Method of intracellular naphthalene-2,3-dicarboxaldehyde derivatization for analysis of amino acids in a single erythrocyte by capillary zone electrophoresis with electrochemical detection

Chemical tagging mass spectrometry: an approach for single-cell omics

Single-cell (SC) analysis offers new insights into the study of fundamental biological phenomena and cellular heterogeneity. The superior sensitivity, high throughput, and rich chemical information provided by mass spectrometry (MS) allow MS to emerge as a leading technology for molecular profiling of SC omics, including the SC metabolome, lipidome, and proteome. However, issues such as ionization suppression, low concentration, and huge span of dynamic concentrations of SC components lead to poor MS response for certain types of molecules. It is noted that chemical tagging/derivatization has been adopted in SCMS analysis, and this strategy has been proven an effective solution to circumvent these issues in SCMS analysis. Herein, we review the basic principle and general strategies of chemical tagging/derivatization in SCMS analysis, along with recent applications of chemical derivatization to single-cell metabolomics and multiplexed proteomics, as well as SCMS imaging. Furthermore, the challenges and opportunities for the improvement of chemical derivatization strategies in SCMS analysis are discussed.

Chemical fixation to arrest phospholipid signaling for chemical cytometry

Chemical cytometry is a powerful tool for measuring biological processes such as enzymatic signaling at the single cell level. Among these technologies, single-cell capillary zone electrophoresis (CZE) has emerged as a powerful tool to assay a wide range of cellular metabolites. However, analysis of dynamic processes within cells remains challenging as signaling pathways are rapidly altered in response to changes in the cellular environment, including cell manipulation and storage. To address these limitations, we describe a method for chemical fixation of cells to stop the cellular reactions to preserve the integrity of key signaling molecules or reporters within the cell and to enable the cell to act as a storage reservoir for the reporter and its metabolites prior to assay by single-cell CZE. Fluorescent phosphatidylinositol 4,5-bisphosphate reporters were loaded into cells and the cells were chemically fixed and stored prior to analysis. The reporter and its metabolites were electrophoretically separated by single-cell CZE. Chemical fixation parameters such as fixative, fixation time, storage solution, storage duration, and extraction solution were optimized. When cells were loaded with a fluorescent C6- or C16-PIP₂ followed by glutaraldehyde fixation and immediate analysis, 24±2% and 139±12% of the lipid was recoverable, respectively, when compared to an unfixed control. Storage of the cells for 24h yielded recoverable lipid of 61±3% (C6-PIP₂) and 55±5% (C16-PIP₂) when compared to cells analyzed immediately after fixation. The metabolites observed with and without fixation were identical. Measurement of phospholipase C activity in single leukemic cells in response to an agonist demonstrated the capability of chemical fixation coupled to single-cell CZE to yield an accurate snapshot of cellular reactions with the probe. This methodology enables cell assay with the reporter to be separated in space and time from reporter metabolite quantification while preserving assay integrity.

Determination of the isomeric forms proportion of fluorogenic naphthalene-2,3-dicarboxaldehyde in a binary mixture of water:methanol using electrochemical methods

The electrochemical response of the fluorogenic label naphthalene-2,3-dicarboxyaldehyde (NDA) in a binary mixture of water/methanol was characterized with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) electrochemical techniques. Naphthalene-2,3-dicarboxyaldehyde does exist in three isomeric forms in aqueous solution: the unhydrated dialdehyde (DA), the acyclic monohydrated (MA) and the cyclic hemiacetal (HAC). The study underlines that the proportion of each of them varies according to the working pH. At low and high pH, the dialdehyde form is in larger proportion than the acyclic monohydrated form. Conversely at intermediate pH, the concentration of the acyclic form is in greater proportion than the dialdehyde form. These results allowed us to determine the optimal pH of 9 for which the labeling of biomolecules could be more efficient due to the base catalyzed regeneration of the unhydrated form. At this pH, the data processing from the analysis of measured currents and estimation of diffusion coefficients of each form according to the semi-empirical models of Wilke-Chang, Scheibel, Reddy-Doraiswamy and Lusis-Ratcliff allowed us to obtain the concentration of dialdehyde (0.28 mM), acyclic monohydrated (0.57 mM) and cyclic hemiacetal monohydrated (0.15 mM) forms starting from 1mM naphthalene-2,3-dicarboxyaldehyde.

