Application of amino acid analysis using hydrophilic interaction liquid chromatography coupled with isotope dilution mass spectrometry for peptide and protein quantification

Study of SI-traceable purity assessment of bovine lactoferrin using mass balance approach and amino acid-based isotope dilution liquid chromatography-mass spectrometry

The accurate measurement of bovine lactoferrin (bLF) attracts wide attention in food and nutraceutical applications as its important physiological and nutritional functions. We present SI traceable procedures for assessing bLF purity using mass balance method and amino acid (AA)-based isotope dilution mass spectrometry (IDMS). The mass balance method was revealed with a purity of 0.938 ± 0.011 g/g by deducting all aspects of impurities, including related structure impurities of 4.60%, ignition residue of 0.28%, Cl^- of 1.10%, SO₄^2- of 0.13%, and moisture of 0.17%. The AA-based IDMS quantitative result was 0.937 ± 0.027 g/g. Hydrolysis conditions were optimized and methodology validation including, accuracy, precision, were studied. Good consistency was achieved between the two independent strategies and bLF purity assigned via the weighted mean value of their results was 0.938 ± 0.015 g/g. These analyses are expected to be applicable to proteins quantification and development of LF certified reference materials.

Analysis of plasma free amino acids in diabetic rat and the intervention of Ginkgo biloba leaves extract using hydrophilic interaction liquid chromatography coupled with tandem mass-spectrometry

Amino acids (AAs) are important metabolites that are related with diabetes. However, their roles in the initiation and development of diabetes mellitus (DM), especially in the treatment of Ginkgo biloba leaves extract (GBE) have not been fully explored. Thus, we investigated the roles that AAs played in the progression and GBE supplementation of DM rat induced by streptozotocin. The rats were randomly divided into a normal control group treated with drug-free solution, a normal control group treated with GBE, a DM group treated with drug-free solution, and DM group treated with GBE; and maintained on this protocol for 9 weeks. Rat plasma was collected from the sixth week to the ninth week and then analyzed with the optimized hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry method. A total of 17 AAs with differential levels were monitored to indicate dysfunction of AAs metabolism to confirm the occurrence and development of DM. Treatment with GBE partially reversed the changes seen in seven AAs including leucine, isoleucine, tyrosine, glutamic acid, asparagines, lysine and alanine in DM rats, indicating that GBE could prevent the occurrence and development of DM by acting on AAs metabolism. The improvement of those AAs metabolism disorders may play a considerable role in the treatment of GBE on the occurrence and development of DM. Those findings potentially promote the understanding of the pathogenic progression of DM and reveal the therapeutic mechanism of GBE against DM.

Procedure providing SI-traceable results for the calibration of protein standards by sulfur determination and its application on tau

Quantitative proteomics is a growing research area and one of the most important tools in the life sciences. Well-characterized and quantified protein standards are needed to achieve accurate and reliable results. However, only a limited number of sufficiently characterized protein standards are currently available. To fill this gap, a method for traceable protein quantification using sulfur isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) was developed in this study. Gel filtration and membrane filtration were tested for the separation of non-protein-bound sulfur in the protein solution. Membrane filtration demonstrated a better performance due to the lower workload and the very low sulfur blanks of 11 ng, making it well suited for high-purity proteins such as NIST SRM 927, a bovine serum albumin (BSA). The method development was accomplished with NIST SRM 927e and a commercial avidin. The quantified mass fraction of NIST SRM 927e agreed very well with the certified value and showed similar uncertainties (3.6%) as established methods while requiring less sample preparation and no species-specific standards. Finally, the developed procedure was applied to the tau protein, which is a biomarker for a group of neurodegenerative diseases denoted “tauopathies” including, e.g., Alzheimer’s disease and frontotemporal dementia. For the absolute quantification of tau in the brain of transgenic mice overexpressing human tau, a well-defined calibration standard was needed. Therefore, a pure tau solution was quantified, yielding a protein mass fraction of (0.328 ± 0.036) g/kg, which was confirmed by amino acid analysis.

