基于FFPE组织切片的膀胱癌N-连接糖链原位酶解及分析

目的 研究膀胱癌FFPE组织切片的N-连接糖链,发现膀胱癌FFPE肿瘤组织的异常N-连接糖链修饰情况。方法 发展基于FFPE组织切片原位提取N-连接糖链的实验流程。通过PNGase F酶切FFPE组织解释放N-连接糖链。对N-连接糖链自由端进行全甲基化修饰。通过MALDI-TOF/TOF-MS检测N-连接糖链的相对含量。进行数据库匹配,确定N-连接糖链的可能糖型。ROC分析用于预测显著差异N-连接糖链作为预测膀胱癌生物标志物的准确度。结果 MALDI-TOF/TOF-MS检测泛甲基化修饰N-连接糖链的数据显示,在16例膀胱癌患者的肿瘤和癌旁组织的3次重复实验中,肿瘤组织中蛋白质高甘露糖型N2H6、N2H7、N2H8、N2H9和复杂型N5H6F1糖链修饰水平显著上升,同时高甘露糖型N2H5、杂合型N3H5以及复杂型N3H4、N4H4、N5H6F1S2糖链修饰水平显著下降。ROC分析显示,双天线型N-连接糖链N3H4（AUC=0.90）和N4H4（AUC=0.91）在单独或者共同区分膀胱癌患者肿瘤组织和癌旁组织中都具有很好的可靠性,可能成为膀胱癌的潜在生物标志物。结论 膀胱癌FFPE肿瘤组织中存在蛋白质异常N-糖基化修饰,N-连接糖链N3H4和N4H4或可成为膀胱癌的潜在生物标志物。     

膀胱癌中唾液酸酶Neu1的异常表达对Toll样受体的影响

哺乳动物细胞内的某些蛋白质或脂类可以被糖基化修饰,而糖链末端往往存在唾液酸化的现象,催化添加唾液酸的酶为糖基转移酶(sialyltransferase,ST),而去除唾液酸的为唾液酸酶(sialidase,SA或称为neuraminidase,NEU).本实验检测了人膀胱正常上皮细胞HCV29、非浸润性膀胱癌细胞KK47和浸润性膀胱癌细胞YTS-1中唾液酸的表达,发现恶性肿瘤细胞中唾液酸的含量高于正常细胞;进一步分析唾液酸酶和唾液酸转移酶的表达,发现唾液酸酶Neu1在正常细胞中表达最高,在良性肿瘤细胞中次之,在恶性肿瘤细胞中表达最低,推测在膀胱癌中Neu1对唾液酸的异常表达起着主要作用.同时,膀胱癌细胞中Toll样受体1,2,3,4(toll-like receptors,TLRs)表达趋势也与Neu1一致.利用TGF-β处理HCV29,使之发生上皮间质转化(epithelial-mesenchymal transition,EMT),细胞中Neu1和TLR3表达明显减少;将Neu1基因沉默后,TLR3表达也明显减少.此外,在YTS-1细胞中过表达Neu1,TLR3表达增高且激活了下游NF-κB通路.这一结果说明膀胱癌中Neu1与TLR3的表达有着密切的关系,这为膀胱癌的分子机理研究提供了工作基础.     

胃癌相关糖蛋白糖链谱的研究进展

胃癌是一类高发病率和高死亡率的恶性肿瘤.研究表明,癌前感染与胃癌的发生发展过程始终伴随着蛋白糖基化的异常.例如在癌前感染阶段糖蛋白糖链发挥的作用:在感染阶段,幽门螺杆菌(Helicobacter pylori,H.pylori)吸附导致的唾液酸化路易斯X抗原的上调,增强了H.pylori的吸附作用使其在胃部定殖并诱发持续的炎症反应;在慢性胃炎和肠上皮化生阶段唾液酸化的Tn抗原表达上调.胃癌发生发展过程中涉及到了血清、组织、细胞中的蛋白糖基化的改变,如核心岩藻糖基化N-糖链表达的下调,β1,6-连接的N-乙酰葡糖胺分支型N-糖链的增加,以及细胞黏附分子糖基化的改变.本文综述了胃癌相关糖蛋白糖链研究的最新进展,阐述了糖基化在胃癌的发生发展中发挥的重要作用及其作为胃癌早期生物标志物与药物靶点的潜在临床应用价值.     

