Research Progress on the Preparation and Anti-inflammatory Mechanism of Oligosaccharides
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摘要: 炎症是机体对各种致炎因素及损伤的一种防御反应,也是许多慢性疾病发生的关键因素。研究表明低聚糖具有抗炎、抗菌、抗氧化和抗癌等多种生理活性。作为新型功能性糖源,低聚糖具有低热量、高溶解度、促进双歧杆菌增殖等特点。本文对近年来具有抗炎活性低聚糖的种类和制备方法进行介绍,并从炎症介质和炎症信号通路两方面对其抗炎机制进行综述,以期为新型抗炎药物开发提供参考。Abstract: Inflammation is a defensive response of the body to various inflammatory factors and injuries, and it is also a key factor in the occurrence of many chronic diseases. The research shows that oligosaccharides have many physiological activities, such as anti-inflammation, anti-biosis, anti-oxidation and anti-cancer. As a new type of functional sugar source, oligosaccharides have the characteristics of low calories, high solubility and promotion of bifidobacterial proliferation. In this article, the types and preparation methods of oligosaccharides with anti-inflammatory activity in recent years are introduced, and the anti-inflammatory mechanisms are reviewed from two aspects of inflammatory mediators and inflammatory signaling pathways, in order to provide the reference for the development of new anti-inflammatory drugs.
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Keywords:
- oligosaccharides /
- preparation methods /
- anti-inflammatory mechanism
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炎症(Inflammation)是具有血管系统的活体组织对致炎因子所发生的防御反应。一般情况下,炎症是有益的,可以清除致炎因子造成的坏死细胞和组织,并修复损伤部位。但当致炎因子持续存在或增加时就会激活炎症信号通路,促进大量促炎因子的产生,炎症会转变成慢性疾病或者向全身扩散,导致病情加重或难以治愈[1-3]。体表的外伤感染和各器官的常见病(如肠炎、肝炎、肾炎等)均属于炎症性疾病。研究发现,许多癌症也是由炎症反应和慢性刺激造成的[4]。
