was determined by performing the experiments in triplicate.

The extracts of 𝘗. 𝘣𝘦𝘵𝘭𝘦 did not show enzyme-inhibitory activity (IC₅₀ > 5000 μg/ml) while the extracts of 𝘖. 𝘰𝘤𝘵𝘢𝘯𝘥𝘳𝘢 had some α-glucosidase-inhibitory activity (IC₅₀, 43-128 μg/ml). The extracts of 𝘊. 𝘻𝘦𝘺𝘭𝘢𝘯𝘪𝘤𝘶𝘮 displayed the highest enzyme-inhibitory and antioxidant activities (IC50: α-amylase, 8-65 μg/ml; α-glucosidase, 3-22 μg/ml; DPPH, 0.2-1.1 μg/ml). All extracts showed considerable antioxidant activity (IC50, 0.2-67.4 μg/ml) compared to butylated hydroxyanisole (IC₅₀, 0.6 μg/ml).