微生物学通报  2017, Vol. 44 Issue (7): 1728−1735

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文章信息

王丹, 夏险, 王革娇, 郑世学
WANG Dan, XIA Xian, WANG Ge-Jiao, ZHENG Shi-Xue
微生物对硒的还原及其产物的应用研究进展——纪念硒发现200周年
Research progress on reduction of selenium by microorganism and application of bio-reducing products of selenium——the 200th anniversary of the selenium discovery
微生物学通报, 2017, 44(7): 1728-1735
Microbiology China, 2017, 44(7): 1728-1735
DOI: 10.13344/j.microbiol.china.170109

文章历史

收稿日期: 2017-02-12
接受日期: 2017-03-17
优先数字出版日期(www.cnki.net): 2017-03-17
微生物对硒的还原及其产物的应用研究进展——纪念硒发现200周年
王丹, 夏险, 王革娇, 郑世学     
华中农业大学生命科学技术学院 农业微生物学国家重点实验室    湖北 武汉    430070
摘要:硒是生命必需的微量元素,以硒代半胱氨酸(Sec,第21位氨基酸)和硒代甲硫氨酸(Se-Met)的形式加入到硒蛋白(酶)中。人畜硒摄入过量或不足均会导致很多疾病。微生物参与了Se(-Ⅱ)、Se(0)、Se(Ⅳ)和Se(Ⅵ)等各种价态间的转化。本文主要综述微生物对硒的还原及其生物学意义。微生物对硒的还原包括同化还原、异化还原以及在还原基础上进行的硒的甲基化。硒的同化还原主要是形成各种硒蛋白,满足微生物自身对硒的需求,食源性微生物对人畜补硒具有重要意义。高浓度硒酸盐和亚硒酸盐则可促使微生物进行异化还原并形成单质纳米硒颗粒。有的微生物会将还原态的Sec和Se-Met进一步转化为挥发态的甲基化硒。硒的异化还原和甲基化都是解毒机制,在硒污染环境的治理中具有重要意义。最后,阐述了单质纳米硒在医药、生物传感器和治理重金属污染等方面的应用前景,以及微生物合成CdSe荧光量子点的应用。
关键词微生物     同化还原     异化还原     单质纳米硒     CdSe荧光量子点    
Research progress on reduction of selenium by microorganism and application of bio-reducing products of selenium——the 200th anniversary of the selenium discovery
WANG Dan, XIA Xian, WANG Ge-Jiao, ZHENG Shi-Xue     
State Key Laboratory of Agricultural Microbiology, College of Life Science and Technology, Huazhong Agricultural University, Wuhan, Hubei 430070, China
Received: February 12, 2017; Accepted: March 17, 2017; Published onlineMarch 17, 2017
Foundation item: National Natural Science Foundation of China (No. 31470227)
*Corresponding author: ZHENG Shi-Xue: Tel: 86-27-87294148; E-mail: zhengsx@mail.hzau.edu.cn.
Abstract: Selenium is an essential trace element for life, incorporates into selenoproteins (seleno-enzymes) in the form of selenocysteine (Sec, also referred to as the 21st protein amino acid) and selenomethionine (Se-Met). Either selenium over-intake or deficiency will lead to the occurrence of many diseases. Microorganisms are involved in the transformation of different selenium speciation including Se(-Ⅱ), Se(0), Se(Ⅳ) and Se(Ⅵ). Here we mainly reviewed the reduction of selenium in microorganisms. The reduction of selenium by microbes include assimilation reduction and dissimilation reduction. Selenoproteins could be produced via the pathway of the selenium assimilation reduction, which is benefit for the selenium enrichment through food chain. Higher concentration of selenate and selenite would promote the process of selenium dissimilation reduction and the formation of selenium nanoparticles in some microbes. Both selenium methylation and the formation of selenium nanoparticles were mechanisms of detoxification, and would provide an economical and "green" solution to the bioremediation of environmental selenium contamination. Finally, the potential applications of biogenic selenium nanoparticles (bio-SeNPs) in medicine, biosensor and heavy metal contamination bioremediation were discussed. The biosynthesis of CdSe quantum dots by microbes and its biological applications were also introduced.
Key words: Microorganism     Assimilation reduction     Dissimilation reduction     Bio-selenium nanoparticle     CdSe quantum dots    