Analysis of inorganic and small organic ions by CE with amperometric detection

Capillary electrophoresis has become a widely useful analytical technology. Amperometric detection is extensively employed in capillary electrophoresis for its many inherent virtues, such as rapid response, remarkable sensitivity, and low cost of both detectors and instrumentations. Analysis of inorganic and small organic ions by capillary electrophoresis is an important research field. This review focuses on the recent developments of capillary electrophoresis coupled with amperometric detection for analysis of inorganic and small organic ions. Advancements in electrophoresis separation modes, amperometric detection modes, working electrodes, and applications of inorganic ions, amino acids, phenols, and amines are discussed.

Simultaneous separation of anionic and cationic proteins by capillary electrophoresis using high concentration of poly(diallyldimethylammonium chloride) as an additive

The simultaneous separation of anionic and cationic proteins has been achieved by addition of high concentration of poly(diallyldimethylammonium chloride) (PDDAC) in capillary electrophoresis. A capillary was filled with PDDAC so that it would act as ion-pair reagents in the separation of anionic proteins. On the other hand, the PDDAC can also be used as coating additives for the analysis of cationic proteins. Increasing the concentration of PDDAC in the separation buffer had the ability to improve the separation efficiency, change the electrophoretic mobility, and alter the separation selectivity; however, this was not true in the case of analyzing proteins by using the PDDAC larger than 1.6%. By both using a buffer containing 1.6% PDDAC and applying pH-stepwise techniques, 13 proteins with a wide range of pI (4.7-11.1) and molecular masses (6.5-198.0 kDa) could be separated within 30 min in a single run. In addition to this separation, we observed not only more peaks from alpha-chymotrypsinogen A and aprotinin but also the bovine serum albumin (BSA) dimer and trimer. With the 50 nL protein injection sample, the limits of detections at signal-to-noise of 3 for proteins are in the range of 0.07-0.79 microM. Except for BSA, the relative standard derivation values of migration time and peak height for all proteins were <1.3 and <6.9%, respectively. We suggested that this proposed method is a promising approach for clinical diagnosis and proteomics applications.

New high-performance liquid chromatographic method for sensitive determination of pheomelanin in biological materials by precolumn fluorescence derivatization with naphthalene-2,3-dicarboxaldehyde

Pheomelanin is an important type of melanin distributed in the skin, eye and hair in the mammal, which is of great social, clinical and cosmetic significance. In this study, a new HPLC method with fluorescence detection is described originally for the sensitive determination of pheomelanin in biological materials. The pheomelanin polymer is decomposed into two specific degradation products, 3-amino-4-hydroxyphenylalanine (3-AHP) and 4-amino-3- hydroxyphenylalanine (4-AHP) with hydriodic acid. Then the two AHP isomers are derivatized using a fluorescent probe naphthalene-2,3-dicarboxaldehyde in the presence of cyanide. The resulting highly stable 2-substituted 1-cyanobenz[f] isoindole derivatives were separated on a 5NH(2)-MS aminopropyl packed HPLC column with binary isocratic elution profile and detected fluorimetrically. The assay shows high sensitivity of 0.11nM (2.2fmol per injection, the lowest reported) at signal-to-noise ratio of 3 for each AHPs, good accuracy and precision (RSDs<3.1%), and linearity (range of 0.02-10microM, r>0.995). The results obtained by using fluorescence detection have been compared with other detection systems (electrochemical and UV). The sensitivity can increase from 100 to respect electrochemical detection and 30000 times respect UV detection. The method has been used for the quantitative determination of pheomelanin in various biological samples, including cell cultures from five types of melanoma cell lines of human and rat origin, hair samples of various colors, melanoma tissue and the urines from human melanoma patients and healthy subjects. This original application of HPLC-fluorescence detection represents a powerful tool for investigating pheomelanin synthesis in vitro and in vivo under physiological and pathophysiological conditions.