糖组学在乳腺癌中的研究进展

乳腺癌仍是备受关注的世界性健康问题,在女性人群中有较高的发病率和致死率.蛋白质的糖基化修饰是一种重要的翻译后修饰,糖基化的改变已被证明与生物学过程密切相关.异常表达的糖链是肿瘤细胞的主要特征之一,对肿瘤的发生、发展,特别是癌细胞的浸润、转移起重要作用.本文从参与调节肿瘤细胞代谢,破坏细胞间黏着,在循环系统中维持肿瘤细胞活性、增强癌细胞与血管内皮细胞黏着及促进血管生成等方面系统阐述了O-连接型N-乙酰葡糖胺、唾液酸化的Lewis抗原、黏蛋白型O-聚糖及包含多聚N-乙酰乳糖胺的β1-6GlcNAc分支型N-糖链等几种异常表达糖链在肿瘤细胞浸润、转移过程中的作用.最后,本文从糖组学角度讨论了与肿瘤相关的异常表达的糖链、糖蛋白、糖基转移酶及针对糖抗原的抗体在乳腺癌临床中的应用前景.     

基于超滤膜辅助的糖蛋白全N-连接糖链的富集和质谱解析

糖基化作为一种常见的蛋白质翻译后修饰,对蛋白质的空间结构、生物功能等具有重要的影响.解析糖蛋白糖链结构有助于更清楚地认识糖蛋白及其功能.本研究建立了一种基于超滤膜富集血清中糖蛋白全N-连接糖链,并利用质谱技术对糖链结构进行分析的方法.根据糖蛋白及其糖链结构之间的分子质量差异,利用Millipore公司的10 ku超滤膜富集血清糖蛋白上酶解(PNGase F)释放的全N-连接糖链,并使用MALDI-TOF/TOF-MS解析糖链结构.通过该技术可以从血清中富集并鉴定到23种独特的N-连接的糖链结构,并且利用二级质谱进行了结构确认.该方法可以被用于从大量生物样本中富集糖蛋白全N-连接糖链,可以达到快速、高通量地解析糖蛋白N-连接糖链的目的.     

糖基转移酶超家族在肿瘤转移中的作用

蛋白质的糖基化修饰主要包括N-连接糖基化、O-连接糖基化和糖基磷脂酰肌醇锚定连接.与核酸和蛋白质不同,糖链的合成过程并不遵循传统的基因信息传递的中心法则,主要由一系列催化糖苷键形成的糖基转移酶完成.异常糖基化修饰被认为与恶性肿瘤的发生发展和临床预后密切相关.研究表明,糖基转移酶的表达及其糖链结构的异常可通过调节肿瘤细胞与细胞外基质的相互作用,继而影响肿瘤转移的关键步骤,如上皮间质转化(E-钙黏着蛋白、N-钙黏着蛋白)、细胞的移动性(整合素β1和α5)、侵袭(基质金属蛋白酶MMPs)、浸润(唾液酸化Lewis抗原sLeX和sLeA).本文主要就唾液酰基转移酶、岩藻糖基转移酶和N-乙酰氨基葡萄糖转移酶等三大糖基转移酶家族的结构和生物学功能及其在肿瘤转移中的作用作一综述,以期为肿瘤转移的预测和诊断提供新思路.     

细胞膜表面糖链结构与肿瘤转移的关系

连接在天冬酰胺上的N-糖链,连接在丝氨酸和苏氨酸上的O-糖链,连接在丝氨酸上的糖胺聚糖,连接在脂类物质上的糖脂等糖基化修饰在真核细胞表面普遍存在,并调节了细胞的各种功能,它们不仅参与生命活动中正常的生理生化活动,而且于疾病的发生发展密不可分。肿瘤细胞表面糖基异常化,如糖链在表达水平上的差异以及特殊糖链结构的出现,均与肿瘤细胞的侵袭和转移有密切关系。本综述主要介绍了肿瘤细胞糖基化的改变:一些主要的糖基化结构如β1,6分支和唾液酸的表达的增加会使某些肿瘤细胞的迁移能力增强。同时对引起这种变化的作用机制进行了介绍,提出一些潜在的抗肿瘤研究靶点,为肿瘤糖生物学的深入研究提供参考。     

