Molecules and elements for quantitative bioanalysis: The allure of using electrospray, MALDI, and ICP mass spectrometry side-by-side.

To understand biological processes, not only reliable identification, but quantification of constituents in biological processes play a pivotal role. This is especially true for the proteome: protein quantification must follow protein identification, since sometimes minute changes in abundance tell the real tale. To obtain quantitative data, many sophisticated strategies using electrospray and MALDI mass spectrometry (MS) have been developed in recent years. All of them have advantages and limitations. Several years ago, we started to work on strategies, which are principally capable to overcome some of these limits. The fundamental idea is to use elemental signals as a measure for quantities. We began by replacing the radioactive 32 P with the "cold" natural 31 P to quantify modified nucleotides and phosphorylated peptides and proteins and later used tagging strategies for quantification of proteins more generally. To do this, we introduced Inductively Coupled Plasma Mass Spectrometry (ICP-MS) into the bioanalytical workflows, allowing not only reliable and sensitive detection but also quantification based on isotope dilution absolute measurements using poly-isotopic elements. The detection capability of ICP-MS becomes particularly attractive with heavy metals. The covalently bound proteins tags developed in our group are based on the well-known DOTA chelate complex (1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid) carrying ions of lanthanoides as metal core. In this review, I will outline the development of this mutual assistance between molecular and elemental mass spectrometry and discuss the scope and limitations particularly of peptide and protein quantification. The lanthanoide tags provide low detection limits, but offer multiplexing capabilities due to the number of very similar lanthanoides and their isotopes. With isotope dilution comes previously unknown accuracy. Separation techniques such as electrophoresis and HPLC were used and just slightly adapted workflows, already in use for quantification in bioanalysis. Imaging mass spectrometry (MSI) with MALDI and laser ablation ICP-MS complemented the range of application in recent years.

Complementarity of molecular and elemental mass spectrometric imaging of Gadovist™ in mouse tissues.

Drug biodistribution analyses can be considered a key issue in pharmaceutical discovery and development. Here, mass spectrometric imaging can be employed as a powerful tool to investigate distributions of drug compounds in biologically and medically relevant tissue sections. Both matrix-assisted laser desorption ionization-mass spectrometric imaging as molecular method and laser ablation inductively coupled plasma-mass spectrometric imaging as elemental detection method were applied to determine drug distributions in tissue thin sections. Several mouse organs including the heart, kidney, liver, and brain were analyzed with regard to distribution of Gadovist™, a gadolinium-based contrast agent already approved for clinical investigation. This work demonstrated the successful detection and localization of Gadovist™ in several organs. Furthermore, the results gave evidence that gadolinium-based contrast agents in general can be well analyzed by mass spectrometric imaging methods. In conclusion, the combined application of molecular and elemental mass spectrometry could complement each other and thus confirm analytical results or provide additional information.

Lipid imaging for visualizing cilastatin amelioration of cisplatin-induced nephrotoxicity.

Nephrotoxicity is a major limitation to cisplatin antitumor therapies. Cilastatin, an inhibitor of renal dehydropeptidase-I, was recently proposed as a promising nephroprotector against cisplatin toxicity, preventing apoptotic cell death. In this work, cilastatin nephroprotection was further investigated in a rat model, with a focus on its effect on 76 renal lipids altered by cisplatin, including 13 new cisplatin-altered mitochondrial cardiolipin species. Lipid imaging was performed with MALDI mass spectrometry imaging (MALDI-MSI) in kidney sections from treated rats. Cilastatin was proved to significantly diminish the lipid distribution alterations caused by cisplatin, lipid levels being almost completely recovered to those of control samples. The extent of recovery of cisplatin-altered lipids by cilastatin turned out to be relevant for discriminating direct or secondary lipid alterations driven by cisplatin. Lipid peroxidation induced by cisplatin was also shown to be reduced when cilastatin was administered. Importantly, significant groups separation was achieved during multivariate analysis of cortex and outer-medullary lipids, indicating that damaged kidney can be discerned from the nephroprotected and healthy groups and classified according to lipid distribution. Therefore, we propose MALDI-MSI as a powerful potential tool offering multimolecule detection possibilities to visualize and evaluate nephrotoxicity and nephroprotection based on lipid analysis.

