Analytical methods involving separation techniques for determination of low-molecular-weight biothiols in human plasma and blood

Smartphone-assisted colorimetric biosensor for the rapid visual detection of natural antioxidants in food samples

Quantitative monitoring of the concentrations of epigallocatechin gallate (EGCG) and cysteine (Cys) is of great significance for promoting human health. In this study, iron/aluminum bimetallic MOF material MIL-53 (Fe, Al) was rapidly prepared under room temperature using a co-precipitation method, followed by investigating the peroxidase-like (POD-like) activity of MIL-53(Fe, Al) using 3,3',5,5'-tetramethylbenzidine (TMB) as a chromogenic substrate. The results showed that the Michaelis -Menten constants of TMB and H2O2 as substrates were 0.167 mM and 0.108 mM, respectively. A colorimetric sensing platform for detecting EGCG and Cys was developed and successfully applied for analysis and quantitative detection using a smartphone. The linear detection range for EGCG was 15∼80 μM (R2=0.994) and for Cys was 7∼95 μM (R2=0.998). The limits of detection (LOD) were 0.719 μM and 0.363 μM for EGCG and Cys, respectively. This work provides a new and cost-effective approach for the real-time analysis of catechins and amino acids.

The emerging application of LC-based metallomics techniques to unravel the bioinorganic chemistry of toxic metal(loid)s

The on-going anthropogenic emission of toxic metal(loid) species into the environment contaminates the food supply and drinking water resources in various parts of the world. Given that inorganic pollutants cannot be degraded, their increased influx into the bloodstream of babies, children and pregnant women is inevitable. Since the ramifications of the ensuing environmental exposure on human health remain poorly defined, fundamentally new insight into their bioinorganic chemistry in organisms is urgently needed. Based on the flow of dietary constituents through organisms, the interaction of toxic metal(loid) species with biomolecules in the bloodstream deserve particular attention as they play an integral role in the mechanisms of their chronic toxicity. Gaining insight into these bioinorganic processes is hampered by the biological complexity of plasma/red blood cells and the low concentrations of the metal(loid) species of interest, but can be overcome by employing LC techniques hyphenated to atomic spectroscopic detectors (i.e. metallomics techniques). This perspective aims to highlight the potential of unconventional hyphenated separation modes to advance our understanding of the bioinorganic chemistry of toxic metal(loid) species in the bloodstream-organ system. Four examples are illustrated. The application of anion-exchange (AEX) and size-exclusion chromatography (SEC) provided new insight into the blood-based bioinorganic mechanisms that direct Cd2+ and MeHg+ to target organs. AEX chromatography also allowed to observe the formation of complexes between Hg2+ and MeHg+ with L-cysteine at pH 7.4, that are implicated in their organ uptake. Lastly, the application of reversed phase (RP) chromatography revealed a possible cytosolic mechanism by which N-acetyl-L-cysteine binds to MeHg+ in the presence of cytosolic glutathione (GSH). New insight into other bioinorganic processes may advance the regulatory framework to better protect public health.

Analytical Methods for Assessing Thiol Antioxidants in Biological Fluids: A Review

Redox metabolism is an integral part of the glutathione system, encompassing reduced and oxidized glutathione, hydrogen peroxide, and associated enzymes. This core process orchestrates a network of thiol antioxidants like thioredoxins and peroxiredoxins, alongside critical thiol-containing proteins such as mercaptoalbumin. Modifications to thiol-containing proteins, including oxidation and glutathionylation, regulate cellular signaling influencing gene activities in inflammation and carcinogenesis. Analyzing thiol antioxidants, especially glutathione, in biological fluids offers insights into pathological conditions. This review discusses the analytical methods for biothiol determination, mainly in blood plasma. The study includes all key methodological aspects of spectroscopy, chromatography, electrochemistry, and mass spectrometry, highlighting their principles, benefits, limitations, and recent advancements that were not included in previously published reviews. Sample preparation and factors affecting thiol antioxidant measurements are discussed. The review reveals that the choice of analytical procedures should be based on the specific requirements of the research. Spectrophotometric methods are simple and cost-effective but may need more specificity. Chromatographic techniques have excellent separation capabilities but require longer analysis times. Electrochemical methods enable real-time monitoring but have disadvantages such as interference. Mass spectrometry-based approaches have high sensitivity and selectivity but require sophisticated instrumentation. Combining multiple techniques can provide comprehensive information on thiol antioxidant levels in biological fluids, enabling clearer insights into their roles in health and disease. This review covers the time span from 2010 to mid-2024, and the data were obtained from the SciFinder® (ACS), Google Scholar (Google), PubMed®, and ScienceDirect (Scopus) databases through a combination search approach using keywords.

Signal-On Detection of Dopamine and Tyrosinase Using Tris(hydroxypropyl)phosphine as a New Lucigenin Chemiluminescence Coreactant

Dopamine (DA) is a very imperative neurotransmitter in our body, since it contributes to several physiological processes in our body, for example, memory, feeling, cognition, cardiovascular diseases, and hormone secretion. Meanwhile, tyrosinase is a critical biomarker for several dangerous skin diseases, including vitiligo and melanoma cancer. Most of the reported chemiluminescent (CL) methods for monitoring DA and tyrosinase are signal-off biosensors. Herein, we introduce a new chemiluminescent "signal-on" system, lucigenin-tris(hydroxypropyl)phosphine (THPP), for the selective determination of DA and tyrosinase. THPP is well known as a versatile and highly water-soluble sulfhydryl-reducing compound that is more highly stable against air oxidation than common disulfide reductants. By employing THPP for the first time as an efficient lucigenin coreactant, the lucigenin-THPP system has shown a high CL response (approximately 16-fold) compared to the lucigenin-H2O2 classical CL system. Surprisingly, DA can remarkably boost the CL intensity of the lucigenin-THPP CL system. Additionally, tyrosinase can efficiently catalyze the conversion of tyramine to DA. Therefore, lucigenin-THPP was employed as an ultrasensitive and selective signal-on CL system for the quantification of DA, tyrosinase, and THPP. The linear ranges for the quantification of DA, tyrosinase, and THPP were 50-1000 nM, 0.2-50 μg/mL, and 0.1-800 μM, respectively. LODs for DA and tyrosinase were estimated to be 24 nM and 0.18 μg/mL, respectively. Additionally, the CL system has been successfully employed for the detection of tyrosinase in human serum samples and the assay of DA in human serum samples as well as in dopamine injection ampules with excellent obtained recoveries.

A label-free and washing-free method to detect biological thiols on a personal glucose meter utilizing glucose oxidase-mimicking activity of gold nanoparticles

We herein developed a label-free and washing-free method to detect biological thiols (biothiols) on a personal glucose meter (PGM) utilizing the intrinsic glucose oxidase (GOx)-mimicking activity of gold nanoparticles (AuNPs). By focusing on the fact that this activity could be diminished by target biothiols through their binding onto the AuNP surface, we correlated the concentration of biothiols with that of glucose readily measurable on a PGM and successfully determined cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) down to 0.116, 0.059, and 0.133 μM, respectively, with high specificity against non-target biomolecules. We further demonstrated its practical applicability by reliably detecting target biothiol in heterogeneous human serum. Due to the meritorious features of PGM such as simplicity, portability, and cost-effectiveness, we believe that this work could serve as a powerful platform for biothiol detection in point-of-care settings.

