Isolation, Purification, and Structural Characterization of Antioxidant Polysaccharides Isolated from the Fruiting Bodies of Cordyceps militaris
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摘要: 本研究以蛹虫草子实体为原料,采用热水浸提法提取蛹虫草子实体粗多糖,然后使用不同体积分数(30%、40%、50%、60%、70%、80%和90%)乙醇分级沉淀得到分级粗多糖Y-1、Y-2、Y-3、Y-4、Y-5、Y-6和Y-7。筛选抗氧化活性最强的分级粗多糖,经DEAE-52纤维素离子交换层析及Sephadex G-150葡聚糖凝胶层析分离纯化后筛选抗氧化活性最强的均一多糖。采用高效凝胶色谱法、气相色谱法及红外光谱法对均一多糖的分子量、单糖组成及官能团等结构表征进行鉴定,并评价其体内抗氧化活性。结果发现,分级多糖Y-4的抗氧化活性最强,其经DEAE-52纤维素离子交换层析及Sephadex G-150葡聚糖凝胶层析分离纯化后得到了抗氧化活性最强的均一多糖STP-4。STP-4是由阿拉伯糖、甘露糖和葡萄糖等聚合而成的α-构型低聚糖,分子量约为1434 Da,具有O-H、C-H、COOH、C=O和C-O-C等官能团结构。另外,STP-4可以显著降低过氧化损伤小鼠血清丙二醛(Malondialdehyde,MDA)含量(P<0.01),提高超氧化物歧化酶(Superoxide dismutase,SOD)(P<0.05)和谷胱甘肽过氧化物酶(Glutathione peroxidase,GSH-Px)活性(P<0.01)。本研究为蛹虫草资源的深度开发和抗氧化活性研究提供了理论基础。Abstract: In the present study, the fruiting bodies of Cordyceps militaris were used as raw materials to extract the crude polysaccharides Y-1, Y-2, Y-3, Y-4, Y-5, Y-6 and Y-7, which fractional precipitation using the ethanol with different volume fractions of 30%, 40%, 50%, 60%, 70%, 80% and 90%. The graded crude polysaccharide with the strongest antioxidant activity was selected to separate and purify the homogeneous polysaccharide with the strongest by DEAE-52 cellulose ion exchange chromatography and Sephadex G-150 gel chromatography. High performance gel chromatography, gas chromatography and infrared spectroscopy were used to identify the molecular weight, monosaccharide composition and functional groups of the homogeneous polysaccharide, and evaluate its antioxidant activity in vivo. The results showed that the graded polysaccharide Y-4 with the strongest antioxidant activity was isolated and purified by DEAE-52 cellulose ion exchange chromatography and Sephadex G-150 gel chromatography, and obtained a homogeneous polysaccharide STP-4 with the strongest antioxidant activity. STP-4 was an α-type configuration oligosaccharide composed of arabinose, mannose, and glucose, with a molecular weight of approximately 1434 Da. It had the functional groups such as O-H, C-H, COOH, C=O, and C-O-C. In addition, STP-4 could significantly reduce the malondialdehyde (MDA) content (P<0.01), increase the superoxide dismutase (SOD) activity (P<0.5), and the glutathione peroxidase (GSH-Px) activity (P<0.01) in the serum of mice with oxidative damage. This study provides a theoretical basis for the in-depth development and antioxidant activity research of Cordyceps militaris resources.
