Advances in Capillary Electrophoresis-Based Enzyme Assays

Atypical applications of transverse diffusion of laminar flow profiles methodology for in-capillary reactions in capillary electrophoresis

Capillary electrophoresis (CE) is a powerful separation technique offering quick and efficient analyses in various fields of bioanalytical chemistry. It is characterized by many well-known advantages, but one, which is perhaps the most important for this application field, is somewhat overlooked. It is the possibility to perform chemical and biochemical reactions at the nL scale inside the separation capillary. There are two basic formats applicable for this purpose, heterogeneous and homogeneous. In the former, one reactant is immobilized onto a particle or monolithic support or directly on the capillary wall, and the other is injected. In the latter, the reactant mixing inside a capillary is based on electromigration or diffusion. One of the diffusion-based methodologies, termed Transverse Diffusion of Laminar Flow Profiles, is the subject of this review. Since most studies utilizing in-capillary reactions in CE focus on enzymes, which are being continuously and exhaustively reviewed, this review covers the atypical applications of this methodology, but still in the bioanalytical field. As can be seen from the demonstrated applications, they are not limited to reactions, but can also be utilized for other biochemical systems.

Immobilized acetylcholinesterase in magnetic nanoparticles for in-line inhibition studies using a capillary electrophoresis system

Enzyme assays can be performed with the capillary electrophoresis technique (CE) in many approaches, such as the immobilized enzyme micro-reactor. Acetylcholinesterase is a promising enzyme to be used when pursuing such a method, as it has already been explored in the proposal of similar methods of miniaturizing enzyme assays. The present work proposes a novel enzyme micro-reactor, based on the anchorage of the enzyme on magnetic nanoparticles of MnFe₂O_4, with chitosan and glutaraldehyde as the cross-linker in the capillary by means of an arrange of neodymium magnets. The calculated K_m of the enzyme evaluated by this method was 1.12 mmol L^-1, comparable to other studies in the literature that utilizes immobilized enzymes. Also, IC₅₀ for neostigmine was assessed in 3 different micro-reactors, with an average of 29.42 ± 3.88 μmol L^-1. In terms of the micro-reactor stability, it was possible to perform at least 25 experiments with assembled micro-reactor. The method was applied to hydroalcoholic extracts of 7 plant species. Plinia cauliflora had the best result, with 42.31 ± 6.81% of enzyme inhibition in a concentration of 100 mg L^-1.

Peptide mapping of proteins by capillary electromigration methods

This review article provides a wide overview of important developments and applications of capillary electromigration methods in the area of peptide mapping of proteins in the period 1997-mid-2022, including review articles on this topic. It deals with all major aspects of peptide mapping by capillary electromigration methods: i) precleavage sample preparation involving purification, preconcentration, denaturation, reduction and alkylation of protein(s) to be analyzed, ii) generation of peptide fragments by off-line or on-line enzymatic and/or chemical cleavage of protein(s), iii) postcleavage preparation of the generated peptide mixture for capillary electromigration separation, iv) separation of the complex peptide mixtures by one-, two- and multidimensional capillary electromigration methods coupled with mass spectrometry detection, and v) a large application of peptide mapping for variable purposes, such as qualitative analysis of monoclonal antibodies and other protein biopharmaceuticals, monitoring of posttranslational modifications, determination of primary structure and investigation of function of proteins in biochemical and clinical research, characterization of proteins of variable origin as well as for protein and peptide identification in proteomic and peptidomic studies.

Capillary electrophoresis for enzyme-based studies: Applications to lipases and kinases

