Automated work-flow for processing high-resolution direct infusion electrospray ionization mass spectral fingerprints

The use of mass spectrometry (MS) is pivotal in analyses of the metabolome and presents a major challenge for subsequent data processing. While the last few years have given new high performance instruments, there has not been a comparable development in data processing. In this paper we discuss an automated data processing pipeline to compare large numbers of fingerprint spectra from direct infusion experiments analyzed by high resolution MS. We describe some of the intriguing problems that have to be addressed, starting with the conversion and pre-processing of the raw data to the final data analysis. Illustrated on the direct infusion analysis (ESI-TOF-MS) of complex mixtures the method exploits the full quality of the high-resolution present in the mass spectra. Although the method is illustrated as a new library search method for high resolution MS, we demonstrate that the output of the preprocessing is applicable to cluster-, discriminant analysis, and related multivariate methods applied directly to mass spectra from direct infusion analysis of crude extracts. This is done to find the relationship between several terverticillate Penicillium species and identify the ions responsible for the segregation.     

Characterization of betaines using electrospray MS/MS

Betaines are an important class of naturally occurring compounds that function as compatible solutes or osmoprotectants. Because of the permanent positive charge on the quaternary ammonium moiety, mass spectrometric analysis has been approached by desorption methods, including fast atom bombardment and plasma desorption mass spectrometry. Here we show that electrospray ionization MS gives comparable results to plasma desorption MS for a range of authentic betaine standards and betaines purified from plant extracts by ion exchange chromatography. A distinct advantage of electrospray ionization MS over plasma desorption MS is the capability of obtaining product ion spectra via MS/MS of selected parent ions, and hence structural information to discriminate between ions of identical mass.     

Quantitative Proteomics Using Reductive Dimethylation for Stable Isotope Labeling

Stable isotope labeling of peptides by reductive dimethylation (ReDi labeling) is a method to accurately quantify protein expression differences between samples using mass spectrometry. ReDi labeling is performed using either regular (light) or deuterated (heavy) forms of formaldehyde and sodium cyanoborohydride to add two methyl groups to each free amine. Here we demonstrate a robust protocol for ReDi labeling and quantitative comparison of complex protein mixtures. Protein samples for comparison are digested into peptides, labeled to carry either light or heavy methyl tags, mixed, and co-analyzed by LC-MS/MS. Relative protein abundances are quantified by comparing the ion chromatogram peak areas of heavy and light labeled versions of the constituent peptide extracted from the full MS spectra. The method described here includes sample preparation by reversed-phase solid phase extraction, on-column ReDi labeling of peptides, peptide fractionation by basic pH reversed-phase (BPRP) chromatography, and StageTip peptide purification. We discuss advantages and limitations of ReDi labeling with respect to other methods for stable isotope incorporation. We highlight novel applications using ReDi labeling as a fast, inexpensive, and accurate method to compare protein abundances in nearly any type of sample.     

Characterization of N-linked oligosaccharides in chorion peroxidase of Aedes aegypti mosquito

A peroxidase is present in the chorion of Aedes aegypti eggs and catalyzes chorion protein cross-linking during chorion hardening, which is critical for egg survival in the environment. The unique chorion peroxidase (CPO) is a glycoprotein. This study deals with the N-glycosylation site, structures, and profile of CPO-associated oligosaccharides using mass spectrometric techniques and enzymatic digestion. CPO was isolated from chorion by solubilization and several chromatographic methods. Mono-saccharide composition was analyzed by HPLC with fluorescent detection. Our data revealed that carbohydrate (D-mannose, N-acetyl D-glucosamine, D-arabinose, N-acetyl D-galactosamine, and L-fucose) accounted for 2.24% of the CPO molecular weight. A single N-glycosylation site (Asn328-Cys- Thr) was identified by tryptic peptide mapping and de novo sequencing of native and PNGase A-deglycosylated CPO using matrix-assisted laser/desorption/ionization time-of-flight mass spectrometry (MALDI/TOF/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS). The Asn328 was proven to be a major fully glycosylated site. Potential tryptic glycopeptides and profile were first assessed by MALDI/TOF/MS and then by precursor ion scanning during LC/MS/MS. The structures of N-linked oligosaccharides were elucidated from the MS/MS spectra of glycopeptides and exoglycosidase sequencing of PNGase A-released oligosaccharides. These CPO-associated oligosaccharides had dominant Man3GlcNAc2 and Man3 (Fuc) GlcNAc2 and high mannose-type structures (Man(4-8)GlcNAc2). The truncated structures, Man2GlcNAc2 and Man2 (Fuc) GlcNAc2, were also identified. Comparison of CPO activity and Stokes radius between native and deglycosylated CPO suggests that the N-linked oligosaccharides influence the enzyme activity by stabilizing its folded state.     

