A New Strategy of Using O18-Labeled Iodoacetic Acid for Mass Spectrometry-Based Protein Quantitation

A new O(18) labeling protocol is designed to assist quantitation of cysteine-containing proteins using LC/MS. Unlike other O(18) labeling strategies, the labeling is carried out at the intact protein level (prior to its digestion) during reduction/alkylation of cysteine side chains using O(18)-labeled iodoacetic acid (IAA). The latter can be easily prepared by exchanging carboxylic oxygen atoms of commercially available IAA in O(18)-enriched water at low pH. Since incorporation of the O(18) label in the protein occurs at the whole protein, rather than peptide level, the quantitation results are not peptide-dependent. The excellent stability of the label in mild pH conditions provides flexibility and robustness needed of sample processing steps following the labeling. In contrast to generally costly isotope labeling reagents, this approach uses only two relatively inexpensive commercially available reagents (IAA and H(2)O(18)). The feasibility of the new method is demonstrated using an 80?kDa human serum transferrin (hTf) as a model, where linear quantitation is achieved across a dynamic range spanning three orders of magnitude. The new approach can be used in quantitative proteomics applications and is particularly suitable for a variety of tasks in the biopharmaceutical sector, ranging from pharmacokinetic studies to quality control of protein therapeutics.     

18O labeling: a tool for proteomics.

An evaluation of the proteolytic labeling and quantification of proteins for diagnostic purposes using trypsin and 18O-enriched H2O is presented. We demonstrate that comparative or relative quantitation can be performed effectively with this approach. We have developed a protocol that allows the conservation of the labeled peptides in natural abundance water without fear of back-exchange providing that pH is sufficiently low to quench the catalytic activity of trypsin, but not so low as to promote chemical back-exchange. Because the labeling efficiency depends on the nature of the peptide, a simple linear relationship between the relative 16O/18O digest buffer mixture content (x) and labeling efficiency (y) does not exist; rather it follows a probability based y = x(2) relationship. As such, the extent of peptide labeling using 16O/18O digest buffer mixture ratios may deviate significantly from that expected based on a linear relationship. The evaluation of the relative Ziptip efficiency indicated a loss in sample recovery as the peptide concentration was reduced using normal conditions, suggesting that there is a limit below which there are diminishing returns. In addition, the adsorptive losses due to Speedvac dry down and recovery indicated modest (20%) losses that may vary widely (0-50%) from peptide to peptide. The in-solution digestion efficiency of standard protein mixtures as a function of concentration revealed a linear decrease with decreasing concentration. This is consistent with enzyme kinetic effects and emphasizes a potential quantitation error that could arise when evaluating differential expression based on peptide detection. The results from our studies demonstrate the power of 18O labeling as an optimization tool for proteomics process development.     

18O同位素标记定量肽段串联体蛋白质结合同位素稀释-多反应监测质谱的蛋白质绝对定量新方法

建立了定量肽段串联体蛋白质(concatamers of Q peptides, QconCATs)结合¹⁸O同位素标记-多反应监测质谱的蛋白质绝对定量新方法。首先对QconCAT重组蛋白质进行了纯度表征,十二烷基硫酸钠-聚丙烯酰胺凝胶电泳(SDS-PAGE)表征结果表明重组蛋白质的纯度在99%以上,相对分子质量约为63.4 kDa。对QconCAT重组蛋白质酶切后的肽段混合物进行质谱分析,并经pFind和pLabel软件处理,验证了目标肽段。还考察了QconCAT重组蛋白质的酶切效率和¹⁸O标记效率,并对QconCAT蛋白质结合¹⁸O标记-同位素稀释-多反应监测质谱方法进行了评价。实验结果表明,采用该方法对腾冲嗜热厌氧菌(Thermoanaerobacter tengcongensis, TTE)中选定蛋白质的肽段进行绝对含量测定时,相对标准偏差小于20%,准确度较高,说明该方法可用于复杂生物样本中蛋白质的绝对定量。更重要的是所建方法不仅解决了细胞培养氨基酸稳定同位素标记(SILAC)技术的重标试剂价格昂贵的问题,也为定量蛋白质组学提供了一种新的方法。     