Capillary electrophoresis (CE) is a powerful technique continuously expanding into new application fields. One of these applications involves the study of enzymes, their catalytic activities and the alteration of this activity by specific ligands. In this review, two model enzymes, lipases and kinases, will be used since they differ substantially in their modes of action, reaction requirements and applications making them perfect subjects to demonstrate the advantages and limitations of CE-based enzymatic assays. Indeed, the ability to run CE in various operation modes and hyphenation to different detectors is essential for lipase-based studies. Additionally, the low sample consumption provided by CE promotes it as a promising technique to assay human and viral nucleoside kinases. Undeniably, these are rarely commercially available enzymes and must be frequently produced in the laboratory, a process which requires special sets of skills. CE-based lipase and kinase reactions can be performed outside the capillary (pre-capillary) where the reactants are mixed in a vial prior to their separation or, inside the capillary (in-capillary) where the reactants are mixed before the electrophoretic analysis. These enzyme-based applications of CE will be compared to those of liquid chromatography-based applications in terms of advantages and limitations. Binding assays based on affinity CE and the compelling microscale thermophoresis (MST) will be briefly presented as they allow a broad understanding of the molecular mechanism behind ligand binding and of the resulting modulation in activity.

Dual electrophoretically-mediated microanalysis in multiple injection mode for the simultaneous determination of acetylcholinesterase and α-glucosidase activity applied to selected polyphenols

Acetylcholinesterase (AChE) and α-glucosidase (α-glu) are key target enzymes in the search for novel strategies in the treatment of Alzheimer's disease and type II diabetes. Therefore, methods to assess the enzyme inhibition are of great value in the research field. Here is proposed a novel a dual electrophoretically-mediated microanalysis for the simultaneous determination of both enzymes' activity. In order to do so, the various solutions required for both assays were introduced in the capillary electrophoresis system using the multiple injections approach. Enzymatic kinetic parameters were tested, K_m for AChE and α-glu were 3.81 and 0.43 mmol L^-1. K_i values were 4.27 μmol L^-1 for neostigmine (an AChE inhibitor) and 0.40 mmol L^-1 for acarbose (an α-glu inhibitor). Results of IC₅₀ (concentration for 50% of inhibition) were 5.11 ± 0.47 μmol L^-1 and 0.58 ± 0,02 mmol L^-1 for neostigmine and acarbose, respectively. All parameters (except for K_i of neostigmine) were comparable with the literature, indicating a good reliability of the proposed method to evaluate these enzymes activity. Total time analysis was approximately 10 min, being possible to perform around 12 enzymatic assays per hour, with low sample and reagent consumption, thus satisfying some of the principles of green chemistry. The method was applied to evaluate 10 phenolic compounds, of wich p-coumaric acid showed the best inhibitory activity for AChE (40.14 ± 4.75% at 10 mg L^-1); and quercetin for α-glu (46.53 ± 4.90% at 10 mg L^-1).

Monitoring the crosstalk between methylation and phosphorylation on histone peptides with host-assisted capillary electrophoresis

Post-translational modifications (PTMs) greatly increase protein diversity and regulate their functions by changing the structures, properties, and molecular interactions of proteins. In peptide regions with high density of PTMs, PTMs can influence modification on residues in proximity or even at distal positions, adding another layer of regulation. Methods that can monitor the activities of PTM enzymes on peptides carrying multiple modifications are valuable tools for better understanding of PTM crosstalk. Herein, we developed a host-assisted capillary electrophoresis (CE) method to separate histone peptides with methylation and phosphorylation and applied it to monitor the crosstalk between serine phosphorylation and lysine methylation when they were added by Aurora B kinase and G9a lysine methyltransferase, respectively. A synthetic receptor molecule, 4-hexasulfonatocalix[6]arene (CX6), was included in the CE buffer to improve the resolution of the corresponding substrates and products. A linear polyacrylamide-coated capillary was employed to effectively reduce wall adsorption of the cationic histone peptides. The peptide substrates were labeled with fluorescein to enhance their detectability during CE separation. Our method successfully revealed that the activity of G9a methyltransferase was completely inhibited by the adjacent phosphorylation, while 25% reduction in the activity of Aurora B kinase was observed with the presence of dimethylation on the nearby residue. The PTM crosstalk was examined not only using a pure peptide substrate, but also in a competitive reaction environment, in which the modified and unmodified peptides were mixed and the enzyme actions on both peptides were monitored simultaneously. Our work demonstrates that host-assisted CE is an effective method for study of PTM crosstalk, which could offer the advantages of fast separation, high resolution, and low sample consumption. Graphical abstract.