硒(Selenium,Se)于1817年由瑞典化学家Berzelius发现,位于元素周期表第四周期Ⅵ主族,化学性质与同一主族的硫和碲相似。自然界中硒以-Ⅱ、0、+Ⅳ和+Ⅵ四种价态存在。硒是生命必需元素,以硒代半胱氨酸(Sec)和硒代甲硫氨酸(Se-Met)的形式加入到硒蛋白中[1]。目前人体中已知的硒蛋白包括谷胱甘肽过氧化物酶、硫氧还蛋白还原酶、脱碘酶、Sec合成酶等25种,发挥的功能包括抗氧化(抗癌)、氧化还原信号转导、硒的转运以及硒蛋白合成等。硒摄入过量或不足均会导致多种疾病。在各种形态的硒化合物中,亚硒酸盐由于其高度的可溶性及化学稳定性而对生命体危害最大,也是环境中硒污染物的主要形式。其对微生物的毒性机理主要表现为引起过氧化损伤[2],不同种类微生物对亚硒酸盐的耐受度可能与其自身抗氧化压力的能力密切相关。

硒在土壤圈中的分布很不均衡,且土壤的pH和Eh对硒形态影响很大。在碱性和通气良好的土壤中硒的主要形态为硒酸盐,在弱酸性及淹水的土壤中硒主要以亚硒酸盐形式存在,而强还原性土壤中硒化物占主导[3]。我国大约有70%土壤属于缺硒土壤(≤0.6 mg/kg),富硒土壤呈点状分布,如浙江、广东、广西、江西、安徽、河北、新疆和青海等省份的局部地区。对我国大部分地区而言要富硒,即增加土壤中有效硒的含量,而对于少部分高硒地区而言则要降低硒的生物可利用性。由于微生物参与了土壤中硒转化的各个方面,因此是调节土壤硒形态的重要因素。

微生物对硒的转化包括氧化(单质硒及硒化物)、对硒化合物的转运、同化利用、异化还原(硒酸盐和亚硒酸盐)以及甲基化和去甲基化过程(图 1)。相对于单质硒而言,高价态的硒氧化物(硒酸盐和亚硒酸盐)具有较高的溶解度和毒性,因此微生物对硒的异化还原过程更多地与解毒相关。而对于那些具有同化硒能力的微生物来说,选择硒同化还是异化还原与硒含量密切相关,但所涉及的调控机制尚不清楚。如Hudman和Glenn采用同位素标记法发现富硒细菌Selenomonas ruminantium除了利用75[Se]标记的亚硒酸盐合成硒代氨基酸之外,同时也有一部分75[Se]-亚硒酸盐最终被还原成了红色单质硒[6]。本文主要介绍微生物对硒的还原过程,包括同化还原、异化还原和还原基础上的甲基化,以及微生物还原硒的产物——单质纳米硒和CdSe量子点的应用。

