α-Glucosidase Inhibitory Activity of Tannat Grape Phenolic Extracts in Relation to Their Ripening Stages

The present study aimed to screen grape extracts as novel alpha-glucosidase inhibitors to prevent type-2 diabetes and hyperglycemia. The total polyphenol content (TPC) was measured by Folin-Ciocalteu assay and the stilbene, anthocyanin and flavan-3-ol compounds were measured by Ultra High-Performance Liquid Chromatography coupled to Mass Spectrometry (UHPLC-MS). The alpha-glucosidase inhibitory of seed and skin Tannat grape extracts at four ripening stages were investigated. The highest TPC values were measured in seeds at the veraison stage (65.29 +/- 5.33 g of Gallic Acid Equivalent (GAE) per kilogram of Fresh Weight (FW)). This was in accordance with the high flavan-3-ol contents measured for these two extracts (43.22 +/- 2.59 and 45.45 +/- 6.48 g/kg of seeds FW, respectively). The skin and seed extracts at the first stage of ripening exerted strong alpha-glucosidase inhibition, exceeding 95% (p< 0.05). A high linear correlation (R = 0.723,p <= 0.05) was observed between flavan-3-ol contents and the alpha-glucosidase inhibitory activity. The stilbene contents and this activity were moderately to strongly anti-correlated (R = -0.828,p <= 0.05 fortrans-resveratrol). The enzyme kinetic studies revealed a mixed type of inhibition. This study brings promising results for the therapeutic potential of seed and skin Tannat grape extracts as a functional food product with anti-diabetic activity.