应用凝集素芯片分析癌细胞膜表面糖链表达

细胞膜表面糖复合物的糖链结构与肿瘤细胞增殖、侵染、转移等发展过程密切相关.凝集素芯片技术的出现实现了对癌症的糖组进行快速、高通量的检测.通过模式细胞系PANC-1证明了构建的凝集素芯片体系的准确性、重复性、特异性,应用这一芯片体系初步检测了几种癌细胞系(HT-29、SGC-7901、BEL-7402、H460)的膜表面糖链表达.这几种癌细胞系表面都有唾液酸、乙酰葡萄糖/葡萄糖、乙酰半乳糖/半乳糖、甘露糖等糖链.根据实验结果,推测它们的细胞膜表面α1-6岩藻糖链表达水平可能较高,而α1-3岩藻糖链表达水平较低;这些聚糖可能是癌症潜在的标志物.凝集素芯片有助于推动癌细胞膜表面糖链的快速分析和筛选出癌症相关的糖链标志物.     

综述: 糖基转移酶超家族在肿瘤转移中的作用

蛋白质的糖基化修饰主要包括N-连接糖基化、O-连接糖基化和糖基磷脂酰肌醇锚定连接．与核酸和蛋白质不同,糖链的合成过程并不遵循传统的基因信息传递的中心法则,主要由一系列催化糖苷键形成的糖基转移酶完成．异常糖基化修饰被认为与恶性肿瘤的发生发展和临床预后密切相关．研究表明,糖基转移酶的表达及其糖链结构的异常可通过调节肿瘤细胞与细胞外基质的相互作用,继而影响肿瘤转移的关键步骤,如上皮间质转化(E-钙黏着蛋白、N-钙黏着蛋白)、细胞的移动性(整合素β1和α5)、侵袭(基质金属蛋白酶MMPs)、浸润(唾液酸化Lewis抗原sLeX和sLeA)．本文主要就唾液酰基转移酶、岩藻糖基转移酶和N-乙酰氨基葡萄糖转移酶等三大糖基转移酶家族的结构和生物学功能及其在肿瘤转移中的作用作一综述,以期为肿瘤转移的预测和诊断提供新思路．     

基于超滤膜辅助的糖蛋白全O-连接糖链的富集和MALDI-TOF/TOF质谱结构解析

哺乳动物中约有50%以上的蛋白质都发生了糖基化修饰.连接在丝氨酸或苏氨酸上的O-连接糖链是常见的蛋白质糖基化修饰方式之一,其主要功能是维持与其连接的蛋白质部分的空间构象,保护其免受蛋白酶水解及覆盖某些抗原决定簇.糖链结构的解析有助于更清楚地认识糖蛋白及其功能.本研究建立了一种基于超滤膜辅助(FASP)富集细胞、血清和尿液中糖蛋白全O-连接糖链的方法,根据糖蛋白与其糖链结构之间的分子质量差异,利用10 KD超滤膜富集蛋白质样品中由β消除反应释放的全O-连接糖链,将糖链甲基化修饰后再使用MALDI-TOF/TOF-MS进行解析,同时利用二级质谱进行结构确认.通过上述方法可从标准糖蛋白mucin、细胞、血清和尿液样本中分别鉴定到83、29、33和85种O-连接糖链结构,利用该方法可以从复杂样品中富集和解析糖蛋白全O-连接糖链,实现快速、高效、高通量地解析糖蛋白O-连接糖链的目的.     

Twoplex 12/13C6 aniline stable isotope and linkage‐specific sialic acid labeling 2D‐LC‐MS workflow for quantitative N‐glycomics

Quantitative glycomics represents an actively expanding research field ranging from the discovery of disease‐associated glycan alterations to the quantitative characterization of N‐glycans on therapeutic proteins. Commonly used analytical platforms for comparative relative quantitation of complex glycan samples include MALDI‐TOF‐MS or chromatographic glycan profiling with subsequent data alignment and statistical evaluation. Limitations of such approaches include run‐to‐run technical variation and the potential introduction of subjectivity during data processing. Here, we introduce an offline 2D LC‐MS^E workflow for the fractionation and relative quantitation of twoplex isotopically labeled N‐linked oligosaccharides using neutral ¹²C₆ and ¹³C₆ aniline (Δmass = 6 Da). Additional linkage‐specific derivatization of sialic acids using 4‐(4,6‐dimethoxy‐1,3,5‐trizain‐2‐yl)‐4‐methylmorpholinium chloride offered simultaneous and advanced in‐depth structural characterization. The potential of the method was demonstrated for the differential analysis of structurally defined N‐glycans released from serum proteins of patients diagnosed with various stages of colorectal cancer. The described twoplex ¹²C₆/¹³C₆ aniline 2D LC‐MS platform is ideally suited for differential glycomic analysis of structurally complex N‐glycan pools due to combination and analysis of samples in a single LC‐MS injection and the associated minimization in technical variation.     