图 1 细菌对硒的还原示意图 Figure 1 A scheme model of selenium reduction in bacteria 注:硒酸盐可被位于周质空间的硒酸盐还原酶复合体转化为亚硒酸盐,也可通过硫酸盐转运通道进入胞内,进而被同化利用转变成有机硒;亚硒酸盐既可被周质空间或细胞内的还原酶类以及硫醇类化合物转化成单质硒,进而形成纳米硒(SeNP),也可以通过同化还原作用转变成有机硒参与硒蛋白的合成;硒酸盐、亚硒酸盐和有机硒还可以通过甲基化途径转变成挥发性的二甲基硒(DMSe)或二甲基联硒(DMDSe)[4];亚硒酸盐在与胞内硫醇类化合物发生反应时会产生活性氧成分(ROS)[5];目前亚硒酸盐进入细菌细胞以及纳米硒排出细胞的机制还不清楚. Note: Selenate could be transformed into selenite by the bacterial periplasmic selenate reductase complex, or enter cytoplasm through the sulfate transporter and then transformed into organic form through the assimilation pathway. Selenite could be reduced into Se0 by the reductases which located in the periplasm and/or cytoplasm, or directly react with thiols and produce Se0. Reactive Oxygen Species will generate from the latter reaction. Se0 will be further assembled into selenium nanoparticles (BioSeNPs). Alternatively, selenite could be further transformed into volatile DMSe or DMDSe through the methylation pathway after reduction. Selenate and organic selenium compounds could also be transformed into DMSe or DMDSe through the methylation pathway. In addition, organic selenium speciation could also be produced from selenite through the bacterial assimilating reduction pathway. However, it remains unclear that how selenite was transported into the cytoplasm or how BioSeNPs was released from the cell.
1 硒的还原和甲基化 1.1 硒的同化还原

Peng等对目前已测序的5 200多个细菌基因组进行了系统性分析,发现潜在具有硒同化能力的细菌约占33%,且这些细菌的分布十分广泛,几乎涉及所有的门[7]。硒蛋白在古菌中的分布则相对较少,目前已完成基因组测序的古菌有500多个,其中仅发现Methanococcales和Methanopyrales这两个产甲烷古菌目含有硒蛋白[1]。Sec是第21位氨基酸,细菌合成Sec以及硒蛋白的过程目前研究得比较清楚,进入胞内的SeO42-可通过硫酸盐还原途径转变为硒化物(HSe-)[1, 8],但SeO32-转变为还原态硒化物的机制还不清楚,该过程可能涉及到硫氧还蛋白还原酶等胞内酶的作用[9]。接着由一系列的酶催化合成Sec,此过程已有相应的文献综述[10-11],本文不作重复。Sec也可进一步通过甲硫氨酸合成途径转化为Se-Met[1]。Sec和硒蛋白在细菌、古菌和真核生物中都被发现,并且几乎无一例外地位于具有还原活性酶类的催化中心,但微生物为何需要Sec并不清楚,因为绝大部分已知微生物体内并没有Sec。目前原核生物中通过生物信息学分析发现的硒蛋白家族已达到90余种[7],其中分布较为广泛的硒蛋白包括起抗氧化作用的甲酸脱氢酶、氢化酶、甘氨酸还原酶和脯氨酸还原酶等[12-13],以及参与铁硫簇生物合成的HesB和IscA[14]。得益于宏基因组测序和分析技术的广泛应用,细菌中许多功能尚不明确的新型硒蛋白正被不断发现,如与哺乳动物中的脱碘酶和硒蛋白W同源的蛋白[15]

利用食品微生物如乳酸菌和酵母菌能同化硒的特点进行食物补硒具有很好的应用前景和较高的经济价值,其菌体中以有机硒为主,人畜吸收性好。富硒酵母在畜牧业和特色食品生产等方面已得到广泛应用[16]。许多乳酸菌和双岐乳杆菌可以富集无机硒并将其转化为包括Sec、Se-Cys2、Se-Met和甲基硒在内的有机硒,如植物乳杆菌(Lactobacillus plantarum)、保加利亚乳杆菌(L. delbrueckii subsp. bulgaricus)、干酪乳杆菌(L. casei)、鼠李糖乳杆菌(L. rhamnosus LB3)、发酵乳杆菌(L. fermentum LB7)、罗伊氏乳杆菌(L. reuteri)、短乳杆菌(L. brevis)、旧金山乳杆菌(L. sanfranciscensis)、布氏乳杆菌(L. buchneri)和双岐乳杆菌中的Bifidobacteria animalis 01及Bifidobacterium lactis LAFTI B94等菌株[17]。乳酸菌中的硒主要以Sec形式存在[18],富硒酵母主要将无机硒转化为Se-Met[19],这两种形态的有机硒均具有很好的生物可利用性。然而,一些益生菌也可将Se-Met进一步转化为挥发性的二甲基硒(DMSe)或二甲基联硒(DMDSe)[20-21]。此外,已有研究提及肠道微生物也可能改变食物中的有机硒形态,从而影响硒的生物可利用性[22],这些不利影响在通过食物补硒时需要加以考虑。肠道微生物对不同形态硒的转化,如何影响机体对硒的吸收,还需要深入研究。

