Electrophoretically mediated microanalysis of beta-galactosidase on microchips

Enzyme kinetics in confined geometries at the single enzyme level

Different confinement, femtoliter chambers and molecular crowders revealed the effects on the catalytic rates of β-galactosidase at the single molecule level.

Design and optimization of a double-enzyme glucose assay in microfluidic lab-on-a-chip

An electrokinetic driven microfluidic lab-on-a-chip was developed for glucose quantification using double-enzyme assay. The enzymatic glucose assay involves the two-step oxidation of glucose, which was catalyzed by hexokinase and glucose-6-phosphate dehydrogenase, with the concomitant reduction of NADP(+) to NADPH. A fluorescence microscopy setup was used to monitor the different processes (fluid flow and enzymatic reaction) in the microfluidic chip. A two-dimensional finite element model was applied to understand the different aspects of design and to improve the performance of the device without extensive prototyping. To our knowledge this is the first work to exploit numerical simulation for understanding a multisubstrate double-enzyme on-chip assay. The assay is very complex to implement in electrokinetically driven continuous system due to the involvement of many species, which has different transport velocity. With the help of numerical simulation, the design parameters, flow rate, enzyme concentration, and reactor length, were optimized. The results from the simulation were in close agreement with the experimental results. A linear relation exists for glucose concentrations from 0.01 to 0.10 g l(-1). The reaction time and the amount of enzymes required were drastically reduced compared to off-chip microplate analysis.

Screening of acetylcholinesterase inhibitors in natural extracts by CE with electrophoretically mediated microanalysis technique

An electrophoretically mediated microanalysis (EMMA) method for screening acetylcholinesterase (AChE) inhibitors in natural extracts is described. In this method, solutions of AChE and the mixture of the substrate and the natural extract were successively injected into the capillary, and mixed electrophoretically by applying a voltage for a short time. Afterwards the voltage was reapplied to separate the product from the unreacted substrate and the natural extract. The measured peak area of the product at UV 230 nm represents the enzyme activity. Since the extract is mixed with the substrate, there is no need to separate the components before testing the inhibition. The inhibitory activity of the natural extract as a whole can be easily found if the peak area of the product is reduced. This makes the present method suitable for screening inhibitors in complex mixtures, such as natural extracts. Compared to the commonly used spectrometric method for screening of AChE inhibitors, the major advantage of the present method is the elimination of Ellman reagent, which is essential for the spectrometric method. This not only simplifies the experimental procedure but also minimizes false-positive results. Moreover, it is an obvious advantage of combining the separation power with the on-column enzyme assay for further investigating which compound(s) is/are responsible for the inhibition. The method was validated using a commercially available AChE inhibitor tacrine and a small chemical library containing four AChE inhibitors and 32 natural extracts. Inhibitors in natural extracts were identified with the present method.

On-chip integrated hydrolysis, fluorescent labeling, and electrophoretic separation utilized for acetylcholinesterase assay

A sensitive on-chip acetylcholinesterase (AChE) assay that serves as a basis for the development of a fully integrated on-chip AChE-inhibitor detection assay is presented. The sequential steps required for the on-chip analysis process were integrated into a microchip. Transport and mixing of the reagents occurred by a combination of electroosmosis and electrophoresis using computer-controlled electrokinetic transport. AChE-catalyzed hydrolysis of acetylthiocholine to thiocholine was determined by on-chip reaction of thiocholine with eosinmaleimide, and the resulting thioether was electrophoretically separated and detected by laser-induced fluorescence (LIF). Enzyme-substrate mixing and reaction by confluent flow of reagents was compared with electrophoretically mediated microanalysis (EMMA), based on injection of an enzyme plug, and the utilization of differences in electrophoretic mobility as a driving force for efficient mixing and reaction. Both methods yielded similar results, however the EMMA-plug technique is preferable. The EMMA-plug technique was optimized for length and pushing time of enzyme plug, length of dyes mixture plug, acetylthiocholine concentration, and detector location. Detection of O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and paraoxon, two AChE inhibitors, was demonstrated by off-chip mixing of the inhibitor and AChE, followed by the on-chip AChE assay. Limit of detection of VX for 5.5 min incubation and of paraoxon for 8 min incubation was 4x10(-10) and 4x10(-7) M, respectively. Utilization of the AChE microchip assay for inhibition kinetics was demonstrated also by evaluation of the inhibitor-enzyme bimolecular reaction constant (k(i)). The evaluated k(i) values for VX and paraoxon for AChE from the electric eel were 3.5 x 10(7) and 1.7 x 10(5) M(-1) min(-1), respectively, conforming well to reported values obtained by bulk methods.