Screening of β-2 agonists and confirmation of fenoterol,orciprenaline, reproterol and terbutaline with gas chromatography–mass spectrometry as tetrahydroisoquinoline derivatives

A new method for a comprehensive screening and confirmation of β-2 agonists in human urine is presented based on gas chromatography–low-resolution mass spectrometry (GC–MS) using electron impact ionisation (EI). After hydrolysis of the conjugates with β-glucuronidase/arylsulfatase a derivatisation step with formaldehyde converts fenoterol, orciprenaline, reproterol and terbutaline to one derivative, a tetrahydroisoquinoline, while the other β-2 agonists remain unchanged. Liquid–liquid extraction and trimethylsilylation follow. The tetrahydroisoquinoline derivatives show good gas chromatographic and mass spectrometric behaviour. The detection limit of these four β-2 agonists in the screening using low-resolution mass spectrometry is 10 ng/ml of urine. The other β-2 agonists are detected as parent compounds with the same recovery after sample preparation with and without formaldehyde. The EI mass spectra of the tetrahydroisoquinoline derivatives are presented.     

Mass spectrometric quantification of the relative amounts of C6 and C3 position phosphorylated glucosyl residues in starch

The quantification of phosphate bound to the C6 and C3 positions of glucose residues in starch has received increasing interest since the importance of starch phosphorylation for plant metabolism was discovered. The method described here is based on the observation that the isobaric compounds glucose-6-phosphate (Glc6P) and glucose-3-phosphate (Glc3P) exhibit significantly different fragmentation patterns in negative ion electrospray tandem mass spectrometry (MS/MS). A simple experiment involving collision-induced dissociation (CID) MS² spectra of the sample and the two reference substances Glc3P and Glc6P permitted the quantification of the relative amounts of the two compounds in monosaccharide mixtures generated by acid hydrolysis of starch. The method was tested on well-characterized potato tuber starch. The results are consistent with those obtained by NMR analysis. In contrast to NMR, however, the presented method is fast and can be performed on less than 1 mg of starch. Starch samples of other origins exhibiting a variety of phosphorylation degrees were analyzed to assess the sensitivity and robustness of the method.     

Rapid Characterization of Microalgae and Microalgae Mixtures Using Matrix-Assisted Laser Desorption Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF MS)

Current molecular methods to characterize microalgae are time-intensive and expensive. Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) may represent a rapid and economical alternative approach. The objectives of this study were to determine whether MALDI-TOF MS can be used to: 1) differentiate microalgae at the species and strain levels and 2) characterize simple microalgal mixtures. A common protein extraction sample preparation method was used to facilitate rapid mass spectrometry-based analysis of 31 microalgae. Each yielded spectra containing between 6 and 56 peaks in the m/z 2,000 to 20,000 range. The taxonomic resolution of this approach appeared higher than that of 18S rDNA sequence analysis. For example, two strains of Scenedesmus acutus differed only by two 18S rDNA nucleotides, but yielded distinct MALDI-TOF mass spectra. Mixtures of two and three microalgae yielded relatively complex spectra that contained peaks associated with members of each mixture. Interestingly, though, mixture-specific peaks were observed at m/z 11,048 and 11,230. Our results suggest that MALDI-TOF MS affords rapid characterization of individual microalgae and simple microalgal mixtures.     

Differential dimethyl labeling of N-termini of peptides after guanidination for proteome analysis