Mixed-acidic cation-exchange material for the separation of underivatized amino acids

Mixed-acidic cation-exchange (MCX) columns with both strongly (SCX) and weakly (WCX) acidic functional groups were developed for the separation of standard amino acids. The resins were prepared by carboxylation of highly crosslinked monodisperse poly(styrene-divinylbenzene) copolymer particles with performic acid and subsequent sulfonation with sulfuric acid. The degree of functionalization was varied independently for each processing step and controlled by measuring pH dependent retention of the obtained resins. A series of mixed-acidic resins with different SCX/WCX-ratios was chromatographically characterized by variation of formic acid and acetonitrile concentration in the aqueous eluent. The overall cation-exchange capacity was varied from 33 to 68 µmol/mL. The comparison with two commercial columns (Metrohm Metrosep C6, WCX and Hamilton PRP X-200, SCX) revealed the additive character of the different functional group properties within MCX columns and a unique selectivity which can be adjusted by both eluent composition and SCX/WCX-ratio of the resin. The retention window between neutral and basic amino acids was altered by varying the amount of sulfonic acid groups attached to the polymer. Orthogonality plots demonstrated constant selectivity for neutral amino acids. Correlating the retention data with log P data demonstrated the influence of non-ionic hydrophobic and π-π-interactions for the separation of amino acids on PS/DVB-based cation-exchangers. An isocratic IC-ESI-MS method was developed to separate and quantitate 20 underivatized standard amino acids within 30 min. Limits of detection were between 4 and 64 nmol L^-1 and a high linearity of calibration curves was obtained for all analytes. The method was validated by comparing a certified reference standard with external calibration data.

Determination of seventeen free amino acids in human urine and plasma samples using quadruple isotope dilution mass spectrometry combined with hydrophilic interaction liquid chromatography - Tandem mass spectrometry

Taking into account the growing demand for new analytical procedures that are appropriate for analysis of complex biological samples with increased sensitivity, accuracy and precision, a novel analytical method was described for the determination of underivatized amino acids in human plasma and urine samples. The presented analytical procedure involved the direct analysis of urine samples and the analysis of plasma samples followed by a simple protein precipitation protocol. Samples were analyzed using a simple and fast chromatographic method developed for the determination of 17 different amino acids by liquid chromatography - tandem mass spectrometry. The limit of detection and quantification values for amino acids were ranged between 0.03-2.26 µmol kg^-1 and 0.09-7.54 µmol kg^-1. Matrix effects of plasma and urine on the quantification of analytes were determined by spiking experiments. The accuracy of method was evaluated by matrix matching and quadruple isotope dilution strategies. Excellent accuracy and precision were obtained with the use isotope labeled amino acids demonstrating the high reliability and reproducibility of the proposed method. The percent recovery values were found to be between 98.70 - 101.68% with%RSD below than 1.62% for human plasma and 99.14 - 101.78% with%RSD below than 2.44% for urine samples.

Determination of purity values of amino acid reference materials by mass balance method: an approach to the quantification of related structure impurities

A systematic procedure for the determination of purity values of amino acid reference materials was developed by use of mass balance method where four categories of impurities (related structure impurities (RSIs), water, organic solvent residue (OSR), and non-volatile residue (NVR)) were quantified separately. The amount of RSIs was determined using a combination of three quantification methods. To ensure metrological traceability in the determination of RSIs, at least one such impurity in each candidate amino acid reference material was quantified using liquid chromatography-isotope dilution tandem mass spectrometry (LC-IDMS/MS). Other RSIs were determined using external calibration liquid chromatography-tandem mass spectrometry (LC-MS/MS) or o-phthaldialdehyde (OPA) derivatization, followed by liquid chromatography-ultraviolet (LC-UV) measurement. As the UV absorption of most RSIs came basically from the same chromophore after OPA derivatization, a relative peak area approach was used in the LC-UV method to quantify the amount of RSIs by comparing their peak areas with that of a reference RSI. The reference RSI was pre-selected and the amount determined by LC-IDMS/MS separately. The absence of D-amino acids was confirmed using Marfey's reagent derivatization, followed by LC-UV analysis. The amounts of water, OSR, and NVR were measured using Karl Fischer coulometry, gas chromatography-mass spectrometry (GC-MS) and thermogravimetry, respectively. By using this procedure, four amino acid (L-valine, L-leucine, L-isoleucine, and L-phenylalanine) certified reference materials (CRMs) were developed from the candidate materials. The homogeneity and stability of the CRMs were demonstrated by use of LC-IDMS/MS or OPA-LC-UV method, following the principles in ISO 17034 and ISO Guide 35.Graphical abstract.

