Pomegranate ellagitannins inhibit in vitro carbohydrate digestion

Pomegranate is an ancient fruit studied for its anti-diabetic properties. The aim of the present work was to identify the α-glucosidase inhibitors in a pomegranate juice polyphenol-rich extract (PPRE). The PPRE, obtained using C18 solid-phase extraction, contained 5.87 ± 0.26 mmol of ellagic acid equivalent (EAE)/L and showed an inhibitor activity on rat intestinal α-glucosidase with an IC50 value of 922.2 ± 7.1 μmol of EAE/L. The PPRE was fractioned in three different fractions using C18 columns and glucosidase inhibitory activity was recovered in an ellagitannins-enriched fraction. After HPLC separations punicalagin, punicalin and ellagic acid were identified as α-glucosidase inhibitors with IC50 values of 140.2 ± 1.1, 191.4 ± 1.3 and 380.9 ± 3.5 μmol/L, respectively. Kinetic analysis showed that the inhibition is of mixed type with a Ki of 903.0 ± 74.7 μmol/L. Pomegranate ellagitannins interact directly with the α-glucosidase and the non-specific binding of ellagitannins may alters the protein structure reducing the velocity of the catalysis and altering the accessibility to the active site of the substrate. The inhibitory effect on carbohydrate hydrolysis of PPRE was tested against a real food system (potatoes) using an in vitro digestion protocol. The presence of PPRE produced a decrease in the amount of released glucose. During digestion punicalin and punicalagin concentration decreased as a consequence of their binding to salivary or potatoes proteins and of their hydrolysis. Despite this loss, the PPRE retained high inhibitory activity. In conclusion, our data are consistent with the hypothesis that pomegranate juice can be considered a very useful food supplement to ameliorate postprandial hyperglycemia linked to type 2 diabetes and hyperglycemia-induced vascular complications.