低聚糖是通过糖苷键将2~10个单糖分子连接而成的化合物总称,又称寡糖或寡聚糖,包括普通低聚糖和功能性低聚糖两种。普通低聚糖易被机体消化吸收,如蔗糖、麦芽糖、环糊精等。功能性低聚糖不易被机体消化吸收,但能促进肠道有益菌增殖和抑制有害菌,并减少有毒发酵产物的形成,如低聚木糖、褐藻胶低聚糖、低聚壳聚糖等。研究证实低聚糖能通过抑制炎症细胞的促炎因子表达和抑制炎症通路的激活[5-9],在急性结肠炎[10-14]、脂肪肝炎[15]、糖尿病肾炎[16]、神经炎[17]等多种炎症性疾病的治疗中发挥作用。低聚糖的原材料获取简单,在食品保鲜、保健品、医药和畜牧业等领域有较好的应用前景。本文综述了近年来抗炎低聚糖的制备方法及抗炎机制的研究进展,旨在为新型抗炎药物开发提供参考。
1. 抗炎低聚糖的种类及其制备方法
低聚糖多来源于自然界,如植物胶、海洋藻类多糖、动物乳汁、甲壳类动物等,一般通过化学法、物理法或者酶法获得。相较于化学法和物理法,酶法具有反应条件温和、绿色环保、产率高和产品均一性好等优点。因此,酶法制备低聚糖已成为近年来低聚糖工业化生产的研究重点。
1.1 来源于植物的低聚糖
1.1.1 低聚半乳糖醛酸
低聚半乳糖醛酸(Oligogalacturonic acid,OGA)是由2~10个半乳糖醛酸通过α-1,4糖苷键线性连接而成的聚合物,一般通过物理法、化学法和酶法降解果胶的主要成分——多聚半乳糖醛酸得到[18]。鉴于物理法和化学法随机性较大,难以控制产物聚合度和结构类型,目前酶法应用比较广泛。研究者们分别从曲霉菌和嗜热腐质霉菌中克隆得到内切多聚半乳糖醛酸酶基因PgaB[19]、果胶裂解酶基因plhy1[20],进而在毕赤酵母中实现高效表达,并对表达条件进行了优化,最终获得不同聚合度的OGA。筛选具有优异的稳定性和高效性的酶,利用低成本的农业食品工业废物(如果胶)进行OGA的工业化生产,在农业食品工业中具有应用前景。
1.1.2 低聚木糖
低聚木糖(Xylo-oligosaccharide,XOS)主要由木聚糖酶降解植物细胞壁中的木聚糖所得,是由2~10个D-木糖分子通过β-1,4糖苷键结合而成的功能性低聚糖,主要有木二糖、木三糖和木四糖[21]。XOS主要通过物理法、化学法和酶法将玉米芯、甘蔗渣等农业副产物中的木聚糖降解获得。酶法因反应条件温和、易控制、转化率较高和环境友好的优点,是目前低聚木糖工业化生产的主要方法[22]。Zhang等[23]采用0.05 mol/L氯化镁和氯化亚铁在140 ℃下将甘蔗渣催化30 min,制备XOS,产率可达54.68%,产物以木二糖和木三糖为主。Nieto-Dominguez等[24]采用GH11家族的内切木聚糖酶XynM降解桦木木聚糖,产物为聚合度2~4的XOS,产率为28.8%。Liu等[25]利用GH10家族的木聚糖酶PbXyn10A可将木糖降解为聚合度2~3的XOS,产率高达75%。近年来,由于XOS广泛应用于食品、医药、农业和畜牧业等领域,木聚糖酶受到了广泛关注,其中GH10和GH11木聚糖酶具有相对较宽的底物特异性,是目前制备XOS的最佳用酶。
1.1.3 低聚果糖
