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母乳低聚糖检测技术研究进展
作者:冯鑫 高芳 别玮 卜汉萍 韩深 汪龙飞 王金花 鲍蕾
冯鑫 高芳 别玮 卜汉萍 韩深 汪龙飞 王金花 鲍蕾
冯 鑫 1 高 芳 2 别 玮 1 卜汉萍 2 韩 深 1 汪龙飞 2 王金花 1 * 鲍 蕾 2#
摘 要 母乳低聚糖(Human Milk Oligosaccharides,HMOs)被认为是婴幼儿生长发育的重要营养成分之一,具有促进益生菌定植和生长、抑制致病菌定植、预防新生儿坏死性结肠炎、调节免疫力和促进脑神经发育等生理功能。部分国家和地区已批准部分HMOs可作为营养强化剂用于婴幼儿配方食品及儿童用调制乳粉。目前已有多种针对母乳或相关产品中HMOs含量的检测方法。本文详细介绍了HMOs的结构和功能,并对多项研究中涉及的检测方法进行归类比较和系统阐述,旨在深入研究HMOs检测方法特性,为促进HMOs检测方法标准化和提升各研究之间结果的可比性提供参考,进而为未来母乳成分研究和乳品质量监管提供技术支撑。
关键词 母乳低聚糖;高效液相色谱仪;离子色谱仪;质谱仪
Advances in Human Milk Oligosaccharides Detection Technology
FENG Xin1 GAO Fang2 BIE Wei1 BU Han-Ping2
HAN Shen1 WANG Long-Fei2 WANG Jin-Hua1* BAO Lei2#
Abstract Human milk oligosaccharides (HMOs) are well known as an important nutritional components of infant growth and development, eliminating pathogens, with physiological functions such as promoting the colonization and growth of probiotics, inhibiting the colonization of pathogens, preventing necrotizing enterocolitis in neonates, regulating immunity, and promoting brain and nerve development. Some regions have approved the use of certain HMOs as nutritional fortifiers in infant formula foods and modified milk powder for children. Various detection methods for HMOs content in human milk or related products have been developed. This paper provides a detailed introduction to the structure and function of HMOs, and systematically discusses and compares the detection methods involved in multiple studies. The aim is to conduct an intensive study of the characteristics of HMOs detection methods, to promote the standardization of HMOs detection methods, and to enhance the comparability of results between studies, thereby providing technical support for future human milk composition research and dairy product quality control.
Keywords human milk oligosaccharides; high-performance liquid chromatography; ion chromatography; mass spectrometry
第一作者:冯鑫(1988—),男,汉族,北京人,硕士,高级工程师,主要从事进出口食品营养与食品卫生检测及标准化工作,E-mail: draconpku@hotmail.com