Exploring human plasma proteome strategies: high efficiency in-solution digestion protocol for multi-dimensional protein identification technology

Multi-dimensional protein identification technology (MudPIT) is becoming a prevalent proteomic approach due to its high-throughput separations and accurate mass detection. Prior to MudPIT analysis, complicated samples required in-solution digestion. Unlike in-gel digestion, in which enzymes work on just a few proteins, in-solution digestion involves simultaneous digestion of hundreds or thousands of proteins. In-solution digestion protocols must therefore be very efficient. Few investigations have evaluated the efficiency of in-solution digestion protocols. The present research compared three such protocols. Results suggest that a protocol utilizing trifluoroethanol (TFE) as denaturant is most efficient.     

Selected reaction monitoring (SRM) mass spectrometry without isotope labeling can be used for rapid protein quantification

The validation of putative biomarker candidates has become the major bottle-neck in protein biomarker development. Conventional immunoaffinity methods are limited by the availability of antibodies and kits. Here we demonstrate the feasibility of using selected reaction monitoring (SRM) without isotope labeling to achieve fast and reproducible quantification of serum proteins. The SRM/MRM assays for three standard serum proteins, including ceruloplasmin (CP), serum aymloid A (SAA) and sex hormone binding globulin (SHBG), have good linear ranges, generally 10(3) to 10(4) . There are almost perfect correlations between SRM intensities and the loaded peptide amounts (R(2) is usually ~0.99). Our data suggest that SRM/MRM is able to quantify proteins within the range of 0.2-2 fmol, which is comparable to the commercial ELISA/LUMINEX kits for these proteins. Excellent correlations between SRM/MRM and ELISA/LUMINEX assays were observed for SAA and SHBG (R(2)=0.928 and 0.851, respectively). However, the correlation between SRM/MRM and ELISA for CP is less desirable (R(2)=0.565). The reproducibility for SRM/MRM assays is generally very good but may depend on the proteins/peptides being analyzed (R(2)=0.931 and 0.882 for SAA and SHBG, and 0.723 for CP). The SRM/MRM assay without isotope labeling is a rapid and useful method for protein biomarker validation in a modest number of samples and is especially useful when other assays such as ELISA or LUMINEX are not available.     

Metal labeling for accurate multiplexed peptide quantification via matrix-assisted laser desorption/ionization mass spectrometry

Two peptide quantification strategies, the isobaric tags for relative or absolute quantitation (iTRAQ) labeling methodology and a metal-chelate labeling approach, were compared using matrix-assisted laser desorption/ionization-TOF/TOF MS and MS/MS analysis. Amino- and cysteine-directed labeling using the rare earth metal chelator 1,4,7,10-tetraazacyclododecane-N,N′,N″,N″′-tetraacetic acid (DOTA) were applied for relative quantification of single peptides and a six-protein mixture. For analyte ratios close to one, iTRAQ and amino-directed DOTA labeling delivered overall comparable results regarding accuracy and reproducibility. In contrast, the MS-based quantification via amino-directed lanthanide-DOTA tags was more accurate for analyte ratios ≥5 and offered an extended dynamic range of three orders of magnitude. Our results show that the amino-directed DOTA labeling is an alternative relative quantification tool offering advantages like flexible multiplexing possibilities and, in particular, large dynamic ranges, which should be useful in sophisticated, targeted issues, where the accurate determination of extremely different protein or peptide concentration becomes relevant.     

Comparison of LFQ and IPTL for Protein Identification and Relative Quantification

(1) Background: Mass spectrometry-based quantitative proteome profiling is most commonly performed by label-free quantification (LFQ), stable isotopic labeling with amino acids in cell culture (SILAC), and reporter ion-based isobaric labeling methods (TMT and iTRAQ). Isobaric peptide termini labeling (IPTL) was described as an alternative to these methods and is based on crosswise labeling of both peptide termini and MS2 quantification. High quantification accuracy was assumed for IPTL because multiple quantification points are obtained per identified MS2 spectrum. A direct comparison of IPTL with other quantification methods has not been performed yet because IPTL commonly requires digestion with endoproteinase Lys-C. (2) Methods: To enable tryptic digestion of IPTL samples, a novel labeling for IPTL was developed that combines metabolic labeling (Arg-0/Lys-0 and Arg-d4/Lys-d4, respectively) with crosswise N-terminal dimethylation (d4 and d0, respectively). (3) Results: The comparison of IPTL with LFQ revealed significantly more protein identifications for LFQ above homology ion scores but not above identity ion scores. (4) Conclusions: The quantification accuracy was superior for LFQ despite the many quantification points obtained with IPTL.     