低聚果糖(Fructo-oligosaccharide,FOS)是蔗糖分子以β-1,2糖苷键与1~3个D-果糖基结合而成的蔗果三糖、蔗果四糖、蔗果五糖及混合物[26]。商品FOS主要通过蔗糖合成或菊糖降解的方法进行制备,后者通过从自然界来源的菊苣、洋蓟、大丽花或龙舌兰的根中水解获得,工艺简单、产品纯度高、价格低廉[27]。Oliveira等[28]发现β-果糖糖苷酶能催化蔗糖发生转移反应,具有很好的稳定性和高效催化性,可长期用于工业FOS和转化糖的生产。Vega等[29]研究发现纤维素酶Rohapect CM在适宜条件下可将蔗糖合成为FOS,产率为63.8%。Rohapect CM是商用食品级酶制剂,能制备较高产率的FOS,可为FOS和果糖的工业化生产提供参考。
1.1.4 低聚甘露糖
低聚甘露糖(Manno-oligosaccharide,MOS)由2~7个甘露糖通过β-1,4糖苷键结合生成的低聚糖,其化学结构因原料来源的不同而异[21]。来源于棕榈粕、椰肉的是低聚直链甘露糖,来源于魔芋、硬质木材的是低聚葡甘露糖,来源于瓜尔豆胶、槐豆胶的是低聚半乳甘露糖,来源于种子胚乳的是低聚半乳葡甘露糖(图1)。目前主要通过物理法、化学法和酶法降解上述原料中的甘露聚糖制备MOS。Liu等[30]采用β-甘露聚糖酶ManAK将刺槐豆胶、魔芋胶和瓜尔胶降解为小分子MOS(<2000 Da),其中将刺槐豆胶和魔芋胶水解成聚合度2~6的MOS产率分别为76.7%、83.3%。Yang等[31]发现β-甘露聚糖酶TcMAN能有效地将魔芋粉中的甘露聚糖水解成聚合度3~7的MOS,产率为97.5%。β-甘露聚糖酶具有良好的酶学特性,可批量生产,为MOS的制备提供了科学依据。
1.1.5 低聚异麦芽糖
低聚异麦芽糖(Isomaltooligosaccharide,IMO),被称为“双歧因子”,是由2~10个葡萄糖通过α-1,6糖苷键结合而成的低聚糖,主要有异麦芽糖、潘糖、异麦芽三糖和异麦芽四糖等。目前IMO主要是以淀粉为原料通过酶法转化生产,常用的酶有α-淀粉酶、β-淀粉酶和α-葡萄糖苷酶等[32]。姚明静等[33]将甘薯与α-淀粉酶共同高温液化,在β-淀粉酶和α-葡萄糖转苷酶的作用下,依次进行糖化转苷反应和面包酵母发酵,最终产物中IMO含量为72.9%,比发酵前提高1倍以上。Chen等[34]以戊二醛为交联剂,壳聚糖为载体,建立了以淀粉为底物,α-葡萄糖苷酶、β-淀粉酶、普鲁兰酶和真菌α-淀粉酶同时糖化转苷制备IMO,产物中IMO含量为41.74%。Huang等[35]研究发现在双酶法(右旋糖酐蔗糖酶和葡聚糖蔗糖酶)合成IMO的体系中,加入麦芽糖受体,可以在一定程度上实现高质量IMO的定向制备,产率为55.56%。酶法制备IMO所涉及的酶均为常用的商用食品级酶,成本低且实用性高。工艺流程一般是先用酶法制备低纯度的IMO,再采用酵母发酵、受体结合或透析等方法获得高纯度IMO。
1.1.6 海藻酸低聚糖
海藻酸低聚糖(Alginate oligosaccharide,AOS),又称褐藻胶低聚糖,是由β-D-甘露糖醛酸(M)和α-L-古罗糖醛酸(G)通过α-1,4糖苷键连接而成的低聚糖,存在三种聚合方式:聚甘露糖醛酸(poly M)、聚古罗糖醛酸(poly G)和杂合褐藻寡糖(poly MG)[36]。AOS是海藻酸盐通过酶解、酸水解和氧化降解等方法解聚得到。海藻酸裂解酶作为一种重要的工具酶被广泛应用于AOS的生产[36]。有研究者分别将海藻酸裂解酶基因Alyw201[37]和AlgA[38]在酵母或者大肠杆菌中实现高效表达,并对表达条件进行优化,最终获得不同聚合度的AOS。酶解法已成为降解海藻酸盐最常用的方法,稳定且高效的海藻酸裂解酶是工业化生产海藻酸低聚糖的关键。
1.1.7 琼胶低聚糖