通信作者:王金花(1968—),女,汉族,北京人,博士,研究员,主要从事进出口食品营养与安全相关研究工作,E-mail: wangjh2008@aliyun.com
共同通信作者:鲍蕾(1973—),女,汉族,河北临西人,博士,研究员,主要从事食品安全检测技术与标准化工作,Lei.bao@rd.nestle.com
1. 中国海关科学技术研究中心 北京 100026
2. 雀巢研发(中国)有限公司雀巢食品安全研究院 北京 100016
1. Science and Technology Research Center of China Customs, Beijing 100026
2. Nestlé Food Safety Institute of China, Nestlé R&D (China) Ltd., Beijing 100016
母乳是新生儿的重要营养来源,包含糖类、蛋白质和脂肪等新生儿发育所必需的营养成分。母乳低聚糖(Human Milk Oligosaccharides,HMOs)的含量在母乳中仅次于乳糖和脂肪,是第三大固体物质[1]。由于低聚糖不易被人体肠道消化吸收,其营养价值曾被忽略。近年来大量细胞实验、动物实验和临床研究发现,HMOs是婴幼儿发育时期不可缺少的营养成分。
婴幼儿配方乳粉原料多来源于牛乳、羊乳或豆类,而三者中的HMOs种类与含量与母乳存在显著差异[2-4],为使婴幼儿配方乳粉的营养功效更加贴近母乳,欧盟、美国、澳大利亚及新西兰和中国分别批准了多种HMOs为新食品原料或营养强化剂,可添加至婴幼儿配方食品及儿童用调制乳粉。目前,2'-岩藻糖基乳糖(2'-FL)已在至少37个国家被允许应用于婴儿配方食品中。截至2023年10月部分国家和地区批准使用情况见表1。
目前针对HMOs的检测集中在两方面。一是针对母乳,主要目的是研究不同地域、不同泌乳时期和不同遗传背景下母乳样品中的HMOs种类及含量的差异,为进一步优化婴幼儿配方食品的配方提供理论依据。二是在婴幼儿配方食品产品层面进行定性及定量分析方法的研发,研究目的是对添加HMOs产品进行质量控制,以保证销售产品满足所在国家或地区对营养成分标签标识等法规的要求。由于母乳成分及婴幼儿配方食品的成分复杂,乳脂肪、乳糖及其他低聚糖类物质的干扰是建立HMOs检测方法的难点。近年来,随着检测技术的进步,多种不同原理的检测方法逐渐被开发,但技术上仍存在许多需要攻克的难题。本文将从HMOs的分型和结构特征出发,结合目前营养学研究,系统梳理不同研究中针对HMOs所建立的检测方法,并分析检测中所面临的问题,旨在为HMOs精准检测的方法及其标准化和不同原理方法的对比提供参考。
1 HMOs的结构与合成路径
HMOs由3~10个重复的单糖单体组成,包括D-半乳糖(D-galactose,Gal)、D-葡萄糖(D-glucose,Glc)、L-岩藻糖(L-fucose,Fuc)、N-乙酰氨基葡萄糖(N-acetylglucosamine,GlcNAc)和唾液酸(sialic acid/N-acetylneuraminic acid,Neu5Ac)5种单体[5]。HMOs单体通过糖苷键相连形成聚糖的核心结构,在此基础上,糖链延伸或者进行唾液酸或岩藻糖基化,构成不同种类的HMOs。在空间结构上,由于糖基序列、链长以及糖基化位点的差异,造成了HMOs结构和数量的多样性。哺乳动物不同物种之间乳汁低聚糖的含量和组成存在很大的差异,人乳中HMOs含量最多且结构最为复杂,目前已经鉴定出来的HMOs约200种[6]。HMOs可分为酸性HMOs和中性HMOs,中性HMOs不含有Neu5Ac。根据是否发生岩藻糖基化又将中性HMOs分为岩藻糖基化HMOs和非岩藻糖基化HMOs,母乳中以中性的岩藻糖基化HMOs为主,约占70%[7]。
HMOs在母乳中的种类和含量存在很大个体差异,主要影响因素为遗传背景以及地域和泌乳时间差异。目前岩藻糖基化的HMOs合成路径较为明确,其中α1-2岩藻糖基转移酶(FUT2)和α1-3/4-岩藻糖基转移酶(FUT3)参与其中[8],人群中个体可同时表达、同时不表达或仅表达其中一种,因此母乳中岩藻糖基化的HMOs含量即存在差异。当个体表达FUT2基因时,母乳中富含2'-FL,LNFP-I等其他需要α1-2岩藻糖基化的HMOs,反之则缺乏该类HMOs;当个体表达FUT3基因时,母乳中富含3-FL,LNFP-II等α1-3或4位的岩藻糖基化HMOs[9-11]。目前关于唾液酸化HMOs的合成路径尚不明确。部分在母乳中含量较高的HMOs结构如图1所示,HMOs末端存在还原型与非还原型两种构型[12],如图2所示。
图2 HMOs非还原型末端与还原型末端构型示意图
Fig.2 Structural elucidation of non-reduced and reduced terminal configurations of HMOs.