Coupling chemiluminescence with capillary electrophoresis to analyze single human red blood cells

In this paper, we describe a new method for determination of hemoglobin of single red blood cells by coupling chemiluminescence with capillary electrophoresis (CL-CE). The chemiluminescent detection is based on the catalytic effects of hemoglobin on the luminol-hydrogen peroxide reaction. The conditions of chemiluminescent reaction and capillary electrophoresis were investigated. Hemoglobin in human blood samples was detected with the present method, the linear range from 1.7 microg mL(-1) to 6.8 microg mL(-1) was tested, and the correlation coefficient of 0.997 and low detection limit of 0.17 microg mL(-1) (approximately 2.2 pg, S/N=3) were obtained. Cell injection procedure was improved, and the method was successfully used to determine hemoglobin of single red blood cells and the statistical result of the average content of hemoglobin in 26 human red blood cells was 23.6 pg. Compared to other current methods, CE with CL system is simple, sensitive and will become an attractive alternative method for single cell analysis.

Intracellular labeling method for chip-based capillary electrophoresis fluorimetric single cell analysis using liposomes

An intracellular derivatization method mediated by liposome was developed for single cell analysis with chip-based capillary electrophoresis (CE) and laser-induced fluorescence (LIF) detection. Liposomes with an average diameter of 100 nm were produced from phosphatidylcholine to encapsulate fluorescent dyes by an ultrasonic method. The encapsulation yield and the vesicle density were determined to be 46+/-5% and 8.8 x 10(14)/mL, respectively. The amount of fluorescent dye that entered the cells was dependent on the duration of incubating cells with liposomes, liposome density, and concentration of the dye solution encapsulated in liposomes. The described method introduced cell membrane nonpermeable fluorescent dyes into living cells without reducing cell viability. Single cell analysis using microfluidic chip-based CE revealed that liposome-membrane fusion occurred after entrance of liposomes into the cells, with release of encapsulated fluorescence dyes and labeling of intracellular species.

High-Throughput Single-Cell Analysis for Enzyme Activity without Cytolysis

A novel high-throughput method without cytolysis for determination of enzyme activity inside single cells was developed by a combination of chemical cell perforation and an intracellular enzyme-catalyzed reaction. Peroxidase (PO) inside human neutrophils was chosen as the model system. Cells were perforated with digitonin to form micropores on the cell membrane. The perforated cells, with physiological buffer saline of pH 7.4 containing hydroquinone (H2Q) and H2O2, were continuously propelled by pressure through a capillary as the microsampler and microreactor. Small molecules H2Q and H2O2 could diffuse into the cell interior through the micropores on the cell membrane, and the large molecule PO remained in the cell interior. Intracellular PO converted H2Q into benzoquinone (BQ). BQ diffused out from the cell interior to the cell surface through the micropores and formed a BQ zone around the cell. The process proceeded in the capillary during cell movement. The BQ zones around every moving perforated cell were continuously delivered to the capillary outlet by hydraulic flow and detected. An average detection rate of >1 cell/min was obtained.

Single-cell analysis by intracellular immuno-reaction and capillary electrophoresis with laser-induced fluorescence detection

A novel method for single-cell analysis was developed by combining electroporation for intracellular immuno-reaction and capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection. Human interferon-gamma (IFN-gamma) in natural killer (NK) cells was chosen as the test antigen. Two forms of IFN-gamma in single cells could be well separated and detected with a limit of detection of zeptomole. In this assay, the anti-IFN-gamma monoclonal antibody labeled with fluorescein isothiocyanate (Ab*) was introduced into NK cells by electrophoration for intracellular immuno-reaction. After completion of the intracellular immuno-reaction, the NK cells were chemically pre-perforated with digitonin to lyse easily. Then, one NK cell containing the complexes of IFN-gamma isoantigens with Ab* was electrokinetically injected into the capillary. The cell adsorbed on the tip of capillary was lysed by ultrasonication. Finally, the complexes of the different forms of IFN-gamma in the cell were separated and detected by CE-LIF detection.

