【作者】 周文斌；

【导师】 马百平； 王林；

【作者基本信息】 中国人民解放军军事医学科学院 ， 药物化学， 2010， 博士

【摘要】 知母皂苷BII为来源于中药知母的呋甾皂苷类单体化合物,可以显著改善多种拟痴呆动物模型的学习记忆功能,其作用机理为上调胆碱能N&M受体,改善脑缺血及缺血损伤。现在正在按照中药、天然药物注册分类Ⅰ进行研究开发,用于痴呆的防治。为了进行构效关系的研究,我们一直在寻找有效的方法对该化合物进行结构修饰。生物转化(Biotransformation)是利用动、植物离体细胞或器官、微生物、或它们所产生的酶等对外源性化合物进行结构修饰而获得有价值产物的生化反应。与有机合成方法相比,生物转化具有工艺简单、选择性强、转化率高、副产物少、条件温和及环境友好等特点。目前为止,有关天然产物的生物转化研究较多,但对像知母皂苷BII这样的甾体皂苷类化合物的生物转化研究相对较少,特别是在其糖基化和C-3位糖链的选择性水解方面。本论文以知母皂苷BII为底物,对本实验室已保存的40多种酶及近百种微生物菌株进行活性筛选,发现一种酶(Toruzyme 3.0L)和两种微生物(黑曲霉Aspergillus niger AS 3.0739;荧光假单胞菌Pseudomonas fluorescens)能够转化知母皂苷BII生成其相应的衍生物。Toruzyme 3.0L是一种商品化环糊精葡萄糖基转移酶( cyclodextrin glucanotransferase, CGTase),发现其可以催化知母皂苷BII生成相对其极性增大的转化产物。经过知母皂苷BII制备级转化,通过大孔吸附树脂、薄层制备、开放C18柱层析及制备液相等分离,共得到了9个转化产物(产物1～9),并利用FAB-MS、HR-ESI-MS及NMR分析,鉴定其全部是知母皂苷BII的糖基化衍生物。并且该糖基化反应是利用非活化的糖源供体,可以在100℃下完成。黑曲霉Aspergillus niger AS 3.0739是一种常见的产糖化酶菌株,发现其在全细胞培养时可以选择性催化知母皂苷BII的C-22位羟基发生脱水反应,生成知母皂苷B(产物B1)。而从其发酵液中提取的全酶,在pH 8.0时,随着转化时间的延长,首先选择性地依次水解知母皂苷BII的C-3位糖链的糖基,得到2个次生呋甾皂苷(产物H1、H2),又水解其C-26糖链,得到1个次生螺甾皂苷(知母皂苷AIII,产物H4)。产物H1的结构为:(25S)-26-O-β-D-吡喃葡萄糖基-22-羟基-5β-呋甾皂苷-3β, 26-二醇;产物H2的结构为:(25S)-26-O-β-D-吡喃葡萄糖基-22-羟基-5β-呋甾-3β, 26-二醇-3-O-β-D-吡喃半乳糖苷。本论文分离鉴定的一株菌-荧光假单胞菌(Pseudomonas fluorescens)可以催化知母皂苷BII生成大极性转化产物(产物H3)。对该大极性转化产物进行制备分离,再通过波谱分析,尤其是二维核磁光谱(1H-1H COSY, HSQC, HMBC)对产物H3的13C和1H进行全归属,确定产物H3的结构是:(25S)-26-O-琥珀酸酯-(1→6)-β-D-吡喃葡萄糖基-22-羟基-5β-呋甾-3β, 26-二醇-3-O-β-D-吡喃葡萄糖基-(1→2)-β-D-吡喃半乳糖苷。实现了知母皂苷BII的琥珀酸酯化,该酯化反应的酯化位点是知母皂苷BII C-26位糖链葡萄糖基的C-6’位羟基。鉴于Toruzyme 3.0L对知母皂苷BII的糖基化反应可以利用非活化糖源供体,且在100℃下还可以完成的新颜性,我们进行了其中糖基化酶的分离纯化,结构分析及酶学特性的研究。以催化知母皂苷BII发生糖基化反应为酶活筛选指标,利用凝胶色谱、离子交换层析等方法,纯化得到了其中的目的酶蛋白;经SDS-PAGE电泳检测为单一条带;然后切割目的蛋白条带,胰蛋白酶降解、MALDI-TOF/TOF和ESI-Q-TOF-MS测定,数据库检索,确定了该糖基化酶的结构,与已报道的来源于Thermoanaerobacterium thermosulfurigenes的环糊精葡萄糖基转移酶(Cyclodextrin-glycosyltransferase,CGTase,EC 2.4.1.19)同源,该酶分子量为78.4 kD,属于GH 13家族(α-淀粉酶家族)。以知母皂苷BII为底物,Toruzyme 3.0L最佳反应pH值为8.0,在pH 4～10范围内相对稳定;最佳反应温度为100℃,在100℃煮沸6 h后仍然能保持60%以上的活性;在最佳反应温度100℃时,反应速度很快,即使使用很少的酶液,也无法检测出该酶的反应速度;在60℃下10min达到最大催化活性。另外,本论文还发现α-淀粉酶和CGTase这些GH 13家族的酶均可以利用非活化糖源供体催化知母皂苷BII发生糖基化反应,而其他种类的淀粉酶,如γ-淀粉酶却没有这样的活性。初步探讨,该酶催化发生糖基化反应的机理可能是:首先水解含有α-1,4葡萄糖苷键的糖源,释放出被激活的葡萄糖基,然后这种葡萄糖基被随机的加成到知母皂苷BII糖链的末端葡萄糖基上。同时还首次发现了黑曲霉Aspergillus niger AS 3.0739的胞外酶在pH 8.0时可以选择性水解知母皂苷BII的糖链;荧光假单胞菌Pseudomonas fluorescens全细胞培养可以转化知母皂苷BII生成相应的琥珀酸酯。这些研究不仅丰富了甾体皂苷类化合物库,为进一步活性筛选及知母皂苷BII构效关系的研究奠定了基础;同时为今后指导甾体皂苷类化合物的定向结构修饰提供了理论依据。更多还原