2 HMOs的主要生理功能
2.1 HMOs的肠道内功能
HMOs在小肠中几乎不被吸收,其代谢和主要功能均发生在结肠。在结肠中HMOs可作为特定菌群的底物并调节肠道微生物菌群,促进婴儿肠道有益细菌的生长[14-16],与乳糖及低聚半乳糖等成分相比,对肠道内条件致病菌的增殖作用不明显,对双歧杆菌的增殖作用更具选择性[17]。HMOs不仅能促进有益菌的生长,还能促进益生菌对肠道上皮细胞的黏附[18-19],有助于刺激人体肠道形成免疫屏障和代谢。一项临床随机双盲实验发现,喂养2'-FL和LNnT营养强化配方奶粉的婴儿,其粪便中双歧杆菌属的丰度与母乳喂养的婴儿粪便情况更接近[20]。
HMOs除益生元效应外,还可以发挥对病原菌的活性抑制作用,或抑制黏附能力病原体与肠黏膜结合。一些研究认为,部分HMOs与肠道上皮细胞上的聚糖类受体结构相似,通过“诱惑”致病菌与HMOs结合从而减少致病菌正在肠道上皮的黏附和定植。一项体内实验发现,母乳中高浓度的2'-FL与婴儿空肠杆菌引发的腹泻发病率降低有相关性[21],同时一项体外细胞实验也发现2'-FL和6'-SL可降低大肠杆菌对Caco-2的黏附作用[22]。
此外,HMOs还可以预防新生儿坏死性结肠炎(Necrotizing Enterocolitis,NEC)的发生。NEC多发于早产儿和低体重儿,发病率及致死率均较高。一项针对200余名产妇与其低体重儿的临床跟踪实验发现,母乳成分中DSLNT浓度越高,NEC的发生率越低[23]。一项大鼠和猪的子代实验中也发现,2'-FL和6'-SL也可降低NEC的发生率[24]。尽管相关机理尚不明确,但目前的临床观点认为通过各种方式为低体重儿补充HMOs是一项正向的医疗措施。
2.2 HMOs的肠道外功能
HMOs可提升婴幼儿免疫力,不仅通过肠道内对致病菌的抑制作用,同时还可以通过调节炎性因子实现对免疫力的调节。Eiwegger等[25]的研究认为酸性HMOs可提升Th-1型细胞因子IFN-γ和Th-2型细胞因子的增加,同时后续研究[26]发现酸性HMOs还可以平衡Th-1/Th-2的表型比例来调节新生儿出生后的过敏免疫反应。此外,2'-FL被证明可以通过抑制CD14的表达来抑制IL-8炎性信号通路,从而减少炎症介质的产生[27]。另一项研究表明,2'-FL可促进上皮凝集素和TGF-β1分泌,从而发挥免疫调节作用[28]。Sprenger等[29]通过临床试验发现,婴儿在出生后一年内喂养含有2'-FL和LNnT成分的配方奶粉,与对照组相比,患下呼吸道感染性疾病的次数更少,可能的原因是岩藻糖基化的HMOs,可增强巨噬细胞活性,增加免疫物质的分泌[30]。
此外,HMOs还具有促进脑神经系统发育的作用。已有明确研究发现婴幼儿接受的喂养方式与脑神经发育情况有直接联系。一项动物实验研究发现,在配方粉中加入3'-SL和6'-SL可提升脑内神经节苷脂含量[31],这是大脑代谢、神经递质传递和神经发育所必需的。也有动物实验研究发现,包括酸性HMOs在内,2'-FL均可通过调节迷走神经的“肠-脑轴”来改善大鼠的学习和记忆能力[32-33]。
3 HMOs的主要检测方法
由于HMOs存在同分异构现象且同一HMOs存在还原型和非还原型末端现象,其定性和定量分析具有一定难度。常用的分析方法主要有离子色谱法(Ion Chromatography,IC)、高效液相色谱法(High Performance Liquid Chromatography,HPLC)、质谱法(mass spectrometry,MS)、毛细管电泳法(capillary electrophoresis,CE)、核磁共振法等。
3.1 离子色谱法