Preparation of thiol-free amino acid plasma matrix for quantification of thiol amino acids and their oxidized forms in human plasma by UPLC-MS/MS

Thiol amino acids, with great physiological significance, are unstable, and have small molecular weights, as well as very low endogenous concentrations. Therefore, to quantitatively and directly analyze them using liquid chromatography-tandem mass spectrometry is difficult. To overcome these problems, we aimed to prepare a thiol-free amino acid plasm matrix as blank sample to reduce the background for the first time. Using compounds with maleimide group that react with classical thiols to generate water-insoluble products. Reducing agents Tris(2-carboxyethyl)phosphine (TCEP) was applied to cooperate with bismaleimide (DM) for elimination of thiol amino acids from plasma 10 min at room temperature and pH 7. Further, the residual TCEP from plasma were removed using an anion exchange resin within 10 min. Methodological validation analysis revealed good performance in linearity, precision, extraction recovery (≥ 82 %), and stability (except oxidized glutathione). This quantitative analysis was successfully applied to blood samples of 9 people in good health. This study provides a foundation for the development of accurate and rigorous quantitative analysis methods targeting thiol amino acids in different body fluids or tissues. Moreover, it paves the way toward realizing several clinical applications.

Simultaneous determination of cystine and other free aminothiols in blood plasma using capillary electrophoresis with pH-mediated stacking

We developed a method of sensitive capillary electrophoresis using UV detection for the determination of certain free aminothiols (reduced cysteinylglycine (rCysGly), cysteine (rCys), glutathione (rGln), and cystine (CysS) in human blood plasma. The reduced thiols were derivatized with N-ethylmaleimide. The plasma was purified from proteins via ultrafiltration. Electrophoretic separation was performed using 115 mM Na phosphate with 7.5% (v/v) polyethylene glycol 600, pH 2.3. The in-capillary concentration of the analytes was achieved with a pH gradient created via the preinjection of triethanolamine and postinjection of phosphoric acid. The separation was carried out using a silica capillary (50 µm i.d.; total/effective separation length 42/35 cm) at a 25 kV voltage. The total analysis/regeneration time was 18 min. The quantification limits varied from 1.3 µM (rCysGly) to 5.4 µM (CysS). The accuracy was 95%-99%, and the repeatability and reproducibility were approximately 1.8%-3.8% and 1.9%-5.0%, respectively. An analysis of plasma samples from healthy volunteers (N = 41) showed that the mean levels of rCysGly, rCys, rGln, and CysS were 1.64, 10.6, 2.58, and 46.2 µM, respectively.

Theory and experiment: The synthesis and drug application of "ON-OFF-ON" fluorescent probes for copper and biothiols detection

Abnormal copper ions (Cu2+) and biothiols have potential impacts on environmental pollution and human health, so the detection of these substances with high selectivity and sensitivity has become an important research topic. In this study, we designed and synthesized two fluorescent probes (L1 and L2) based on naphthalene and anthracene derivatives that could specifically detect Cu2+ and biothiols. Owing to the paramagnetic effect of Cu2+, the strong fluorescent intensity was quenched after the addition of Cu2+. When biothiols were added to the solution (L-Cu2+), the fluorescence intensity was significantly enhanced and recovered. So, the interaction process was accompanied with "ON-OFF-ON" phenomenon in fluorescent intensity. Two complexes (L-Cu2+) showed low limit of detection for biothiols (Cys was 3.4 ×10-5 M and GSH was 2.0 ×10-5 M) and weak cytotoxicity (< 150 μg/mL). Theoretical investigation analysis revealed that the intramolecular hydrogen bond existed in the structure of probes and the roles of molecular frontier orbitals in molecular interplay. In addition, two probes also showed good applicability in actual drug Atomolan. The GSH content in the tested Atomolan reached over 99.9% of the labeling which was accord with the percentage of pharmacopoeia. Therefore, two probes have the real application value in the detection of Cu2+, biothiols and drug efficacy in various environments.

Equivalent Response Strategy for Sensing Total Biothiols in Human Serums and Living Cells Using a Hemicyanine-Based Self-Immolative Probe

Biothiols including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are crucial in maintaining the redox balance in the body, and the metabolism and transportation of biothiols rely on the coreaction of diverse proteins and enzymes. The abnormal concentrations and metabolism of biothiols are closely associated with many diseases. However, due to the same active reaction site of the sulfydryl group in biothiols, it is inevitable to bear a confused signal of mutual influence on both nonselective detection and discriminate detection, which presents a serious challenge of accurately sensing or imaging the three biothiols. By assigning an α,β-unsaturated ketone moiety as a Michael acceptor to trigger thiols to complete the irreversible equivalent domino response processes of nucleophilic addition, olefinic bond migration, and self-immolation, a targeted strategy was rationally pointed out, and herein, a hemicyanine-based probe CyOCy was prepared as a proof of strategy demonstration. The new probe could be equivalently lit up by Cys, Hcy, GSH, and even biothiol combinations (Cys/Hcy, Cys/GSH, Hcy/GSH, or Cys/Hcy/GSH) with unified linear ranges, detection limits, and response times. The probe CyOCy has been successfully used for the accurate quantification of total biothiols in the serum samples of healthy persons and coronary heart disease patients. In addition, the probe has been applied for cell screening, exogenous biothiol imaging, and monitoring drug-induced biothiol fluctuations. The purposive thinking of this work may provide an effective avenue for the accurate sensing of multicomponent samples.

One-pot sample preparation procedure for the determination of protein N-linked homocysteine by HPLC-FLD based method

The report presents robust and high throughput method, based on liquid chromatography coupled with fluorescence detection (HPLC-FLD), for the determination of total protein N-linked homocysteine (Hcy) in human plasma. The assay involves simultaneous proteins precipitation with perchloric acid and removal of any other form of Hcy, except protein N-linked Hcy, via disulfides reduction with tris(2-carboxyethyl)phosphine (TCEP) and plasma protein pellet wash with perchloric acid followed by liberation of N-linked Hcy from proteins by hydrochloric acid hydrolysis, drying under vacuum and residue reconstitution in diluted hydrochloric acid. The chromatographic separation of resulting in this way Hcy-thiolactone (HTL) is achieved within 3 min at room temperature on PolymerX RP-1 (150 × 4.6 mm, 5.0 µm) column using isocratic elution with eluent, consisted of o-phthaldialdehyde (OPA) in sodium hydroxide and acetonitrile (ACN), delivered at a flow rate 1 mL/min. The analyte is quantified by monitoring fluorescence at 480 nm using excitation at 370 nm, in a linear range from 0.25 to 10 µmol/L in plasma, while the limit of quantification (LOQ) equals 0.25 µmol/L. The method was successfully applied to plasma samples delivered by fifteen apparently healthy donors showing that the HPLC-FLD assay is suitable for screening of human plasma.

