Different Polyphenolic Components of Soft Fruits Inhibit α-Amylase and α-Glucosidase

Polyphenol-rich extracts from soft fruits were tested for their ability to inhibit α-amylase and α-glucosidase. All extracts tested caused some inhibition of α-amylase, but there was a 10-fold difference between the least and most effective extracts. Strawberry and raspberry extracts were more effective α-amylase inhibitors than blueberry, blackcurrant, or red cabbage. Conversely, α-glucosidase was more readily inhibited by blueberry and blackcurrant extracts. The extent of inhibition of α-glucosidase was related to their anthocyanin content. For example, blueberry and blackcurrant extracts, which have the highest anthocyanin content, were the most effective inhibitors of α-glucosidase. The extracts most effective in inhibiting α-amylase (strawberry and raspberry) contain appreciable amounts of soluble tannins. Other tannin-rich extracts (red grape, red wine, and green tea) were also effective inhibitors of α-amylase. Indeed, removing tannins from strawberry extracts with gelatin also removed inhibition. Fractionation of raspberry extracts on Sephadex LH-20 produced an unbound fraction enriched in anthocyanins and a bound fraction enriched in tannin-like polyphenols. The unbound anthocyanin-enriched fraction was more effective against α-glucosidase than the original extract, whereas the α-amylase inhibitors were concentrated in the bound fraction. The LH-20 bound sample was separated by preparative HPLC, and fractions were assayed for inhibition of α-amylase. The inhibitory components were identified as ellagitannins using LC-MS-MS. This study suggests that different polyphenolic components of fruits may influence different steps in starch digestion in a synergistic manner.

Keywords: α-Amylase; α-glucosidase; anthocyanins; ellagitannins; inhibition; starch digestion