离子色谱仪是检测样品中单糖、双糖和聚糖成分的常用仪器之一[34-36],由于糖类物质缺乏紫外和荧光发光基团,因此脉冲安培检测器这种通用型检测器则被用于糖类物质的检测,基本方式是在检测时使用Au电极作为工作电极,Ag/AgCl电极作为参比电极,糖分子中还原性基团在工作电极上被氧化从而产生电流变化信号支持检测。HMOs作为糖类亦可采用离子色谱法开展检测,原理是HMOs的pKa值多数在12~14之间,在碱性条件下可通过离子色谱分离后再通过脉冲安培检测器进行定量。但本检测方法也存在部分局限:(1)脉冲安培检测器作为通用型检测器,样本中具备氧化-还原性的成分均可被检出,对目标化合物的检测形成干扰,仅靠离子色谱分离的保留时间进行定性是不够的;(2)仪器检出限不足,使低含量的成分不易被检出;(3)由于流动相中使用强碱性物质,因此无法与质谱检测器联用。
随着色谱和质谱技术的发展,使用离子色谱法的研究逐渐减少,但仍有部分研究人员开发了更有效的离子色谱分析方法,对样本中HMOs含量开展测定并研究其健康效应,如Haselberger等[37]利用离子色谱仪开发出测定婴幼儿配方食品中6种HMOs的方法,Sprenger等[38]采用离子色谱法分析新加坡34份母乳样本中2'-FL、LNT和LNnT浓度变化并对新生儿生长发育情况进行追踪。Paganini等[39]利用离子色谱法对75例肯尼亚籍母乳样品中的12种HMOs含量进行分析,Wu等[40]利用离子色谱法分析222例样品中2'-FL含量与神经损伤之间的联系。
3.2 高效液相色谱法
液相色谱仪可通过不同组分在多种固定相和流动相中保留能力及分配系数等特性的差异实现对目标化合物和杂质成分的分离,通过搭配不同类型色谱柱和检测器实现定性和定量分析,使检测方法更具多样性和灵活性,是近年来进行HMOs检测和分析的主要方式之一。
由于糖类物质缺乏紫外和荧光基团,通常采用酰胺基类色谱柱分离定性,采用示差折光检测器[41]或蒸发光散射检测器[42]进行定量检测的方式,但无法同时检测多种类型的HMOs。有研究发现,通过衍生化反应可使HMOs被标记,使其具备紫外或荧光基团,并通过改变其极性使同分异构体之间具备更佳的分离效果,Asakuma等[43]采用1-苯基-3-甲基-5-吡唑啉酮(PMP)进行衍生,通过液相色谱分离紫外检测器检测,对20份母乳样本中8种HMOs进行分析,此外更多研究中将2-氨基苯甲酸[44-45]和2-氨基苯甲酰胺[46-49]等作为发色活性标记物与HMOs进行衍生,使HMOs带上荧光基团,再通过高效液相色谱仪或超高效液相色谱仪分离后采用荧光检测器进行定量分析,获得比紫外检测器更佳的灵敏度。此类方法可对母乳中20余种HMOs进行检测,但对婴幼儿配方乳粉中多种HMOs的分析时,则需要开展更复杂的前处理步骤,以排除多种外源性营养强化剂对检测的干扰,如婴幼儿配方乳粉中添加低聚半乳糖和麦芽糊精时,需要采用β-半乳糖苷酶和淀粉葡萄糖苷酶等多步骤酶解过程实现对婴幼儿配方食品中多种HMOs的定量检测[49]。
3.3 质谱法
质谱检测器具备更佳的分析能力,与液相色谱结合可同时开展100余种HMOs的检测,提升了检测的定性和定量能力,也可大幅减少前处理复杂程度,同时部分研究采用多孔石墨碳色谱柱分离结合质谱检测,可根据HMOs的结构轮廓进行分离。
采用高分辨质谱仪可实现对不同HMOs的精准定性。Nijman等[50]采用Nano-LC-QTOF技术对10份样本中的133种HMOs进行定性分析,Marino等[51]采用HILIC色谱柱结合Q-TOF实现37种HMOs的分离。但高分辨质谱价格昂贵且存在精准定量问题较弱的问题,因此部分研究采用三重四极杆质谱进行HMOs检测,如Mank等[52]、Tonon等[53]、Xu等[54]、Zhang等[55]和Li等[56]团队均建立了多种HMOs的检测方法,但针对同一化合物,各方法之间存在正负离子扫描模式差异和离子对差异,差异可能来自于仪器间、色谱柱类别和流动相,这种差异增加了样本检测结果之间比较的难度。同时由于质谱检测存在基质效应的问题,会影响部分化合物的离子化效率,使检测结果出现偏离,通常会通过使用同位素内标消除质谱体系的基质效应,但目前HMOs尚无商品化同位素内标,因此暂无有效解决基质效应的手段。