【Abstract】 Timosaponin BII, a furostanol saponin purified from Anemarrhena asphodeloides Bunge, can significantly improve the learning and memory abilities, up-regulate the number of nAChRs and increase the rCBF (regional cerebral blood flow) of rat in our previous studies. Recently Timosaponin BII has been studying as a new candidate for anti-vascular dementia and anti-alzheimer’s disease drug. In order to find new target compounds that having high activity and low toxicity with similar structure to timosaponin BII for structure-activity relationship, we have been looking for an effective method to modify structural of this compound. Biotransformation is a biochemical reaction to modify the structure of the xenobiotics by vegetal celluar or organ, animal cellular, microorganism and its cellular, and isolated enzyme, which is mainly enzyme-catalyzed reaction. Advantages often associated with biotransformation are pronounced exquisite chemoselectivity, regioselectivity, stereoselectivity, less by-product, and easy to operate under mild conditions. Biotransformation is a technology which has potentially application and market value for widening diversity of natural product, searching for lead compounds, enhancing the sustainability of the rare species of natural resources, improving preparation efficiency and reducing the costs. However, there are not many reports on biotransformation of steroidal saponin and its relevant research, especially on the hydrolysis and glycosylation at C3-sugar chain.In this thesis, timosaponin BII as substrate, over 40 enzymes and a hundred microorganisms were screened, and an enzyme (Toruzyme 3.0L) and two microorganisms (Aspergillus niger AS 3.0739, Pseudomonas fluorescens) were found to be able to transform BII into its corresponding derivatives.Toruzyme 3.0L, a commercial CGTase from Thermoanaerobacter sp., was found that it could synthesize glucosylation derivatives for timosaponin BII. Nine products (Product 1～9) with different degrees of glucosylation were purified and their structures were elucidated on the basis of 13C-NMR, HR-ESI-MS and FAB-MS spectra data. In this work, we found that Toruzyme 3.0L was able to use non-activated sugar donor to synthesize glucosylation derivatives, and showed a high thermal tolerance with the most favorable enzymatic activity at 100℃.Aspergillus niger AS 3.0739, a common strain producing glucoamylase, was found to had the ability of hydrolyzing C-22 hydroxyl of timosaponin BII in whole-cell culture to generate timosaponin B (Product B1). While its crude enzymes could selectively hydrolyze the glycosyl groups of C-3 sugar chain at pH 8.0 to generate two de-glycosyl furostanol saponins (Product H1, H2), and then hydrolyze the glucosyl group at C-26 position of timosaponin BII to generate one de-glycosyl spirostanol saponin (timosaponin AIII, Product H4). The structure of Product H1 was determined to be (25S)-26-O-β-D-glucopyranosyl-22, 3-hydroxy-5β-furostside. The structure of Product H2 was determined to be (25S)-26-O-β-D-glucopyranosyl-22-hydroxy-5β-furost-3β, 26-diol-3-O-β-D-galactopyranoside;A Pseudomonas fluorescens (idenfied by China