Capillary Electrophoresis/Dynamic Frontal Analysis for the Enzyme Assay of 4-Nitrophenyl Phosphate with Alkaline Phosphatase

A substrate of 4-nitrophenyl phosphate was enzymatically hydrolyzed by alkaline phosphatase (ALP) in a capillary tube, while an injected zone of the substrate was electrophoretically migrating in the separation buffer containing the enzyme by capillary electrophoresis (CE). During CE migration of the substrate from the start time of the electrophoresis to the detection time of the substrate, the substrate was continuously hydrolyzed by ALP to form a product of 4-nitrophenolate, and a plateau signal of 4-nitrophenolate was detected as a result of the zero-order kinetic reaction. The height of the plateau signal was directly related to the reaction rate, and it was used for the determination of a Michaelis-Menten constant through Lineweaver-Burk plots. Since the plateau signal is attributed to the dynamic formation of the product by the enzymatic reaction in CE, this analysis method is named as capillary electrophoresis/dynamic frontal analysis (CE/DFA). In CE/DFA, the CE separation is included on detecting the plateau signal, and the hydrolysis product before the sample injection is resolved from the dynamically and continuously formed product. The inhibition of the enzyme with the product is also eliminated in CE/DFA by the CE separation.

Magnetic nanoparticles in nanomedicine: a review of recent advances

Nanomaterials, in addition to their small size, possess unique physicochemical properties that differ from bulk materials, making them ideal for a host of novel applications. Magnetic nanoparticles (MNPs) are one important class of nanomaterials that have been widely studied for their potential applications in nanomedicine. Due to the fact that MNPs can be detected and manipulated by remote magnetic fields, it opens a wide opportunity for them to be used in vivo. Nowadays, MNPs have been used for diverse applications including magnetic biosensing (diagnostics), magnetic imaging, magnetic separation, drug and gene delivery, and hyperthermia therapy, etc. Specifically, we reviewed some emerging techniques in magnetic diagnostics such as magnetoresistive (MR) and micro-Hall (μHall) biosensors, as well as the magnetic particle spectroscopy, magnetic relaxation switching and surface enhanced Raman spectroscopy (SERS)-based bioassays. Recent advances in applying MNPs as contrast agents in magnetic resonance imaging and as tracer materials in magnetic particle imaging are reviewed. In addition, the development of high magnetic moment MNPs with proper surface functionalization has progressed exponentially over the past decade. To this end, different MNP synthesis approaches and surface coating strategies are reviewed and the biocompatibility and toxicity of surface functionalized MNP nanocomposites are also discussed. Herein, we are aiming to provide a comprehensive assessment of the state-of-the-art biological and biomedical applications of MNPs. This review is not only to provide in-depth insights into the different synthesis, biofunctionalization, biosensing, imaging, and therapy methods but also to give an overview of limitations and possibilities of each technology.

Capillary electrophoresis-immobilized enzyme microreactors for acetylcholinesterase assay with surface modification by highly-homogeneous microporous layer

We propose a new capillary electrophoresis (CE)-based open-tubular immobilized enzyme microreactor (OT-IMER) and its application in acetylcholinesterase (AChE) assays. The IMER is fabricated at the capillary inlet (reactor length of ∼1 cm) with the inner surface modified by a micropore-structured layer (thickness of ∼220 nm, pore size of ∼15-20 nm). The use of IMER accomplishes the enzymatic reaction and separation/detection of the products in the same capillary within 3 min. The feasibility of the proposed method is evaluated via online analysis of the activity and inhibition of AChE enzymes. Such method exhibits good reproducibility with relative standard deviation (RSD) of less than 4% for 20 runs, and the enzyme remains over 82% of the initial activity after usage of 7 days. The IMERs are successfully applied to detect the organophosphorus pesticide, paraoxon, in three types of vegetable juice samples with a limit of detection of as low as 61 ng mL^-1. Results show that the spiked samples are in the range of 89.6-105.9% with RSD less than 2.7%, thereby indicating its satisfactory level of accurate and reliable analysis of real samples by using the proposed method. Our study indicates that, with combination of advantages of both porous-layer capillary and CE OT-IMER, the proposed method is capable to enhance enzymatic reactions and to achieve rapid analysis with simple instrumentation and operation, thus would pave the way for extensive application of CE-based IMERs in a variety of bioanalysis.