Simultaneous determination of 2-(3-hydroxy-5-phosphonooxymethyl-2-methyl-4-pyridyl)-1,3-thiazolidine-4-carboxylic acid and main plasma aminothiols by HPLC-UV based method

The report presents the first method for simultaneous determination of plasma 2-(3-hydroxy-5-phosphonooxymethyl-2-methyl-4-pyridyl)-1,3-thiazolidine-4-carboxylic acid (HPPTCA), an adduct of cysteine (Cys) and active form of vitamin B6 pyridoxal 5'-phosphate (PLP), as well as total low molecular-weight thiols content, including Cys, homocysteine (Hcy), cysteinyl-glycine (Cys-Gly), and glutathione (GSH). The assay is based on high performance liquid chromatography coupled with ultraviolet detection (HPLC-UV) and involves disulfides reduction with tris(2-carboxyethyl)phosphine (TCEP), derivatization with 2-chloro-1-methylquinolinium tetrafluoroborate (CMQT) followed by sample deproteinization with perchloric acid (PCA). The chromatographic separation of obtained stable UV-absorbing derivatives is achieved on ZORBAX SB-C18 (150 × 4.6 mm, 5.0 µm) column using gradient elution with eluent consisted of 0.1 mol/L trichloroacetic acid (TCA), pH 1.7 and acetonitrile (ACN), delivered at a flow rate 1 mL/min. Under these conditions, the analytes are separated within 14 min at room temperature, and quantified by monitoring at 355 nm. Regarding HPPTCA, the assay linearity was demonstrated within a 1-100 µmol/L in plasma and the lowest concentration on the calibration curve was recognized as the limit of quantification (LOQ). The accuracy ranged from 92.74 to 105.57% and 95.43 to 115.73%, while precision varied from 2.48 to 6.99% and 0.84 to 6.98% for intra- and inter-day measurements, respectively. The utility of the assay was proved by application to plasma samples delivered by apparently healthy donors (n = 18) in which the HPPTCA concentration ranged from 19.2 to 65.6 µmol/L. The HPLC-UV assay provides complementary tool for routine clinical analysis, facilitating further studies on the role of aminothiols and HPPTCA in living systems.

Covalent organic framework-MnO2 nanoparticle composites for shape-selective sensing of bithiols

Covalent organic frameworks (COFs) for detecting biological macromolecules in water or biological environments are generally challenging. In this work, a composite material IEP-MnO₂ is obtained by combining manganese dioxide (MnO₂) nanocrystals and a fluorescent COF (IEP), which is synthesized by using 2,4,6-tris(4-aminophenyl)-s-triazine and 2,5-dimethoxyterephthalaldehyde. By the addition of biothiols, such as glutathione, cysteine or homocysteine with different sizes, the fluorescence emission spectra of IEP-MnO₂ changed ("turn-on" or "turn-off") via different mechanisms. The fluorescence emission of IEP-MnO₂ increased in the presence of GSH by the elimination of the FRET (Förster resonance energy transfer) effect between MnO₂ and IEP. Surprisingly, due to the formation of a hydrogen bond between Cys/Hcy and IEP, the fluorescence quenching for IEP-MnO₂ + Cys/Hcy may be explained via the photoelectron transfer (PET) process, which endows IEP-MnO₂ with specificity in distinguishing the detection of GSH and Cys/Hcy compared to other MnO₂ complex materials. Therefore, IEP-MnO₂ was used to detect GSH and Cys in human whole blood and serum, respectively. The limit of detection for GSH in whole blood and Cys in human serum was calculated to be 25.58 μM and 4.43 μM, which indicates that IEP-MnO₂ can be used to investigate some diseases related to GSH and Cys concentration. Moreover, the research expands the application of covalent organic frameworks in the fluorescence sensing field.

Manipulating the monomer-dimer transformation of a heptamethine cyanine ligand: near infrared chromogenic recognition of biothiols

A novel absorbance recovery method has been developed for the determination of biothiols with a near-infrared reagent. This method employs a two-reagent system composed of cation heptamethine cyanine (CyL) and Hg²⁺. The absorbance of CyL, with a maximum peak at 760 nm, was decreased due to addition of Hg²⁺, but recovered when biothiols were added. Under optimal conditions, the reciprocal extent of recovered absorbance was proportional to the concentration of biothiols. The calibration curves are linear over the range of (0.3-7.0) × 10^-6 M for cysteine, (1.0-10.0) × 10^-6 M for homocysteine and (1.0-9.0) × 10^-6 M for glutathione. Because of the specific affinity of Hg²⁺for biothiols, there is minimal interference from other amino acids. This method has been successfully applied to the determination of homocysteine in human urine samples with satisfactory results.

Structural Characterization of Toxicologically Relevant Cd2+-L-Cysteine Complexes

The exposure of humans to Cd exerts adverse human health effects at low chronic exposure doses, but the underlying biomolecular mechanisms are incompletely understood. To gain insight into the toxicologically relevant chemistry of Cd²⁺ in the bloodstream, we employed an anion-exchange HPLC coupled to a flame atomic absorption spectrometer (FAAS) using a mobile phase of 100 mM NaCl with 5 mM Tris-buffer (pH 7.4) to resemble protein-free blood plasma. The injection of Cd²⁺ onto this HPLC-FAAS system was associated with the elution of a Cd peak that corresponded to [CdCl₃]^-/[CdCl₄]^2- complexes. The addition of 0.1-10 mM L-cysteine (Cys) to the mobile phase significantly affected the retention behavior of Cd²⁺, which was rationalized by the on-column formation of mixed CdCys_xCl_y complexes. From a toxicological point of view, the results obtained with 0.1 and 0.2 mM Cys were the most relevant because they resembled plasma concentrations. The corresponding Cd-containing (~30 μM) fractions were analyzed by X-ray absorption spectroscopy and revealed an increased sulfur coordination to Cd²⁺ when the Cys concentration was increased from 0.1 to 0.2 mM. The putative formation of these toxicologically relevant Cd species in blood plasma was implicated in the Cd uptake into target organs and underscores the notion that a better understanding of the metabolism of Cd in the bloodstream is critical to causally link human exposure with organ-based toxicological effects.

Use of metal nanoparticles for preconcentration and analysis of biological thiols

Metal nanoparticles (NPs) exhibit several unique physicochemical properties, including redox activity, surface plasmon resonance, ability to quench fluorescence, biocompatibility, or a high surface-to-volume ratio. They are being increasingly used in analysis and preconcentration of thiol containing compounds, because they are able to spontaneously form a stable Au/Ag/Cu-S dative bond. They thus find wide application in environmental and particularly in medical science, especially in the analysis of biological thiols, the endogenous compounds that play a significant role in many biological systems. In this review article, we provide an overview of various types of NPs that have been applied in analysis and preconcentration of biological thiols, mainly in human biological fluids. We first discuss shortly the types of NPs and their synthesis, properties, and their ability to interact with thiol compounds. Then we outline the sample preconcentration and analysis methods that were used for this purpose with special emphasis on optical, electrochemical, and separation techniques.