We describe an enabling technique for proteome analysis based on isotope-differential dimethyl labeling of N-termini of tryptic peptides followed by microbore liquid chromatography (LC) matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS). In this method, lysine side chains are blocked by guanidination to prevent the incorporation of multiple labels, followed by N-terminal labeling via reductive amination using d(0),(12)C-formaldehyde or d(2),(13)C-formaldehyde. Relative quantification of peptide mixtures is achieved by examining the MALDI mass spectra of the peptide pairs labeled with different isotope tags. A nominal mass difference of 6 Da between the peptide pair allows negligible interference between the two isotopic clusters for quantification of peptides of up to 3000 Da. Since only the N-termini of tryptic peptides are differentially labeled and the a(1) ions are also enhanced in the MALDI MS/MS spectra, interpretation of the fragment ion spectra to obtain sequence information is greatly simplified. It is demonstrated that this technique of N-terminal dimethylation (2ME) after lysine guanidination (GA) or 2MEGA offers several desirable features, including simple experimental procedure, stable products, using inexpensive and commercially available reagents, and negligible isotope effect on reversed-phase separation. LC-MALDI MS combined with this 2MEGA labeling technique was successfully used to identify proteins that included polymorphic variants and low abundance proteins in bovine milk. In addition, by analyzing a mixture of two equal amounts of milk whey fraction as a control, it is shown that the measured average ratio for 56 peptide pairs from 14 different proteins is 1.02, which is very close to the theoretical ratio of 1.00. The calculated percentage error is 2.0% and relative standard deviation is 4.6%.     

AMASS: software for automatically validating the quality of MS/MS spectrum from SEQUEST results

Time-consuming and experience-dependent manual validations of tandem mass spectra are usually applied to SEQUEST results. This inefficient method has become a significant bottleneck for MS/MS data processing. Here we introduce a program AMASS (advanced mass spectrum screener), which can filter the tandem mass spectra of SEQUEST results by measuring the match percentage of high-abundant ions and the continuity of matched fragment ions in b, y series. Compared with Xcorr and DeltaCn filter, AMASS can increase the number of positives and reduce the number of negatives in 22 datasets generated from 18 known protein mixtures. It effectively removed most noisy spectra, false interpretations, and about half of poor fragmentation spectra, and AMASS can work synergistically with Rscore filter. We believe the use of AMASS and Rscore can result in a more accurate identification of peptide MS/MS spectra and reduce the time and energy for manual validation.     

基于液相色谱质谱联用的蛋白质组非标记定量研究策略的建立及应用

定量蛋白质组研究是蛋白质组研究的热点和难点,而液相色谱质谱技术已经被广泛地应用于蛋白质的定性和定量研究.该研究建立和优化了一种基于液相色谱质谱联用技术的蛋白质组非标记定量方法,并对两种肽段质谱检测计数的归一化算法进行了比较,结果发现ASC法要优于Rsc法.最后,将建立的方法应用于肝癌细胞模型HepG2和HepG2-HBx细胞系的差异蛋白质组表达研究.质谱鉴定结果用聚类分析软件cluster3.0进行分析,最后鉴定出107个重叠蛋白,其中9个蛋白质表达上调(Ratio>1.75),6个蛋白质表达下调(Ratio<0.5),这些蛋白质均与肝癌发生和恶化密切相关.结果表明,该技术操作简单、方便,具有较高的灵敏度和动态范围,利用该方法进行差异蛋白质组研究和发现生物标志物在理论和临床上具有十分重要的意义.     

A micro method involving micro high-performance liquid chromatography-mass spectrometry for the structural characterization of neutral glycosphingolipids and monosialogangliosides

A micro method involving high-performance liquid chromatography-fast atom bombardment mass spectrometry (HPLC/FAB/MS) has been developed for the sensitive structural characterization of neutral glycosphingolipids and monosialogangliosides. The method involves a micro silica gel column (0.3 mm i.d. x 100 mm) and a micro HPLC apparatus working at a flow rate of 6 microliters/min. All injected materials can be structurally characterized by mass spectrometry without the splitting or wasting of materials, which was not possible with our previous method [M. Suzuki et al. (1990) J. Biochem. 108, 92-98]. A mixture containing 160 ng each of five neutral glycosphingolipids (GlcCer, LacCer, Gb3Cer, Gb4Cer, and IV3 alpha GalNAc-Gb4Cer) and a mixture containing 160 ng each of three monosialogangliosides [GM3(NeuAc), GM2(NeuAc), and GM1(NeuAc)] were injected into the micro HPLC with programmed elution with isopropanol-n-hexane-water with or without ammonium hydroxide. Each glycosphingolipid was separated by mass chromatography and the obtained mass spectra were suitable for structural characterization. Thus, the characterization of glycosphingolipids was achieved with small amounts of materials, 160 ng each, and in mixtures.     