1.2 硒的异化还原

微生物对硒酸盐和亚硒酸盐的还原广泛发生于土壤、沉积物和水体环境中。在已经发现的硒还原菌中,同化还原只占整个硒形态的很少一部分,大部分硒酸盐和亚硒酸盐最终通过异化还原途径转变成了单质硒。一般认为异化过程是清除水体环境中可溶性硒酸盐和亚硒酸盐的重要途径。这类还原在硒污染治理中发挥着重要作用,对这些硒还原菌的研究也越来越多,因为这类研究可以为硒污染治理提供一种经济有效的手段[23]。一些特定种类的微生物,可利用醇类、糖类、有机酸、腐殖质以及氢气等物质作为异化还原过程中的电子供体,从而获取能量进行硒还原[24]

硒还原微生物的分布十分广泛,在细菌、古菌和真菌中都有发现,但一个有趣的现象是目前已知的亚硒酸盐还原菌大多分布于变形菌门如Pseudomonas spp.[25-26]Rhizobium sp.[27]Burkholderia fungorum[28]Paenirhodobacter enshiensis[29]Comamonas testosteroni[30],厚壁菌门如Bacillus spp.[31-32]Paenibacillus spp.[33-34],放线菌门如Streptomyces sp.[35],这可能与这3个门的细菌含有丰富的硫醇类化合物有关。与可进行亚硒酸盐还原的微生物相比,能还原硒酸盐的微生物种类较少。Kuroda等[36]发现硒酸盐还原为亚硒酸盐和亚硒酸盐还原为单质硒是两个独立的过程。一些细菌具有将硒酸盐和亚硒酸盐还原为单质硒的双重能力,如大肠杆菌和施氏假单胞菌[25, 37],而大部分细菌只能完成亚硒酸盐到单质硒的单一还原过程[38-39]。其中厌氧条件下硒酸盐还原为亚硒酸盐的机制已研究得较为透彻,在以Bacillus selenatarsenatis为代表的革兰氏阳性菌和以Thauera selenati为代表的革兰氏阴性菌中均发现了特异性的硒酸盐还原酶。这些硒酸盐还原酶均由含多个4[Fe-S]的3个亚单位组成,并且需要钼离子的参与。但好氧条件下的硒酸盐还原为亚硒酸盐的机制还不清楚。

国内外关于微生物对亚硒酸盐的还原机制研究依然比较活跃。亚硒酸盐还原发生的部位因不同细菌而异,目前的结果表明亚硒酸盐的还原在胞外、周质空间和胞内均可发生,而且好氧还原菌和厌氧还原菌均有报道。其还原机制有酶促反应和非酶促反应两大类。早前的研究发现亚硫酸盐还原酶、周质亚硝酸盐还原酶和二甲亚砜(DMSO)还原酶等还原酶类具有亚硒酸盐还原功能[24]。在硒还原菌Pseudomonas selenitipraecipitans CA-5中通过非变性凝胶酶谱分析的方法表明谷胱甘肽还原酶和硫氧还蛋白还原酶可能参与了亚硒酸盐的还原[40],而在根瘤菌Rhizobium selenitireducens中采用同样的方法则鉴定出一个属于old-yellow-enzymes (OYE)家族的黄素蛋白可以将亚硒酸盐还原成单质硒[41]。值得一提的是不同种类的硒还原菌对亚硒酸盐的还原机制并不相同,甚至在同一种菌中可能存在多种机制共同起作用,如敲除Shewanella oneidensis MR-1中的一个延胡索酸盐还原酶编码基因fccA之后该菌的亚硒酸盐还原能力仅丧失了60%[42],预示着该菌还存在着其它的亚硒酸盐还原路径。

