A novel derivatization reagent possessing a bromoquinolinium structure for biological carboxylic acids in HPLC-ESI-MS/MS

Derivatization of carboxylic groups prior to their LC analysis - A review

Carboxylic acids (CAs) represent a large group of important molecules participating in various biologically significant processes. Analytical study of these compounds is typically performed by liquid chromatography (LC) combined with various types of detection. However, their analysis is often accompanied by a wide variety of problems depending on used separation system or detection method. The dominant ones are: i) poor chromatographic behavior of the CAs in reversed-phase LC; ii) absence of a chromophore (or fluorophore); iii) weak ionization in mass spectrometry (MS). To overcome these problems, targeted chemical modification, and derivatization, come into play. Therefore, derivatization still plays an important and, in many cases, irreplaceable role in sample preparation, and new derivatization methods of CAs are constantly being developed. The most commonly used type of reaction for CAs derivatization is amidation. In recent years, an increased interest in the isotopic labeling derivatization method has been observed. In this review, we comprehensively summarize the possibilities and actual trends in the derivatization of CAs that have been published over the past decade.

Liquid Chromatography-Mass Spectrometry (LC-MS) Derivatization-Based Methods for the Determination of Fatty Acids in Biological Samples

Fatty acids (FAs) play pleiotropic roles in living organisms, acting as signaling molecules and gene regulators. They are present in plants and foods and may affect human health by food ingestion. As a consequence, analytical methods for their determination in biological fluids, plants and foods have attracted high interest. Undoubtedly, mass spectrometry (MS) has become an indispensable technique for the analysis of FAs. Due to the inherent poor ionization efficiency of FAs, their chemical derivatization prior to analysis is often employed. Usually, the derivatization of the FA carboxyl group aims to charge reversal, allowing detection and quantification in positive ion mode, thus, resulting in an increase in sensitivity in determination. Another approach is the derivatization of the double bond of unsaturated FAs, which aims to identify the double bond location. The present review summarizes the various classes of reagents developed for FA derivatization and discusses their applications in the liquid chromatography-MS (LC-MS) analysis of FAs in various matrices, including plasma and feces. In addition, applications for the determination of eicosanoids and fatty acid esters of hydroxy fatty acids (FAHFAs) are discussed.

Differential Bile Acid Detection in Refractory GERD Patient Saliva Using a Simple and Sensitive Liquid Chromatography Tandem Mass Spectrometry Approach

GOALS

The aim was to measure bile acids in human saliva using a sensitive ultraperformance liquid chromatography tandem mass spectrometry analysis method to distinguish quantitative differences in refractory gastroesophageal reflux disease (GERD) patients as compared with proton pump inhibitor (PPI) controlled GERD patients and healthy volunteers.

STUDY

Human saliva samples were analyzed from 2 separate studies. The first a meal-controlled pilot, in which premeal and postmeal saliva samples were analyzed from 20 healthy subjects and 20 patients with GERD symptoms controlled by PPIs. In a subsequent exploratory study, saliva was collected from 34 patients with continuing GERD symptoms despite PPI treatment (refractory GERD), 30 healthy subjects, and 30 PPI-controlled GERD patients at ≥4 hours postmeal.

RESULTS

In the meal-controlled pilot study, both healthy subjects and patients with PPI-controlled GERD, had total saliva bile acid increase for the first hour after consumption of a meal and returned to baseline levels 4 hours later. There was no difference in bile acid levels between the 2 groups. In the exploratory study, the saliva from patients with refractory GERD had statistically significant higher levels of total bile acid concentration compared with those of healthy volunteers and patients with PPI-controlled GERD (P=0.0181).

CONCLUSIONS

Bile acids can be detected and accurately quantitated in human saliva using a sensitive ultraperformance liquid chromatography tandem mass spectrometry assay. Increases above threshold could indicate an underlying disease.This method could potentially be used to evaluate biliary reflux as an underlying pathophysiology of refractory GERD.