Negative nucleotide ions as sensitive probes for energy specificity in collision-induced fragmentation in mass spectrometry.

RATIONALE: The most commonly used fragmentation methods in tandem mass spectrometry (MS/MS) are collision-induced dissociation (CID) and higher energy collisional dissociation (HCD). While in CID the preselected ions in the trap are resonantly (and m/z exclusively) excited, in HCD the entire m/z range experiences the dissociative acceleration. The different excitation is reflected in different fragment distributions. METHODS: As a test-bed for particularly pronounced fragmentation specificity, here MS/MS experiments on several 4-mer oligonucleotides were conducted employing both collision methods and the results were thoroughly compared. Oligonucleotides are shown to be sensitive probes to subtle changes, especially in the negative ion mode. A detailed analysis of these differences reveals insight into the dissociation mechanics. RESULTS: The differences are represented in heat-maps, which allow for a direct visual inspection of large amounts of data. In these false colour representations the, sometimes subtle, changes in the individual dissociation product distributions become distinct. Another advantage of these graphic plots can be found in the formation of systematic patterns. These patterns reflect trends in dissociation specificity which allow for the formulation of general rules in fragmentation behavior. CONCLUSIONS: Instruments equipped with two different excitation schemes for MS/MS are today widely available. Nonetheless, direct comparisons between the individual results are scarcely made. Such comparative studies bear a powerful analytical potential to elucidate fragmentation reaction mechanism.

A new strategy for metal labeling of glycan structures in antibodies.

Quantitative analysis of complex proteins is a challenging task in modern bioanalytical chemistry. Commonly available isotope labels are still suffering from limitations and drawbacks, whereas new metal labels open numerous possibilities in mass spectrometric analyses. In this work, we have developed a new metal labeling strategy to tag glycan structures of proteins, more particularly antibodies. The oligosaccharide glycans were selectively trimmed to the last N-acetylglucosamine to which an artificial azide containing galactose residue was bound. This azide can be used for subsequent cycloaddition of an alkyne. Therefore, we developed a lanthanide-containing macrocyclic reagent to selectively connect to this azido galactose. In summary, the glycan structures of an antibody can be labeled with a metal functionality using this approach. Furthermore, the functionality of the antibodies can be fully maintained by labeling the Fc glycans instead of using labeling reagents that target amino or thiol groups. This approach enables the possibility of using elemental, besides molecular mass spectrometry, for quantitative analyses or imaging experiments of antibodies in complex biological samples. Graphical abstract Antibody labeling at sugar moieties with rare earth elements to enable application in elemental mass spectrometry.

Charge-induced geometrical reorganization of DNA oligonucleotides studied by tandem mass spectrometry and ion mobility.

Mass spectrometry is applied as a tool for the elucidation of molecular structures. This premises that gas-phase structures reflect the original geometry of the analytes, while it requires a thorough understanding and investigation of the forces controlling and affecting the gas-phase structures. However, only little is known about conformational changes of oligonucleotides in the gas phase. In this study, a series of multiply charged DNA oligonucleotides (n = 15-40) has been subjected to a comprehensive tandem mass spectrometric study to unravel transitions between different ionic gas-phase structures. The nucleobase sequence and the chain length were varied to gain insights into their influence on the geometrical oligonucleotide organization. Altogether, 23 oligonucleotides were analyzed using collision-induced fragmentation. All sequences showed comparable correlation regarding the characteristic collision energy. This value that is also a measure for stability, strongly correlates with the net charge density of the precursor ions. With decreasing charge of the oligonucleotides, an increase in the fragmentation energy was observed. At a distinct charge density, a deviation from linearity was observed for all studied species, indicating a structural reorganization. To corroborate the proposed geometrical change, collisional cross-sections of the oligonucleotides at different charge states were determined using ion mobility-mass spectrometry. The results clearly indicate that an increase in charge density and thus Coulomb repulsion results in the transition from a folded, compact form to elongated structures of the precursor ions. Our data show this structural transition to depend mainly on the charge density, whereas sequence and size do not have an influence.