琼脂糖是由D-半乳糖和3, 6-内醚-L-半乳糖通过β-1,4与α-1,3糖苷键交替连接而成的直链线性高分子聚合物。琼胶低聚糖(Agaro oligosaccharide,AGO)是琼脂糖水解后生成的聚合度2~10的低聚糖,包括琼脂寡糖和新琼脂寡糖两种类型。AGO的制备方法有化学水解法和酶法,目前多采用酶法生产。α-琼胶酶能作用于α-1, 3糖苷键,生成琼二糖、四糖等寡糖,β-琼胶酶可作用于β-1, 4糖苷键生成新琼二糖、新琼四糖等寡糖。目前只有5种α-琼胶酶被生化鉴定[39]。研究者们分别克隆了α-琼胶酶基因AgaWS5 [40]和β-琼胶酶基因Aga3027 [41],构建表达载体并在大肠杆菌中进行表达,获得重组蛋白。重组Aga WS5具有耐寒性,低温(10 ℃)条件下仍可维持40%以上活性,可将琼脂糖降解为琼二糖、琼四糖和琼六糖。重组Aga3027具有良好的热稳定性和pH稳定性,可将琼脂糖水解为新琼脂四糖和新琼脂六糖。目前琼胶酶在营养和食品工业中有较大的应用潜力。
1.2 来源于动物的低聚糖
1.2.1 低聚半乳糖
低聚半乳糖(Galactooligosaccharide,GOS)是来源于动物乳汁中的非人工合成的低聚糖,分子结构一般是在半乳糖或葡萄糖分子上连接1~7个半乳糖基[42]。目前GOS的制备主要通过β-半乳糖苷酶的转糖苷功能催化乳糖合成[43]。自然界中许多微生物都可以产生β-半乳糖苷酶,如黑曲霉[44]、米曲霉[45]、芽孢杆菌[46]、乳酸酵母[47]等。Gao等[48]以米曲霉Aspergillus oryzae RIB40为出发菌株,通过原生质体诱变获得了三株β-半乳糖苷酶产量较高的米曲霉突变菌株:N140C、W806F和N140C/W806F。通过对突变体及野生型β-半乳糖苷酶生产GOS的条件进行优化,发现三株突变体生产GOS的产率分别为50.7%(N140C)、49.3%(W806F)和59.8%(N140C/W806F),均比野生型(35.7%)有较大提高。其中双突变体N140C/W806F具有更优异的性能,适合作为工业催化剂。通过诱变获取β-半乳糖苷酶的突变型菌株,进而筛选高GOS产率的菌株,为COS的制备方法提供新策略。
1.2.2 低聚壳聚糖
低聚壳聚糖(Chitosan oligosaccharide,COS)又称壳寡糖,是2-氨基-2-脱氧-D-吡喃葡萄糖通过β-1,4糖苷键连接而成的功能性低聚糖,也是壳聚糖的水解产物[49]。COS可以通过酶法、物理方法和化学方法制备。酶法是通过专一性酶和非专一性酶降解高聚壳聚糖制备COS的方法,专一性酶主要指壳聚糖酶,非专一性酶包括纤维素酶、溶菌酶、脂肪酶、蛋白酶、淀粉酶等。目前COS主要是通过非专一性酶法获得,但降解率和产率较低;而壳聚糖酶能克服上述缺点,是制备COS的理想有效的方法[50]。
研究者分别将壳聚糖酶基因Csn[51]和BaCsn46B[52]在毕赤酵母中实现高效表达,重组Csn可将不同脱乙酰度的壳聚糖水解为壳二糖~壳五糖;重组BaCsn46B水解壳聚糖的产物主要为壳二糖和壳三糖。为了获得丰富的壳聚糖酶,且该酶需具有较高的溶解性,才可用于大规模生产COS,目前筛选了许多具有壳聚糖酶活性的微生物,为大规模的壳聚糖酶制备COS奠定基础。
2. 低聚糖的抗炎作用及机制
2.1 对炎症介质的影响
2.1.1 对细胞因子的影响