Identifying the Metabolic Signatures of PPARD-Overexpressing Gastric Tumors

Peroxisome proliferator-activated receptor delta (PPARD) is a nuclear receptor known to play an essential role in regulation of cell metabolism, cell proliferation, inflammation, and tumorigenesis in normal and cancer cells. Recently, we found that a newly generated villin-PPARD mouse model, in which PPARD is overexpressed in villin-positive gastric progenitor cells, demonstrated spontaneous development of large, invasive gastric tumors as the mice aged. However, the role of PPARD in regulation of downstream metabolism in normal gastric and tumor cells is elusive. The aim of the present study was to find PPARD-regulated downstream metabolic changes and to determine the potential significance of those changes to gastric tumorigenesis in mice. Hyperpolarized [1-13C] pyruvate magnetic resonance spectroscopy, nuclear magnetic resonance spectroscopy, and liquid chromatography-mass spectrometry were employed for metabolic profiling to determine the PPARD-regulated metabolite changes in PPARD mice at different ages during the development of gastric cancer, and the changes were compared to corresponding wild-type mice. Nuclear magnetic resonance spectroscopy-based metabolomic screening results showed higher levels of inosine monophosphate (p = 0.0054), uracil (p = 0.0205), phenylalanine (p = 0.017), glycine (p = 0.014), and isocitrate (p = 0.029) and lower levels of inosine (p = 0.0188) in 55-week-old PPARD mice than in 55-week-old wild-type mice. As the PPARD mice aged from 10 weeks to 35 weeks and 55 weeks, we observed significant changes in levels of the metabolites inosine monophosphate (p = 0.0054), adenosine monophosphate (p = 0.009), UDP-glucose (p = 0.0006), and oxypurinol (p = 0.039). Hyperpolarized [1-13C] pyruvate magnetic resonance spectroscopy performed to measure lactate flux in live 10-week-old PPARD mice with no gastric tumors and 35-week-old PPARD mice with gastric tumors did not reveal a significant difference in the ratio of lactate to total pyruvate plus lactate, indicating that this PPARD-induced spontaneous gastric tumor development does not require glycolysis as the main source of fuel for tumorigenesis. Liquid chromatography-mass spectrometry-based measurement of fatty acid levels showed lower linoleic acid, palmitic acid, oleic acid, and steric acid levels in 55-week-old PPARD mice than in 10-week-old PPARD mice, supporting fatty acid oxidation as a bioenergy source for PPARD-expressing gastric tumors.

Options of the Main Derivatization Approaches for Analytical ESI and MALDI Mass Spectrometry

The inclusion of preliminary chemical labeling (derivatization) in the analysis process by such powerful and widespread methods as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is a popular and widely used methodological approach. This is due to the need to remove some fundamental limitations inherent in these powerful analytic methods. Although a number of special reviews has been published discussing the utilization of derivatization approaches, the purpose of the present critical review is to comprehensively summarize, characterize and evaluate most of the previously developed and practically applied, as well as recently proposed representative derivatization reagents for ESI-MS and MALDI-MS platforms in their mostly sensitive positive ion mode and frequently hyphenated with separation techniques. The review is focused on the use of preliminary chemical labeling to facilitate the detection, identification, structure elucidation, quantification, profiling or MS imaging of compounds within complex matrices. Two main derivatization approaches, namely the introduction of permanent charge-fixed or highly proton affinitive residues into analytes are critically evaluated. In situ charge-generation, charge-switch and charge-transfer derivatizations are considered separately. The potential of using reactive matrices in MALDI-MS and chemical labeling in MS-based omics sciences is given.

Liquid-Chromatographic Methods for Carboxylic Acids in Biological Samples

Carboxyl-bearing low-molecular-weight compounds such as keto acids, fatty acids, and other organic acids are involved in a myriad of metabolic pathways owing to their high polarity and solubility in biological fluids. Various disease areas such as cancer, myeloid leukemia, heart disease, liver disease, and lifestyle diseases (obesity and diabetes) were found to be related to certain metabolic pathways and changes in the concentrations of the compounds involved in those pathways. Therefore, the quantification of such compounds provides useful information pertaining to diagnosis, pathological conditions, and disease mechanisms, spurring the development of numerous analytical methods for this purpose. This review article addresses analytical methods for the quantification of carboxylic acids, which were classified into fatty acids, tricarboxylic acid cycle and glycolysis-related compounds, amino acid metabolites, perfluorinated carboxylic acids, α-keto acids and their metabolites, thiazole-containing carboxylic acids, and miscellaneous, in biological samples from 2000 to date. Methods involving liquid chromatography coupled with ultraviolet, fluorescence, mass spectrometry, and electrochemical detection were summarized.