Absolute quantitative analysis of intact and oxidized amino acids by LC-MS without prior derivatization

The precise characterization and quantification of oxidative protein damage is a significant challenge due to the low abundance, large variety, and heterogeneity of modifications. Mass spectrometry (MS)-based techniques at the peptide level (proteomics) provide a detailed but limited picture due to incomplete sequence coverage and imperfect enzymatic digestion. This is particularly problematic with oxidatively modified and cross-linked/aggregated proteins. There is a pressing need for methods that can quantify large numbers of modified amino acids, which are often present in low abundance compared to the high background of non-damaged amino acids, in a rapid and reliable fashion. We have developed a protocol using zwitterionic ion-exchange chromatography coupled with LC-MS to simultaneously quantify both parent amino acids and their respective oxidation products. Proteins are hydrolyzed with methanesulfonic acid in the presence of tryptamine and purified by strong cation exchange solid phase extraction. The method was validated for the common amino acids (excluding Gln, Asn, Cys) and the oxidation products 3-chlorotyrosine (3-ClTyr), 3-nitrotyrosine (3-NO₂Tyr), di-tyrosine, N^ε-(1-carboxymethyl)-l-lysine, o,o’-di-tyrosine, 3,4,-dihydroxyphenylalanine, hydroxy-tryptophan and kynurenine. Linear standard curves were observed over ~3 orders of magnitude dynamic range (2–1000 pmol for parent amino acids, 80 fmol–20 pmol for oxidation products) with limit-of-quantification values as low as 200 fmol (o,o’-di-tyrosine). The validated method was used to quantify Tyr and Trp loss, and formation of 3-NO₂Tyr on the isolated protein anastellin treated with peroxynitrous acid, and for 3-ClTyr formation (over a 2 orders of magnitude range) in cell lysates and complex protein mixtures treated with hypochlorous acid.

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Identification and quantification of oxidative protein damage is a major challenge.

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A versatile LC-MS assay is reported that involves hydrolysis to free amino acids.

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Quantification is possible for both parent amino acids and products in single runs.

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A dynamic range of 2-3 orders of magnitude is available for most analytes.

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Example of use with pure proteins, extracellular matrix and cell lysates are given.

Validation of a HILIC UHPLC-MS/MS Method for Amino Acid Profiling in Triticum Species Wheat Flours

Amino acids are essential nutritional components as they occur in foods either in free form or as protein constituents. An ultra-high-performance (UHPLC) hydrophilic liquid chromatography (HILIC)-tandem Mass Spectrometry (MS) method has been developed and validated for the quantification of 17 amino acids (AA) in wheat flour samples after acid hydrolysis with 6 M HCl in the presence of 4% (v/v) thioglycolic acid as a reducing agent. The developed method proved to be a fast and reliable tool for acquiring information on the AA profile of cereal flours. The method has been applied and tested in 10 flour samples of spelt, emmer, and common wheat flours of organic or conventional cultivation and with different extraction rates (70%, 90%, and 100%). All the aforementioned allowed us to study and evaluate the variation of the AA profile among the studied flours, in relation to other quality characteristics, such as protein content, wet gluten, and gluten index. Significant differences were observed in the AA profiles of the studied flours. Moreover, AA profiles exhibited significant interactions with quality characteristics that proved to be affected based mainly on the type of grain. A statistical and multivariate analysis of the AA profiles and quality characteristics has been performed, as to identify potential interactions between protein content, amino acids, and quality characteristics.

Ultrasensitive Quantification of Recombinant Proteins Using AAA-MS

Recombinant proteins are essential components of therapeutic, biotechnological, food, and household products. In some cases, recombinant proteins must be purified and their quantity and/or concentration precisely determined. In this chapter, we describe a protocol for the quantification of purified recombinant proteins. The protocol is based on a microwave-assisted acidic hydrolysis of the target protein followed by high-resolution mass spectrometry (HRMS) analysis of the hydrolytic products. Absolute quantification is obtained by adding controlled amounts of labeled amino acids that serve as standards.

Development and validation of a simple LC-MS/MS method for the simultaneous quantitative determination of trimethylamine-N-oxide and branched chain amino acids in human serum

Serum branched chain amino acids and trimethylamine-N-oxide are monitored as potential indicators of diabetes and cardiovascular health respectively. A rapid method for their simultaneous determination using liquid chromatography and tandem mass spectrometry is described here. Branched chain amino acids and trimethylamine-N-oxide were quantified based on their specific MS/MS fragments using a selected reaction monitoring approach. A number of columns were tested for their ability to separate the analytes. A C18-PFP column separated the analytes in just 4 minutes, and resulted in excellent peak shape and retention time repeatability, and was therefore chosen as the optimal column. A second column, the Intrada Amino Acid column, was chosen for comparison and validation experiments as it provided an orthogonal separation mechanism. The intra-day and inter-day precision and accuracy were less than 12% for trimethylamine-N-oxide and less than 6% for the branched chain amino acids. Recoveries, where serum was spiked with three different concentrations of the analytes, ranged from 97 to 113%. The LODs and LOQs for trimethylamine-N-oxide were 1 and 6 ng/mL, for leucine and isoleucine were 4 and 8 ng/mL, and for valine were 5 and 15 ng/mL, respectively. The C18-PFP column method was validated using the Intrada Amino Acid column method and percentage agreement for all four analytes was within 10%. Sample preparation was minimal, and use of labelled internal standards accounted for matrix effects. The method was successfully applied to human plasma samples. Graphical abstract ᅟ.