A Competitive "On-Off-Enhanced On" AIE Fluorescence Switch for Detecting Biothiols Based on Hg2+ Ions and Gold Nanoclusters

In this study, a novel "on-off-enhanced on" approach to highly sensitive rapid sensing of biothiols was developed, based on competitive modulation of gold nanoclusters (AuNCs) and Hg2+ ions. In our approach, the AuNCs were encapsulated into a zeolite imidazole framework (ZIF) for predesigned competitive aggregation-induced luminescence (AIE) emission. To readily operate this approach, the Hg2+ ions were selected as mediators to quench the fluorescence of AuNCs. Then, due to the stronger affinities between the interactions of Hg2+ ions with -SH groups in comparison to the AuNCs with -SH groups, the quenched probe of AuNCs@ZIF-8/Hg2+ displayed enhanced fluorescence after the Hg2+ ions were competitively interacted with -SH groups. Based on enhanced fluorescence, the probe for AuNCs@ZIF-8/Hg2+ had a sensitive and specific response to trace amounts of biothiols. The developed fluorescence strategy had limit of quantification (LOQ) values of 1.0 μM and 1.5 μM for Cys and GSH molecules in serum, respectively. This competitive AIE strategy provided a new direction for developing biological probes and a promising method for quantifying trace amounts of biothiols in serum. It could promote progress in disease diagnosis.

In Situ Formation of o-Phenylenediamine Cascade Polymers Mediated by Metal-Organic Framework Nanozymes for Fluorescent and Photothermal Dual-Mode Assay of Acetylcholinesterase Activity

A fluorescent and photothermal dual-mode assay method was established for the detection of acetylcholinesterase (AChE) activity based on in situ formation of o-phenylenediamine (oPD) cascade polymers. First, copper metal-organic frameworks of benzenetricarboxylic acid (Cu-BTC) were screened out as nanozymes with excellent oxidase-like activity and confinement catalysis effect. Then, an ingenious oPD cascade polymerization strategy was proposed. That is, oPD was oxidized by Cu-BTC to oPD oligomers with strong yellow fluorescence, and oPD oligomers were further catalyzed to generate J-aggregation, which promotes the formation of oPD polymer nanoparticles with a high photothermal effect. By utilizing thiocholine (enzymolysis product of acetylthiocholine) to inhibit the Cu-BTC catalytic effect, AChE activity was detected through the fluorescence-photothermal dual-signal change of oPD oligomers and polymer nanoparticles. Both assay modes have low detection limitation (0.03 U L^-1 for fluorescence and 0.05 U L^-1 for photothermal) and can accurately detect the AChE activity of human serum (recovery 85.0-111.3%). The detection results of real serum samples by fluorescent and photothermal dual modes are consistent with each other (relative error ≤ 5.2%). It is worth emphasizing that this is the first time to report the high photothermal effect of oPD polymers and the fluorescence-photothermal dual-mode assay of enzyme activity.

Colorimetric determination of biothiols based on peroxidase-mimicking Ag nanoparticles decorated Ti3C2 nanosheets

Ag nanoparticle-decorated Ti₃C₂ nanosheets (AgNPs@Ti₃C₂ NSs) were facilely synthesized via a self-reduction approach, in which Ti₃C₂ NSs acted as both reductant and supporter. The AgNPs@Ti₃C₂ NS nanocomposite exhibited excellent peroxidase-like activity with o-phenylenediamine (OPD) and H₂O₂ as substrates. The catalytic behavior followed the typical Michaelis-Menten kinetics; Michaelis constant (K_m) and maximum initial velocity (V_max) for OPD were 0.263 mM and 43.2 × 10^-8 M^-1 s, indicating high affinity and high catalytic efficiency towards OPD. The catalytic mechanism was revealed to be an accelerated electron transfer process. Based on the inhibition effect on the peroxidase-like activity of AgNPs@Ti₃C₂ NSs, a simple, fast, and sensitive colorimetric method for detection of low-weight biothiols (cysteine (Cys), homocysteine (Hcy), and glutathione (GSH)) was developed by measuring the absorbance at 425 nm. The colorimetric method displayed wide linear range (50 nM to 50 μM for Cys, 10 nM to 250 μM for Hcy, 10 nM to 50 μM for GSH), low limit of detection (48.5 nM for Cys, 5.5 nM for Hcy, 7.0 nM for GSH), and good selectivity and short assay time (3 min). Moreover, the feasibility of this colorimetric sensor was demonstrated by accurately determining Cys in diluted human serum samples; good recovery (95.9-101.0%) and low relative standard deviations (2.8-4.9%) were obtained, showing great promise for point-of-care test in clinical samples.

Plasmonic Azobenzene Chemoreporter for Surface-Enhanced Raman Scattering Detection of Biothiols

Low molecular weight thiols (biothiols) are highly active compounds extensively involved in human physiology. Their abnormal levels have been associated with multiple diseases. In recent years, major efforts have been devoted to developing new nanosensing methods for the low cost and fast quantification of this class of analytes in minimally pre-treated samples. Herein, we present a novel strategy for engineering a highly efficient surface-enhanced Raman scattering (SERS) spectroscopy platform for the dynamic sensing of biothiols. Colloidally stable silver nanoparticles clusters equipped with a specifically designed azobenzene derivative (AzoProbe) were generated as highly SERS active substrates. In the presence of small biothiols (e.g., glutathione, GSH), breakage of the AzoProbe diazo bond causes drastic spectral changes that can be quantitatively correlated with the biothiol content with a limit of detection of ca. 5 nM for GSH. An identical response was observed for other low molecular weight thiols, while larger macromolecules with free thiol groups (e.g., bovine serum albumin) do not produce distinguishable spectral alterations. This indicates the suitability of the SERS sensing platform for the selective quantification of small biothiols.

Gold-activated luminol chemiluminescence for the selective determination of cysteine over homocysteine and glutathione

This work reports a chemiluminescence assay for the highly selective determination of cysteine in biological fluids without separation techniques. The method is based on the ability of cysteine to selectively enhance the metal-catalyzed chemiluminescence generated by the oxidation of luminol from gold tetrachloride anions under alkaline conditions. The selectivity of the method stems from the fact that, under strongly alkaline conditions, the formation of the four-membered ring transition state of cysteine is less favorable as compared to the formation of the respective 5- and 9- membered ring transition states of homocysteine and glutathione, respectively. These transition states exert stronger hindrance and hydrophobic interactions repelling the negatively charged luminol dianion and possibly exhibit lower reducing ability for dissolved oxygen, towards the formation of superoxide radicals, thus reducing the oxidation of luminol. Under the optimum experimental conditions, the linear range of the method extended from 0.5 to 20 μΜ while cysteine could be determined at concentrations as low as 0.5 μM, with good reproducibility (<3.5%) and recoveries between 80 and 93% in artificial and real biological fluids.