Capillary electrophoretic assay of human acetyl-coenzyme A carboxylase 2

Human acetyl-coenzyme A carboxylase 2 catalyzes the carboxylation of acetyl coenzyme A to form malonyl coenzyme A, along with the conversion of magnesium-adenosine triphosphate complex to magnesium-adenosine diphosphate complex. A simple off-column capillary electrophoresis assay for human acetyl-coenzyme A carboxylase 2 was developed based on the separation of magnesium-adenosine triphosphate complex, magnesium-adenosine diphosphate complex, acetyl coenzyme A and malonyl coenzyme A with detection by ultraviolet absorption at 256 nm. When Mg²⁺ was absent from the separation buffer, the zones due to magnesium-adenosine triphosphate complex and magnesium-adenosine diphosphate complex both split and migrated as two separate peaks. With Mg²⁺ added to the separation buffer, magnesium-adenosine triphosphate complex and magnesium-adenosine diphosphate complex produced single peaks, and the reproducibility of peak shape and area improved for human acetyl-coenzyme A carboxylase 2 assay components. The final separation buffer used was 30.0 mM HEPES, 3.0 mM MgCl₂ , 2.5 mM KHCO₃ , and 2.5 mM potassium citrate at pH 7.50. The same buffer was used for the enzyme-catalyzed reaction (off-column). Inhibition of human acetyl-coenzyme A carboxylase 2 by CP-640186, a known inhibitor, was detected using the capillary electrophoresis assay.

Capillary electrophoresis integrated immobilized enzyme reactor for kinetic and inhibition assays of β-secretase as the Alzheimer's disease drug target

Capillary electrophoresis integrated immobilized enzyme reactors are becoming an increasingly popular alternative for enzyme kinetic and inhibition assays thanks to their unique set of features including cost effectiveness, repeated use of the enzyme, minuscule sample consumption, rapid analysis time and easy automation. In this work we present the development and application of a capillary electrophoresis integrated immobilized enzyme reactor based on magnetic particles for kinetic and inhibition studies of β-secretase, a key enzyme in the development of Alzheimer's disease and a promising drug target. We document the optimization of the immobilization procedure, characterization of immobilized β-secretase, optimization of a mutually compatible incubation protocol and separation method as well as the production of the capillary electrophoresis integrated immobilized enzyme reactor. The applicability of the capillary electrophoresis integrated immobilized enzyme reactor was demonstrated by kinetic assay with an unlabelled substrate and by inhibition assays using three structurally different reference inhibitors. The resulting kinetic and inhibition parameters clearly support the applicability of the herein presented method as well as document the fundamental phenomena which need to be taken in account when comparing the results to other methods.

Determination of acetylcholinesterase and α-glucosidase inhibition by electrophoretically-mediated microanalysis and phenolic profile by HPLC-ESI-MS/MS of fruit juices from Brazilian Myrtaceae Plinia cauliflora (Mart.) Kausel and Eugenia uniflora L

Alzheimer's disease and diabetes mellitus are contemporary diseases of great concern. Phenolic compounds are linked to several health benefits and could lead to novel strategies to combat these ailments. The objective of this study was to evaluate by electrophoretically-mediated microanalysis the potential inhibitory activity of the fruit juices from Plinia cauliflora ("jaboticaba") and Eugenia uniflora ("pitanga") toward acetylcholinesterase (AChE) and α-glucosidase, target enzymes in strategies for the treatment of these diseases. The phenolic profiles of the samples were also investigated. Jaboticaba and pitanga juices inhibited 85.90 ± 1.73 and 52.67 ± 1.24% of AChE activity at 5 mg mL^-1, and 57.91 ± 2.60 and 69.47 ± 2.89% of α-glucosidase activity at 1 mg mL^-1, respectively. Total phenolic content of the juices were 303.54 ± 28.28 and 367.00 ± 11.42 mgGA L^-1, respectively. The observed inhibitory activity can be explained, at least in part, by the presence of the phenolic compounds.