A rapid UHPLC-HILIC method for algal guanosine 5'-diphosphate 3'-diphosphate (ppGpp) and the potential separation mechanism

A fast and facile hydrophilic interaction liquid chromatography (HILIC) method was developed and applied to quantify physiologically important ppGpp and its analogues in a tough sample, the astaxanthin-accumulating alga Hameatococcus pluvialis. The method is able to analyze simultaneously seven nucleotides, including ppGpp at the order of pmol g^-1 cells within 12 min. Mechanism on the elution order was investigated. It was found that 1) phosphate salt competed for the amide groups on the HILIC column with the phosphate groups of the nucleotides; 2) intramolecular hydrogen bonds might contribute to the elution order by offsetting and reducing the number of free hydrogen acceptor/donor of the nucleotide molecules interacting with the amide groups. This is the first HILIC method for ppGpp, which is feasible and applicable to a wide range of samples, especially tough samples, e.g., algae and plants.

A novel potential primary method for quantification of enantiomers by high performance liquid chromatography-circular dichroism

Primary methods play an important role in metrology. They can be used for the value assignment of certified reference materials, enabling the accuracy and comparability of the measurement. A novel potential primary method for enantiomer quantitation based on high-performance liquid chromatography-circular dichroism is described using L-phenylalanine as an example. The optimal quantitation range of L-Phe was from 0.1 mg/g to 1.2 mg/g, where both the relative bias and method variance were lower than 1%. The LOD and LOQ were 4 μg/g and 30 μg/g, respectively. The proposed method was also applied to the determination of the mass fraction of pure porcine insulin in solid. The average mass fraction obtained was 0.922 g/g with a RSD of 1.5%, and the associated relative uncertainty is 3.8% (k = 2), which agreed well with that obtained from the traditional isotope dilution mass spectrometry method. The LOD and LOQ for insulin quantitation were found to be 0.12 mg/g and 0.44 mg/g, respectively. The proposed method can be entirely described and understood by equations and a complete uncertainty statement can be defined in SI units.Therefore, it may be a potential primary method useful for the quantification of chiral compounds and proteins, and a supplementary method to the traditional isotope dilution mass spectrometry approach.

Implementing a Reference Measurement System for C-Peptide: Successes and Lessons Learned

BACKGROUND

Assessment of endogenous insulin secretion by measuring C-peptide concentrations is widely accepted. Recent studies have shown that preservation of even small amounts of endogenous C-peptide production in patients with type 1 diabetes reduces risks for diabetic complications. Harmonization of C-peptide results will facilitate comparison of data from different research studies and later among clinical laboratory results at different sites using different assay methods.

CONTENT

This review provides an overview of the general process of harmonization and standardization and the challenges encountered with implementing a reference measurement system for C-peptide.

SUMMARY

Efforts to harmonize C-peptide results are described, including those by the National Institute of Diabetes and Digestive and Kidney Diseases-led C-peptide Standardization Committee in the US, activities in Japan, efforts by the National Institute for Biological Standards and Control in the UK, as well as activities led by the Bureau International des Poids et Mesures and the National Metrology Institute in China. A traceability scheme is proposed along with the next steps for implementation. Suggestions are made for better collaboration to optimize the harmonization process for other measurands.

High-Speed Quantitative UPLC-MS Analysis of Multiple Amines in Human Plasma and Serum via Precolumn Derivatization with 6-Aminoquinolyl-N-hydroxysuccinimidyl Carbamate: Application to Acetaminophen-Induced Liver Failure