Accelerated 18O-labeling in urinary proteomics

Proteolytic (18)O-labeling of peptides has been studied and optimized in order to improve the labeling efficiency and to accelerate the process without increasing the degree of incomplete labeling. Using peptides generated from tryptic digested bovine serum albumin (BSA) and cytochrome c as model proteins, it was shown that complete labeling was achieved after 2 h at pH 6. To increase the sample throughput in a bottom-up proteomic setup, tryptic digestion of proteins in-solution was replaced with tryptic digestion using immobilized trypsin. As a result, an integrated approach was made possible, where both digestion (pH 8) and (18)O/(16)O-labeling of the resulting peptides (pH 6) were done using immobilized trypsin beads. This simplified the sample handling and reduced the overall reaction time significantly: the setup enabled tryptic digestion and (18)O/(16)O-labeling without sample transfer steps within 3.5 h with average (18)O/(16)O-ratios of 0.96±0.13 in aqueous buffer. The initial results were confirmed with a more complex matrix, by spiking urine with the model proteins, yielding results comparable with the ratios obtained in buffer. Satisfying ratios were also achieved regarding urinary proteins identified in a full scale bottom-up experiment. Average (18)O/(16)O-peptide ratios of 0.83±0.13 and 0.91±0.27 indicated good performance in a highly relevant matrix for biomarker discovery.     

Development of an absolute quantification method targeting growth hormone biomarkers using liquid chromatography coupled to isotope dilution mass spectrometry

A method to perform absolute quantification of two biomarkers (IGF-1 and IGFBP-3) of growth hormone abuse has been developed. Isotope dilution is used with synthetically labelled peptides as internal standards. Peptide selection and multiple reaction monitoring design are discussed. A simple sample preparation based on the reduction and alkylation of cysteine residues followed by tryptic digestion provides a sufficient digestion of proteins. Serum samples fortified with increasing amounts of target proteins are analysed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) on a triple quadrupole mass spectrometer. Specificity is ensured by the selection of sequences with no homology in BLAST, as well as retention time deviation check, and ion ratio monitoring. Linearity is studied in terms of calibration curves. These curves for IGFBP-3 and IGF-1 are generated with mean slopes of 0.055 and 0.065, intercepts of 0.107 and -0.011, and with coefficients of correlation of 0.95 and 0.98, respectively. These curves result from the addition of proteins to the serum. Risks of variations related to potential matrix effects are therefore reduced, as well as probable variations related to the digestion steps. The working concentration ranges are 4-10 ng/microl for IGFBP-3 and 2-8 ng/microl for IGF-1. Preliminary data regarding repeatability show that relative standard deviations (RSDs) range between 13 and 32% for IGFBP-3 and between 7 and 29% for IGF-1.     

Comparative quantification and identification of phosphoproteins using stable isotope labeling and liquid chromatography/mass spectrometry

A new liquid chromatography/mass spectrometry (LC/MS) method is described for relative quantification of phosphoproteins to simultaneously compare the phosphorylation status of proteins under two different conditions. Quantification was achieved by beta-elimination of phosphate from phospho-Ser/Thr followed by Micheal addition of ethanethiol and/or ethane-d(5)-thiol selectively at the vinyl moiety of dehydroalanine and dehydroamino-2-butyric acid. The method was evaluated using the model phosphoprotein alpha(S1)-casein, for which three phosphopeptides were found after tryptic digestion. Reproducibility of the relative quantification of seven independent replicates was found to be 11% SD. The dynamic range covered two orders of magnitude, and quantification was linear for mixtures of 0 to 100% alpha(S1)-casein and dephospho-alpha(S1)-casein (R(2) = 0.986). Additionally, the method allowed protein identification and determination of the phosphorylation sites via MS/MS fragmentation.     