Polyphenols from Tamarix nilotica: LC⁻ESI-MSn Profiling and In Vivo Antifibrotic Activity.

Tamarix nilotica (Ehrenb.) Bunge (Tamaricaceae), an indigenous plant to the Middle East region, is well-known as a medicinal plant for treating many human ailments. The current study aimed at exploring the polyphenol profile of the alcohol soluble fraction of aqueous T. nilotica extract, assessing its in vivo antifibrotic activity and the possible underlying mechanism, to unravel the impact of quantitative difference of sulphated polyphenols content on the antifibrotic activity of T. nilotca grown in two different habitats. Polyphenol profiling of T. nilotica extracts was performed using HPLC-HRESI-QTOF-MS-MS. The major polyphenol components included sulphated flavonoids, phenolic acids and free aglycones. The antifibrotic activity was evaluated through carbon tetrachloride-induced liver fibrosis in rats. Biochemical evaluations revealed that both fractions ameliorated the increased levels of hepatic aminotransferases, lipid peroxidation, hydroxyproline, α-smooth muscle actin (α-SMA), tumor necrosis factor-α (TNF-α), cyclooxygenase-2 (COX-2) and nuclear factor kappa B (NF-κB). Moreover, both fractions reduced catalase activity (CAT) and enhanced hepatic glutathione (GSH) content. Histopathological imaging undoubtedly confirmed such results. In conclusion, the T. nilotica polyphenol-rich fraction exhibited potential antifibrotic activity in rats. Significant alterations in GSH levels were recorded based on the sulphated polyphenol metabolite content.

Dual Internal Standards with Metals and Molecules for MALDI Imaging of Kidney Lipids.

The quest for internal standards useful in MALDI imaging studies goes on to get not only lateral distribution but also reliable relative quantitative information. We developed a method based on application of matrix and dual internal standards to allow intra- and intersample normalization of lipids intensities in kidney sections of control and cisplatin-treated Wistar rats. An inkjet printer was used to deposit a custom-prepared ink with DHB as MALDI matrix, a primary lipids-based internal standard, and a spiked lanthanide as a secondary internal standard. We applied different laser energy and varied the amounts of matrix-internal standards mixture to evaluate the normalization potential of the internal standards. Successful correction of intensity artifacts caused by instrumental drifts was possible, but not those resulting from uneven matrix application. ICP-MS absolute quantification of the lanthanide in the printed layer ensured the reproducibility of the matrix and internal standards application with RSD of 10-15%. Internal standard-normalized data allowed intrasample modification of the MALDI image to make it compatible with the optical image. Normalization to internal standards corrected a 2-fold difference in lipids intensity, which allowed a meaningful comparison of tissue lipids in control and cisplatin-treated kidneys. More importantly, normalization of lipid relative abundances based on the same adduct type (H+, Na+, and K+) for analyte and internal standard corrected for different ionization efficiencies showing a realistic signal level and enabling reliable comparison of different samples on relative quantitative basis.

Comprehensive Molecular Characterization of a Cisplatin-Specific Monoclonal Antibody.

Despite their immense and rapidly increasing importance as analytical tools or therapeutic drugs, the detailed structural features of particular monoclonal antibodies are widely unknown. Here, an antibody already in use for diagnostic purposes and for molecular dosimetry studies in cancer therapy with very high affinity and specificity for cisplatin-induced DNA modifications was studied extensively. The molecular structure and modifications as well as the antigen specificity were investigated mainly by mass spectrometry. Using nano electrospray ionization mass spectrometry, it was possible to characterize the antibody in its native state. Tandem-MS experiments not only revealed specific fragments but also gave information on the molecular structure. The detailed primary structure was further elucidated by proteolytic treatment with a selection of enzymes and high resolution tandem-MS. The data were validated by comparison with known antibody sequences. Then, the complex glycan structures bound to the antibody were characterized in all detail. The Fc-bound oligosaccharides were released enzymatically and studied by matrix-assisted laser desorption/ionization mass spectrometry. Overall 16 different major glycan structures were identified. The binding specificity of the antibody was investigated by applying synthetic single and double stranded DNA oligomers harboring distinct Pt adducts. The antibody-antigen complexes were analyzed by mass spectrometry under native conditions. The stability of the complex with double stranded DNA was also investigated.