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蛹虫草(Cordyceps militaris)又被叫做北虫草,是虫草属的模式种菌,属于真菌门(Eumycom)、子囊菌亚门(Ascomycotina)、核菌纲(Pyrenomycetes)、球壳菌目(Sphaeriales)、麦角菌科(Clavicipitaceae)、虫草属(Cordyceps)[1]。蛹虫草富含多糖、生物碱、氨基酸、虫草素、虫草酸、甾醇等活性物质,具有保湿、抗氧化、抗炎、抗菌、抗病毒、抗肿瘤、降血糖以及免疫调节等功效[2−5],具有广阔的开发应用潜力。蛹虫草于2009年被批准为新资源食品[6],目前已开发蛹虫草猪肉脯、蛹虫草发酵菌茶、发酵米制品、发酵面制品、发酵杂粮制品、发酵饮料、发酵调味品及虫草膏等药膳产品和保健食品[7−13]。因此,深入探究蛹虫草活性成分,明确其生理活性及作用机制,对促进蛹虫草在药品、食品及保健品等方面的开发应用具有重要的意义。
多糖是蛹虫草的主要活性成分之一,蛹虫草多糖可以通过多种方式发挥降血糖、抗肿瘤调节机体免疫力、抗炎、改善血脂、抑菌及保护肝肾等生物学作用。杨爽等[14]发现蛹虫草多糖可以降低α-葡萄糖苷酶活性,同时能明显升高SOD活性,降低MDA含量,增加糖尿病小鼠的糖耐量。Jing等[15]证明蛹虫草多糖可以有效抑制HT-29人结肠癌细胞和K562人慢性髓系白血病细胞的生长。Zhang等[16]研究发现虫草多糖能显著降低RAW264.7细胞中脂多糖诱导的NO、TNF-α和IL-6的分泌,具有良好的抗炎活性。蛹虫草多糖还可以通过减少脂多糖产生菌和增加短链脂肪酸产生菌重塑肠道菌群改善高脂血症[17]。秦令祥等[18]研究发现蛹虫草多糖对大肠杆菌和金黄色葡萄球菌均具有一定的抑制作用。韩瑜玮等[19]发现蛹虫草多糖对乙醇所致的小鼠肝损伤具有明显的保护作用。然而,目前针对蛹虫草多糖的相关研究主要以粗多糖为主,而对发挥活性的均一多糖组分进行分离鉴定及活性分析的研究较少。
多糖的生物活性与其单糖组成、糖苷键类型和分子量等结构表征密切相关[20]。低分子量多糖因存在还原性基团而具有更好的抗氧化活性[21]。此外,多糖的单糖组成对确定其结构特征和生物学功能具有重要意义。沙棘多糖PBP-3由Glc、Xyl、Gal组成,摩尔比为2.15:1:0.28,比沙棘多糖PBP-1和PBP-2具有更好的抗氧化活性[22]。另外,沙棘多糖HRPs/SBPs含有-COOH、-OH等官能团,可以与自由基相互作用发挥抗氧化活性[23]。因此,阐明多糖结构特征对于进一步探索其生物活性及解析其构效关系具有重要意义。研究发现,蛹虫草多糖具有较好的抗氧化活性[24],然而目前对蛹虫草粗多糖发挥抗氧化作用的均一成分及其结构表征研究较少。本研究以蛹虫草为原料,通过分级醇沉的方法得到不同的分级多糖,创新性的依据粗多糖的抗氧化能力逐一筛选并分离纯化单一多糖。对纯化后的单一多糖的分子量、单糖组成和官能团等结构表征进行解析,利用氧化损伤小鼠模型对多糖的抗氧化作用进行评估。本研究将为蛹虫草的高值化开发利用提供理论依据。
1. 材料与方法
1.1 材料与仪器
蛹虫草 本实验室培养并采集;95%乙醇、葡萄糖、正丁醇、硫酸、苯酚、1,1-二苯基-2-三硝基苯肼(2,2-Diphenyl-1-picrylhydrazyl,DPPH)、氯仿、正丁醇、抗坏血酸、溴化钾、三氟乙酸、硼氢化钠、乙酸、正丙胺、碳酸钠、乙酸酐、吡啶、鼠李糖、葡萄糖、木糖、半乳糖、阿拉伯糖、甘露糖 均为国产分析纯,南京晚晴化玻仪器有限公司;磷酸二氢钠、叠氮化钠、Dextran系列标准品 色谱纯,美国Sigma公司;DEAE-52纤维素 北京瑞达恒辉科技发展有限公司;Sephadex G-150葡聚糖凝胶 上海源叶生物科技有限公司;清洁级昆明雄性小鼠,5周龄,24±2 g 济南朋悦实验动物繁育有限公司,许可证号:SCXK(鲁)20230002。
