Dansyl-peptides matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) and tandem mass spectrometric (MS/MS) features improve the liquid chromatography/MALDI-MS/MS analysis of the proteome

Simultaneous quantitation of cytokinin bases and their glycoconjugates with stable isotope labelling ultrahigh performance liquid chromatography mass spectrometry

Cytokinins (CKs) are one class of important phytohormones widely investigated in most aspects of plant life. Similar to other phytohormones, CKs and their glycoconjugates are hydrophilic. Their ionization efficiencies for mass spectrometry (MS) detection are rather poor, whereas their retention and separation on reverse phase liquid chromatography (RPLC) are often unsatisfying. Chemical isotope labelling LC-MS analysis methods have been developed for most other phytohormones, enhancing their LC separations and quantitative sensitivity. However, there are currently no reports for chemical-labelled CKs. Here, we report a new chemical isotope labelling LC-MS analytical method for one-pot derivatization of CK bases and their glycoconjugates, based on differential benzylation labelling of the adenine skeleton of CKs with benzyl bromide and its deuterium isotope-labelled reagent. Benzylation alters the hydrophilicity of CKs and their glycoconjugates, improving their retention and separation on RPLC. The developed method demonstrated enhanced sensitivity, as the CKs and their glycoconjugates could be analysed with LODs within the range of 0.62-25.9 pg/mL. The method also demonstrated good intra- and inter-day precisions with standard deviations in the range of 1.9%-13.0%, and acceptable accuracy with recoveries in the range of 84.0%-119.9%. The developed method was employed on the quantitation of CKs in the fresh roots of Astragalus membranaceus collected from both fertilized and unfertilized fields. The significant impact that fertilizers had on endogenous CKs metabolism was observed. As such, monitoring endogenous CKs and their metabolites might be promising to control fertilizer abuse.

MALDI-TOF mass spectrometry-based quantification of C-peptide in diabetes patients

BACKGROUND

Serum C-peptide concentrations reflect insulin secretion and beta cell function and can be used to diagnose and distinguish type-1 and type-2 diabetes. C-peptide is a more accurate indicator of insulin status than direct insulin measurement for monitoring patients with diabetes. However, the current methods available for C-peptide quantification exhibit poor reproducibility, are costly, and require highly trained laboratory personnel. Here, we have developed and evaluated a matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS)-based assay to standardize C-peptide measurements, providing highly accurate and comparable results across testing systems and laboratories.

METHODS

C-peptide from human serum was enriched using antibody-conjugated magnetic beads. The eluted isolates were further modified with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) to enhance the ionization of naturally acidic C-peptide. After desalting with ZipTips, the samples were subjected to MALDI-TOF MS analysis. Recombinant human C-peptide was used to develop the assay, and a heavy isotope labeled human C-peptide was used as an internal standard for quantification.

RESULTS

The MALDI-TOF MS method was validated in accordance with the restrictions of the device, with a limit of quantitation of 25 pmol/L. A correlation between the MAL-DI-TOF MS assay and a reference method was conducted using patient samples. The resulting regression revealed good agreement.

CONCLUSIONS

A simple, high-throughput, cost effective and quantitative MALDI-TOF MS C-peptide assay has been successfully developed and validated in clinical serum samples.

Rapid detection of insulin by immune-enrichment with silicon-nanoparticle-assisted MALDI-TOF MS

BACKGROUND

Insulin is central to regulating fat and carbohydrate metabolism in the body. However, it is difficult to detect insulin using mass spectrometry (MS). The integration of nanotechnology with mass spectrometry for selective and sensitive detection is an important research area. Our aim was to establish a method to detect insulin using silicon nanoparticle-assisted high-throughput MS.

METHODS

Different nanomaterials with the potential for use as MALDI components to enhance the MS signal by increasing peptide ionization were investigated in the present study. Insulin in samples was enriched with antibody-coated silicon nanoparticles and then analyzed by MALDI-TOF MS. Method validation was performed in the present study.