除酶促因素外,亚硒酸盐的还原还存在由非酶促因素介导的模式,如硫化物介导的还原模式[43]和硫醇类化合物如谷胱甘肽(GSH)介导的还原模式[5]。此外,细菌分泌的一些胞外化合物如吩嗪-1-羧酸(phenazine-1-carboxylic acid,PCA)也可还原亚硒酸盐[44]

相对于硒酸盐和亚硒酸盐的还原,有关微生物将单质硒还原成硒化物(Se2-)的报道还较少[45-46],这可能与硒化物较高的生物毒性和化学活性有关,但这些微生物所具有的将不可溶的单质硒转化成可溶态硒的能力值得关注,发掘这类微生物资源对于提高缺硒地区土壤和水体可溶态硒含量,从而促进植物富硒同样具有重要意义。

1.3 与还原相关的硒甲基化

许多细菌、古菌和真菌都可以将硒酸盐或亚硒酸盐以及有机态的含硒氨基酸转化成挥发性的二甲基硒(DMSe)或二甲基联硒(DMDSe) (图 1)[18-20, 47]。硒甲基化的分子机制已很清楚,在相关中文文献中[10-11]已有综述,本文不再赘述。由于DMSe和DMDSe中的硒均为还原态Se(–Ⅱ),因而硒甲基化过程必然涉及到硒的还原,Eswayah等关于微生物硒甲基化途径的总结也提及了这一点[4]。此外,超聚硒高等植物中的Sec被甲基化发生于质体中[48],而绝大部分细菌并不含Sec,由此推测细菌中的硒甲基化是一种古老的现象。微生物和植物对硒的甲基化,在硒污染环境的治理中具有重要的应用价值。

1.4 CdSe的合成

除了通过异化还原途径将高价态的硒化合物转化为单质纳米硒以外,一些微生物还可利用硒化合物和镉化合物合成另外一种具有重要应用价值的纳米材料——CdSe量子点,但微生物合成这类荧光量子点的机制还不清楚。Kumar等[49]报道了利用真菌Fusarium oxysporum在室温条件下体外合成CdSe量子点。Yan等[50]则报道了一种利用大肠杆菌合成CdSe量子点的方法,但具体合成机制未涉及。Li等的研究表明谷胱甘肽在酿酒酵母CdSe荧光量子点的合成过程中起重要作用,并且CdSe的最终合成存在一种时序控制机制[51]。Cui等的体外实验结果表明Sec也参与了CdSe合成过程[52]

2 硒还原产物的应用 2.1 单质纳米硒的应用

微生物还原硒酸盐和亚硒酸盐的终产物大部分为红色单质硒,且产物颗粒大小从17 nm-500 nm不等[28, 30, 35, 53]。这类由微生物代谢产生的纳米至微米级别的颗粒兼具纳米材料的光电子特性以及多种抗生物活性,常被称为生物纳米硒(BioSeNP)。由于微生物还原并生产纳米硒一般在温和条件下如发酵罐中进行,因此与用化学还原法制备单质纳米硒相比,用微生物还原法制备单质纳米硒具有低能耗、低成本和低污染的优点。不同微生物形成的生物纳米硒颗粒的大小不同,可能意味着不同种类的微生物对于控制纳米硒大小的机制存在差异。生物纳米硒的形成机制目前尚不清楚,除了在T. selenatis中发现的特异性纳米硒包装蛋白SefA之外[54],一些研究表明含有半胱氨酸残基的蛋白质如乙醇脱氢酶(Adh)[55]可能参与纳米硒的形成。此外,Gonzalez-Gil等[56]发现位于细胞膜上的孔蛋白和细胞质延伸因子Tu蛋白等可以与纳米硒结合,表明纳米硒的形成还存在非特异性因素。