3.4 毛细管电泳法
有部分研究采用毛细管电泳仪对样品中的多种HMOs进行分离。毛细管电泳以高压直流电场为驱动,对样品中的带电荷组分进行有效分离,特别是针对唾液酸化的HMOs具有较好的分离效果[57]。但相关研究发现,该方法对部分同分异构体间的分离效果有一定的局限,如Galeotti等[58]研究中发现LNFP-Ⅱ和LNFP-Ⅲ以及3'-SL和6'-SL无法分离,而Olivares等[59]的研究发现3'-SL和6'-SL、2'-FL和3-FL、LNT和LNnT、LNFP-Ⅰ和LNFP-Ⅲ间均存在共流出,无法有效分离。
3.5 核磁共振法
有研究人员[60-63]采用核磁共振仪对样本中的HMOs进行了分析。核磁共振能够提供更多关于HMOs的结构信息和其中各基团的连接方式,可对各同分异构体成分进行明确辨析,在低聚糖的结果判定中起到重要作用。但由于核磁共振仪所需样品量大、要求纯度高,分析过程也较为复杂,因此并不适合高通量检测。
4 展望
本文介绍了母乳低聚糖的结构、主要功能和目前相关检测方法的基本情况。针对样品中HMOs检测方法的研究已开展多年,多数方法已实现部分样品中不同种类HMOs的准确定性和定量,但仍面临以下三个困难的挑战:(1)HMOs之间存在同分异构现象,如2'-FL与3-FL、3'-SL与6'-SL等,检测方法需实现同分异构体分离;(2)HMOs作为碳水化合物,其结构中缺少发光基团,使得液相色谱紫外或荧光检测技术充满挑战;(3)HMOs存在还原型和非还原型末端两种形态,使用色谱分离存在同一化合物出现两个甚至多个色谱峰的情况。
目前我国已批准2种HMOs可作为营养强化剂用于婴幼儿配方食品,配方中含有母乳低聚糖的进口婴幼儿配方食品将在未来大量注册并逐步上市,对于相关产品的监管及检测方法的标准化将被提上日程。通过方法间比对、实验室内方法学验证和实验室间验证等过程,实现婴幼儿配方食品中HMOs检测的标准化,是从标签监管层面确保产品质量符合标准要求的必要步骤,是海关和其他监管机构积极维护消费者合法权益的执法依据和保障。
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表1 部分国家和地区批准HMOs在婴配乳粉中的最大使用量
Table 1 Allowable addition range of HMOs in infant formulas in different countries and regions
化合物名称 | 最大使用量 (g/L) | |||||
欧盟 | 美国 | 澳大利亚及新西兰 | 中国 | 韩国 | 加拿大 | |
2 '-岩藻糖基乳糖 (2 '-FL) | 1.2 | 2.4 | 2.4 | 2.4 | 2.0 | 1.2 |
3-岩藻糖基乳糖 (3-FL) | 0.85 | 0.44 | — | — | — | — |
乳糖N-四糖 (LNT) | 0.8 | 0.8 | — | — | — | — |
乳糖-N-新四糖 (LNnT) | 0.6 | 0.6 | 0.6 | 0.6 | — | — |
3 '-唾液酸乳糖 (3 '-SL) | 0.2 | 0.2 | — | — | — | — |
6 '-唾液酸乳糖 (6 '-SL) | 0.4 | 0.4 | — | — | — | — |
2 '-岩藻糖基乳糖/二岩藻糖基乳糖 (2 '-FL/DFL) | 1.6 | 1.6 | — | — | — | — |
注: “—”表示暂未批准使用, 部分数据源自“食品伙伴网”
图1 部分HMOs结构示意图[13]
Fig.1 Structural elucidation of HMOs