Mass spectrometry-based proteomics turns quantitative

The field of proteomics is built on technologies to analyze large numbers of proteins--ideally the entire proteome--in the same experiment. Mass spectrometry (MS) has been successfully used to characterize proteins in complex mixtures, but results so far have largely been qualitative. Two recently developed methodologies offer the opportunity to obtain quantitative proteomic information. Comparing the signals from the same peptide under different conditions yields a rough estimate of relative protein abundance between two proteomes. Alternatively, and more accurately, peptides are labeled with stable isotopes, introducing a predictable mass difference between peptides from two experimental conditions. Stable isotope labels can be incorporated 'post-harvest', by chemical approaches or in live cells through metabolic incorporation. This isotopic handle facilitates direct quantification from the mass spectra. Using these quantitative approaches, precise functional information as well as temporal changes in the proteome can be captured by MS.     

Two-dimensional mass spectra generated from the analysis of 15N-labeled and unlabeled peptides for efficient protein identification and de novo peptide sequencing

Protein identification has been greatly facilitated by database searches against protein sequences derived from product ion spectra of peptides. This approach is primarily based on the use of fragment ion mass information contained in a MS/MS spectrum. Unambiguous protein identification from a spectrum with low sequence coverage or poor spectral quality can be a major challenge. We present a two-dimensional (2D) mass spectrometric method in which the numbers of nitrogen atoms in the molecular ion and the fragment ions are used to provide additional discriminating power for much improved protein identification and de novo peptide sequencing. The nitrogen number is determined by analyzing the mass difference of corresponding peak pairs in overlaid spectra of (15)N-labeled and unlabeled peptides. These peptides are produced by enzymatic or chemical cleavage of proteins from cells grown in (15)N-enriched and normal media, respectively. It is demonstrated that, using 2D information, i.e., m/z and its associated nitrogen number, this method can, not only confirm protein identification results generated by MS/MS database searching, but also identify peptides that are not possible to identify by database searching alone. Examples are presented of analyzing Escherichia coli K12 extracts that yielded relatively poor MS/MS spectra, presumably from the digests of low abundance proteins, which can still give positive protein identification using this method. Additionally, this 2D MS method can facilitate spectral interpretation for de novo peptide sequencing and identification of posttranslational or other chemical modifications. We envision that this method should be particularly useful for proteome expression profiling of organelles or cells that can be grown in (15)N-enriched media.     

Accurate mass comparison coupled with two endopeptidases enables identification of protein termini

Protein termini play important roles in biological processes, but there have been few methods for comprehensive terminal proteomics. We have developed a new method that can identify both the amino and the carboxyl termini of proteins. The method independently uses two proteases, (lysyl endopeptidase) Lys-C and peptidyl-Lys metalloendopeptidase (Lys-N), to digest proteins, followed by LC-MS/MS analysis of the two digests. Terminal peptides can be identified by comparing the peptide masses in the two digests as follows: (i) the amino terminal peptide of a protein in Lys-C digest is one lysine residue mass heavier than that in Lys-N digest; (ii) the carboxyl terminal peptide in Lys-N digest is one lysine residue mass heavier than that in Lys-C digest; and (iii) all internal peptides give exactly the same molecular masses in both the Lys-C and the Lys-N digest, although amino acid sequences of Lys-C and Lys-N peptides are different (Lys-C peptides end with lysine, whereas Lys-N peptides begin with lysine). The identification of terminal peptides was further verified by examining their MS/MS spectra to avoid misidentifying pairs as termini. In this study, we investigated the usefulness of this method using several protein and peptide mixtures. Known protein termini were successfully identified. Acetylation on N-terminus and protein isoforms, which have different termini, was also determined. These results demonstrate that our new method can confidently identify terminal peptides in protein mixtures.     

MudPIT: multidimensional protein identification technology

Large-scale biology emerged out of the efforts to sequence genomes of important organisms. Based on resources created by whole genome sequencing, large-scale analyses of messenger RNA (mRNA) and protein expression are now possible. With the availability of large amounts of genomic sequence information, a convenient method for the identification and analysis of proteins based on proteolytic digestion into peptides emerged. Processes to fragment peptides using collision-activated dissociation (CAD) in tandem mass spectrometers and computer algorithms to match the tandem mass spectra of peptides to sequences in databases enable rapid identification of amino acid sequences, and hence proteins, present in mixtures. The inherent complexity of the peptide mixtures has necessitated improvements in methodology for mass spectrometry (MS) analysis of peptides.     

