Abstract
Purpose: To investigate the effects of (-)-epigallocatechin gallate (EGCG) and quercetin on the activity and structure of α-amylase.
 Methods: The inhibitory effects of 7 functional factors were compared by measuring half maximal inhibitory concentration (IC50) values. Lineweaver-Burk plots were used to determine the type of inhibition exerted by EGCG and quercetin against α-amylase. The effect of EGCG and quercetin on the conformation of α-amylase was investigated using fluorescence spectroscopy.
 Results: Quercetin and EGCG inhibited α-amylase with IC50 values of 1.36 and 0.31 mg/mL, respectively, which were much lower than the IC50 values of the other compounds (puerarin, paeonol, konjac glucomannan and polygonatum odoratum polysaccharide). The Lineweaver−Burk plots indicated that EGCG and quercetin inhibited α-amylase competitively, with ki values of 0.23 and 1.28 mg/mL, respectively. Fluorescence spectroscopy revealed that treatment with EGCG and quercetin led to formation of a loosely-structured hydrophobic hydration layer.
 Conclusion: This study has unraveled the mechanism underlying the inhibition of α-amylase activity by EGCG and quercetin in vitro. This should make for better understanding of the mechanisms that underlie the antidiabetic effects of EGCG and quercetin in vivo.
Highlights
Diabetes mellitus is a chronic metabolic disorder characterized by high level of fasting blood glucose
The IC50 value of epigallocatechin gallate (EGCG) (0.31mg/mL) was much lower than that of acarbose (0.45 mg/mL), indicating that EGCG strongly suppressed α-amylase activity, indicating that it could possibly be utilized for controlling postprandial hyperglycemia
The findings suggest that EGCG and quercetin may limit the release of simple sugars from the gut, thereby alleviating postprandial hyperglycemia
Summary
Diabetes mellitus is a chronic metabolic disorder characterized by high level of fasting blood glucose. Studies have shown that tea polyphenols and flavonoids effectively inhibit the activity of αamylase [6,7]. Epidemiological studies have shown that the intake of certain types of flavonoids, including quercetin and myricetin is inversely associated with the risk of type 2 diabetes [10]. Flavonoids are beneficial for reducing the risk of metabolic syndrome. In addition to their antioxidant effects, flavonoids have been reported to prevent diabetes in vivo [11]. Studies on the inhibitory effects of isolated flavonoid compounds against α-glucosidase and α-amylase revealed that quercetin inhibited αamylase with IC50 of 4.8mM [6,7]. The effect of quercetin on α-amylase conformation has not been demonstrated
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