Anti-hyperglycaemic effects of Burdock (Arctium lappa L.) leaf flavonoids through inhibiting α-amylase and α-glucosidase

Burdock (Arctium lappa L.) leaves are a polyphenol-rich and underutilised by-product of burdock. To exploit the use of burdock leaves, inhibitory effects of burdock leaf flavonoids (BLF) on alpha-amylase (alpha-A) and alpha-glucosidase (alpha-G) activities and starch digestion were investigated. The findings revealed that BLF exhibited promising alpha-A (IC50 value of 92.01 mu g mL(-1)) and alpha-G (IC50 value of 29.49 mu g mL(-1)) inhibitions in a mixed-type manner. Fourteen compounds in BLF were identified, and apigenin glycosides, morin, kaempferol and quercetin 3-O-rhamnoside were the predominant compounds in BLF. The key active compound existing in BLF was morin which possessed the highest inhibition of alpha-A and alpha-G activities followed by quercetin 3-O-rhamnoside and kaempferol. Kinetic and spectroscopic studies verified that morin could bind to the two carbohydrate digestive enzymes to form the complexes, causing declines in alpha-A and alpha-G activities. The results of starch digestion in vitro showed that the addition of 4% BLF and morin significantly reduced the content of rapidly digestible starch in wheat flour. The above results suggest that BLF can become a new functional ingredient which may help in the design of lower glycaemic-response foods.