Validation and Application of a Derivatization-Free RP-HPLC-DAD Method for the Determination of Low Molecular Weight Salivary Metabolites

Saliva is an interesting, non-conventional, valuable diagnostic fluid. It can be collected using standardized sampling device; thus, its sampling is easy and non-invasive, it contains a variety of organic metabolites that reflect blood composition. The aim of this study was to validate a user-friendly method for the simultaneous determination of low molecular weight metabolites in saliva. We have optimized and validated a high throughput, direct, low-cost reversed phase liquid chromatographic method with diode array detection method without any pre- or post-column derivatization. We indexed salivary biomolecules in 35 whole non-stimulated saliva samples collected in 8 individuals in different days, including organic acids and amino acids and other carbonyl compounds. Among these, 16 whole saliva samples were collected by a single individual over three weeks before, during and after treatment with antibiotic in order to investigate the dynamics of metabolites. The concentrations of the metabolites were compared with the literature data. The multianalyte method here proposed requires a minimal sample handling and it is cost-effectiveness as it makes possible to analyze a high number of samples with basic instrumentation. The identification and quantitation of salivary metabolites may allow the definition of potential biomarkers for non-invasive "personal monitoring" during drug treatments, work out, or life habits over time.

Analysis of bile acids in human biological samples by microcolumn separation techniques: A review

Bile acids are a group of compounds essential for lipid digestion and absorption with a steroid skeleton and a carboxylate side chain usually conjugated to glycine or taurine. Bile acids are regulatory molecules for a number of metabolic processes and can be used as biomarkers of various disorders. Since the middle of the twentieth century, the detection of bile acids has evolved from simple qualitative analysis to accurate quantification in complicated mixtures. Advanced methods are required to characterize and quantify individual bile acids in these mixtures. This article overviews the literature from the last two decades (2000-2020) and focuses on bile acid analysis in various human biological samples. The methods for sample preparation, including the sample treatment of conventional (blood plasma, blood serum, and urine) and unconventional samples (bile, saliva, duodenal/gastric juice, feces, etc.) are shortly discussed. Eventually, the focus is on novel analytical approaches and methods for each particular biological sample, providing an overview of the microcolumn separation techniques, such as high-performance liquid chromatography, gas chromatography, and capillary electrophoresis, used in their analysis. This is followed by a discussion on selected clinical applications.

Analysis of major bile acids in saliva samples of patients with Barrett's esophagus using high-performance liquid chromatography-electrospray ionization-mass spectrometry

A fast, non-invasive, high-performance liquid chromatographic screening method with electrospray ionization mass spectrometric detection was developed for the analysis of three major glycine-conjugated bile acids in human saliva. Using a mobile phase composed of 80% methanol and 0.1% formic acid, glycocholic, glycodeoxycholic, and glycochenodeoxycholic acids were separated in less than 4 minutes with sensitivity in the low nM range. Bile acids are thought to contribute to the pathology of various complications in gastroesophageal reflux disease, for instance, Barrett's esophagus, which may eventually lead to esophageal carcinoma. In this pilot study, samples of saliva obtained from 15 patients with Barrett's esophagus of various severities were compared to saliva samples from 10 healthy volunteers. Glycochenodeoxycholic acid was significantly elevated in the patients and principal component analysis of all bile acids could distinguish the most severe Barrett's esophagus patients. We also reported on the detection of glycochenodeoxycholic acid in exhaled breath condensate for the first time. The promising results of this pilot study warrant future investigation, aiming at non-invasive diagnostics of Barrett's esophagus susceptibility in patients with gastroesophageal reflux disease.