Glycan reductive isotope labeling for quantitative glycomics

Many diseases and disorders are characterized by quantitative and/or qualitative changes in complex carbohydrates. Mass spectrometry methods show promise in monitoring and detecting these important biological changes. Here we report a new glycomics method, termed glycan reductive isotope labeling (GRIL), where free glycans are derivatized by reductive amination with the differentially coded stable isotope tags [¹²C₆]aniline and [¹³C₆]aniline. These dual-labeled aniline-tagged glycans can be recovered by reverse-phase chromatography and can be quantified based on ultraviolet (UV) absorbance and relative ion abundances. Unlike previously reported isotopically coded reagents for glycans, GRIL does not contain deuterium, which can be chromatographically resolved. Our method shows no chromatographic resolution of differentially labeled glycans. Mixtures of differentially tagged glycans can be directly compared and quantified using mass spectrometric techniques. We demonstrate the use of GRIL to determine relative differences in glycan amount and composition. We analyze free glycans and glycans enzymatically or chemically released from a variety of standard glycoproteins, as well as human and mouse serum glycoproteins, using this method. This technique allows linear relative quantitation of glycans over a 10-fold concentration range and can accurately quantify sub-picomole levels of released glycans, providing a needed advancement in the field of glycomics.     

A study of lipid- and protein- bound sialic acids for the diagnosis of bladder cancer and their relationships with the severity of malignancy

Background:

The gold standard for detection of bladder cancer is cystoscopy, which is an invasive and complicated procedure. Our study was conducted to find a tumor marker with high specificity, sensitivity, and accuracy for the diagnosis of bladder cancer.

Methods:

Serum samples were collected from 58 bladder cancer patients and 60 healthy control subjects. Levels of lipid-bound sialic acid (LBSA), and protein-bound sialic acid (PBSA) were measured spectrophotometrically by Aminoff’s method.

Results:

Mean levels of both markers were found to be significantly higher in the patients than the healthy controls. Positive correlations were observed between serum levels of lipid- (r=0.283, p<0.05) and protein- bound (r=0.56, p<0.05) sialic acids and the grade of malignancy. To differentiate patients with bladder tumors from healthy controls, cut-offpoints were determined for each of the two parameters based on Receiver Operating Characteristic (ROC) curve analysis (LBSA=21.25 mg/dL, PBSA=6.15 mg/dL). The data showed good sensitivities (LBSA=89%, PBSA=79%), specificities (LBSA=70%, PBSA=70%) and accuracies (LBSA=83%, PBSA=81%) for both markers.

Conclusion:

Measuring serum LBSA and PBSA by this simple, reproducible, noninvasive, and inexpensive method can accurately discriminate cancer patients from healthy individuals. Key Words: Urinary Bladder Neoplasms, N-Acetylneuraminic Acid, Tumor Markers     

Oligosaccharide-derivatized dendrimers: defined multivalent inhibitors of the adherence of the cholera toxin B subunit and the heat labile enterotoxin of E. coli to GM1

Poly(propylene imine) dendrimers having four or eight primary amino groups and a Starburst^TM (PAMAM) dendrimer having eight primary amino groups were used as core molecules, to which phenylisothiocyanate derivatized (PITC) galβ1-3galNAcβ1-4[sialic acidβ2-3]-galβ1-4glc (oligo-GM1) residues were covalently attached to yield multivalent oligosaccharides. The synthesis of the oligo-GM1-PITC derivatized dendrimers was monitored using high performance thin layer chromatography, infrared spectroscopy, sialic acid content, and mass spectroscopy. The ability of multivalent oligo-GM1-PITC dendrimers to inhibit the binding of ¹²⁵I-labeled cholera toxin B subunit and the heat labile enterotoxin of E. coli to GM1-coated microtiter wells was determined. IC₅₀s obtained for the oligo-GM1-PITC dendrimers, GM1, and the oligosaccharide moiety of GM1 indicated that the derivatized dendrimers inhibited binding of the choleragenoid and the heat labile enterotoxin to GM1-coated wells at a molar concentration five- to 15-fold lower than native GM1 and more than 1,000-fold lower than that of the free oligosaccharide. This revised version was published online in November 2006 with corrections to the Cover Date.     