A targeted reversed-phase gradient UPLC-MS/MS assay has been developed for the quantification /monitoring of 66 amino acids and amino-containing compounds in human plasma and serum using precolumn derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AccQTag Ultra). Derivatization of the target amines required minimal sample preparation and resulted in analytes with excellent chromatographic and mass spectrometric detection properties. The resulting method, which requires only 10 μL of sample, provides the reproducible and robust separation of 66 analytes in 7.5 min, including baseline resolution of isomers such as leucine and isoleucine. The assay has been validated for the quantification of 33 amino compounds (predominantly amino acids) over a concentration range from 2 to 20 and 800 μM. Intra- and interday accuracy of between 0.05 and 15.6 and 0.78-13.7% and precision between 0.91 and 16.9% and 2.12-15.9% were obtained. A further 33 biogenic amines can be monitored in samples for relative changes in concentration rather than quantification. Application of the assay to samples derived from healthy controls and patients suffering from acetaminophen (APAP, paracetamol)-induced acute liver failure (ALF) showed significant differences in the amounts of aromatic and branched chain amino acids between the groups as well as a number of other analytes, including the novel observation of increased concentrations of sarcosine in ALF patients. The properties of the developed assay, including short analysis time, make it suitable for high-throughput targeted UPLC-ESI-MS/MS metabonomic analysis in clinical and epidemiological environments.

A hydrophilic interaction chromatography-tandem mass spectrometry method for amino acid profiling in mussels

A UHPLC-HILIC-tandem MS method has been developed and validated for the quantification of 21 amino acids (20 protein amino acids and cystine) in their free form (FAA) and as protein constituents (total amino acids, TAA) in a rich protein food matrix such as lyophilized mussels (Mytilus galloprovincialis) samples. FAA were analyzed after suspending the samples in the presence of trichloroacetic acid in order to prevent dissolving the proteins, while TAA were determined after acid hydrolysis with 6M HCl in the presence of 4% v/v thioglycolic acid as a reducing agent. In hydrolysed samples 17 amino acids could be determined since tryptophan, cysteine, cystine and asparagine were degraded during acid hydrolysis. Linear regression coefficients (R²) were above 0.99 for all amino acids. Accuracy and precision, expressed as recovery (%) and relative standard deviation (RSD, %) were in acceptable levels, ranging from 78.2 to 123.3% and below 15%, respectively for both FAA and TAA. Uncertainty was also below 12% for FAA and below 22% for TAA. Sensitivity of the method was high with LOD values ranging from 0.003 to 0.034g/100g for FAA and 0.001 to 0.004g/100g for TAA, while LOQ ranged from 0.009 to 0.104g/100g for FAA and 0.002 to 0.011g/100g for TAA. The method proved to be a fast and reliable tool for acquiring information on free and total amino acids profile in high protein content foodstuffs such as mussels.

Secondary Structure Alterations of Histones H2A and H2B in X-Irradiated Human Cancer Cells: Altered Histones Persist in Cells for at Least 24 Hours

We measured and compared the circular dichroism (CD) spectra and secondary structures of histone proteins H2A, H2B and their variants extracted from X-irradiated and unirradiated human HeLa cells. Compared to unirradiated cells, a relative increase in α-helix structure and decrease in other secondary structures was observed in X-irradiated cells. These structural alterations persisted for at least 24 h, which is substantially longer than the 2 h generally known to be required for DNA double-strand break repair.

Supercritical synthesis of poly (2-dimethylaminoethyl methacrylate)/ferrite nanocomposites for real-time monitoring of protein release

A supercritical carbon dioxide (SCC)-assisted process was developed to synthesize protein-supported poly (2-dimethylaminoethyl methacrylate)/ferrite nanocomposites (PNCs). The process involve 2,2-azobisisobutyronitrile-initiated in situ polymerization of 2-dimethylaminoethyl methacrylate in presence of ferrite nanoparticles and bisacrylamide at 90 ± 1 °C, 1200 psi over 6 h in SCC. This was followed by subsequent loading of bovine serum albumin (BSA) as a model protein over PNCs in phosphate buffer (PBS, pH 7.4) at 1200 psi, 35 ± 1 °C over additional 2 h in SCC. The formation of PNCs was ascertained through ultraviolet-visible, Fourier transform-infrared, X-ray diffraction spectra, transmission electron, atomic force microscopy and magnetometry. The developed process extends large scale production of nanomagnetic PNCs suitable as carrier for protein release applications with optimal release properties. The release of protein from PNCs under in vitro in PBS down to nanomolar range with high temporal resolution, speed and reproducibility was quantified through square wave voltammetry.