JYT-2药物天平 上海医用激光仪器厂;FA2004B电子天平 上海越平科学仪器有限公司;101-1ES电热鼓风干燥箱 北京市永光明医疗仪器厂;UV-1900PC紫外可见分光光度计 皓缇仪器(上海)有限公司;SHB-ⅢS循环水式多用真空泵 郑州长城科工贸有限公司;HH-4数显式电热恒温水浴锅 国华电器有限公司;TGL-16G台式高速离心机、DL-5低速大容量离心机 上海安亭科学仪器厂;L550台式低速离心机 湖南湘仪实验室仪器开发有限公司;SENCOR201L旋转蒸发器 上海申生科技有限公司;0-50/50-100酒精计 浙江余姚仪表二厂;MDF-594超低温冰箱 日本SANYO电子有限公司;LGJ-10真空冷冻干燥机 上海比朗仪器有限公司;HL-2蠕动泵 上海嘉鹏科技有限公司;BS-100A自动部分收集器 上海沪西分析仪器厂有限公司;1200高效液相色谱仪,配有TSKgel® G5000PWXL凝胶色谱柱(7.8 mm×300 mm)和G1352A RID检测器、7890A-5975C气相色谱-质谱联用仪,配有DB-5色谱柱(30 m×0.25 mm×0.25 μm) 安捷伦科技有限公司;WGH-30/6双光束红外分光光度计 天津港东科技发展有限公司。
1.2 实验方法
1.2.1 蛹虫草子实体粗多糖提取工艺
参考于士军等[25]的方法,干燥蛹虫草子实体使用高速粉碎机粉碎,过80目筛后收集子实体粉末。粉末按照料液比1:20与去离子水混合,90 ℃水浴3 h,随后对浸提液进行抽滤,收集滤液并使用旋转蒸发仪进行减压浓缩。将浓缩液分为7份,每份浓缩液中加入95%乙醇分别至终浓度为30%、40%、50%、60%、70%、80%和90%,4 ℃静置24 h后4000 r/min离心10 min,收集沉淀。采用Sevage法去除分级多糖的杂蛋白[26]。用去离子水溶解沉淀,按照体积比1:1加入氯仿正丁醇混合液(氯仿:正丁醇=4:1),剧烈振荡20 min后,4000 r/min离心10 min。将上清液置于透析袋中4 ℃透析48 h。透析液浓缩后冷冻干燥得到蛹虫草子实体分级粗多糖Y-1、Y-2、Y-3、Y-4、Y-5、Y-6和Y-7。
1.2.2 DPPH自由基清除率的测定
参考Jang等[27]的实验方法并稍作修改,配制浓度分别为0.2、0.4、0.6、0.8、1.0、2.0和3.0 mg/mL的蛹虫草分级粗多糖溶液。取1.5 mL多糖溶液与1.5 mL 0.2 mmol/L的DPPH-乙醇溶液混合后避光反应30 min。以VC作阳性对照,517 nm下检测样品吸光值,然后按照公式(1)计算DPPH自由基清除率(%)。
DPPH自由基清除率(%)=(1−A1−A2A0)×100 (1) 式中:A0为不含多糖溶液的DPPH-乙醇溶液的吸光值;Al为多糖溶液与DPPH-乙醇溶液的吸光值;A2为多糖溶液与不含DPPH的无水乙醇溶液的吸光值。
1.2.3 Fe2+螯合力测定
参考席丽琴等[28]的实验方法并稍作修改,配制浓度分别为0.2、0.4、0.6、0.8、1.0、2.0和3.0 mg/mL的蛹虫草分级粗多糖溶液。分别取2 mL多糖溶液,加入3.71 mL去离子水混匀,然后再加入2 mmol/L的FeCl2溶液0.1 mL,5 mmol/L菲咯嗪0.2 mL,漩涡分离器混和后室温反应10 min。以VC作阳性对照,562 nm下检测样品吸光值,然后按照公式(2)计算Fe2+螯合率(%)。
Fe2+螯合率(%)=A0−A1A0×100 (2) 其中:A0为未添加多糖的空白组溶液的吸光值;A1为多糖样品溶液的吸光值。