A simple route to [11C]N-Me labeling of aminosuberic acid for proof of feasibility imaging of the xC− transporter

Oxidative stress has been implicated in a variety of conditions, including cancer, heart failure, diabetes, neurodegeneration and other diseases. A potential biomarker for oxidative stress is the cystine/glutamate transporter, system x_C⁻. l-Aminosuberic acid (l-ASu) has been identified as a system x_C⁻ substrate. Here we report a facile method for [¹¹C]N-Me labeling of l-ASu, automation of the radiochemical process, and preliminary PET imaging with EL4 tumor bearing mice. The results demonstrate uptake in the tumor above background, warranting further studies on the use of radiolabeled analogs of l-ASu as a PET imaging agent for system x_C⁻.     

Stable isotopic labeling‐based quantitative targeted glycomics (i‐QTaG)

Mass spectrometry (MS) analysis combined with stable isotopic labeling is a promising method for the relative quantification of aberrant glycosylation in diseases and disorders. We developed a stable isotopic labeling‐based quantitative targeted glycomics (i‐QTaG) technique for the comparative and quantitative analysis of total N‐glycans using matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS). We established the analytical procedure with the chemical derivatizations (i.e., sialic acid neutralization and stable isotopic labeling) of N‐glycans using a model glycoprotein (bovine fetuin). Moreover, the i‐QTaG using MALDI‐TOF MS was evaluated with various molar ratios (1:1, 1:2, 1:5) of ¹³C₆/¹²C₆‐2‐aminobenzoic acid‐labeled glycans from normal human serum. Finally, this method was applied to direct comparison of the total N‐glycan profiles between normal human sera (n = 8) and prostate cancer patient sera (n = 17). The intensities of the N‐glycan peaks from i‐QTaG method showed a good linearity (R² > 0.99) with the amount of the bovine fetuin glycoproteins. The ratios of relative intensity between the isotopically 2‐AA labeled N‐glycans were close to the theoretical molar ratios (1:1, 1:2, 1:5). We also demonstrated that the up‐regulation of the Lewis antigen (～82%) in sera from prostate cancer patients. In this proof‐of‐concept study, we demonstrated that the i‐QTaG method, which enables to achieve a reliable comparative quantitation of total N‐glycans via MALDI‐TOF MS analysis, has the potential to diagnose and monitor alterations in glycosylation associated with disease states or biotherapeutics. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:840–848, 2015     

1H and 13C NMR assignments for the glycans in glycoproteins by using 2H/13C-labeled glucose as a metabolic precursor

In order to understand the role of the glycans in glycoproteins in solution, structural information obtained by NMR spectroscopy is obviously required. However, the assignment of the NMR signals from the glycans in larger glycoproteins is still difficult, mainly due to the lack of appropriate methods for the assignment of the resonances originating from the glycans. By using [U-¹³C₆,²H₇]glucose as a metabolic precursor, we have successfully prepared a glycoprotein whose glycan is uniformly labeled with ¹³C and partially with D at the sugar residues. The D to H exchange ratios at the C1-C6 positions of the sugar residues have been proven to provide useful information for the spectral assignments of the glycan in the glycoprotein. This is the first report on the residue-specific assignment of the anomeric resonances originating from a glycan attached to a glycoprotein by using the metabolic incorporation of hydrogen from the medium into a glycan labeled with [U-¹³C₆,²H₇]glucose.     

