Abstract
Certain flavonoids can influence glucose metabolism by inhibiting enzymes involved in carbohydrate digestion and suppressing intestinal glucose absorption. In this study, four structurally-related flavonols (quercetin, kaempferol, quercetagetin and galangin) were evaluated individually for their ability to inhibit human α-glucosidases (sucrase, maltase and isomaltase), and were compared with the antidiabetic drug acarbose and the flavan-3-ol(−)-epigallocatechin-3-gallate (EGCG). Cell-free extracts from human intestinal Caco-2/TC7 cells were used as the enzyme source and products were quantified chromatographically with high accuracy, precision and sensitivity. Acarbose inhibited sucrase, maltase and isomaltase with IC50 values of 1.65, 13.9 and 39.1 µM, respectively. A similar inhibition pattern, but with comparatively higher values, was observed with EGCG. Of the flavonols, quercetagetin was the strongest inhibitor of α-glucosidases, with inhibition constants approaching those of acarbose, followed by galangin and kaempferol, while the weakest were quercetin and EGCG. The varied inhibitory effects of flavonols against human α-glucosidases depend on their structures, the enzyme source and substrates employed. The flavonols were more effective than EGCG, but less so than acarbose, and so may be useful in regulating sugar digestion and postprandial glycaemia without the side effects associated with acarbose treatment.
Highlights
One of the earliest signs of type 2 diabetes (T2D) is elevated and erratic postprandial glycaemia that promotes oxidative stress at various sites within the body [1]
Using an enzyme preparation from these cells, we have evaluated sucrase, maltase and isomaltase inhibition by several flavonols and compared them to acarbose and (−)-epigallocatechin-3-gallate (EGCG), a flavan-3-ol known for its inhibitory activity on sucrase and maltase of various sources [27]
The peak areas from mixed sugar standards prepared in sodium phosphate assay buffer (SPB) were used to plot standard curves (0–10 μg/mL), with excellent linearity for all sugars in this range (Figure 2b)
Summary
One of the earliest signs of type 2 diabetes (T2D) is elevated and erratic postprandial glycaemia that promotes oxidative stress at various sites within the body [1]. One way is by slowing down carbohydrate digestion and glucose absorption in the intestine via the inhibition of salivary/pancreatic α-amylases and membrane-bound brush-border α-glucosidases. There are four relevant types of digestive α-glucosidases in humans, maltase (α1,4-glucosidase; EC 3.2.1.20), glucoamylase (exo-1,4-α-glucosidase; EC 3.2.1.3), sucrase (α-glucohydrolase; EC 3.2.1.48) and isomaltase (oligo-1,6-glucosidase or α-dextrinase; EC 3.2.1.10). Maltase and glucoamylase have a unique, high α-1,4 hydrolytic activity for longer chain maltooligosaccharides to produce glucose [2], and are referred to as maltase/glucoamylase (MGAM) [3]. Sucrase hydrolyses α-1,2-glycosidic bonds in sucrose to produce glucose and fructose. Isomaltase is the only enzyme able to hydrolyze the α-1,6-glycosidic linkage in α-limit dextrins to produce glucose. MGAM and SI complexes are located along the entire small intestine [5,6] and function to catalyze the production of glucose and fructose from disaccharides, dextrins and dietary polysaccharides. Digested and absorbed glucose in the intestine results in a sharp increase in plasma glucose, which is regulated by insulin-stimulated uptake of glucose into tissues
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