Quantitative proteomics for cancer biomarker discovery

The mass spectrometry (MS)-based quantitative proteomics is powerful to discover disease biomarkers that can provide diagnostic, prognostic and therapeutic targets, and it also can address important problems in clinical and translational medical research. The current status of MS-based quantification strategy and technical advances of several main quantitative assays (two-dimensional (2-D) gel-based methods, stable isotope labeling with amino acids in cell culture (SILAC), isotope-coded affinity tag (ICAT), the isobaric tags for relative and absolute quantification (iTRAQ), 1?O labeling, absolute quantitation and label-free quantitation) have been summarized and reviewed. At present, except 2-D gel-based methods, several stable isotope labeling quantitative techniques, including SILAC, ICAT and iTRAQ, etc, have been widely applied in identification of differential expression of proteins, post-translational modifications and protein-protein interactions in order to look for novel candidate cancer biomarkers from different physiological states of cells, body fluids or tissue samples. Also, the advantages and challenges of different quantitative proteomic approaches are discussed in identification and validation of candidate targets.     

A Stable Isotopic Pre-digestion Labeling Method for Protein Quantitative Analysis Using Matrix-assisted Laser Desorption/ionization Mass Spectrometry

As an extension of our previous work, here a strategy was demonstrated for protein identification and quantification analyses utilizing a combination of stable isotope chemical labeling with subsequent denaturation, enzymatic digestion and matrix assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS). Using [d₀]‐ and [d₆]‐4,6‐dimethoxy‐2‐(methylsulfonyl)pyrimidine ([d₀]‐/[d₆]‐DMMSP), stable isotopic labels were incorporated before digestion. The comparative samples were combined before labeling after digestion, thus biases resulting from differences in sample digestion were avoided and the higher accuracy of quantification could be attained. The labeling was spatial‐selective to particular residues of cysteine, lysine, and tyrosine before denaturation, which could lead to a better universality of the strategy for cysteine‐free proteins. In addition, some lysine residues were blocked after labeling, the partly destroyed recognition sites could simplify the trypsin hydrolysates and hence facilitate the MS complexity. Together, our one‐step labeling strategy combined several desirable properties such as spatial‐selective labeling, reliability of quantitative results, simplification of analysis of complex systems and direct analysis with minimum sample handling. Our results demonstrate the usefulness of the method for analyzing lysozyme in egg white. The method was expected to provide a new powerful tool for comparative proteome research.     

Improving off-line accelerated tryptic digestion. Towards fast-lane proteolysis of complex biological samples

Off-line digestion of proteins using immobilized trypsin beads is studied with respect to the format of the digestion reactor, the digestion conditions, the comparison with in-solution digestion and its use in complex biological samples. The use of the filter vial as the most appropriate digestion reactor enables simple, efficient and easy-to-handle off-line digestion of the proteins on trypsin beads. It was shown that complex proteins like bovine serum albumin (BSA) need much longer time (89 min) and elevated temperature (37 degrees C) to be digested to an acceptable level compared to smaller proteins like cytochrome c (5 min, room temperature). Comparing the BSA digestion using immobilized trypsin beads with conventional in-solution digestion (overnight at 37 degrees C), it was shown that comparable results were obtained with respect to sequence coverage (>90%) and amount of missed cleavages (in both cases around 20 peptides with 1 or 2 missed cleavages were detected). However, the digestion using immobilized trypsin beads was considerable less time consuming. Good reproducibility and signal intensities were obtained for the digestion products of BSA in a complex urine sample. In addition to this, peptide products of proteins typically present in urine were identified.     

A nanoporous reactor for efficient proteolysis

A nanoreactor based on mesoporous silicates is described for efficient tryptic digestion of proteins within the mesochannels. Cyano-functionalized mesoporous silicate (CNS), with an average pore diameter of 18 nm, is a good support for trypsin, with rapid in situ digestion of the model proteins, cytochrome c and myoglobin. The generated peptides were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Proteolysis by trypsin-CNS is much more efficient than in-solution digestion, which can be attributed to nanoscopic confinement and concentration enrichment of the substrate within the mesopores. Proteins at concentrations of 2 ng muL(-1) were successfully identified after digestion for 20 min. A biological complex sample extracted from the cytoplasm of human liver tissue was digested by using the CNS-based reactor. Coupled with reverse-phase HPLC and MALDI-TOF MS/MS, 165 proteins were identified after standard protein data searching. This nanoreactor combines the advantages of short digestion time with retention of enzymatic activity, providing a promising way to advance the development of proteomics.     