1.2.4 总还原力测定
参考Oyaizu等[29]的实验方法并稍作修改,配制浓度分别为0.2、0.4、0.6、0.8、1.0、2.0和3.0 mg/mL的蛹虫草分级粗多糖溶液。分别取2 mL多糖溶液,加入2 mL 0.2 mol/L的磷酸缓冲液(pH6.6),随后加入2.0 mL 1%(w/v)的铁氰化钾溶液并充分混匀。50 ℃水浴20 min后加入2.0 mL 10%的三氯乙酸溶液。充分混匀后4000 r/min离心10 min。取2.0 mL上清液加入0.4 mL 1%的FeCl3溶液以及2 mL去离子水,充分混匀后在波长700 nm处测定其吸光度。以VC作阳性对照,计算总还原力。
1.2.5 多糖的分离纯化
称取0.1 g抗氧化活性最强的分级粗多糖样品溶于5 mL去离子水中。0.45 μm滤膜过滤后加入到DEAE-52离子交换柱(2.6 cm×30 cm)进行纯化。依次用浓度为0、0.1、0.2、0.3、0.4、0.5、0.6和0.7 mol/L NaCl溶液以1.0 mL/min的流速洗脱样品,每10 mL收集1管,每个梯度收集30管。采用苯酚-硫酸法[30]隔管检测样品吸光值并绘制洗脱曲线。根据洗脱曲线合并相同组分的洗脱液。洗脱液经透析和减压浓缩,滤膜过滤后通过Sephadex G-150葡聚糖凝胶柱(1.5 cm×80 cm)进一步纯化。使用去离子水以0.5 mL/min的流速洗脱样品,每5 mL收集1管,共收集120管。绘制洗脱曲线,根据洗脱曲线合并相同组分的洗脱液。洗脱液经减压浓缩和冷冻干燥后得到纯化后蛹虫草子实体多糖。
1.2.6 蛹虫草子实体多糖结构表征分析
1.2.6.1 分子量测定
多糖分子量使用高效凝胶色谱(High Performance Gel Permeation Chromatography,HPGPC)法进行检测[31]。称取一定量Dextran标准品,使用流动相将其配制成2 mg/mL的溶液后逐一进行高效液相色谱(High Performance Liquid Chromatography,HPLC)检测。绘制标准品保留时间-分子量标准曲线。称取蛹虫草子实体多糖,加入流动相将其配制成浓度1 mg/mL的溶液,Millipore 0.45 μm滤膜过滤后,使用HPGPC方法进行检测,然后根据样品保留时间计算多糖分子量。色谱条件:TSKgel® G5000PWXL凝胶色谱柱(7.8 mm×300 mm),柱温20 ℃,0.002 mol/L磷酸二氢钠(含0.05% NaN3)为流动相,流速0.6 mL/min。
1.2.6.2 单糖组成测定
参考白娣斯等[32]多糖糖醇醋酸盐衍生物的方法测定样品的单糖组成。称取蛹虫草子实体多糖5 mg,加入2 mL 2 mol/L的三氟乙酸,99 ℃反应5 h。旋蒸除酸后加入4%的硼氢化钠溶液0.5 mL,室温放置1.5 h,滴加乙酸至无气泡产生。样品浓缩后真空干燥,加吡啶及正丙胺各1 mL,55 ℃水浴30 min,真空干燥,加吡啶及乙酸酐各0.5 mL,95 ℃水浴1 h,氮气吹干,真空干燥后以氯仿溶解,进行GC-MS分析。GC-MS条件:色谱柱:DB-5(30 m×0.25 mm×0.25 μm);检测器:质谱检测器;进样口温度:250 ℃;检测器温度:280 ℃;氦气流速:0.6 mL/min;分流比:20:1;进样量:5 μL;升温程序:200 ℃保持2 min,以3 ℃/min的速率升至245 ℃,再以10 ℃/min的速率升至270 ℃,保持2 min。
1.2.6.3 紫外及红外吸收光谱分析