生物纳米硒在医药、生物传感器和治理重金属污染等方面具有广阔的应用前景。生物纳米硒的抗微生物活性与其浓度和粒径大小有关,并且对革兰氏阴性菌和阳性菌具有同样的抑制效果[57]。Zonaro等[58]发现生物纳米硒具有降解生物膜的功能。此外,生物纳米硒还可通过抑制孢子形成的方式抑制皮肤真菌如Malassezia sympodialisMalassezia furfur的生长[59]。生物纳米硒对有鞭毛的婴儿利什曼原虫(Leishmania infantum)和无鞭毛的巨大利什曼虫(L. major)的半抑制浓度范围仅为1-25 mg/L,因而有可能被用于利什曼虫感染的治疗[60-61]

大量研究表明纳米硒具有抗多种癌细胞如肾、乳腺、肺以及骨肉瘤的活性[62-63],因此可作为潜在的抗癌药物。但是纳米硒的抗癌机理尚不很清楚,目前提出了多种假设:(1) 增加癌细胞氧化压力,加强致癌物脱毒以及机体免疫监视;(2) 诱导体细胞和线粒体介导的细胞凋亡;(3) 抑制肿瘤细胞的侵入和血管生成;(4) 将细胞周期阻滞在S合成期;(5) 抑制金属蛋白的表达;(6) 增加内源性铜的移动性[64]

目前已有将生物单质纳米硒应用于检测过氧化氢的生物感应器的报道[65],这种经过纳米硒修饰的传感器具有很高的灵敏度,在以生物纳米硒作为基质时检出限可达到8×10-8 mol/L[66]。此外,由于纳米硒与Hg0具有高度亲和性,因此可应用于汞污染环境的修复[67]

2.2 CdSe荧光量子点的应用

荧光量子点(Quantum dots,QD)具有吸光度强、荧光亮度高、发射光谱狭窄并且匀称、激发范围广和光稳定性高等诸多优点,因而在生物传感器、生物成像、生物分析、靶向给药、新型诊疗试剂开发和纳米药物等领域具有很好的应用前景[68]。将量子点应用于生物材料的标记、示踪及成像等方面已经有许多工作,其中最早将量子点应用于生物材料的标记出现于1998年[69]。量子点特别适合用于免疫标记、细胞运动、原位杂交以及活细胞的标记。工程化的水溶性量子点可与蛋白质、多肽、寡核苷酸和抗体等生物大分子交联从而形成特异性靶标。活细胞内的量子点容易聚集并且富集在内含体和溶酶体中,因此可利用激光共聚焦显微镜、全内反射显微镜、广角荧光显微镜和荧光计等仪器观察它们的存在。然而,将量子点应用于生物体还面临诸多挑战,包括生物兼容性、非特异性结合以及细胞毒性等问题。此外,量子点的稳定生产及维持纳米晶体大小的均一性也是一大难题。

3 展望

微生物硒代谢的研究进展很快,但仅就硒的还原而言依然有许多问题尚待阐释。硒的同化还原方面,细菌硒蛋白的发掘及其功能阐释还有许多工作要做;肠道微生物如何影响机体对不同形态硒的吸收与转化尚不清楚;好氧微生物还原硒的分子机制尚未很好阐明;微生物还原硒过程中可能涉及到的调控机制还未开展研究;微生物控制单质纳米硒和CdSe荧光量子点合成的机制远未阐明;生物纳米硒的安全性也有待检验。此外,微生物在土壤等环境中转化硒的生态学过程也很少研究,尽管这与植物富硒或硒污染的治理直接关联。解决了这些问题,才能为人类合理、安全有效地利用硒提供充分的理论依据和保障。

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