Mass spectrometry combined with affinity probes for the identification of CP4 EPSPS in genetically modified soybeans

Sample preparation methods used for genetically modified organisms (GMOs) analysis are often time consuming, require extensive manual manipulation, and result in limited amounts of purified protein, which may complicate the detection of low‐abundance GM protein. A robust sample pretreatment method prior to mass spectrometry (MS) detection of the transgenic protein (5‐enolpyruvylshikimate‐3‐phosphate synthase [CP4 EPSPS]) present in Roundup Ready soya is investigated. Liquid chromatography‐multiple reaction monitoring tandem MS (nano LC‐MS/MS‐MRM) was used for the detection and quantification of CP4 EPSPS. Gold nanoparticles (AuNPs) and concanavalin A (Con A)‐immobilized Sepharose 4B were used as selective probes for the separation of the major storage proteins in soybeans. AuNPs that enable the capture of cysteine‐containing proteins were used to reduce the complexity of the crude extract of GM soya. Con A‐sepharose was used for the affinity capture of β‐conglycinin and other glycoproteins of soya prior to enzymatic digestion. The methods enabled the detection of unique peptides of CP4 EPSPS at a level as low as 0.5% of GM soya in MRM mode. Stable‐isotope dimethyl labeling was further applied to the quantification of GM soya. Both probes exhibited high selectivity and efficiency for the affinity capture of storage proteins, leading to the quantitative detection at 0.5% GM soya, which is a level below the current European Union's threshold for food labeling. The square correlation coefficients were greater than 0.99. The approach for sample preparation is very simple without the need for time‐consuming protein prefractionation or separation procedures and thus presents a significant improvement over existing methods for the analysis of the GM soya protein.     

Proteomics based on selecting and quantifying cysteine containing peptides by covalent chromatography

This paper describes a procedure in which cysteine containing peptides from tryptic digests of complex protein mixtures were selected by covalent chromatography based on thiol-disulfide exchange. identified by mass spectrometry, and quantified by differential isotope labeling. Following disruption of disulfide bridges with 2,2'-dipyridyl disulfide, all proteins were digested with trypsin and acylated with succinic anhydride. Cysteine containing peptides were then selected from the acylated digest by disulfide interchange with sulfhydryl groups on a thiopropyl Sepharose gel. Captured cysteine containing peptides were released from the gel with 25 mM dithiothreitol (pH 7.5) containing 1 mM (ethylenedinitrilo)tetraacetic acid disodium salt and alkylated with iodoacetic acid subsequent to fractionation by reversed-phase liquid chromatography (RPLC). Fractions collected from the RPLC column were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Based on isotope ratios of peptides from experimental and control samples labeled with succinic and deuterated succinic anhydride, respectively, it was possible to determine the relative concentration of each peptide species between the two samples. Peptides obtained from proteins that were up-regulated in the experimental sample were easily identified by an increase of the relative amount of the deuterated peptide. The results of these studies indicate that by selecting cysteine containing peptides, the complexity of protein digest could be reduced and database searches greatly simplified. When coupled with the isotope labeling strategy for quantification it was possible to determine proteins that were up-regulated in plasmid bearing Escherichia coli when expression of plasmid proteins was induced. Up-regulation of several proteins of E. coli origin was also noted.     