Physiologically relevant hCys concentrations mobilize MeHg from rabbit serum albumin to form MeHg-hCys complexes

Methylmercury (MeHg) is one of the most potent neurotoxins to which humans are exposed via the consumption of fish from which it is effectively absorbed via the gastrointestinal tract into the bloodstream. Its interactions with plasma proteins, small molecular weight (SMW) molecules, and red blood cells, however, are incompletely understood, but critical as they determine if and how much MeHg reaches target organs. To better define the role that SMW thiols play in the delivery of MeHg to known transporters located at the placental and blood-brain barrier, we have employed size exclusion chromatography-inductively coupled plasma-atomic emission spectroscopy to analyze MeHg-spiked rabbit plasma in the absence and presence of SMW thiols dissolved in the PBS-buffer mobile phase. While 300 µM L-methionine did not affect the binding of MeHg to rabbit serum albumin (RSA), cysteine (Cys), homocysteine (hCys) and glutathione (GSH) resulted in the elution of the main Hg-peak in the SMW elution range. In addition, 50 µM of hCys or Cys in the mobile phase resulted in the mobilization of MeHg from RSA in rabbit plasma and from pure RSA in solution. The Hg-peak that eluted in the SMW elution range (50 µM of hCys) was identified by electrospray ionization-mass spectrometry as a MeHg-hCys complex. Since L-type amino acid transporters are present at the blood brain barrier (BBB) which facilitate the uptake of MeHg-Cys species into the brain, our results contribute to establish the bioinorganic mechanisms that deliver MeHg to the BBB, which is critical to predict organ-based adverse health effects.

Separation and determination of cysteine enantiomers in plasma after derivatization with 4-fluoro-7-nitrobenzofurazan

The importance of D-amino acids in mammals associated with enantio-dependent biological functions has been increasingly highlighted. In addition to naturally occurring, D-amino acid supplementation could have a positive biological impact, including cytoprotective implications. In this context, supplementation with D-cysteine has revealed beneficial effects. Quantification of cysteine enantiomers in rodent plasma has been achieved by using 4-fluoro-7-nitrobenzofurazan derivatization of the target analytes. Cystine, the main form of cysteine in the plasma, was initially reduced to cysteine using DL-dithiothreitol. Baseline enantioseparation was then achieved in less than 3 min using a (R,R)-Whelk-O 1 stationary phase and isocratic elution using CH₃OH-H₂O 90:10 (v/v) with 15 mM ammonium formate (apparent pH 6.0) at 0.5 mL/min. The derivatives were then detected using negative ESI-MS in SRM mode. An external calibration was employed for D-cysteine, while L-cysteine quantification, as an endogenous analyte, was addressed using a background subtraction strategy. The method was validated. Response functions were obtained from 0 to 300 µM and from 0 to 125 µM for D-cysteine and L-cysteine, respectively. The trueness ranged from 96% to 105% for both enantiomers with repeatability and intermediate precision lower than 8% and 15% for the D-form and the endogenous L-form, respectively. The method was successfully applied for determining D- and L-cysteine in mouse plasma after D-cysteine administration.

Nanoparticle-based colorimetric sensors to detect neurodegenerative disease biomarkers

Neurodegenerative disorders (NDDs) are progressive, incurable health conditions that primarily affect brain cells, and result in loss of brain mass and impaired function. Current sensing technologies for NDD detection are limited by high cost, long sample preparation, and/or require skilled personnel. To overcome these limitations, optical sensors, specifically colorimetric sensors, have garnered increasing attention towards the development of a cost-effective, simple, and rapid alternative approach. In this review, we evaluate colorimetric sensing strategies of NDD biomarkers (e.g. proteins, neurotransmitters, bio-thiols, and sulfide), address the limitations and challenges of optical sensor technologies, and provide our outlook on the future of this field.

Targeting Enrichment and Correlation Studies of Glutathione and Homocysteine in IgAVN Patient Urine Based on a Core-Shell Zr-Based Metal-Organic Framework

Aminothiols are closely related to chronic kidney disease, but little is known regarding levels of related aminothiols in the urine of immunoglobulin A vasculitis with nephritis (IgAVN) patients. Herein, a well-defined core-shell Zr-based metal-organic framework (Zr-MOF) composite SiO₂@50Benz-Cys was constructed as a mercury ion affinity material via a solvent-assisted ligand exchange strategy for the selective extraction and enrichment of low-concentration aminothiols in IgAVN patient urine. SiO₂@50Benz-Cys was competent to enrich the total glutathione (GSH) and total homocysteine (Hcy) in virtue of the excellent affinity after chelation with mercury ions. The extraction efficiencies were closely related to the pH, dithiothreitol amount, and the dose of functional Zr-MOF. Coupled with HPLC-MS/MS in optimized conditions, GSH and Hcy were determined with low detection limits of 0.5 and 1 nmol L^-1, respectively. The recoveries of GSH and Hcy for the urine sample at three spiked levels were in the range of 85.3-105% and 79.5-103%, which showed good precision and accuracy. Benefiting from the matrix interference elimination in the process of extraction, the simultaneous detection of aminothiols in the urine of the healthy group and immunoglobulin A vasculitis (IgAV) and IgAVN patients was successfully carried out, suggesting that the Zr-MOF and the robust method together provided a potential application in the analysis of urinary biomolecules. The analysis of variance (ANOVA) showed that the levels of GSH and Hcy had significant differences between the patients and the control. This work is very valuable as it provides a better understanding of concentration alterations of GSH and Hcy in urine involved with IgAVN for clinical research.

A simple and accurate HFCF-UF method for the analysis of homocysteine, cysteine, cysteinyl-glycine, and glutathione in human blood

The presence of reduced aminothiols, including homocysteine (Hcy), cysteine (Cys), cysteinyl-glycine (CG), and glutathione (GSH), is significantly increased in the pathological state. However, there have been no reports on the relationship between reduced aminothiols (Hcy, Cys, CG, and GSH) and different genders, ages, and drug combinations in human blood. The accurate quantification of these reduced thiols in biological fluids is important for monitoring some special pathological conditions of humans. However, the published methods typically not only require cumbersome and technically challenging processing procedures to ensure reliable measurements, but are also laborious and time-consuming, which may disturb the initial physiological balance and lead to inaccurate results. We developed a hollow fiber centrifugal ultrafiltration (HFCF-UF) method for sample preparation coupled with a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method and used it to determine four reduced aminothiols (Hcy, Cys, CG, and GSH) in human blood for the first time. A total of 96 clinical patients were enrolled in our study. The influence of different genders, ages, and drug combinations on the levels of four reduced thiols in human blood was also discussed by SPSS 24.0. The sample preparation was simplified to a single 5 min centrifugation step in a sealed system that did not disturb the physiological environment. The validation parameters for the methodological results were excellent. The procedure was successfully applied to monitoring the concentrations of four reduced aminothiols (Hcy, Cys, CG, and GSH) in 96 clinical blood samples. There were no significant differences in Hcy, Cys, CG, or GSH for the different genders, ages, or combinations with methotrexate or vancomycin (P > 0.05). However, there was a significant increase in Hcy concentration in patients treated with valproic acid who were diagnosed with epilepsy (p=0.0007). It is advisable to measure reduced Hcy level in patients taking valproic acid. The developed HFCF-UF method was simple and accurate. It can be easily applied in clinical research to evaluate oxidative stress in further study.

