Black Tea Inhibits Small Intestinal α-Glucosidase Activity in db/db Mouse

High-throughput screening and investigation of the inhibitory mechanism of α-glucosidase inhibitors in teas using an affinity selection-mass spectrometry method

An affinity selection-mass spectrometry method was applied for high-throughput screening of α-glucosidase (AGH) inhibitors from teas. Fourteen out of nineteen screened AGH inhibitor candidates were clustered as galloylated polyphenols (GPs). "AGH-GPs" interaction studies, including enzyme kinetics, fluorescence spectroscopy, circular dichroism, and molecular docking, jointly suggested that GPs noncompetitively inhibit AGH activity by interacting with amino acid residues near the active site of AGH and inducing changes in AGH secondary structure. Representative GPs and white tea extract (WTE) showed comparable AGH inhibition effects in Caco2 cells and postprandial hypoglycemic efficacy in diabetic mice as acarbose. The area under the curve of oral sucrose tolerance test was lower by 8.16%, 6.17%, and 7.37% than control group in 15 mg/kg EGCG, 15 mg/kg strictinin, and 150 mg/kg WTE group, respectively. Our study presents a high-efficiency approach to discover novel AGH inhibitors and elucidates a potential mechanism by which tea decreases diabetes risks.

Novel Potent Hypoglycemic Compounds from Euonymus laxiflorus Champ. and Their Effect on Reducing Plasma Glucose in an ICR Mouse Model

α-Glucosidase inhibitors (aGIs) have been used as an effective therapy for type-2 diabetes, which remains a global health issue. The aim of this study was to achieve bioactivity-guided isolation, identification and evaluation of hypoglycemic compounds from Euonymus laxiflorus Champ. trunk bark (ELCTB). Eleven active compounds were isolated and identified as walterolactone A/B β-d-pyranoglucoside (1), 1-β-d-glucopyranosyloxy-3,5-dimethoxy-4-hydroxybenzene (9), (−)-gallocatechin (10), schweinfurthinol 9-O-β-d-pyranoglucoside (11), 1-O-(3-methyl)-butenoyl-myo-inositol (12), leonuriside (14), (+)-catechin (19), methyl galloate (20), (−)-catechin (23), and condensed tannins (5 and 18). Of these 11, novel 4 compounds (1, 11, 12, and 14) were found as new α-glucosidase inhibitors. Notably, in vitro results indicated that compounds 1, 5, 10–12, 18, and 19 showed potent activity (IC₅₀ = 0.076−31 µg/mL), and their activities were at a higher level than that of acarbose, a commercial inhibitor (IC₅₀ = 1345 µg/mL). In animal tests, the major inhibitor, condensed tannin (18), demonstrated significant reduction of plasma glucose in mice with no symptoms of diarrhea at the dose of 100 mg/kg bw. The results suggest that Euonymus laxiflorus Champ. is a rich source of bioactive compounds for development as health food or drugs with potent hypoglycemic effect. The results of this study also enriched the current novel biological activities of constituents from Euonymus laxiflorus species.

Inhibitory effect of black tea and its combination with acarbose on small intestinal α-glucosidase activity

ETHNOPHARMACOLOGICAL RELEVANCE

It is said that black tea is effective against type 2 diabetes mellitus because it can help modulate postprandial hyperglycemia. However, the mechanism underlying its therapeutic and preventive effects on type 2 diabetes mellitus is unclear. In this study, we focused on the effect of black tea on the carbohydrate digestion and absorption process in the gastrointestinal tract. We examined whether black tea can modulate postprandial hyperglycemia.

MATERIALS AND METHODS

The freeze-dried powder of the aqueous extract of black tea leaves (JAT) was used for in vitro studies of α-amylase activity, α-glucosidase activity, and glucose uptake by glucose transporters in Caco-2 cells; ex vivo studies of small intestinal α-glucosidase activity; and in vivo studies of oral sugar tolerance in GK rats, an animal model of nonobese type 2 diabetes mellitus.

RESULTS

Half maximal inhibitory concentration values indicated that JAT significantly reduced α-glucosidase activity, but weakly reduced α-amylase activity. Kinetic studies of rat small intestinal α-glucosidase activity revealed that the combination of JAT and the α-glucosidase inhibitor, acarbose, showed a mixed-type inhibition. JAT had no effect on the uptake of 2'-deoxy-d-glucose by glucose transporter 2 (GLUT2) and the uptake of α-methyl-d-glucose by sodium-dependent glucose transporter 1 (SGLT1). In the oral sucrose tolerance test in GK rats, JAT reduced plasma glucose levels in a dose-dependent manner compared with the control group. The hypoglycemic action of JAT was also confirmed: JAT, in combination with acarbose, produced a synergistic inhibitory effect on plasma glucose levels in vivo. In contrast to the oral sucrose tolerance test, JAT showed no effect in the oral glucose tolerance test.

CONCLUSIONS

JAT was demonstrated to inhibit the degradation of disaccharides into monosaccharides by α-glucosidase in the small intestine. Thereby indirectly preventing the absorption of the dietary source of glucose mediated by SGLT1 and GLUT2 transporters localized at the apical side of enterocytes in the small intestine. The results indicate that black tea could be useful as a functional food in the dietary therapy for borderline type 2 diabetes mellitus that could modulate postprandial hyperglycemia.