细胞因子是一类具有生物活性的小分子蛋白或多肽的总称,可分为促炎因子(TNF-α、IL-1β、IL-6和IL-8等)和抑炎因子(IL-4、IL-10、IL-37、IL-38等)。研究表明,中波紫外线(Ultraviolet B,UVB)对体外培养的人永生化表皮角质形成细胞(HaCaT)辐射损伤后,TNF-α、IL-6大量分泌,引发细胞内的炎症反应并加速细胞损伤[5],山楂果胶低聚半乳糖醛酸提取物能通过抑制TNF-α、IL-6分泌,从而减轻UVB辐射对HaCaT细胞的损伤[53]。壳寡糖可通过抑制结肠组织中TNF-α、IL-6的分泌,从而减轻葡聚糖硫酸钠(Dextran sulfate sodium,DSS)诱导的小鼠急性结肠炎[10]。低聚甘露糖能有效缓解DSS所致急性结肠炎小鼠的临床症状,与其下调促炎介质(IL-1α、IL-1β、IL-6、KC、G-CSF和MCP-1)有关[11]。在炎症反应中,低聚糖可以通过抑制促炎因子的释放发挥抗炎作用。
2.1.2 对热休克蛋白的影响
血红素氧合酶1(Hemoxigenase-1,HO-1)是一种能将血红素分解生成一氧化碳、铁和胆红素的限速酶,间接抑制促炎细胞因子、一氧化氮(Nitric oxide,NO)的产生和防止炎症反应的恶化[54]。Enoki等[54]研究发现,在脂多糖(Lipopolysaccharide,LPS)诱导的人单核细胞体外炎症模型中,琼脂寡糖能上调HO-1蛋白表达;在LPS诱导的活化巨噬细胞体外炎症模型中,琼脂寡糖通过上调HO-1蛋白表达而抑制亚硝酸盐和前列腺素E2(Prostaglandin E2,PGE2)的产生。在LPS诱导的RAW 264.7巨噬细胞炎症模型中,壳寡糖可上调HO-1蛋白表达;HO-1抑制剂锌原卟啉(ZnPP)可部分逆转壳寡糖对LPS诱导RAW 264.7巨噬细胞炎症反应的抑制作用[8]。琼脂寡糖和壳寡糖能上调HO-1蛋白表达,间接抑制促炎介质的产生,防止炎症的恶化。
2.1.3 对其他炎性介质的影响
环氧化酶(Cyclooxygenase,COX)、诱导型一氧化氮合酶(Inducible nitric oxide synthase,iNOS)、前列腺素(Prostaglandins,PGs)和丙二醛(Malondialdehyde,MDA)都与氧化应激的机制有关,在炎症反应过程中也起到重要作用。COX是将花生四烯酸代谢为前列腺素家族的炎症介质,有COX-1和COX-2两种亚型。海藻酸低聚糖可通过降低NO和PGE2的生成,下调iNOS和COX-2 mRNA及蛋白表达,从而抑制LPS和β-淀粉样蛋白(β-amyloid,Aβ)诱导的BV2小胶质细胞神经炎症[17]。魔芋寡糖可以改善2,4,6-三硝基苯磺酸(TNBS)诱导的溃疡性结肠炎大鼠的体重,降低结肠组织中MDA、iNOS和COX-2水平[13]。Wang等[7]研究发现在LPS诱导的RAW 264.7巨噬细胞炎症模型中,新琼脂低聚糖能抑制iNOS的mRNA表达。低聚糖能降低COX-2、iNOS、PGE2和MDA等促炎介质水平,减轻炎症。
2.2 对炎症信号通路的影响
2.2.1 对NF-κB信号通路的影响
核转录因子-κB(NF-κB)是细胞中重要的转录调节因子,NF-κB信号通路广泛参与机体的非特异性免疫过程和炎症反应,激活该通路可激活免疫分子的表达,参与炎症反应的调节过程[55]。Wang等[7]发现在LPS诱导的RAW 264.7巨噬细胞炎症模型中,新琼脂低聚糖能显著下调NF-κB p65和IKK蛋白表达。Huang等[12]研究发现日粮添加壳寡糖后可明显缓解LPS诱导的仔猪肠道损伤,下调小肠组织中p-NF-κB p65、IKKα/β和IκB蛋白表达。Chu等[14]研究发现在DSS诱导的小鼠结肠炎模型中,低聚半乳糖能降低结肠组织中IL-6、IL-18、IL-13和IL-33分泌及其mRNA表达,抑制结肠中p-IκBα和p-NF-κB p65蛋白表达。综上,新琼脂低聚糖、壳寡糖、低聚半乳糖能通过抑制NF-κB信号通路从而发挥抗炎作用。