Determination of enzymatic activity by capillary electrophoresis

Enzymes are biological catalysts that play an important role in biochemical reactions necessary for normal growth, maturation and reproduction through whole live world. Their accurate quantitation in biological samples is important in many fields of biochemistry, not only in routine biochemistry and in fundamental research, but also in clinical and pharmacological research and diagnosis. Since the direct measurement of enzymes by masses is impossible, they must be quantified by their catalytic activities. Many different methods have been applied for this purpose so far. Although photometric methods are undoubtedly the most frequently used, separation methods will further gain their position in this field. The article reviews different possibilities for the assay of enzymatic activity by means of capillary electrophoresis (CE). Both the off-line and on-line enzyme assays based on CE are discussed.

On-column labeling for capillary electrophoretic analysis of individual mitochondria directly sampled from tissue cross sections

This technical note reports on a new procedure to on-column-label organelles sampled from a tissue cross section into a fused silica capillary. These organelles are then analyzed by capillary electrophoresis with postcolumn laser-induced fluorescence detection. In this procedure, the fluorescent label does not come in contact with the tissue, which facilitates visualization of the sampled tissue cross section. In addition, on-column labeling allows for better control of the reaction time and fluorescent label concentrations. As a proof-of-principle, we show results of mitochondria from rat gastrocnemius muscle cross sections that were on-column-labeled with 10-N-nonyl acridine orange (NAO), a mitochondrion-specific probe, and compare them with results for NAO in-tissue labeling of the same tissue. The new organelle labeling procedure reported here may easily be extended to the analysis of individual organelles in other biological samples and may become a valuable tool in studies investigating the role of mitochondria in muscle aging and exercise physiology.

Increasing the efficiency of in-capillary electrophoretically mediated microanalysis reactions via rapid polarity switching

We report herein a new approach to enhance the sensitivity or speed of CE-based methods that involve in-line reactions. Rapid polarity switching (RPS) is used as a novel means for in-line mixing of two reactant solutions via rapid (1-5 s) and sequential switching of the applied potential field. By employing the RPS approach with a model chemical reaction, that between creatinine and alkaline picrate, significant enhancement in sensitivity (or a decrease in analysis time) is realized. Both increased convection and electrophoretic stacking of the ionic reagent appear to contribute to the rise in apparent reaction rate. When coupled with in-line chemistry of the Jaffe method for creatinine, the RPS methodology allows for 3-fold faster determination of creatinine in the concentration range needed for the analysis of clinical blood serum specimens. The new approach also allows the analysis to be performed without the need for the cumbersome and problematic enhanced sensitivity cell.

On-column capillary electrophoretic monitoring of rapid reaction kinetics for determination of the antioxidative potential of various bioactive phenols

An on-column capillary electrophoretic procedure for the determination of the antioxidative potential of various bioactive phenols, found in plant, fruit, and vegetable extracts, is described. The assay is based on a rapid mixing of phenols or phenolic extracts before the capillary, followed by pressurized injection of the reaction mixture into the capillary. After incubation of the reaction mixture inside the capillary, high voltage is switched on and separation of reactants and products is performed. Using hydrogen peroxide as a stressor, the kinetics of the oxidation of various bioactive phenols was studied (rutin, chlorogenic acid, quercetin, caffeic acid, gallic acid, and combinations of these) and compared with the oxidation rate of L-ascorbic acid as a reference. The concept was demonstrated for the determination of the antioxidative potential of various polyphenol mixtures and of the methanol extract of the sea buckthorn (Hippophae rhamnoides L.). In most cases quercetin has the highest rate constant of oxidation among the tested phenolic compounds. However, in the mixture L-ascorbic acid/quercetin, the oxidation rate of L-ascorbic acid was enhanced and oxidation of quercetin was strongly inhibited compared with the other combinations of tested polyphenols.