Pharmacy Microbial Culture Collection, CPCC), isolated from a unknown microorganism, incubated with BII to produce product with bigger polarity than that of BII product (Product H3). The big polarity product was separated and identified as (25S)-26-O-succinate-(1→6)-β- D-glucopyranosyl-22-hydroxy-5β-furostanol-3β,26-diol-3-O-β-D-glucopyranosyl-(1→2)-β-D-galactopyranoside based on the data of spectra.Whereas the specificity of toruzyme 3.0L with high efficiency and thermostability for glucosylation in timosaponin BII and understanding its protein structure, the specific enzyme was isolated to electrophoretic homogeneity by Gel filtration (S-200 HR), Anion-exchange (Q-HP) and Cation-exchange chromatography (SP-HP). After SDS-PAGE analysis, the protein was detected as a single band. The purified protein band on SDS-PAGE separation was submitted to the MS Laboratory, Center for Research of Proteome (Beijing, China), for protein sequencing. After the protein was digested with trypsin and the peptides were separated by the AB 4800 Plus MALDI-TOF/TOF? Proteomics Analyzer and Q-TOF2 (Waters Micromass, USA). These protein sequences were compared by the Basic Local Alignment Search Tool of National Center for Biotechnology Information (NCBI) and displayed the highest similarity to the Cyclodextrin-glycosyltransferase (CGTase, EC 2.4.1.19) from Thermoanaerobacterium thermosulfurigenes. The molecular mass of the protein was 78.4 kDa. The characteristics of purified enzyme was systematically investigated, and its optimal pH value was 8.0, and it had a very broad pH activity range; the optimum temperature for maximal enzyme activity was detected at 100°C. The enzyme showed a high thermal stability. After 6 h boiling, it still retained more than 60% of maximum activity.It is the found that theα-amylases and CGTase, ie. GH13 family enzyme could catalyze the glucosylation of steroidal saponin in one step. But, other kinds of amylases, such asγ-amylase (GH15 family), had no such the activity under the same reaction conditions. The mechanism of transglycosylation was hypothesized as follows: sugar-donor (dextrin) was first hydrolyzed to an activated glucose, and then the activated glucose was attached to the glucose-acceptor. We also first discovered that crude enzymes from Aspergillus. niger AS 3.0739 could selectively hydrolyze BII into its deglycosyl derivatives at pH 8.0. The whole-cell culture of Pseudomonas fluorescens could synthesize succinate derivatives in timosaponin BII. These studies not only enriched compound libraries of steroidal saponins with structural diversity, but also established a foundation for the research of structure-function relationship of timosaponin BII and guiding the transformation of steroidal saponins.更多还原