Facile derivatization of ultratrace carboxylic acids in saliva for quantification by HPLC-MS/MS

It remains an issue to directly quantify trace biologically important carboxyl compounds in body fluids. Herein we propose an innovative method to determine α-lipoic acid, 2-(β-carboxyethyl)-6-hydroxy-2,7,8-trimethylchroman, prostaglandin E₂, cholic acid, and chenodeoxycholic acid in saliva. The method consists of two successive steps: fast and direct labeling of the target analytes with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide followed by ultrahigh-performance liquid chromatography-tandem mass spectrometry analysis. The method exhibited a wide linear range from 2.5 to 2500 pg/mL, with linear coefficients greater than 0.9963 and limits of detection and quantification as low as 0.10 and 0.33 pg/mL, respectively. The method precision was evaluated, with relative standard deviations ranging from 2.12% to 10.63% for intraday assays and from 2.98% to 12.88% for interday assays. The recoveries were measured by our spiking saliva samples with standards at three different levels, and ranged from 72.5% to 98.0%. Real applicability was validated by direct quantification of trace target analytes in human saliva, with simple pretreatment, use of a small sample volume, and a short analysis time. Graphical abstract Sequential steps to extract, label, and determine the ultratrace carboxylic acids in saliva. CDCA chenodeoxycholic acid, γ-CEHC 2-(β-carboxyethyl)-6-hydroxy-2,7,8-trimethylchroman, α-LA α-lipoic acid, PGE₂ prostaglandin E₂, UHPLC-MS/MS ultrahigh-performance liquid chromatography-tandem mass spectrometry.

A New Derivatization Reagent for HPLC-MS Analysis of Biological Organic Acids

Small molecules containing carboxylic acid functional groups are ubiquitous throughout biology, playing vital roles in biological chemistry ranging from energy metabolism to cellular signaling. This paper describes a new derivatization reagent, 4-bromo-N-methylbenzylamine, which was selected for its potential to derivatize mono-, di- and tri-carboxylic acids, such as the intermediates of the tricarboxylic acid (TCA) cycle. This derivatization procedure facilitated the use of positive electrospray ionization (ESI) tandem mass spectrometry (MS/MS) detection of derivatized species allowing for clear identification thanks to the easily recognizable isotope pattern of the incorporated bromine. A liquid chromatography (LC)-MS/MS method was developed which provided limits of detection between 0.2 and 44 μg L⁻¹ in under 6 min, depending on the analyte and total analysis time. This method was successfully applied in both in vitro and in vivo models.

Unconjugated bile acids in rat brain: Analytical method based on LC/ESI-MS/MS with chemical derivatization and estimation of their origin by comparison to serum levels

Although some studies have revealed the implication of bile acids (BAs) and neurological diseases, the levels and origin of the BAs in the brain are not fully understood. In this study, we first developed and validated a sensitive and specific method for the determination of three unconjugated BAs [cholic acid (CA), chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA)] in the rat brain by liquid chromatography/electrospray ionization-tandem mass spectrometry combined with chemical derivatization. The measured brain concentrations (mean±standard deviation, n=10) of normal rats were 58.7±48.8, 14.2±11.7 and 13.2±8.7ng/g tissue for CA, CDCA and DCA, respectively. For their origin, we developed the hypothesis that they might be mostly derived from the periphery. To test this hypothesis, the brain BA levels were compared with the serum levels. The brain levels had high correlations with the serum levels, and were always lower than the serum levels for the three unconjugated BAs. Furthermore, the higher brain-to-serum concentration ratios were found for the BAs with higher logD values (higher lipophilicity). Moreover, the brains of the rats intraperitoneally administered with deuterium-labeled CA and CDCA were also analyzed; the deuterium-labeled BAs were detected in the brain of the rats administered with these compounds. Based on all the results, we concluded that the BAs found in the brain are mostly derived from the periphery and the major mechanism for the transportation of the unconjugated BAs to the brain is by passive diffusion.

Caveolin-1 regulates hormone resistance through lipid synthesis, creating novel therapeutic opportunities for castration-resistant prostate cancer