Elemental labelling and mass spectrometry for the specific detection of sulfenic acid groups in model peptides: a proof of concept.

Oxidative transformation of cysteine thiol groups into different functional groups is considered a significant posttranslational modification of great importance in pathological and physiological processes. A cysteine sulfenic acid (SA) residue is the transient state for thiol group oxidation and it can react with free thiols to form disulfide bonds or can be further oxidized with reactive oxygen/reactive nitrogen species (ROS/RNS) to form sulfinic and sulfonic acids. The increase in ROS/RNS concentrations is correlated to age-related diseases such as cancer and Alzheimer's disease. Since the formation of SA represents a transient state of oxidation of thiols, its formation can be considered a redox-sensitive sensor for the presence of ROS/RNS. Thereby, the detection of the short-lived SA will provide greater insight into the redox-mediated events that alter the structure and function of peptides and proteins. The aim of this study is to provide a new strategy for the highly sensitive and specific detection of SA in peptides as a proof of concept. For this aim, SA was firstly generated in model peptides on oxidation with H2O2 and then captured by the linear alkyne ß-ketoester (KE) previously linked to a lanthanide (Ln)-containing chelator (Ln-DOTA, where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The linking of the KE to DOTA was performed by click chemistry, resulting in a new reagent (Ln-DOTA-KE) that permits highly sensitive elemental (inductively coupled plasma) and molecular (electrospray) mass spectrometric detection. The new reagent (Ln-DOTA-KE) reacts specifically with SA, offering improved reactivity at physiological pH, facile derivatization and a cell-membrane-permeable compound that has promising future applications. Graphical Abstract A new derivatizing reagent for specific detection of sulphenic acid (SA) generated in model peptides by oxidation of cysteine groups is presented in this work.

Bridging the gap between molecular and elemental mass spectrometry: higher energy collisional dissociation (HCD) revealing elemental information.

Molecular mass spectrometry has been applied to simultaneously obtain molecular and elemental information from metal-containing species. Energy tuning of the higher-energy collision dissociation (HCD) fragmentation cell allows the controlled production of typical peptide fragments or elemental reporter ions informing about the metallic content of the analyzed species. Different instrumental configurations and fragmentation techniques have been tested, and the efficiency extracting the elemental information has been compared. HCD fragmentation operating at very high energy led to the best results. Platinum, lanthanides, and iodine reporter ions from peptides interacting with cisplatin, peptides labeled with lanthanides-MeCAT-IA, and iodinated peptides, respectively, were obtained. The possibility to produce abundant molecular and elemental ions in the same analysis simplifies the correlation between both signals and open pathways in metallomics studies enabling the specific tracking of metal-containing species. The proposed approach has been successfully applied to in solution standards and complex samples. Moreover, interesting preliminary MALDI-imaging experiments have been performed showing similar metal distribution compared to laser ablation (LA)-ICPMS.

A shotgun approach for the identification of platinum-protein complexes.