使用紫外可见分光光度计分析多糖样品中核酸和蛋白质[33]。配制浓度为1 mg/mL的蛹虫草子实体多糖溶液,使用紫外分光光度计在200~400 nm范围内进行全波长扫描。采用傅里叶变换红外光谱(Fourier Transform Infrared Spectroscopy, FT-IR)分析多糖官能团结构[34]。称取蛹虫草子实体多糖2 mg和溴化钾200 mg,压制成片,空白对照采用溴化钾粉末压片而成。采用WGH-30/6型双光束红外分光光度计于400~4000 cm−1范围内进行扫描。
1.2.7 动物实验
将20只清洁级昆明雄性小鼠在温度(22±2 ℃),相对湿度(50%±20%),光照/黑暗周期各12 h,自由摄食和饮水条件下适应性喂养7 d。动物实验由徐州医科大学实验动物伦理委员会审议通过,审批编号L20210226457。随后小鼠随机分为正常组、阴性对照组、阳性对照组和多糖处理组,每组5只。参考辛炫英等[35]的方法,采用D-半乳糖皮下注射法建造小鼠亚急性过氧化损伤模型。阴性对照组、阳性对照组和多糖处理组每天皮下注射5% D-半乳糖(1000 mg/kg·BW),正常组每天注射0.2 mL生理盐水。另外,阳性对照组每天灌胃VC(200 mg/kg·BW),多糖处理组每天灌胃蛹虫草子实体多糖溶液(200 mg/kg·BW),正常组和阴性对照组每天灌胃等量生理盐水,连续30 d。小鼠眼球取血收集血液,随后血液室温静置3 h,5000 r/min离心20 min分离血清。使用ELISA试剂盒测定小鼠血清中SOD、GSH-Px及MDA含量。
1.3 数据处理
数据采用平均值±标准差表示。使用Graph Pad 8.0.2软件进行绘图及单因素方差分析,P<0.05表示为具有显著性差异。
2. 结果与分析
2.1 蛹虫草子实体分级醇沉多糖的抗氧化能力分析
采用酶法辅助热水、超声辅助低共熔溶剂法及双频逆流聚能式超声波辅助法等提取的蛹虫草多糖均呈现较好的抗氧化活性,然而,这些研究并未对发挥抗氧化作用的均一成分进行深入分析[18,24,36]。本研究得到的乙醇分级粗多糖Y-1、Y-2、Y-3、Y-4、Y-5、Y-6、Y-7的抗氧化能力分析如图1所示。结果发现,当浓度在0.2~3.0 mg/mL范围时,分级多糖的DPPH自由基清除率、Fe2+螯合率及总还原力随着浓度增加而升高。当浓度超过0.6 mg/mL时,Y-4的DPPH自由基清除率明显优于其他分级多糖,当浓度为3.0 mg/mL时Y-4的DPPH自由基清除率达到86.70%±2.03%(图 1a)。当浓度在0.2~0.6 mg/mL范围时,Y-3和Y-5的Fe2+螯合率较高。然而当浓度超过1.0 mg/mL时,Y-4的Fe2+螯合率明显优于其他分级多糖。当浓度为3.0 mg/mL时Y-4的Fe2+螯合率为60.21%±1.55%(图 1b)。当浓度在0.2~1.0 mg/mL范围时,Y-4的总还原力优于其他分级多糖,浓度超过2 mg/mL时,其他分级多糖的总还原力与Y-4逐渐接近,但Y-4的总还原力还是稍优于其他分级多糖。当浓度为3.0 mg/mL时Y-4的吸光值为0.56±0.015(图 1c)。综合以上结果,本研究选择Y-4进行后续的分离纯化。
2.2 Y-4的DEAE-52纤维素离子交换层析及其分离多糖的抗氧化活性检测
何宝林[37]利用DEAE-52层析柱及Sephadex G-100、G-200柱层析对蛹虫草粗多糖进行分离纯化,获得CMP-Ⅰ、CMP-Ⅱ和CMP-Ⅲ三个组分。其中CMP-Ⅲ能显著促进巨噬细胞NO和免疫相关细胞因子IL-6、TNF-α的分泌发挥免疫调节活性。这表明蛹虫草粗多糖分离纯化后的均一多糖具有不同的生物活性。本研究首先采用DEAE-52纤维素对多糖Y-4进行分离纯化,洗脱曲线见图2a。根据洗脱曲线合并多糖溶液,经透析和减压浓缩后得到多糖TP-1、TP-2和TP-3。