The role of light in soybean seed filling metabolism

Soybean (Glycine max) yields high levels of both protein and oil, making it one of the most versatile and important crops in the world. Light has been implicated in the physiology of developing green seeds including soybeans but its roles are not quantitatively understood. We have determined the light levels reaching growing soybean embryos under field conditions and report detailed redox and energy balance analyses for them. Direct flux measurements and labeling patterns for multiple labeling experiments including [U‐¹³C₆]‐glucose, [U‐¹³C₅]‐glutamine, the combination of [U‐¹⁴C₁₂]‐sucrose + [U‐¹⁴C₆]‐glucose + [U‐¹⁴C₅]‐glutamine + [U‐¹⁴C₄]‐asparagine, or ¹⁴CO₂ labeling were performed at different light levels to give further insight into green embryo metabolism during seed filling and to develop and validate a flux map. Labeling patterns (protein amino acids, triacylglycerol fatty acids, starch, cell wall, protein glycan monomers, organic acids), uptake fluxes (glutamine, asparagine, sucrose, glucose), fluxes to biomass (protein amino acids, oil), and respiratory fluxes (CO₂, O₂) were established by a combination of gas chromatography‐mass spectrometry, ¹³C‐ and ¹H‐NMR, scintillation counting, HPLC, gas chromatography‐flame ionization detection, C:N and amino acid analyses, and infrared gas analysis, yielding over 750 measurements of metabolism. Our results show: (i) that developing soybeans receive low but significant light levels that influence growth and metabolism; (ii) a role for light in generating ATP but not net reductant during seed filling; (iii) that flux through Rubisco contributes to carbon conversion efficiency through generation of 3‐phosphoglycerate; and (iv) a larger contribution of amino acid carbon to fatty acid synthesis than in other oilseeds analyzed to date.     

Glycomic profiling of carcinoembryonic antigen isolated from human tumor tissue

Background

Carcinoembryonic antigen (CEA) is a protein commonly found in human serum, with elevated CEA levels being linked to the progression of a wide range of tumors. It is currently used as a biomarker for malign tumors such as lung cancer and colorectal cancer [Urol Oncol: Semin Orig Invest 352: 644–648, 2013 and Lung Cancer 80: 45-49, 2013]. However, due to its low specificity in clinical applications, CEA can be used for monitoring only, rather than tumor diagnosis. The function of many glycoproteins is critically dependent on their glycosylation pattern, which in turn has the potential to serve in tumor detection. However, little is known about the detailed glycan patterns of CEA.

Methods

To determine these patterns, we isolated and purified CEA proteins from human tumor tissues using immunoaffinity chromatography. The glycan patterns of CEA were then analyzed using a Matrix-Assisted Laser Desorption/Ionization-Time of Flight-Mass Spectrometry³ (MALDI-TOF-MS³) approach.

Results

We identified 61 glycoforms in tumor tissue, where CEA is upregulated. These glycosylation entities were identified as bi-antennary, tri-antennary and tetra-antennary structures carrying sialic acid and fucose residues, and include a multitude of glycans previously not reported for CEA.

Conclusion

Our findings should facilitate a more precise tumor prediction than currently possible, ultimately resulting in improved tumor diagnosis and treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-015-9088-3) contains supplementary material, which is available to authorized users.     

The repair of the surface structure of animal cells

Experiments were performed to determine if animal cells in culture possess specific mechanisms to repair surface molecules damaged by enzymes. The surface membranes of a primary cell culture, chick fibroblasts, a permanent hamster cell line, BHK₂₁/C₁₃, and its virally transformed counterpart, C₁₃/B₄ were damaged by exposure to trypsin or to neuraminidase. Following digestion with trypsin, the incorporation of radioactive amino acids or sugars into purified surface membrane of cells was monitored. No differences were noted in rates of incorporation when control and trypsin-damaged cells were compared. Neuraminidase damage to the surface of BHK₂₁/C₁₃ and C₁₃/B₄ cells was evidenced by altered gel filtration profiles of surface glycopeptides, i.e., delayed elution because of reduction in size. By labelling cells with ¹⁴C-L-fucose prior to neuraminidase treatment and following the incorporation of ³H-L-fucose into cell surface glycopeptides after neuraminidase digestion, we were able to monitor the synthesis and turnover of fucose-containing glycopeptides in the same cells. Gel filtration profiles indicated that little or no desialylated glycoproteins were resialylated (repaired) by specific replacement of sialic acid. Comparing neuraminidase-digested and control cells we observed no difference in rates of ³H-L-fucose incorporation or of ¹⁴C-L-fucose loss from these cells; nor did we find differences in the rate of incorporation of isotopic glucosamine into sialic acid. Neuraminidase treatment failed to alter the rate of cell growth or the pattern of isotopic incorporation into various cell surface components. These results support the suggestion that return of sialic acid (repair) was effected by turnover which serves as a non-specific repair mechanism to replace damaged cell surface molecules (Warren and Glick '68; Warren, '69).