Simultaneous Determination of Human Serum Albumin and Low-Molecular-Weight Thiols after Derivatization with Monobromobimane

Biothiols are extremely powerful antioxidants that protect cells against the effects of oxidative stress. They are also considered relevant disease biomarkers, specifically risk factors for cardiovascular disease. In this paper, a new procedure for the simultaneous determination of human serum albumin and low-molecular-weight thiols in plasma is described. The method is based on the pre-column derivatization of analytes with a thiol-specific fluorescence labeling reagent, monobromobimane, followed by separation and quantification through reversed-phase high-performance liquid chromatography with fluorescence detection (excitation, 378 nm; emission, 492 nm). Prior to the derivatization step, the oxidized thiols are converted to their reduced forms by reductive cleavage with sodium borohydride. Linearity in the detector response for total thiols was observed in the following ranges: 1.76–30.0 mg mL⁻¹ for human serum albumin, 0.29–5.0 nmol mL⁻¹ for α-lipoic acid, 1.16–35 nmol mL⁻¹ for glutathione, 9.83–450.0 nmol mL⁻¹ for cysteine, 0.55–40.0 nmol mL⁻¹ for homocysteine, 0.34–50.0 nmol mL⁻¹ for N-acetyl-L-cysteine, and 1.45–45.0 nmol mL⁻¹ for cysteinylglycine. Recovery values of 85.16–119.48% were recorded for all the analytes. The developed method is sensitive, repeatable, and linear within the expected ranges of total thiols. The devised procedure can be applied to plasma samples to monitor biochemical processes in various pathophysiological states.

Simultaneous Determination of Chiral Thiol Compounds and Monitoring of Dynamic Changes in Human Urine after Drinking Chinese Korean Ethnic Rice Wine

Chinese Korean ethnic rice wine, a traditional fermented wine made from rice or corn, has antioxidant and antihypertensive activities. Although the determination of amino acids and other nutrients in rice wine has been reported, the existence of chiral thiol compounds has not been published in the literature. Therefore, we established a highly sensitive and selective ultrahigh-performance liquid chromatography-high-resolution mass spectrometry method for simultaneous determination and chiral separation of dl-Cys-GSH, dl-Cys-Cys, and dl-Cys-Hcy based on (R)-(5-(3-isothiocyanatopyrrolidin-1-yl)-5-oxopentyl) triphenylphosphonium derivatization. Three thiol diastereomers were completely separated on a YMC Triart C18 (2.0 × 150 mm, 1.9 μm) column with a resolution value (Rs) ≥ 1.52. The correlation coefficients were ≥0.9996, limit of detection was 2.40-7.20 fmol, and mean recoveries were 83.33-98.59%. Furthermore, fitted curves for dynamic changes in three kinds of chiral thiols in 10 human urine samples after drinking rice wine were drawn. Meanwhile, the metabolic changes in d/l-thiol compounds in human urine were investigated.

Quantification of Intracellular Thiols by HPLC-Fluorescence Detection

Biothiols, such as cysteine and glutathione, play important roles in various intracellular reactions represented by the redox equilibrium against oxidative stress. In this study, a method for intracellular thiol quantification using HPLC-fluorescence detection was developed. Thiols were derivatized with a thiol-specific fluorescence derivatization reagent, viz. ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate (SBD-F), followed by reversed-phase separation on an InertSustain AQ-C18 column. Six different SBD-thiols (homocysteine, cysteine, cysteinylglycine, γ-glutamylcysteine, glutathione, and N-acetylcysteine as an internal standard) were separated within 30 min using a citric buffer (pH 3.0)/MeOH mobile phase. The calibration curves of all the SBD-thiols had strong linearity (R² > 0.999). Using this developed method, the thiol concentrations of human chronic myelogenous leukemia K562 cell samples were found to be 5.5–153 pmol/1 × 10⁶ cells. The time-dependent effect of a thiol scavenger, viz. N-ethyl maleimide, on intracellular thiol concentrations was also quantified. This method is useful for elucidating the role of intracellular sulfur metabolism.

Gold-Modified Micellar Composites as Colorimetric Probes for the Determination of Low Molecular Weight Thiols in Biological Fluids Using Consumer Electronic Devices

This work describes a new, low-cost and simple-to-use method for the determination of free biothiols in biological fluids. The developed method utilizes the interaction of biothiols with gold ions, previously anchored on micellar assemblies through electrostatic interactions with the hydrophilic headgroup of cationic surfactant micelles. Specifically, the reaction of AuCl4− with the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) produces an intense orange coloration, due to the ligand substitution reaction of the Br− for Cl− anions, followed by the coordination of the AuBr4− anions on the micelle surface through electrostatic interactions. When biothiols are added to the solution, they complex with the gold ions and disrupt the AuBr4−–CTAB complex, quenching the initial coloration and inducing a decrease in the light absorbance of the solution. Biothiols are assessed by monitoring their color quenching in an RGB color model, using a flatbed scanner operating in transmittance mode as an inexpensive microtiter plate photometer. The method was applied to determine the biothiol content in urine and blood plasma samples, with satisfactory recoveries (i.e., >67.3–123% using external calibration and 103.8–115% using standard addition calibration) and good reproducibility (RSD < 8.4%, n = 3).

Biothiol modulated growth and aggregation of gold nanoparticles and their determination in biological fluids using digital photometry

This work describes a novel and easy to use method for the determination of biologically important thiols that relies on their ability to inhibit the catalytic enlargement of AuNP seeds in the presence of ACl₄^- ions and trigger their aggregation. UV-vis spectroscopic monitoring of the plasmon resonance bands of the formed AuNPs showed that the spectral and color transitions depend both on the concentration and the structure of biothiols. The colorimetric changes induced by biothiols were quantified in the concentration range from 5 to 300 μM in the RGB color system with digital photometry using a commercially available flatbed scanner as detector. On the basis of these results, the applicability of the method was tested to the determination of glutathione in red blood cells and cysteine in blood plasma with satisfactory recoveries (88.7-96.5%), low detection limits (1.0 μM), good selectivity against major biomolecules under physiologically relevant conditions and satisfactory reproducibility (<8%). The method requires minimum technical expertise, is easy to use and is performed without scientific equipment, holding promise as a simple assay of biothiol testing even by non-experts.

Simultaneous determination of three endogenous chiral thiol compounds in serum from humans at normal and stress states using ultrahigh-performance liquid chromatography coupled to quadrupole-Orbitrap high resolution mass spectrometry

Measurement of chiral thiol compounds such as glutathione (GSH), cysteine (Cys), and homocysteine (Hcy) in human serum plays an important role in the early diagnosis and warning of cardiovascular disease, neurodegenerative disease, and cancer. We developed a novel chiral mass spectrometry derivatization reagent, (R)-(5-(3-isothiocyanatopyrrolidin-1-yl)-5-oxopentyl) triphenylphosphonium (NCS-OTPP), with triphenylphosphine (TPP) as a basic structure carrying a permanent positive charge for the diastereomeric separation of chiral thiol compounds by ultrahigh-performance liquid chromatography coupled to quadrupole-Orbitrap high resolution mass spectrometry (UHPLC-Q-Orbitrap HRMS). A novel method was developed for simultaneous determination of three kinds of chiral thiol compounds based on the NCS-OTPP derivatization method. Three kinds of chiral thiol compounds on a YMC Triart C18 (2.0 × 150 mm, 1.9 μm) column with Rs were 1.56-1.68. The protonated precursor to product ion transitions monitored for GSH was m/z 780.16→747.24/473.18, Cys was m/z 594.20→561.18/473.18, and Hcy was m/z 608.21→575.19/473.18. An excellent linearity for all the analytes with correlation coefficients ≥ 0.9995 and suitable precision with inter-day and intra-day coefficients of variation RSDs was 0.83-4.06% and 0.95-3.11%. Satisfactory accuracy with recoveries between 83.73 and 103.35% was observed. The limit of detection (S/N = 3) was 2.4-7.2 fmol. Furthermore, the method was successfully applied to the simultaneous determination of three kinds of free and total thiol compounds in serum from 10 healthy volunteers at normal and stress states.