苯酚-硫酸法检测TP-1、TP-2和TP-3浓度,以TP-1、TP-2和TP-3当中多糖质量浓度最低的TP-2为标准(图2b),将TP-1和TP-3多糖质量浓度稀释成与TP-2相同质量浓度(0.016 mg/mL),检测DPPH自由基清除率、Fe2+螯合率及总还原力。结果发现,虽然Fe2+螯合率在三组之间无显著性差异,然而TP-1(8.20%±0.33% )的DPPH自由基清除能力极显著高于TP-2(5.15%±0.19%,P<0.01)和TP-3(5.39%±0.18%,P<0.01)(图2c~d)。另外,TP-1(0.067±0.010 )的总还原力也显著高于TP-2(0.036±0.013,P<0.05)和TP-3(0.034±0.0065,P<0.01)(图2e)。故使用Sephadex G-150葡聚糖凝胶柱层析对TP-1进一步分离纯化。
图 2 Y-4的DEAE-52纤维素离子交换层析洗脱曲线及分离多糖TP-1、TP-2和TP-3抗氧化活性注:a:Y-4经DEAE-52纤维素离子交换层析后的洗脱曲线;b:TP-1、TP-2和TP-3的浓度;c:DPPH自由基清除能力;d:Fe2+螯合率;e:总还原力;*表示与TP1组相比P<0.05;**表示与TP1组相比P<0.01;***表示与TP1组相比P<0.001。Figure 2. DEAE-52 cellulose ion exchange chromatography elution curve of Y-4 and the antioxidant activity of the isolated polysaccharides TP-1, TP-2 and TP-32.3 TP-1的Sephadex G-150葡聚糖凝胶柱层析及其分离多糖抗氧化活性分析
采用Sephadex G-150葡聚糖凝胶柱层析对多糖TP-1进行进一步分离纯化,洗脱曲线见图3a。根据洗脱曲线合并多糖溶液,经减压浓缩后得到多糖STP-1、STP-2、STP-3和STP-4。苯酚-硫酸法检测STP-1、STP-2、STP-3和STP-4浓度(图3b),将STP-1、STP-3和STP-4多糖的质量浓度稀释成与STP-2同一质量浓度(0.021 mg/mL),检测DPPH自由基清除率、Fe2+螯合率及总还原力。结果发现,STP-4(7.55%±0.22%)的DPPH自由基清除能力极显著优于STP-1(4.31%±0.13%,P<0.001)、STP-2(3.23%±0.18%,P<0.001)和STP-3( 6.20%±0.39%,P<0.01),STP-4(3.28%±0.27%)的Fe2+螯合率显著高于STP-1(2.13%±0.65%,P<0.05),STP-4(0.05±0.0089)总还原力显著高于STP-1(0.026±0.0038,P<0.05)和STP-2( 0.026±0.0047,P<0.05)(图3c~e)。随后对STP-4再次进行Sephadex G-150葡聚糖凝胶层析,结果发现样品的洗脱曲线只有一个峰值,这说明STP-4纯度较高(图3f)。随后将STP-4冷冻干燥后进行结构表征分析。
图 3 TP-1的Sephadex G-150葡聚糖凝胶层析洗脱曲线及分离多糖STP-1、STP-2、STP-3和STP-4抗氧化活性注:a:TP-1经Sephadex G-150葡聚糖凝胶层析后的洗脱曲线;b:STP-1、STP-2、STP-3和STP-4的浓度;c:DPPH自由基清除能力;d:Fe2+螯合率;e:总还原力;f:STP-4经Sephadex G-150葡聚糖凝胶层析后的洗脱曲线;*表示与STP4组相比P<0.05;**表示与STP4组相比P<0.01;***表示与STP4组相比P<0.001。Figure 3. Sephadex G-150 gel chromatography elution curve of TP-1 and antioxidant activity of the isolated polysaccharides STP-1, STP-2, STP-3 and STP-42.4 STP-4分子量及单糖组成分析