RESULTS

A platform for insulin detection with small sample volumes (100 μL) and a simplified procedure was successfully developed. The silicon nanoparticle-MS assay exhibited high sensitivity (LOQ, 0.1 nM) and good linear correlation of MS intensity with insulin concentration (R² = 0.99). Intra-assay precision (% coefficient of variation) ranged from 1.81 to 4.53%, and interassay precision ranged from 2.71 to 8.09%. In addition, a correlation between the MALDI assay and a chemiluminescence immunoassay (CIA) was completed in patient samples, and the resulting Deming regression revealed good agreement (R² = 0.981).

CONCLUSIONS

In our study, we found that the insulin signal could be enhanced with silicon nanoparticles. A new insulin determination method, immunoaffinity-based mass spectrometry, that saves time and involves simple processes, has been successfully established. The present assay was validated to detect insulin with low limits of detection.

Whole-cell-based identification of electrochemically active bacteria in microbial fuel cells by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

RATIONALE

Electrochemically active bacteria (EAB) that are capable of producing electricity from renewable biomass and organic wastes have been of particular interest in recent years. Methods for selective enrichment, accurate identification and easy acquisition of EAB fingerprints for phylogenetic characterization would facilitate utilization of these bioenergy-producing species in practical environmental engineering applications.

METHODS

Electricigens/exoelectrogens were selectively enriched from domestic wastewater in a microbial fuel cell (MFC). Whole EAB cell-derived mass spectra were obtained with simple agar incubation for 24 h and subsequent release of proteins by 25% formic acid (FA) and ultrasonication. Mass fingerprints of EAB were obtained by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/TOF-MS) and species-specific analyses were completed by using the Spectral ARchive and Microbial Identification System (SARAMIS).

RESULTS

EAB could be discriminated by clustering of MALDI-TOF/TOF-MS results. Different species in mixtures originating from domestic sewage could be identified unambiguously at 99.90% confidence. Five species, namely Klebsiella oxytoca (K. oxytoca), Comamonas testosterone (C. testosterone), Pseudomonas putida (P. putida), Klebsiella pneumonia (K. pneumonia) and Raoultella ornithinolytica (R. ornithinolytica), that are known to be of clinical significance, were found to be enriched in MFCs and determined as high power-producing species. By using an agglomerative clustering algorithm to compute spectral similarity and diversity, a dendrogram was constructed to illustrate the phylogenetic relationships for EAB on the basis of mass spectral analyses.

CONCLUSIONS

An integrated method based on MFC-enrichment, agar-cultivation and MALDI-TOF/TOF-MS identification of whole-cell-extracted proteins has been proved to be a simple, rapid and reliable approach for rapid identification and routine inspection of EAB. Mixed phyla can be analyzed at species level to provide phylogenetic information on the highly efficient bacteria generating electricity from domestic wastewater.

Development of a novel imidazolium-based aromatic quaternary ammonium tag: synthesis and application to the efficient analysis of cysteinyl-peptides by mass spectrometry

RATIONALE

Chemical derivatization is a very promising technique for improving analysis of peptides by mass spectrometry (MS). In this study, a novel kind of imidazolium-based aromatic quaternary ammonium tag, 1-[3-[(2-iodo-1-oxoethyl)amino]propyl]-3-butylimidazolium bromide (IPBI), designed with strong gas-phase basicity and a permanent positive charge, was firstly synthesized and further used for derivatization of cysteinyl-peptides with improved ionization efficiency and higher charge states.

METHODS

Both the model peptides and tryptic digests of proteins were used to evaluate the effect of IPBI derivatization on the MS performance of the derivatized peptides, and the results were further compared with the commonly used iodoacetamide (IAA) tag. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)-MS and electrospray ionization (ESI)-MS were used to evaluate the ionization efficiency and charge states of the derivatized peptides.

RESULTS

With model peptides as samples, a nearly 100% derivatization efficiency and superior stability were achieved via IPBI derivatization. By further analysis of both standard peptides and tryptic protein digests, the ionization efficiency and charge states of IPBI-derivatized peptides could be remarkably improved. For example, for protein bovine serum albumin, compared with the commercial available IAA tag, the identification efficiency of cysteinyl-peptides was increased about 67% by combining with IPBI derivatization.

CONCLUSIONS

The results indicated that the novel tag is an effective derivatization reagent for cysteinyl-peptide identification. We hope it could be further used for high-efficiency cysteinyl-peptide identification in proteome research, especially those with low abundance and poor ionization efficiency.