Physicochemical Properties,Antioxidant And Hypoglycemic Activities Of The Polysaccharides From Oolong Tea

In last two decades, tea polysaccharides (TPS) attacted a lot of attention for its healthy functions, especially antioxidant and antidiabetic properties. Anxi Tieguanyin, Fenghuangdancong and Wuyi Dahongpao were three typical oolong teas in China, with the degree of fermentation from lightest to highest accordingly. This study was carried out to isolate the TPS from these three teas, named TTPS, FTPS and DTPS respectively, analysis their physicochamical properties, antioxidant and hypoglycemic effects in vitro. Then, according to their bioactivities, DTPS was chosen for further in vivo research and tried to discover the mechanisms of hypoglycemic effects. Meanwhile, physicochemical characteristics, in vitro and in vivo bioactivities of the purifications of DTPS were studied to discover the primary structure-activity relationship of TPS. The results were shown in following:1. Three polysaccharides named TTPS, FTPS and DTPS, were isolated from Anxi Tieguanyin, Fenghuangdancong and Wuyi Dahongpao, respectively. Analysis of the chemical compositions (uronic acid, neutral sugar and protein) revealed that they are all belonged to acid heteropolysaccharides bound with protein. The total contents of uronic acid, neutral sugar and protein were all above 70%, of which uronic acid content was the most, and protein content was the lest. But the contents of protein were significantly different in these three TPS, which in proportionally increased with degree of fermentation, highest in DTPS (9.30%), lowest in TTPS (5.57%). Analysis of neutral sugar compositions showed that they all contained L-rhamnose, D-fucose. L-arabinose, D-xylose, D-mannose, D-glucose and D-galactose these seven monosaccharides with different molar ratio. Gel chromatography was applied to test the molecular weight of these OTPS (oolong tea polysaccharides), and the results indicated that these OTPS had quite large molecuar weight ranging from 104 to 106 Da.Furthermore, measurements of antioxidant activities by assays of DPPH, ABTS and ferric-reducing antioxidant power (FRAP) in vitro showed that DTPS was the most potent, FTPS the next and TTPS the least respectively. Similarly, DTPS possessed the strongest inhibitory potential against a-glucosidase, followed by FTPS and then TTPS. The potencies of the bioactivities of these three OTPS were apparently proportional to their contents of protein and unonic acid, suggesting that the higher degree of fermentation of oolong teas produced higher contents of protein-bound acid heteropolysaccharides accompanied with higher antioxidant and a-glucosidase inhibitory activities.2. Due to the results above, the most active DTPS was chosen for further study. Effect of DTPS on postprandial blood glucose showed that DTPS could slow down the increasing speed and lowering the level of blood glucose in alloxan-induced diabetic mice, which resulted in shortening the time of high blood glucose. After administration of DTPS for 4 weeks, DTPS could relieve the typical symptoms of diabetes, improving the quality of alloxan-induced diabetic mice. Also, it could reduce the injury to pancreas by increasing the number and area of islet. What’s more, the fasting blood glucose in diabetic mice was reduced by feeding DTPS, which showen the best effetct in 2nd week. In vivo antioxidant expriments releaved that DTPS (40 mg/kg BW) could significantly decrease the content of MDA in diabetic mice’s in serum and liver, accompanied by significantly improving the activity of SOD and GSH-Px.Making comprehensive analysis of antioxidant effects in vivo and in vitro, it could be concluded that DTPS was a kind of effective antioxidant. It worked by directily scavenging the free radicals or improving the activities of antioxidant enzymes, to prevent body from lipid peroxidation. Furthermore, the results of in vivo and in vitro hypoglycemic effects suggested DTPS showed positive preventive and therapeutic effect to diabetics, and the possible mechanisms were presented as following:(1) it could ruduce the absorption of glucose in intestine by inhibitory activity to a-glucosidase; (2) it could increase the number and area of islet to produce more insulin in body, which might result from the less injury to pancreas; (3) its antioxidant effect could prevent the β cells from the injury of free radicals.3. DTPS was purified throngh ion-exchange chromatography of DEAE-52 cellulose, and got 4 parts, named DTPS-0, DTPS-1, DTPS-2 and DTPS-3, of which the yield of DTPS-2 was the most (31.9%). After elution, the protein content of these 4 parts was sharply declined to 0.21%-0.83%, while uronic acid, neutral sugar became the main compounds in these purifications. HPLC analysis of neutral sugar compositions showed that they all contained L-rhamnose, D-mannose, D-glucose, D-galactose, L-arabinose, D-xylose and galacturonic acid these seven monosaccharides, but with different molar ratio.IR, SEM and AFM were applied to measure the stuctures of these 4 parts. The results indicated that they all had the characteristic absorptions of polysaccharide and protein, and DTPS-0 contained the furanose while DTPS-1, DTPS-2 and DTPS-3 had the pyranose. A plurality of sugar chains synthesized to form polysaccharide molecule of 50-100 nm in diameter, and polysaccharide molecules twisted each other into polysaccharide particle of 500 nm in diameter, then these particles grouped to form different kinds of 3-D structures, like multi-level flake, random coil shape and beaded shape and so on.The more content of uronic acid and protein, the more complex the structure. In vitro analysis of the 4 parts’ antioxidant activities under DPPH, ABTS and FRAP assays exhibited that they presented quite weak antioxidant effects due to the sharp decrease of protein content. While measuring the a-glucosidase inhibitory effect, DTPS-2 and DTPS-3 showed some inhibitory effects but much weaker than DTPS. Furthermore, the antioxidant and hypoglycemic effects of DTPS-2 in alloxan-induced diabetic mice were also not evident, mitht due to the less content of protein. So the protein in polysaccharides played a key role in its bioactivity.