A shotgun approach including peptide-based OFFGEL-isoelectric focusing (IEF) fractionation has been developed with the aim of improving the identification of platinum-binding proteins in biological samples. The method is based on a filter-aided sample preparation (FASP) tryptic digestion under denaturing and reducing conditions of cisplatin-, oxaliplatin-, and carboplatin-protein complexes, followed by OFFGEL-IEF separation of the peptides. Any risk of platinum loss is minimized throughout the procedure due to the removal of the reagents used after each stage of the FASP method and the absence of thiol-based reagents in the focusing buffer employed in the IEF separation. The platinum-peptide complexes stability after the FASP digestion and the IEF separation was confirmed by size exclusion chromatography-inductively coupled plasma-mass spectrometry (SEC-ICP-MS). The suitability of peptide-based OFFGEL-IEF fractionation for reducing the sample complexity for further nano-liquid chromatography-electrospray ionization-tandem mass spectrometry (nLC-ESI-MS/MS) analysis has been demonstrated, allowing the detection of platinum-containing peptides, with significantly lower abundance and ionization efficiency than unmodified peptides. nLC-MS/MS analysis of selected OFFGEL-IEF fractions from tryptic digests with different complexity degrees: standard human serum albumin (HSA), a mixture of five proteins (albumin, transferrin, carbonic anhydrase, myoglobin, and cytochrome-c) and human blood serum allowed the identification of several platinum-peptides from cisplatin-HSA. Cisplatin-binding sites in HSA were elucidated from the MS/MS spectra and assessed considering the protein three-dimensional structure. Most of the potential superficial binding sites available on HSA were identified for all the samples, including a biologically relevant cisplatin-cross-link of two protein domains, demonstrating the capabilities of the methodology.

Acylated flavonol diglucosides from Ammania auriculata.

Chemical investigation of the extract of the whole Ammania auriculata plant resulted in the identification of 13 polyphenols, including the hitherto unknown flavonoids, kaempferol 3-O-ß-(6″-galloylglucopyranoside)-7-O-ß-glucopyranoside, and its quercetin analogue. The structures of all isolates were elucidated by conventional methods, spectroscopic analysis, including 1D and 2D NMR, and by HRESI-MS as well.

DNA quantification via ICP-MS using lanthanide-labeled probes and ligation-mediated amplification.

The combination of lanthanide-tagged oligonucleotide probes with inductively coupled plasma mass spectrometry (ICP-MS) as the detection technique is a novel labeling and analysis strategy for heterogeneous nucleic acid quantification assays. We describe a hybridization assay based on biotin-streptavidin affinity using lanthanide-labeled reporter probes and biotinylated capture probes. For the basic sandwich type assay, performed in streptavidin-coated microtitration wells, the limit of detection (LOD) was 7.2 fmol of DNA target, corresponding to a final concentration of 6 pM terbium-labeled probes detectable by ICP-MS after elution from the solid support. To improve the sensitivity and sequence specificity of the approach, it was combined with established molecular biological techniques, i.e., elution with a restriction endonuclease and signal and target amplification by the ligase detection reaction (LDR) and ligase chain reaction (LCR), respectively. Initial experiments showed that the enzymes facilitated the discrimination of single-base mismatches within the recognition or ligation site. Furthermore, LCR as a target amplification step resulted in a 6000-fold increase of sensitivity, and finally an LOD of 2.6 amol was achieved with an artificial double-stranded DNA target.

Inductively coupled plasma mass spectrometry-based method for the specific quantification of sulfenic acid in peptides and proteins.

A robust ICPMS-based method is introduced to obtain relative and absolute quantification of sulfenic acid (SA) in peptides and proteins. A new metal-containing reagent (Ln-DOTA-Dimedone) devised to react specifically with SA has been developed. The lanthanide-containing metal-coded affinity tag (Ln-MeCAT) was used to quantify thiol residues. We presented two approaches which allow the parallel and consecutive determination of SA and thiols in peptide and protein samples. The high sensitivity, structure-independent signal, and multiplexing capabilities of ICPMS together with the specificity of Ln-DOTA-Dimedone and Ln-MeCAT toward sulfenic acid and thiol residues, respectively, allow the characterization of various biological states and offer closer insight onto thiol-sulphenic acid equilibria which are involved in intracellular redox-mediated events altering structure and function of proteins in important diseases.

Solid phase synthesis of short peptide-based multimetal tags for biomolecule labeling.