Recent applications of microextraction sample preparation techniques in biological samples analysis

Analysis of biological samples is affected by interfering substances with chemical properties similar to those of the target analytes, such as drugs. Biological samples such as whole blood, plasma, serum, urine and saliva must be properly processed for separation, purification, enrichment and chemical modification to meet the requirements of the analytical instruments. This causes the sample preparation stage to be of undeniable importance in the analysis of such samples through methods such as microextraction techniques. The scope of this review will cover a comprehensive summary of available literature data on microextraction techniques playing a key role for analytical purposes, methods of their implementation in common biological samples, and finally, the most recent examples of application of microextraction techniques in preconcentration of analytes from urine, blood and saliva samples. The objectives and merits of each microextration technique are carefully described in detail with respect to the nature of the biological samples. This review presents the most recent and innovative work published on microextraction application in common biological samples, mostly focused on original studies reported from 2017 to date. The main sections of this review comprise an introduction to the microextraction techniques supported by recent application studies involving quantitative and qualitative results and summaries of the most significant, recently published applications of microextracion methods in biological samples. This article considers recent applications of several microextraction techniques in the field of sample preparation for biological samples including urine, blood and saliva, with consideration for extraction techniques, sample preparation and instrumental detection systems.

Colorimetric detection of individual biothiols by tailor made reactions with silver nanoprisms

We herein described a rapid, sensitive, and selective colorimetric sensing platform for biothiols in human serum, which relies on the dual functions of biothiols as anti-etching and aggregating agent for silver nanoprisms (AgNPRs). In principle, the target biothiols that bind to the surface of AgNPRs through Ag–S covalent interactions protect the AgNPRs from being etched by chloride ion (Cl⁻) in human serum, thus exhibiting the blue/purple color that is indicative of AgNPRs. On the other hand, the color of AgNPRs turned to yellow in the absence of biothiols or the presence of non-sulfur-containing amino acids, indicating the formation of small silver nanoparticles (AgNPs). Importantly, we found that individual biothiols (Hcy, Cys, and GSH) exert not only the anti-etching effect, but also the aggregating effect on AgNPRs, which can be modulated by simply tuning the pH conditions, and this consequently allows for the discriminative detection of each biothiol. Based on this simple and cost-effective strategy, we successfully determined the Hcy, Cys, and GSH in human serum with high sensitivity and selectivity within 10 min, demonstrating the diagnostic capability and potential in practical applications.

Simultaneous determination of DL-cysteine, DL-homocysteine, and glutathione in saliva and urine by UHPLC-Q-Orbitrap HRMS: Application to studies of oxidative stress

A high-sensitivity and -selectivity mass spectrometry derivatization reagent, (R)-(5-(3-isothiocyanatopyrrolidin-1-yl)-5-oxopentyl) triphenylphosphonium (NCS-OTPP), was developed for the enantiomeric separation of chiral thiol compounds as prospectively important diagnostic markers for oxidative stress-related diseases. Complete separation of GSH, DL-Cys, and DL-Hcy was achieved. The parent ions of all derivatives had a fragment of m/z 473.18 and a structure of m/z 75.95 (R-S = C-S-R'), conducive to qualitative and quantitative analysis. Good linear relationships were obtained for all analytes (R²≥ 0.9995). The intra-day and inter-day precision were 0.82-5.16 % and 1.02-4.18 % in saliva, and 0.81-3.45 % and 0.99-6.47 % in urine, with mean recoveries of 83.31-105.66 % and 84.09-101.11 %, respectively. The limit of detection (S/N = 3) was 19.20-57.60 nM. Free and total GSH, DL-Cys, and DL-Hcy were detected simultaneously in saliva and urine from 10 volunteers in the normal, stressed, and stable states by UHPLC-Q-Orbitrap HRMS. The thiol compounds were quantitatively related to oxidative stress state changes.

A near-infrared phosphorescent iridium(iii) complex for fast and time-resolved detection of cysteine and homocysteine

Thiol-containing amino acids, cysteine (Cys) and homocysteine (Hcy), play crucial roles in the biosystem; their abnormal contents in the cells are linked to many diseases. Herein, we designed and synthesized a novel near-infrared (NIR) phosphorescent iridium(iii) complex-based probe (FNO1) that can detect Cys and Hcy in real-time in the biosystem. Due to the advantages of the iridium complex, the FNO1 probe had excellent chemical stability and photostability, high luminescence efficiency, and long luminescence lifetime. In addition, the probe showed a fast response, high sensitivity, and low cytotoxicity. As verified by high resolution mass spectra (HR-MS) and density functional theory (DFT) calculations, the detection was achieved through the addition of the α,β-unsaturated ketone group in FNO1 by the nucleophilic thiol group in Cys and Hcy. Through time-resolved emission spectroscopy (TRES) and in the presence of a strongly fluorescent dye rhodamine B, the FNO1 probe could detect Cys and Hcy due to its long luminescence lifetime (260/197 ns). Finally, owing to its NIR-emitting properties, the FNO1 probe was successfully applied in the imaging of Cys and Hcy in living cells, zebrafish, and mice.

In Situ Generation of Prussian Blue by MIL-53 (Fe) for Point-of-Care Testing of Butyrylcholinesterase Activity Using a Portable High-Throughput Photothermal Device

Butyrylcholinesterase (BuChE), the primary source of serum cholinesterase activity, is an indispensable biochemical marker for clinical diagnosis of liver function and organophosphorus poisoning. The requirement for bulky and expensive instruments represents a huge hindrance for point-of-care testing (POCT) of BuChE, especially in resource-limited settings. Herein, an easy-operated, economic, and portable photothermal (PT) biosensing platform for high-throughput BuChE detection was rationally designed. BuChE could "light up" the PT signal through in situ generation of Prussian blue (PB) by MIL-53 (Fe), which allowed us to translate biological signals into temperature signals. Such temperature change signals could be monitored at high throughput (six samples for a single measurement) by a miniature self-made integrated PT device via combining separable 96-well plates, a three-dimensional (3D) printed sample bracket, 808 nm lasers, and thermometers, satisfying the requirement for rapid on-site detection in a large batch with low cost. In addition, the large specific surface area, 3D network structure, and high porosity of MIL-53 (Fe) offered a beneficial platform for its reaction with enzymatic hydrolysate, resulting in high sensing sensitivity and low detection limit (0.3 U L^-1), which was at least 20 000 times lower than the normal human serum BuChE activity. This facile, affordable, and broad applicability PT sensing platform provides a beneficial reference for the rational design of other disease diagnostic approaches suitable for POCT.