使用HPGPC法分析STP-4的分子量。由图4a可知,STP-4洗脱曲线只有一个独立的峰,表明样品的纯化效果良好。根据标准品保留时间-分子量标准曲线y=-0.4453x+10.808,可知STP-4分子量约为1434 Da。研究表明,高分子量多糖由于水溶性差或活性位点较少等原因,导致其抗氧化能力弱于低分子量多糖[38]。从图4b可以看出,STP-4多糖样品有三个峰,与标准糖乙酰化图谱(图4c)作对比,发现STP-4多糖是由阿拉伯糖、甘露糖和葡萄糖组成,其摩尔比为0.72:4.81:0.43。研究表明,由甘露糖和Ara等单糖组成的沙棘叶多糖,对DPPH自由基、ABTS+自由基和羟基自由基都具有很强的清除活性[39]。另外,江琦等[40]采用微波辅助提取蛹虫草粗多糖,分离纯化后得到α-构型的中性多糖CMP,其单糖组成为甘露糖、葡萄糖和半乳糖,摩尔比为1:3.90:1.51。这表明提取分离方式对均一多糖结构有较大的影响。由于单糖种类与多糖的抗氧化活性紧密相关[41],这也直接导致均一多糖显出不同的生物活性。
2.5 STP-4光谱分析
紫外光谱分析表明STP-4在260 nm和280 nm处没有明显的吸收峰,表明STP-4中不存在核酸和蛋白质(图5a)。FT-IR分析显示STP-4多糖样品在3412 cm−1处出现宽并且强的吸收现象,这是由于分子内部或者分子之间的O-H伸缩振动造成[42],而且O-H的伸缩振动吸收峰是多糖的典型特征峰[43]。在2938 cm−1处有一个弱的吸收峰主要是由于C-H的伸缩振动[44]。在2140 cm−1处有一个较弱的吸收峰,这是由于COOH弯曲振动导致的[45]。在1605 cm−1处的吸收峰是由羰基C=O伸缩振动引起[46]。在1407 cm−1处的吸收峰是C-H的变角振动[47],它与C-H的伸缩振动一起组成了糖环的特性吸收。在987~1152 cm−1处出现宽而且最强吸收现象,这是由于甘露糖环的醚(C-O-C)伸缩振动造成的[38]。在942 cm−1处和846 cm−1处的吸收现象说明STP-4多糖存在α-型和β-型糖苷键[48](图5b)。研究表明,存在C-H、COOH、C=O和C-O-C等官能团的白芨多糖BSPs-A显示出较好的DPPH和ABTS+自由基清除活性和还原能力[45]。综合分子量、单糖组成及红外光谱等结构信息表明STP-4具有较强的抗氧化活性潜能。
2.6 STP-4对氧化损伤小鼠氧化水平的影响
与正常组相比,阴性对照组小鼠血清中MDA含量(17.77±3.40 nmol/mL,P<0.001)极显著上升,SOD(263.16±45.57 U/mL,P<0.001)和GSH-Px活力(3023.93±620.28 U/mL,P<0.001)极显著降低(图6)。与阴性对照组小鼠相比,STP-4可以高度显著降低MDA含量(30.31±3.70 nmol/mL,P<0.01),提高SOD( 83.16±20.90 U/mL,P<0.05)和GSH-Px活力(747.77±229.23 U/mL,P<0.01)(图6)。本研究结果表明,STP-4作用于D-半乳糖诱导的氧化损伤小鼠后,可以通过改善小鼠血清中MDA、SOD及GSH-Px水平增强细胞稳定性,表现出对衰老动物的抗氧化作用。雷燕妮等[49]研究发现仅经过DEAE-52初步纯化的蛹虫草多糖能够提高衰老模型小鼠血清和肝脏组织中的SOD、CAT和GSH-Px活力,同时能显著降低MDA含量。这也验证了蛹虫草多糖具有较好的抗氧化活性,这为蛹虫草资源的充分利用和科学应用提供了理论支撑。