We describe an unprecedented solid phase peptide synthesis (SPPS) of short peptide-based multimetal tags designated as elemental tags for the quantification of biomolecules via inductively coupled plasma mass spectrometry (ICP-MS). The macrocyclic chelator 1,4,7,10-tetraazacyclododecane N,N',N″,N‴-tetra acetic acid (DOTA) was attached to the side chain of N-α-(9-fluorenylmethoxycarbonyl)-l-lysine (Fmoc-Lys-OH) and metalated with a lanthanide to provide a building block for Fmoc-based SPPS. Thereby, in contrast to existing strategies for the synthesis of DOTA-peptide conjugates, an already metalated DOTA-amino acid was used as a building block for SPPS. The DOTA-lanthanide complex was stable throughout the whole SPPS, even during the final cleavage in concentrated trifluoroacetic acid. This indicates that the strategy to first metalate the Fmoc-Lys(DOTA)-OH and to utilize the metal coordination to protect the carboxyl groups of DOTA offers an alternative to conventional synthetic routes using tert-butyl protected DOTA. Several small peptides containing up to four metal ions were synthesized, among them peptides carrying defined metal sequences consisting of two different lanthanides. The peptides were N-terminally maleimide-functionalized, thus introducing a moiety for conjugation to thiol-bearing biomolecules. The final objective of this work was the signal enhancement in ICP-MS-based DNA quantification assays. To evaluate the performance of the multimetal peptide tags in assay, they were applied to label thiol-modified 15mer DNA oligonucleotide probes. These served as reporter probes in a model sandwich-type hybridization assay. Thereby, we found that the ICP-MS signal increased linearly with the number of lanthanide ions attached to the reporter probe.

MeCAT--comparing relative quantification of alpha lactalbumin using both molecular and elemental mass spectrometry.

Chemical tagging with stable isotopes is one of the best established methods for the quantification of proteins using mass spectrometry, especially in non-proliferating cells and tissue. The absolute quantification of proteins is still a challenge. Metal-coded affinity tagging (MeCAT), used to label proteins and peptides with lanthanide ions, allows both, relative and absolute, quantitative determination. MeCAT loaded with lanthanide ions allows the use of inductively coupled plasma mass spectrometry (ICP-MS) enabling very accurate and sensitive quantification of peptides and proteins based on the metal ion signal. Furthermore, multiplex assays are possible that are not limited to 4- or 8-plex analyses when using different lanthanides. Naturally, different lanthanides also lead to different molecular masses for the same labelled peptides which can be distinguished easily. This enables the relative quantification in electrospray MS based on the relative signal intensities of the differentially labelled peptides. We have studied MeCAT labelled peptides, using LC/ESI-MS and LC/ESI-MS/MS with infrared multiphoton dissociation (IRMPD) to show that both the molecular masses and the specific fragments resulting from the MS/MS experiments can be used for relative quantification. The results are compared with high performance liquid chromatography (HPLC)/ICP-MS and direct ICP-MS analysis as standard methods. We show that the ESI and IRMPD based methods deliver quantitative results comparable to ICP-MS.

Origanum majorana L. protects against neuroinflammation-mediated cognitive impairment: a phyto-pharmacological study.

BACKGROUND: Neuroinflammation and oxidative stress are critical players in the pathogenesis of numerous neurodegenerative diseases, such as Alzheimer's disease (AD) which is responsible for most cases of dementia in the elderly. With the lack of curative treatments, natural phenolics are potential candidates to delay the onset and progression of such age-related disorders due to their potent antioxidant and anti-inflammatory effects. This study aims at assessing the phytochemical characteristics of Origanum majorana L. (OM) hydroalcohol extract and its neuroprotective activities in a murine neuroinflammatory model. METHODS: OM phytochemical analysis was done by HPLC/PDA/ESI-MSn. Oxidative stress was induced in vitro by hydrogen peroxide and cell viability was measured using WST-1 assay. Swiss albino mice were injected intraperitoneally with OM extract at a dose of 100 mg/kg for 12 days and with 250 µg/kg LPS daily starting from day 6 to induce neuroinflammation. Cognitive functions were assessed by novel object recognition and Y-maze behavioral tests. Hematoxylin and eosin staining was used to assess the degree of neurodegeneration in the brain. Reactive astrogliosis and inflammation were assessed by immunohistochemistry using GFAP and COX-2 antibodies, respectively. RESULTS: OM is rich in phenolics, with rosmarinic acid and its derivatives being major constituents. OM extract and rosmarinic acid significantly protected microglial cells against oxidative stress-induced cell death (p < 0.001). OM protected against the LPS-induced alteration of recognition and spatial memory in mice (p < 0.001) and (p < 0.05), respectively. Mice that received OM extract prior to the induction of neuroinflammation showed comparable histology to control brains, with no overt neurodegeneration. Furthermore, OM pre-treatment decreased the immunohistochemistry profiler score of GFAP from positive to low positive and COX-2 from low positive to negative in the brain tissue, compared to the LPS group. CONCLUSION: These findings highlight the potential preventive effects of OM phenolics against neuroinflammation and pave the way toward drug discovery and development for neurodegenerative disorders.