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.

A novel colorimetric and fluorometric probe for biothiols based on MnO2 NFs-Rhodamine B system

Herein, a novel bimodal ratiometric probe for sensitive and selective detection of biothiols (including glutathione (GSH), cysteine (Cys) and homocysteine (Hcys)) was constructed, which was based on the redox reaction between manganese dioxide nanoflakes (MnO₂ NFs) and rhodamine (RhB) and biothiols. When MnO₂ NFs was added into RhB solution, RhB was oxidized to a series of derivatives, accompanying with the colorimetric color changing from purple to light pink and fluorescence changing from red to green. In the presence of GSH, Cys or Hcys, they could reduce MnO₂ NFs to Mn²⁺, thereby preventing the following oxidization of RhB and the corresponding color and fluorescence changes. The absorption intensity ratio and fluorescence intensity ratio showed good linear relationships with the concentrations of biothiols. The colorimetric detection limits for GSH, Cys and Hcys were 0.057 μM, 0.140 μM and 0.087 μM, respectively. And the fluorescence detection limits were 0.177 μM, 0.282 μM and 0.161 μM. More importantly, this probe was successfully applied to monitor the concentration of GSH/Cys/Hcys in human serum samples, with satisfactory recovery. Thus, this MnO₂ NFs-RhB platform can potentially be a candidate for the detection of biothiols.

Analysis of Total Thiols in the Urine of a Cystathionine β-Synthase-Deficient Mouse Model of Homocystinuria Using Hydrophilic Interaction Chromatography

Homocysteine and related thiols (cysteine, cysteinylglycine, and glutathione) in the urine of a cystathionine β-synthase (CBS)-deficient mouse model were quantified using hydrophilic interaction chromatography with fluorescence detection. Urine samples were incubated with tris(2-carboxyethyl) phosphine to reduce disulfide bonds into thiols. After deproteinization, thiols were fluorescently derivatized with ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate (SBD-F). Homocysteine, cysteine, cysteinylglycine, and glutathione in mouse urine were analyzed using an amide-type column with a mobile phase of acetonitrile/120 mM ammonium formate buffer (pH 3.0) (81:19). The developed method was well-validated. Thiol concentrations in the urine of CBS-wild type (-WT), -heterozygous (-Hetero), and -knockout (-KO) mice were quantified using the developed method. As expected, total homocysteine concentration in CBS-KO mice was significantly higher than that in CBS-WT and CBS-Hetero mice. The developed method shows promise for diagnoses in preclinical and clinical studies.

Micelle-dominated distribution strategy for non-matrix matched calibration without an internal standard: "Extract-and-shoot" approach for analyzing hydrophilic targets in blood and cell samples

The analysis of trace hydrophilic targets in complex aqueous-rich matrices is considerably challenging, generally requiring matrix-matched calibration, internal standard, or time-and-labor-intensive sample preparation. To address this analytical bottleneck, a non-matrix-matched calibration strategy without using internal standard was reported for the first time to analyze complicated biosamples such as whole blood, plasma, serum, and cell samples. This strategy, termed micelle-dominated distribution, also aimed at realizing the simple "extract-and-shoot" analytical process for such complex matrices. The micelle-matrix interaction was found to efficiently eliminate the matrix effect by dominating phase separation and analyte distribution between the extraction and matrix phases. Thus, calibration linear curves prepared in water were applicable to the analysis of all the above-mentioned sample types. Rapid distribution equilibrium within 4 min was achieved. This strategy could tolerate direct large volume injection, thereby providing two-order-of-magnitude enhancement in the sensitivity of ion-pair chromatography. The analytical method integrated cell rupture, matrix cleanup, analyte extraction, and on-column preconcentration into a fast and high-throughput operation. The successful application to the determination of exogenous pesticides and endogenous glutathione exhibited low limits of detection (0.0085-0.015 μg mL^-1 for pesticides; 0.52 μg mL^-1 for glutathione), wide linear ranges (0.028-50 μg mL^-1 and 0.049-50 μg mL^-1 for pesticides; 1.7-1000 μg mL^-1 for glutathione), good linearies (R² = 0.9994-0.9999), excellent accuracy (recoveries of 91.3-105.2%), and good precision (0.7-6.2% at the levels of 0.028 (or 0.049), 0.1, 0.5, and 50 μg mL^-1 for pesticides; 0.5-8.7% at 1.7, 500, and 1000 μg mL^-1 for glutathione).

Development of C60-based labeling reagents for the determination of low-molecular-weight compounds by matrix assisted laser desorption ionization mass spectrometry (II): Determination of thiols in human serum

Perturbation of thiol homeostasis in biological fluids are thought to be associated with several diseases, and reliable analytical methods for the determination of low molecular weight (LMW) thiols in human plasma or serum are thus required. In this study, a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) method is described for high throughput determination of four LMW thiols (glutathione, cysteine, homocysteine and cysteinylglycine) in human serum. It is based on the use of a bromoacetyl functionalized C60 (Br-C60) as a derivatization reagent to label thiols. The Br-C60 labeling can add an 832-Da tag to thiols, which moves thiol signals to high mass region and effectively avoids the signal interference generated by the traditional MALDI matrix below 800 Da. The labeling can be completed within 5 min under microwave-assisted condition. Thereby, the Br-C60 labeling based MALDI-TOF MS analytical method can achieve high throughput analysis of LMW thiols in serum. Good linearities of the method for the thiols in human serum were obtained in the range of 0.5-500.0 μM with correlation coefficient (R) greater than 0.9960. The limit of detection is in the range of 0.07-0.18 μM for the investigated thiols in human serum with relative standard deviations of lower than 13.5% and recoveries ranging from 81.9 to 117.1%. Using the method, four thiols in microliter serum samples of breast cancer (BC) patients were determined. The result showed that the contents of the four thiols in BC serum samples significantly changed compared to the healthy control (HC).

Microbial Redox Regulator-Enabled Pulldown for Rapid Analysis of Plasma Low-Molecular-Weight Biothiols

Although low-molecular-weight (LMW) biothiols function as a disease indicator in plasma, rapidly and effectively analyzing them remains challenging in the extracellular oxidative environment due to technical difficulties. Here, we report a newly designed, affinity pulldown platform using a Bacillus subtilis-derived organic hydroperoxide resistance regulatory (OhrR_BS) protein and its operator dsDNA for rapid and cost-effective analyses of plasma LMW biothiols. In the presence of organic hydroperoxide, LMW biothiols triggered the rapid dissociation of FAM-labeled dsDNA from FLAG-tagged OhrR_BS via S-thiolation of OhrR_BS on anti-FLAG antibody-coated beads, which led to a strong increase of fluorescence intensity in the supernatant after pulldown. This method was easily extended by using a reducing agent to detect free and total LMW biothiols simultaneously in mouse plasma. Unlike free plasma LMW biothiols, total plasma LMW biothiols were more elevated in ΔLDLR mice than those in normal mice. Owing to the rapid dissociation of OhrR/dsDNA complexes in response to LMW biothiols, this pulldown platform is immediately suitable for monitoring rapid redox changes in plasma LMW biothiols as well as studying oxidative stress and diseases in blood.