Rapid fabrication of glass/PDMS hybrid µIMER for high throughput membrane proteomics

Mass spectrometry (MS) based proteomics has brought a radical approach to systems biology, offering a platform to study complex biological functions. However, key proteomic technical challenges remain, mainly the inability to characterise the complete proteome of a cell due to the thousands of diverse, complex proteins expressed at an extremely wide concentration range. Currently, high throughput and efficient techniques to unambiguously identify and quantify proteins on a proteome-wide scale are in demand. Miniaturised analytical systems placed upstream of MS help us to attain these goals. One time-consuming step in traditional techniques is the in-solution digestion of proteins (4-20 h). This also has other drawbacks, including enzyme autoproteolysis, low efficiency, and manual operation. Furthermore, the identification of α-helical membrane proteins has remained a challenge due to their high hydrophobicity and lack of trypsin cleavage targets in transmembrane helices. We demonstrate a new rapidly produced glass/PDMS micro Immobilised Enzyme Reactor (μIMER) with enzymes covalently immobilised onto polyacrylic acid plasma-modified surfaces for the purpose of rapidly (as low as 30 s) generating peptides suitable for MS analysis. This μIMER also allows, for the first time, rapid digestion of insoluble proteins. Membrane protein identification through this method was achieved after just 4 min digestion time, up to 9-fold faster than either dual-stage in-solution digestion approaches or other commonly used bacterial membrane proteomic workflows.     

A method combining SPITC and 18O labeling for simultaneous protein identification and relative quantification

The relative quantification and identification of proteins by matrix‐assisted laser desorption ionization time‐of‐flight MS is very important in /MS is very important in protein research and is usually conducted separately. Chemical N‐terminal derivatization with 4‐sulphophenyl isothiocyanate facilitates de novo sequencing analysis and accurate protein identification, while ¹⁸O labeling is simple, specific and widely applicable among the isotopic labeling methods used for relative quantification. In the present study, a method combining 4‐sulphophenyl isothiocyanate derivatization with ¹⁸O isotopic labeling was established to identify and quantify proteins simultaneously in one experiment. Reaction conditions were first optimized using a standard peptide (fibrin peptide) and tryptic peptides from the model protein (bovine serum albumin). Under the optimized conditions, these two independent labeling steps show good compatibility, and the linear relativity of quantification within the ten times dynamic range was stable as revealed by correlation coefficient analysis (R² value = 0.998); moreover, precursor peaks in MS/MS spectrum could provide accurate quantitative information, which is usually acquired from MS spectrum, enabling protein identification and quantification in a single MS/MS spectrum. Next, this method was applied to native peptides isolated from spider venoms. As expected, the de novo sequencing results of each peptide matched with the known sequence precisely, and the measured quantitative ratio of each peptide corresponded well with the theoretical ratio. Finally, complex protein mixtures of spider venoms from male and female species with unknown genome information were analyzed. Differentially expressed proteins were successfully identified, and their quantitative information was also accessed. Taken together, this protein identification and quantification method is simple, reliable and efficient, which has a good potential in the exploration of peptides/proteins from species with unknown genome. Copyright © 2014 John Wiley & Sons, Ltd.     

Integrating Lys-N proteolysis and N-terminal guanidination for improved fragmentation and relative quantification of singly-charged ions

Department of Molecular Biology, Princeton University, Princeton, New Jersey, USA The study of isolated protein complexes has greatly benefited from recent advances in mass spectrometry instrumentation and quantitative, isotope labeling techniques. The comprehensive characterization of protein complex components and quantification of their relative abundance relies heavily upon maximizing protein and peptide sequence information obtained from MS and tandem MS studies. Recent work has shown that using a metalloendopeptidase, Lys-N, for proteomic analysis of biological protein mixtures produces complementary protein sequence information compared with trypsin digestion alone. Here, we have investigated the suitability of Lys-N proteolysis for use with MALDI mass spectrometry to characterize the yeast Arp2 complex and E. coli PAP I protein interactions. Although Lys-N digestion resulted in an average decrease in protein sequence coverage of ∼30% compared with trypsin digestion, CID analysis of singly-charged Lys-N peptides yielded a more extensive b-ions series compared with complementary tryptic peptides. Taking advantage of this improved fragmentation pattern, we utilized differential ¹⁵N/¹⁴N guanidination of Lys-N peptides and MALDI-MS/MS analysis to relatively quantify the changes in PAP I associations due to deletion of sprE, previously shown to regulate PAP I-dependent polyadenylation. Overall, this Lys-N/guanidination integrative approach is applicable for functional proteomic studies utilizing MALDI mass spectrometry analysis, as it provides an effective and economical mean for relative quantification of proteins in conjunction with increased sensitivity of detection and fragmentation efficiency.