3. 结论
本研究采用热水浸提法提取蛹虫草多糖,利用分级乙醇(30%、40%、50%、60%、70%、80%、90%)沉淀得到抗氧化活性最强的分级多糖Y-4。Y-4经DEAE-52纤维素柱分离纯化得到抗氧化活性最优的多糖TP-1,TP-1经Sephadex G-150葡聚糖凝胶柱分离纯化得到抗氧化活性最强的均一多糖STP-4。结构表征分析表明STP-4是由阿拉伯糖、甘露糖和葡萄糖聚合而成的α-型差相异构低聚糖,分子量约为1434 Da,具有O-H、C-H、COOH、C=O和C-O-C等官能团结构。另外,STP-4可以显著降低氧化损伤小鼠血清MDA含量,提高SOD和GSH-Px活性。本研究创新性的根据蛹虫草粗多糖的抗氧化能力逐一筛选并分离纯化得到了具有良好抗氧化活性的均一多糖STP-4,为蛹虫草资源的深度开发和抗氧化活性研究提供了理论支撑。
本研究表明STP-4具有清除自由基,缓解小鼠过氧化损伤的功效,然而,尚存在一些问题需要解决。首先,STP-4的结构和功能之间的关系尚不完全清楚,由于多糖结构的复杂性,STP-4的构效关系仍需进一步阐明,包括糖苷键、分支度、高级构象和化学修饰等[50]。另外,活性物质多糖可以通过不同的机制发挥抗氧化活性。研究发现,Nrf2-ARE信号通路是微藻多糖调节细胞氧化应激的重要途径[51]。白芍多糖作为一种膳食补充剂,可以通过Nrf2/Keap1信号通路调节细胞代谢,预防肠道氧化应激[52]。因此,与STP-4抗氧化作用相关的靶点和信号通路仍有待阐明。
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图 2 Y-4的DEAE-52纤维素离子交换层析洗脱曲线及分离多糖TP-1、TP-2和TP-3抗氧化活性
注:a:Y-4经DEAE-52纤维素离子交换层析后的洗脱曲线;b:TP-1、TP-2和TP-3的浓度;c:DPPH自由基清除能力;d:Fe2+螯合率;e:总还原力;*表示与TP1组相比P<0.05;**表示与TP1组相比P<0.01;***表示与TP1组相比P<0.001。
Figure 2. DEAE-52 cellulose ion exchange chromatography elution curve of Y-4 and the antioxidant activity of the isolated polysaccharides TP-1, TP-2 and TP-3
图 3 TP-1的Sephadex G-150葡聚糖凝胶层析洗脱曲线及分离多糖STP-1、STP-2、STP-3和STP-4抗氧化活性
注:a:TP-1经Sephadex G-150葡聚糖凝胶层析后的洗脱曲线;b:STP-1、STP-2、STP-3和STP-4的浓度;c:DPPH自由基清除能力;d:Fe2+螯合率;e:总还原力;f:STP-4经Sephadex G-150葡聚糖凝胶层析后的洗脱曲线;*表示与STP4组相比P<0.05;**表示与STP4组相比P<0.01;***表示与STP4组相比P<0.001。
Figure 3. Sephadex G-150 gel chromatography elution curve of TP-1 and antioxidant activity of the isolated polysaccharides STP-1, STP-2, STP-3 and STP-4
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