Caveolin-1 (Cav-1) is overexpressed in aggressive and metastatic prostate cancer (PCa) and induces PCa cell proliferation. Androgens mediate lipid synthesis through acetyl-CoA carboxylase-1 (ACC1) and fatty acid synthase (FASN). We investigated the Cav-1-mediated lipid synthesis in the development of castration resistance, and identified novel therapeutic opportunities. Using the PBCre+;Ptenloxp/loxp;PBCav-1+ mouse model we found that Cav-1 induction increased cancer incidence and growth, and ACC1-FASN expression in intact and castrated mice. We demonstrated that Cav-1 regulated ACC1 and FASN expression in an AR-independent way and increased palmitate synthesis using western blot analysis, qRT-PCR and mass spectrometry in vitro. By using FASN siRNA and C-75, we found that FASN inhibition was more effective in Cav-1-overexpressing cells. This inhibition was abrogated by ACC1si RNA, revealing the role of malonyl-CoA, an ACC1 product, as a mediator of cytotoxicity. Cav-1 was associated with ACC1 in human tumors and ACC1, FASN, and Cav-1 expression were increased in metastatic PCa compared to primary tumors and normal prostate epithelium. Palmitoleate and oleate levels were higher in BMA from patients with metastatic PCa who responded poorly to abiraterone acetate. Our findings suggest that Cav-1 promotes hormone resistance through the upregulation of ACC1-FASN and lipid synthesis under androgen deprivation, suggesting that FASN inhibition could be used to treat PCa that demonstrates Cav-1 overexpression.

Profiling of 3-hydroxy fatty acids as environmental markers of endotoxin using liquid chromatography coupled to tandem mass spectrometry

3-Hydroxy acids are constituents of the lipid A part of lipopolysaccharides and may potentially be used as chemical markers of endotoxin. While commercial enzymatic assays, such as the widely used Limulus amebocyte lysate (LAL) assay, commonly detect merely the water-soluble fraction of the bioactive endotoxin, the chemical approach aims to estimate the total amount of endotoxin present in a sample. Our objective was to develop a simple method for quantitative profiling of 3-hydroxy fatty acids in occupational and environmental samples based on detection with HPLC-MS/MS. We included eleven 3-hydroxy fatty acids (3-hydroxyoctanoic acid to 3-hydroxyoctadecanoic acid) in the HPLC-MS/MS based method, which involved base hydrolysis of filter samples using 1M sodium hydroxide and removal of the base as well as concentration of the fatty acids using solid-phase extraction on a functionalized polystyrene-divinylbenzene polymer. Recovery trials from spiked glass fiber filters, using threo-9,10-dihydroxyhexadecanoic acid as internal standard, gave an overall recovery of 54-86% for 3-hydroxy fatty acids of medium chain length (3-hydroxynonanoic to 3-hydroxypentadecanoic acid). 3-Hydroxyoctanoic acid and the longer chain fatty acids were more problematic yielding overall spike recoveries of 11-39%. While the 3-hydroxy fatty acid profile of pure lipopolysaccharides was dominated by 3-hydroxydecanoic, 3-hydroxydodecanoic and 3-hydroxytetradecanoic acid the aqueous phase from drilling mud contained in addition relatively high amounts of 3-hydroxyoctanoic and 3-hydroxynonanoic acid. Endotoxin activity as measured by the LAL assay was reasonably correlated (R(2)=0.54) to the sum of 3-hydroxydecanoic acid, 3-hydroxydodecanoic acid and 3-hydroxytetradecanoic acid in these samples.

HPLC-MRM relative quantification analysis of fatty acids based on a novel derivatization strategy

Fatty acids (FAs) are associated with a series of diseases including tumors, diabetes, and heart diseases. As potential biomarkers, FAs have attracted increasing attention from both biological researchers and the pharmaceutical industry. However, poor ionization efficiency, extreme diversity, strict dependence on internal standards and complicated multiple reaction monitoring (MRM) optimization protocols have challenged efforts to quantify FAs. In this work, a novel derivatization strategy based on 2,4-bis(diethylamino)-6-hydrazino-1,3,5-triazine was developed to enable quantification of FAs. The sensitivity of FA detection was significantly enhanced as a result of the derivatization procedure. FA quantities as low as 10 fg could be detected by high-performance liquid chromatography coupled with triple-quadrupole mass spectrometry. General MRM conditions were developed for any FA, which facilitated the quantification and extended the application of the method. The FA quantification strategy based on HPLC-MRM was carried out using deuterated derivatization reagents. "Heavy" derivatization reagents were used as internal standards (ISs) to minimize matrix effects. Prior to statistical analysis, amounts of each FA species were normalized by their corresponding IS, which guaranteed the accuracy and reliability of the method. FA changes in plasma induced by ageing were studied using this strategy. Several FA species were identified as potential ageing biomarkers. The sensitivity, accuracy, reliability, and full coverage of the method ensure that this strategy has strong potential for both biomarker discovery and lipidomic research.