Alkaline conditions in hydrophilic interaction liquid chromatography for intracellular metabolite quantification using tandem mass spectrometry

Modeling of metabolic networks as part of systems metabolic engineering requires reliable quantitative experimental data of intracellular concentrations. The hydrophilic interaction liquid chromatography-electrospray ionization-tandem mass spectrometry (HILIC-ESI-MS/MS) method was used for quantitative profiling of more than 50 hydrophilic key metabolites of cellular metabolism. Without prior derivatization, sugar phosphates, organic acids, nucleotides, and amino acids were measured under alkaline and acidic mobile phase conditions with pre-optimized multiple reaction monitoring (MRM) transitions. Irrespective of the polarity mode of the acquisition method used, alkaline conditions achieved the best quantification limits and linear dynamic ranges. Fully 90% of the analyzed metabolites presented detection limits better than 0.5pmol (on column), and 70% presented 1.5-fold higher signal intensities under alkaline mobile phase conditions. The quality of the method was further demonstrated by absolute quantification of selected metabolites in intracellular extracts of Escherichia coli. In addition, quantification bias caused by matrix effects was investigated by comparison of calibration strategies: standard-based external calibration, isotope dilution, and standard addition with internal standards. Here, we recommend the use of alkaline mobile phase with polymer-based zwitterionic hydrophilic interaction chromatography (ZIC-pHILIC) as the most sensitive scenario for absolute quantification for a broad range of metabolites.

Applications of hydrophilic interaction chromatography to amino acids, peptides, and proteins

This review summarizes the recent advances in the analysis of amino acids, peptides, and proteins using hydrophilic interaction chromatography. Various reports demonstrate the successful analysis of amino acids under such conditions. However, a baseline resolution of the 20 natural amino acids has not yet been published and for this reason, there is often a need to use mass spectrometry for detection to further improve selectivity. Hydrophilic interaction chromatography is also recognized as a powerful technique for peptide analysis, and there are a lot of papers showing its applicability for proteomic applications (peptide mapping). It is expected that its use for peptide mapping will continue to grow in the future, particularly because this analytical strategy can be combined with reversed-phase liquid chromatography, in a two-dimensional setup, to reach very high resolving power. Finally, the interest in hydrophilic interaction chromatography for intact proteins analysis is less evident due to possible solubility issues and a lack of suitable hydrophilic interaction chromatography stationary phases. To date, it has been successfully employed only for the characterization of membrane proteins, histones, and the separation of glycosylated isoforms of an intact glycoprotein. From our point of view, the number of hydrophilic interaction chromatography columns compatible with intact proteins (higher upper temperature limit, large pore size, etc.) is still too limited.

A novel amino acid analysis method using derivatization of multiple functional groups followed by liquid chromatography/tandem mass spectrometry

We have developed a novel amino acid analysis method using derivatization of multiple functional groups (amino, carboxyl, and phenolic hydroxyl groups).

Determination of Cystatin C in human serum by isotope dilution mass spectrometry using mass overlapping peptides

We propose a peptide-based isotope dilution mass spectrometry approach for Cystatin C determination in human serum samples, a clinical marker for renal status for which backup by a mass spectrometry based primary method has been missing so far. In contrast to common protocols, the isotope labelled version of the proteotypic signature peptide is designed such as keeping the isotopic difference as little as possible with respect to the peptide released from the protein. Peptides labelled in two (13)C atoms are added to the serum samples just before proteolysis. After two steps of chromatographic purification the sample is measured by selected reaction monitoring using a LC-MS/MS. Resolution of the first quadrupole is reduced to transmit the whole parent ion cluster to the collision cell for monitoring accurate isotopic distributions of the molecular fragments. Molar fractions of labelled and natural abundance peptides are directly obtained from the experimental mass spectra of the in-cell fragment ions. Thus, the natural abundance protein concentration is obtained from the fragment-ion spectrum of the sample without resorting to extra calibration runs. Applicability of the approach is demonstrated by the measurement of the serum concentration of Cystatin C in Reference Material ERM R-DA471/IFCC and real samples.

BIOLOGICAL SIGNIFICANCE

Cystatin C is used as an alternative marker instead of, or in combination with creatinine for non-invasive determination of glomerular filtration rates. Advantages advocating in favour of Cystatin C in diagnosis of chronic kidney diseases are the lower variability of its serum level and, particularly, virtual independence on sex, age and muscle mass. However, in order to capitalize, accuracy of measurement has to be in proportion with the predictive power of the marker. Though there are label-free methods available for screening purposes or high-throughput analysis, achieving high levels of reliability and accuracy in quantitative proteomics takes reference to isotope labelled materials. Present routine assays (mainly nephelometry, turbidimetry and ligand-binding assays) are known to leave improvement to be desired in that respect. Absolute quantification based on enzymatic signature-peptides provides a method principle establishing traceability to the International System of Units on the level of primary methods. The kind of technique is capable, by this way, of high accuracy value-assignment to matrix materials needed for calibration of present routine assays, where not completely replacing them. Cystatin C measurement by isotope dilution mass spectrometry is developed in this study with the aim of making available this tool to support diagnostics of kidney function in the same way.