Dual labeling of biomolecules using MeCAT and DOTA derivatives: application to quantitative proteomics.

In this study, single and dual labeling of primary amino and thiol groups of target peptides is presented as a proof of concept. The proposed method allows flexible, independent and sequential labeling of the mentioned residues using lanthanides introduced via DOTA-complexes (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The efficiency of the method was optimized using cysteine-containing standard peptides and then applied to bovine serum albumin (BSA) and human serum albumin (HSA) to demonstrate qualitative and quantitative aspects of this strategy. For amino labeling, cysteinyl peptides were immobilized on Sepharose-6B resin and labeled with DOTA-NHS ester followed by metallation with lanthanides. Thiol labeling was carried out using lanthanide-containing metal-coded affinity tags (MeCAT) after elution of peptides from the resin. Complete dual labeling of the standard peptides was demonstrated by liquid chromatography electrospray ionization mass spectrometry, whereas more than 80% of the detected peptides of BSA and HSA were completely dual-labeled. Parallel detection by LC coupled to inductively coupled plasma mass spectrometry (ICP-MS) delivered reliable quantitative information. Thus, the versatile lanthanide choice in both labeling steps allowed estimating primary amino and thiol stoichiometries for the studied samples using different lanthanides. On the other hand, enhancement of ICP-MS signal was achieved as expected when all positions were labeled with the same lanthanide. Finally, linear calibrations of the signal for most of the labeled peptides by standard additions of digested BSA showed a suitable behaviour for quantitative applications and demonstrated the pre-concentration capability of the employed resin.

Absolute protein quantification by LC-ICP-MS using MeCAT peptide labeling.

Nowadays, the most common strategies used in quantitative proteomics are based on isotope-coded labeling followed by specific molecule mass spectrometry. The implementation of inductively coupled plasma mass spectrometry (ICP-MS) for quantitative purposes can solve important drawbacks such as lack of sensitivity, structure-dependent responses, or difficulties in absolute quantification. Recently, lanthanide-containing labels as metal-coded affinity tag (MeCAT) reagents have been introduced, increasing the interest and scope of elemental mass spectrometry techniques for quantitative proteomics. In this work one of the first methodologies for absolute quantification of peptides and proteins using MeCAT labeling is presented. Liquid chromatography (LC) interfaced to ICP-MS has been used to separate and quantify labeled peptides while LC coupled to electrospray ionization mass spectrometry served for identification tasks. Synthetic-labeled peptides were used as standards to calibrate the response of the detector with compounds as close as possible to the target species. External calibration was employed as a quantification technique. The first step to apply this approach was MeCAT-Eu labeling and quantification by isotope dilution ICP-MS of the selected peptides. The standards were mixed in different concentrations and subjected to reverse-phase chromatography before ICP-MS detection to consider the column effect over the peptides. Thus, the prepared multi-peptide mix allowed a calibration curve to be obtained in a single chromatographic run, correcting possible non-quantitative elutions of the peptides from the column. The quantification strategy was successfully applied to other labeled peptides and to standard proteins such as digested lysozyme and bovine serum albumin.