Identification and quantitation of fatty acid double bond positional isomers: a shotgun lipidomics approach using charge-switch derivatization

The specific locations of double bonds in mammalian lipids have profound effects on biological membrane structure, dynamics and lipid second messenger production. Herein, we describe a shotgun lipidomics approach that exploits charge-switch derivatization with N-(4-aminomethylphenyl) pyridinium (AMPP) and tandem mass spectrometry for identification and quantification of fatty acid double bond positional isomers. Through charge-switch derivatization of fatty acids followed by positive-ion mode ionization and fragmentation analysis, a marked increase in analytic sensitivity (low fmol/μL) and the identification of double bond positional isomers can be obtained. Specifically, the locations of proximal double bonds in AMPP-derivatized fatty acids are identified by diagnostic fragment ions resulting from the markedly reduced 1,4-hydrogen elimination from the proximal olefinic carbons. Additional fragmentation patterns resulting from allylic cleavages further substantiated the double bond position assignments. Moreover, quantification of fatty acid double bond positional isomers is achieved by the linear relationship of the normalized intensities of characteristic fragment ions vs the isomeric compositions of discrete fatty acid positional isomers. The application of this approach for the analysis of fatty acids in human serum demonstrated the existence of two double bond isomers of linolenic acid (i.e., Δ(6,9,12) 18:3, γ-linolenic acid (GLA), and Δ(9,12,15) 18:3, α-linolenic acid (ALA)). Remarkably, the isomeric ratio of GLA vs ALA esterified in neutral lipids was 3-fold higher than the ratio of their nonesterified moieties. Through this developed method, previously underestimated or unidentified alterations in fatty acid structural isomers can be determined facilitating the identification of novel biomarkers and maladaptive alterations in lipid metabolism during disease.

Fatty acidomics: global analysis of lipid species containing a carboxyl group with a charge-remote fragmentation-assisted approach

Charge-remote fragmentation has been well recognized as an effective approach for dissociation of long aliphatic chains. Herein, we exploited this approach for structural identification of all fatty acids including saturated, unsaturated, and modified ones by using electrospray ionization tandem mass spectrometry after one-step derivatization of a charge-carried reagent through an amidation reaction. We tested the approach with different charge-carried reagents with respect to the hydrophobicity, charge strength, and distance from the charge to the carboxyl group. We found that all of the derivatives with these reagents could yield informative charge-remote fragmentation patterns regardless of the different chemical and physical properties of the reagents. These informative fragmentation patterns all could be effectively used for structural elucidation of lipid species containing a carboxyl group. We further found that the distinguished charge-remote fragmentations of fatty acid isomers enabled us to determine the composition of these isomers without any chromatographic separation. Finally, the abundant fragments yielded from an individual derivatized moiety enabled us to sensitively quantify the individual species containing a carboxyl group. The described approach was a great extension to the multidimensional mass-spectrometry-based shotgun lipidomics for global analysis of fatty acids including isomers and modifications. We believe that this approach could greatly facilitate identification of the biochemical mechanisms underlying numerous pathological conditions.

Separation and characterization of sucrose esters from Oriental tobacco leaves using accelerated solvent extraction followed by SPE coupled to HPLC with ion-trap MS detection

Sucrose esters (SEs) were successfully extracted from Oriental tobacco leaves using a new methodology based on accelerated solvent extraction followed by hydrophilic-lipophilic balanced cartridge cleanup step. The SEs were detected by HPLC with ion-trap MS detection using an electrospray interface operated in the positive ion mode. This methodology combines the high efficiency of extraction provided by a pressurized fluid and the highly sensitive characterization offered by ion-trap MS. Under the optimized conditions, 14 SEs were first identified among a total of 23 SEs found in Oriental tobacco leaves. Under the same conditions, only four new SEs were extracted by using traditional ultrasound-assisted extraction and liquid-solid extraction methods. The present method might be potentially useful in high-efficiency extraction and sensitive characterization of SEs from complex matrices such as tobacco leaves.