A novel quantification method for analysis of twenty natural amino acids in human serum based on N-phosphorylation labeling using reversed-phase liquid chromatography-tandem mass spectrometry

A novel method based on the strategy of N-phosphorylation labeling is described for quantification of twenty natural amino acids in human serum by reversed-phase liquid chromatography-electrospray tandem mass spectrometry (RP-LC/ESI-MS). The derivatization reaction was easily performed in one-pot reaction under mild conditions within 30min. The reaction mixture was then evaporated to dryness, redissolved, desalted by C18 SPE. The twenty N-phosphoryl amino acids were separated on an RP-C18 column within 20min by isocratic elution (0.1% formic acid-acetonitrile, v/v 7:3). At the same time, multiple reaction monitoring (MRM) MS enabled quantitation of twenty natural amino with the LOD of 0.0005-0.15μM and LOQ of 0.0020-0.5μM in human serum. The linear range was from 0.025 to 25μM (except Cys and Trp) with R>0.99. The recovery range was determined to be 85.5-117.4% with the relative standard deviation (RSD) in the range of 1.3-13.9%. All twenty amino acids were successfully detected in human serum samples with the concentration from 5.7 to 577.9μM, which indicates potential of the developed method for determination of amino acids in complex biological samples, hence for screening of amino acid metabolite related diseases.

Development of an SI-Traceable HPLC-Isotope Dilution Mass Spectrometry Method To Quantify β-Lactoglobulin in Milk Powders

β-Lactoglobulin (β-LG) is one of the major allergenic proteins in milk. There is an urgent demand for an accurate and traceable method to develop β-LG certified reference material (CRM). In this work, β-LG was enzymatically digested and a specific peptide was chosen for quantitation by isotope-dilution mass spectrometry (IDMS). With amino acid CRMs as standards, the results could be traced to SI unit. By the proposed method, the recovery ranged from 86.0% to 118.3% with CVs <9.0%. The LOD and LOQ were 4.8 × 10^-5 g/g and 1.6 × 10^-4 g/g of β-LG in milk powder, respectively. Ten samples from domestic market were analyzed with CVs <5.6%, and the relative expanded uncertainties ranged from 4.2% to 5.9% (k = 2). With the CRMs, it is expected that the comparability of β-LG quantitation results will be improved among different laboratories.

Precise measurement for the purity of amino acid and peptide using quantitative nuclear magnetic resonance

Precise measurement for the purity of organic compounds will fundamentally improve the capabilities and measurement services of the organic chemical analysis. Quantitative nuclear magnetic resonance (qNMR) is an important method to assess the purity of organic compounds. We presented a precise measurement method for the purity of small molecule with identification of impurities. In addition, the qNMR was rarely applied to purity of large compounds such as peptide, for which qNMR peaks are too crowded. Other than general idea of qNMR, we removed unwanted exchangeable peaks by proton exchange, as a new approach for qNMR, to make the quantitative protons of peptide isolated, which can ensure precise measurement. Moreover, a suitable internal standard, acesulfame potassium, was applied. The analytes were valine and peptide T5, due to their importance for protein analysis. For valine, the intraday CV was 0.052%, and the interday CV during 8 months was 0.071%. For peptide T5, simpler operation, shorter analytical time (1h vs. 3 days) and smaller CV (0.36% vs. 0.93%) were achieved by qNMR, compared with a traditional method (amino acid based isotope labeled mass spectrometry) via a hydrolysis reaction. This method has greatly increased the quantitative precision of qNMR for small compounds, and extended application scope of qNMR from small compounds to peptides.

Standardised methods for amino acid analysis of food

Amino acids (AA) are essential nutritional components of a balanced diet and occur in foods in either the free AA form or as the building blocks of proteins. The analysis of AAs in foods is composed of a number of unit operations; the release of the AAs from the food matrix, the separation of the individual AAs and their quantification using calibration standards. Each of these steps has their own idiosyncrasies, e.g. different hydrolysis conditions are required for the optimal release of different AAs and there are a diverse number and type of food matrices, such that most laboratories adapt methods to best suit their applications. There is currently no official standardised method for AA analysis, although the Association of Analytical Communities (AOAC) has validated methods for a number of individual AA components. The established analytical techniques of HPLC (ion exchange or reversed phase) and GC-MS have recently been supplemented by a number of new methods. These include capillary electrophoresis MS and Ultra HPLC-MS, and LC with other detectors. This review will address the intricacies and concerns of the protein hydrolysis step, discuss what specifications or prerequisites need to be placed on the existing and new methods and laboratories using these methods, comment on whether one method can successfully satisfy the exacting requirements of the various unit operations, and finally pose the question ‘Is there any merit in ‘developing’ a validated (e.g. AOAC) official method of analysis for AAs in food?’