Proteome analysis of Escherichia coli using high-performance liquid chromatography and Fourier transform ion cyclotron resonance mass spectrometry

The basic problem of complexity poses a significant challenge for proteomic studies. To date two-dimensional gel electrophoresis (2-DE) followed by enzymatic in-gel digestion of the peptides, and subsequent identification by mass spectrometry (MS) is the most commonly used method to analyze complex protein mixtures. However, 2-DE is a slow and labor-intensive technique, which is not able to resolve all proteins of a proteome. To overcome these limitations gel-free approaches are developed based on high performance liquid chromatography (HPLC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The high resolution and excellent mass accuracy of FT-ICR MS provides a basis for simultaneous analysis of numerous compounds. In the present study, a small protein subfraction of an Escherichia coli cell lysate was prepared by size-exclusion chromatography and proteins were analyzed using C4 reversed phase (RP)-HPLC for pre-separation followed by C18 RP nanoHPLC/nanoESI FT-ICR MS for analysis of the peptide mixtures after tryptic digestion of the protein fractions. We identified 231 proteins and thus demonstrated that a combination of two RP separation steps - one on the protein and one on the peptide level - in combination with high-resolution FT-ICR MS has the potential to become a powerful method for global proteomics studies.     

A novel quantitative proteomics workflow by isobaric terminal labeling

Quantification by series of b, y fragment ion pairs generated from isobaric-labeled peptides in MS2 spectra has recently been considered an accurate strategy in quantitative proteomics. Here we developed a novel MS2 quantification approach named quantitation by isobaric terminal labeling (QITL) by coupling (18)O labeling with dimethylation. Trypsin-digested peptides were labeled with two (16)O or (18)O atoms at their C-termini in H(2)(16)O or H(2)(18)O. After blocking all ε-amino groups of lysines through guanidination, the N-termini of the peptides were accordingly labeled with formaldehyde-d(2) or formaldehyde. These indistinguishable, isobaric-labeled peptides in MS1 spectra produce b, y fragment ion pairs in the whole mass range of MS2 spectra that can be used for quantification. In this study, the feasibility of QITL was first demonstrated using standard proteins. An accurate and reproducible quantification over a wide dynamic range was achieved. Then, complex rat liver samples were used to verify the applicability of QITL for large-scale quantitative analysis. Finally, QITL was applied to profile the quantitative proteome of hepatocellular carcinoma (HCC) and adjacent non-tumor liver tissues. Given its simplicity, low-cost, and accuracy, QITL can be widely applied in biological samples (cell lines, tissues, and body fluids, etc.) for quantitative proteomic research.     

Oxygenated di- and tricyclic diterpenoids of southern hemisphere conifers

A range of conifers of the southern hemisphere have been analysed (using gas chromatography–mass spectrometry (GC–MS)) for oxygenated cyclic diterpenoids. Parts of the trees examined include resins, cones, shoots, woods and barks. Trimethylsilyl (TMS) derivatives were mainly used for identification purposes. Identifications were facilitated by using previous work based on classical isolation procedures on certain species. Rules for interpreting the mass spectra of such compounds in the derivatised form were derived and are described. A compilation of such compounds with mass spectra and retention times now exists.     

An analytical system for the characterization of highly heterogeneous mixtures of N-linked oligosaccharides

A novel system for characterizing complex N-linked oligosaccharide mixtures that uses a combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), capillary electrophoresis (CE), and high-performance liquid chromatography (HPLC) has been developed. In this study, oligosaccharides released from recombinant TNK-tPA (tissue plasminogen activator) were derivatized with 5-amino-2-naphthalenesulfonic acid (ANSA). The negative charge imparted by the ANSA label facilitated the analysis of the oligosaccharides by MALDI-TOF MS by allowing the observation of both neutral and sialylated oligosaccharides in a single negative ion mode spectrum. Labeling with ANSA was also determined to be advantageous in the characterization of oligosaccharides by both HPLC and CE. The ANSA label was demonstrated to provide superior resolution over the commonly used label 8-aminopyrene-1,3,6-trisulfonic acid (APTS) in both the CE and HPLC analysis of oligosaccharides. To date, no other labels that enable the analysis of complex oligosaccharide mixtures in a single mass spectral mode, while also enabling high-resolution chromatographic and electrophoretic separation of the oligosaccharides, have been reported. By integrating the structural information obtained by MALDI-TOF MS analysis with the ability of CE and HPLC to discriminate between structural isomers, the complete characterization of complex oligosaccharide mixtures is possible.