C-glucosidic ellagitannins and galloylated glucoses as potential functional food ingredients with anti-diabetic properties: a study of α-glucosidase and α-amylase inhibition

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia, which can be counteracted by inhibition of alpha-glucosidase and alpha-amylase, both involved in the carbohydrate metabolism. Fourteen C-glucosidic ellagitannins and three galloylated glucoses were studied as potential alpha-glucosidase and alpha-amylase inhibitors. Most of the compounds were found to be moderate inhibitors of alpha-amylase, but potent inhibitors of alpha-glucosidase, showing low-micromolar IC50 values, far lower than that of the antidiabetic drug acarbose. This selectivity can be an advantage for their possible application as functional food ingredients with anti-diabetic properties because strong alpha-amylase inhibition generally causes undesired side effects. The best inhibitors were selected for further studies. Intrinsic fluorescence measurements confirmed their high affinity towards alpha-glucosidase, highlighting a static quenching mechanism. Circular dichroism measurements and kinetics of inhibition indicated that the most active C-glucosidic ellagitannin roburin D (RobD) is a competitive inhibitor, whereas alpha-pentagalloylglucose (alpha-PGG) acts as a mixed-type inhibitor.