2.2.2 对MAPK信号通路的影响
丝裂原活化蛋白激酶(MAPK)是炎症的另一条重要信号通路,MAPK家族成员包括有胞外信号调节激酶1/2(ERK1/2)、c-Jun氨基末端激酶(JNK)和丝裂原活化蛋白激酶(p38)三条途径[56]。在LPS诱导RAW 264.7巨噬细胞炎症模型中,壳寡糖能诱导NF-E2相关因子2(Nrf2)的核转位及上调HO-1、ERK1/2、JNK和p38MAPK蛋白表达;而ERK1/2、JNK和p38的特异性抑制剂可抑制Nrf2核转位及下调HO-1蛋白表达,表明壳寡糖可能通过Nrf2/MAPK信号通路介导HO-1蛋白表达发挥抗炎作用[8]。在LPS诱导的RAW 264.7巨噬细胞炎症模型中,新琼脂低聚糖能下调p38MAPK、ERK1/2、JNK蛋白表达,表明其通过抑制MAPK和Ras/MEK/ERK信号通路发挥抗炎作用[7]。Yeh等[6]研究发现在D-半乳糖诱导大鼠衰老模型中,低聚果糖能下调Jun、JNK蛋白表达,表明低聚果糖可能通过抑制JNK/Jun通路的激活而改善衰老大鼠肺组织炎症和纤维化。Lim等[16]研究发现木二糖能下调2型糖尿病小鼠肝脏组织p-SAPK/JNK、p-ERK1/2、p-p38 MAPK蛋白表达,改善肝脏的胰岛素抵抗,提示木二糖的作用机制与抑制肝脏中MAPK信号通路有关。综上,壳寡糖、新琼脂寡糖、低聚果糖可能通过下调ERK1/2、JNK和p38蛋白表达,抑制MAPK信号通路的激活,从而发挥抗炎作用。
2.2.3 对PI3K/Akt信号通路的影响
磷脂酰肌醇3-激酶/蛋白质丝氨酸苏氨酸激酶(PI3K/Akt)是细胞内信号转导通路,参与细胞的生长、增殖和凋亡等活动,与癌症相关的炎症性疾病密切相关。Liu等[9]研究发现在LPS诱导人脐静脉内皮细胞体外炎症模型中,壳寡糖可下调激活蛋白-1(Activator protein-1,AP-1)、NF-κB、p38MAPK和Akt蛋白表达,而p38MAPK或PI3K抑制剂可逆转炎症细胞中IL-8 mRNA的过度表达,表明壳寡糖可能通过抑制MAPK和PI3K/Akt信号通路缓解细胞炎症反应。刘美思[57]研究发现壳寡糖能抑制LPS诱导的RAW264.7巨噬细胞异常增殖,下调p-Akt和p-PI3K蛋白表达水平,提示壳寡糖通过抑制PI3K/Akt信号通路,从而减轻LPS所致巨噬细胞炎症损伤。
3. 前景及展望
研究发现,海藻酸低聚糖对LPS和Aβ诱导的BV2小胶质细胞神经炎症反应有抑制作用,是一种潜在的治疗阿尔兹海默症或其他神经退行性疾病的营养制剂。低聚果糖可以改善生物合成障碍,从而延缓非酒精性脂肪性肝炎的发生,可作为一种潜在的用于防治非酒精性脂肪肝的膳食配料。低聚壳聚糖对体外细胞炎性损伤模型有明显的干预作用,而且低聚壳聚糖、低聚甘露糖和低聚半乳糖均能改善急性结肠炎小鼠的临床症状。上述低聚糖在抑制炎症反应方面具有较大的开发潜力。
低聚糖是一种新型功能性糖源,广泛应用于食品、保健品、畜牧业和医药等领域。低聚糖因具有双歧因子的功能,可作为益生元而调节肠道菌群,因其具有抗菌活性可作为天然食品保鲜剂,其抗炎、抗癌等生理活性也得到广泛的认可。但目前低聚糖的应用还存在一定瓶颈:低聚糖缺乏标准化的制备方法和检测方法,难以大规模生产高纯度、低成本、聚合度稳定的低聚糖产品;低聚糖普遍具有还原性和吸水性,较易吸潮,难以长期稳定储存;低聚糖的抗炎机制研究还不够深入。随着人们对低聚糖抗炎机制的深入研究,将为筛选出高效无毒的低聚糖抗炎药物提供依据。
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