Using nile red-adsorbed gold nanoparticles to locate glutathione within erythrocytes

An aqueous solution of Nile Red (NR)-absorbed 32-nm gold nanoparticles (AuNPs) have been used to sense glutathione (GSH). When the NR product is displaced by GSH on the AuNP surface, the fluorescence of the solution increases and the AuNPs aggregate. To determine the concentration and distribution of GSH within erythrocyte cells, a homemade fluorescence and scattering microscope was constructed. This system allows monitoring, within individual cells, of the uptake and transportation of the NRAuNPs and the displacement of the NR product from the NRAuNP surface by GSH. The fluorescence and scattering images clearly indicate the location of GSH inside the cells; these findings are supported by images recorded using 2,3-naphthalenedicarboxaldehyde, which is a highly selective fluorogenic reagent for GSH. Microscopic fluorescence measurements of the NRAuNPs revealed that the GSH concentration inside erythrocyte cells is 1.30 +/- 0.31 mM. To confirm this result, lysed erythrocyte cells were analyzed by applying capillary electrophoresis in conjunction with laser-induced fluorescence using NRAuNPs; accordingly, the average GSH concentration in a single erythrocyte cell was determined to be 1.32 +/- 0.06 mM.

Catalysis−Electrochemical Determination of Zeptomole Enzyme and Its Application for Single-Cell Analysis

A novel electrochemical method for determination of zeptomole amounts of enzyme was developed by a combination of on-capillary enzyme-catalyzed reaction and electrochemical detection. A limit of detection (LOD) of zeptomole (zmol, 10(-)(21) mol) was achieved by monitoring the product of the enzyme-catalyzed reaction. In this method, after enzyme molecules were electrokinetically injected into the capillary, they were electromigrated to the section of the capillary immersed in a warm water bath, where the enzyme molecules reacted with the enzyme substrates in the running buffer in the presence of the activator of the enzyme-catalyzed reaction. Then the electroactive product zone of the enzyme-catalyzed reaction was electromigrated to the horn-shaped outlet of the capillary and electrochemically detected by a carbon fiber disk bundle electrode at a constant potential. Glucose-6-phosphate dehydrogenase (G6PDH) was chosen as the model enzyme. A LOD of 1.3 zmol was achieved. This method was applied to determine zeptomoles of G6PDH in individual human erythrocytes.

Simultaneous Determination of Tryptophan and Glutathione in Individual Rat Hepatocytes by Capillary Zone Electrophoresis with Electrochemical Detection at a Carbon Fiber Bundle−Au/Hg Dual Electrode

A method for single-cell analysis was developed by combining capillary zone electrophoresis (CZE) with electrochemical detection (ECD) using a dual electrode consisting of two different types of electrode material (carbon fiber and Au/Hg). In this method, the parallel mode was used. Different potentials were applied to both electrodes of the dual electrode. Tryptophan and glutathione, which could not be simultaneously detected by normal CZE-ECD, could be simultaneously and selectively detected by CZE-ECD at the dual electrode in one run, respectively. The CZE-ECD system with the dual electrode was applied to determine them in individual rat hepatocytes.

Recent progress in derivatization methods for LC and CE analysis

The derivatization procedure with a suitable fluorescence or chemiluminescence reagent is performed for the purpose of increasing the detection sensitivity and selectivity, in high-performance liquid chromatography (HPLC) and/or capillary electrophoresis (CE). In this article, recent derivatization methods and their applications to biosamples are described. In HPLC, femto mol order of mass detection limits are obtained by derivatization. Regarding the fluorescence reagents, the use of water-soluble reagents has been effective to avoid an undesired adsorption in the process of determination of peptides. In CE, the advantages of having extremely low mass detection limits (ranging from atto to yocto mol level) and requiring only a very short analysis time (less than a few minutes) are made possible by using laser-induced fluorescence or near infra-red detections.