High performance affinity chromatography and related separation methods for the analysis of biological and pharmaceutical agents

The last few decades have witnessed the development of many high-performance separation methods that use biologically related binding agents. The combination of HPLC with these binding agents results in a technique known as high performance affinity chromatography (HPAC). This review will discuss the general principles of HPAC and related techniques, with an emphasis on their use for the analysis of biological compounds and pharmaceutical agents. Various types of binding agents for these methods will be considered, including antibodies, immunoglobulin-binding proteins, aptamers, enzymes, lectins, transport proteins, lipids, and carbohydrates. Formats that will be discussed for these methods range from the direct detection of an analyte to indirect detection based on chromatographic immunoassays, as well as schemes based on analyte extraction or depletion, post-column detection, and multi-column systems. The use of biological agents in HPLC for chiral separations will also be considered, along with the use of HPAC as a tool to screen or study biological interactions. Various examples will be presented to illustrate these approaches and their applications in fields such as biochemistry, clinical chemistry, and pharmaceutical research.

Recent developments in capillary and microchip electroseparations of peptides (2015-mid 2017)

The review brings a comprehensive overview of recent developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) to analysis, microscale isolation, purification, and physicochemical and biochemical characterization of peptides in the years 2015, 2016, and ca. up to the middle of 2017. Advances in the investigation of electromigration properties of peptides and in the methodology of their analysis (sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, and detection) are described. New developments in particular CE and CEC methods are presented and several types of their applications to peptide analysis are reported: qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC methods to provide important physicochemical characteristics of peptides are demonstrated.

Screening Platform Based on Robolid Microplate for Immobilized Enzyme-Based Assays

A facile, cost-effective, and high-throughput screening method was developed for enzyme-based assays based on Robolid/Microplate (RLMP) platform. The RLMP platform is constructed by immobilizing enzyme on commercial robolids and combining it with a standard 96-well microplate to achieve high-throughput analysis. The initiation and quenching of enzymatic reaction can be performed by simply sandwiching or unsealing the enzyme-immobilized robolids and the sample-containing microplate. This platform enables measurements of multiple target analytes simultaneously based on immobilized enzymatic reactions, with analysis time independent of the number of wells in the microplate. Using urea as the model analyte, we have shown that the RLMP platform exhibits large linear detection range of up to 10 mM, fast analysis time of 30 min/96 samples, as well as good reproducibility and stability. Measurements of urea in human urine and serum samples were performed using the RLMP platform and were compared with the commercial urea test kit. A good correlation was found between the two methods. This study shows that the present RLMP platform has promising prospects for detection of clinical markers and application in disease diagnosis and biochemical analysis.

Screening of tyrosinase inhibitors by capillary electrophoresis with immobilized enzyme microreactor and molecular docking

A new method for screening tyrosinase inhibitors from traditional Chinese medicines (TCMs) was successfully developed by capillary electrophoresis with reliable online immobilized enzyme microreactor (IMER). In addition, molecular docking study has been used for supporting inhibition interaction between enzyme and inhibitors. The IMER of tyrosinase was constructed at the outlet of the capillary by using glutaraldehyde as cross-linker. The parameters including enzyme reaction, separation of the substrate and product, and the performance of immobilized tyrosinase were investigated systematically. Because of using short-end injection procedure, the product and substrate were effectively separated within 2 min. The immobilized tyrosinase could remain 80% active for 30 days at 4°C. The Michaelis-Menten constant of tyrosinase was determined as 1.78 mM. Kojic acid, a known tyrosinase inhibitor, was used as a model compound for the validation of the inhibitors screening method. The half-maximal inhibitory concentration of kojic acid was 5.55 μM. The method was successfully applied for screening tyrosinase inhibitors from 15 compounds of TCM. Four compounds including quercetin, kaempferol, bavachinin, and bakuchiol were found having inhibitory potentials. The results obtained in this work were supported by molecular docking study.