An UPLC-ESI-MS/MS Assay Using 6-Aminoquinolyl-N-Hydroxysuccinimidyl Carbamate Derivatization for Targeted Amino Acid Analysis: Application to Screening of Arabidopsis thaliana Mutants

In spite of the large arsenal of methodologies developed for amino acid assessment in complex matrices, their implementation in metabolomics studies involving wide-ranging mutant screening is hampered by their lack of high-throughput, sensitivity, reproducibility, and/or wide dynamic range. In response to the challenge of developing amino acid analysis methods that satisfy the criteria required for metabolomic studies, improved reverse-phase high-performance liquid chromatography-mass spectrometry (RPHPLC-MS) methods have been recently reported for large-scale screening of metabolic phenotypes. However, these methods focus on the direct analysis of underivatized amino acids and, therefore, problems associated with insufficient retention and resolution are observed due to the hydrophilic nature of amino acids. It is well known that derivatization methods render amino acids more amenable for reverse phase chromatographic analysis by introducing highly-hydrophobic tags in their carboxylic acid or amino functional group. Therefore, an analytical platform that combines the 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) pre-column derivatization method with ultra performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) is presented in this article. For numerous reasons typical amino acid derivatization methods would be inadequate for large scale metabolic projects. However, AQC derivatization is a simple, rapid and reproducible way of obtaining stable amino acid adducts amenable for UPLC-ESI-MS/MS and the applicability of the method for high-throughput metabolomic analysis in Arabidopsis thaliana is demonstrated in this study. Overall, the major advantages offered by this amino acid analysis method include high-throughput, enhanced sensitivity and selectivity; characteristics that showcase its utility for the rapid screening of the preselected plant metabolites without compromising the quality of the metabolic data. The presented method enabled thirty-eight metabolites (proteinogenic amino acids and related compounds) to be analyzed within 10 min with detection limits down to 1.02 × 10-11 M (i.e., atomole level on column), which represents an improved sensitivity of 1 to 5 orders of magnitude compared to existing methods. Our UPLC-ESI-MS/MS method is one of the seven analytical platforms used by the Arabidopsis Metabolomics Consortium. The amino acid dataset obtained by analysis of Arabidopsis T-DNA mutant stocks with our platform is captured and open to the public in the web portal PlantMetabolomics.org. The analytical platform herein described could find important applications in other studies where the rapid, high-throughput and sensitive assessment of low abundance amino acids in complex biosamples is necessary.

Hydrophilic interaction ultra performance liquid chromatography retention prediction under gradient elution

The development and application of new separation mechanisms such as hydrophilic interaction chromatography (HILIC) is of high importance for the simultaneous analysis of polar molecules such as primary metabolites. However the retention mechanism in HILIC is not fully understood and as a result retention prediction tools are not at hand for this chromatographic approach. In the present report we study the utility of a simple algorithm, based on a simple linear and/or a simple logarithmic retention model, for retention prediction in HILIC gradient separation of a mixture of 23 selected compounds including (poly)amines, amino acids, saccharides, and other molecules. Utilizing two types of gradient elution programs with or without an isocratic part, retention data were collected in order to build prediction models. Starting from at least three gradient runs the prediction of analyte retention was very satisfactory for all gradient programs tested, providing useful evidence of the value of such retention time prediction methodologies.

Amino acid analysis by hydrophilic interaction chromatography coupled with isotope dilution mass spectrometry

Here, we describe an amino acid analysis that is based on the use of hydrophilic interaction liquid chromatography coupled with isotope dilution mass spectrometry for the accurate quantification of underivatized amino acids from hydrolyzed peptide/protein. Twelve underivatized amino acids were separated and detected during an 88-min runtime. The absolute limits of detection and limits of quantification (on column) of the four amino acids (isoleucine, phenylalanine, proline, and valine) were in the range of 6-80 and 20-200 fmol, respectively. As little as 25 pmol of peptide or protein hydrolysates is sufficient for determining absolute content.