On-Line Coupling of Microdialysis Sampling with Microchip-Based Capillary Electrophoresis

Microdialysis sampling is a technique that has been used for in vivo and in vitro monitoring of compounds of pharmaceutical, biomedical, and environmental interest. The coupling of a commercially available microdialysis probe to a microchip-based capillary electrophoresis (CE) system is described. A continuously flowing dialysate stream from a microdialysis probe was introduced into the microchip, and discrete injections were achieved using a valveless gating approach. The effect of the applied voltage and microdialysis flow rate on device performance was investigated. It was found that the peak area varied linearly with the applied voltage. Higher voltages led to lower peak response but faster separations. Perfusion flow rates of 0.8 and 1.0 microL/min were found to provide optimal performance. The on-line microdialysis/microchip CE system was used to monitor the hydrolysis of fluorescein mono-beta-d-galactopyranoside (FMG) by beta-d-galactosidase. A decrease of the FMG substrate with an increase in the fluorescein product was observed. The temporal resolution of the device, which is dependent on the CE separation time, was 30 s. To the best of our knowledge, this is the first reported coupling of a microdialysis sampling probe to a microchip capillary electrophoresis device.

Separation methods applicable to the evaluation of enzyme-inhibitor and enzyme-substrate interactions

Enzymes catalyze a rich variety of metabolic transformations, and do so with very high catalytic rates under mild conditions, and with high reaction regioselectivity and stereospecificity. These characteristics make biocatalysis highly attractive from the perspectives of biotechnology, analytical chemistry, and organic synthesis. This review, containing 128 references, focuses on the use of separation techniques in the elucidation of enzyme-inhibitor and enzyme-substrate interactions. While coverage of the literature is selective, a broad perspective is maintained. Topics considered include chromatographic methods with soluble or immobilized enzymes, capillary electrophoresis, biomolecular interaction analysis tandem mass spectrometry (BIA-MS), phage and ribosomal display, and immobilized enzyme reactors (IMERs). Examples were selected to demonstrate the relevance and application of these methods for determining enzyme kinetic parameters, ranking of enzyme inhibitors, and stereoselective synthesis and separation of chiral entities.

Recent highlights in stationary phase design for open-tubular capillary electrochromatography

This review examines the most recent innovations made to achieve high performance in open-tubular capillary electrochromatography (OT-CEC) separations, focusing on the ingenious chemical and physical solutions made to increase the surface area and equip the stationary phase with exploitable selectivity. Among the approaches taken are chemically bonded ligands, etching with chemical bonding, sol-gels, molecularly imprinted polymers, porous layers, physically attached or adsorbed phases, and nanoparticle coatings. Particularly noteworthy are modern developments with macrocyclic receptor ligands, nanoparticles and open channel electrochromatography on-chip.

Electrophoretically mediated microanalysis

This review describes the existing developments in the use of the capillary electrophoretic microanalytical technique for the in-line study of enzyme reaction, electrophoretically mediated microanalysis (EMMA). The article is divided into a number of parts. After an introduction, the different modes, basic principle, procedure, and some mathematical treatments of EMMA methodology are discussed and illustrated. The applications of EMMA for enzyme assay and for non-enzymatic determination are summarized into two tables. In addition to classical capillary electrophoresis (CE) instrument EMMA, special emphasis is given to a relatively new technique: EMMA on CE microchip. Finally, conclusions are drawn.

Native fluorescence detection of flavin derivatives by microchip capillary electrophoresis with laser-induced fluorescence intensified charge-coupled device detection

To widen the scope of laser-induced fluorescence (LIF) for detection in microchip capillary electrophoresis (CE), a microchip CE LIF-ICCD (intensified charge-coupled device) system based on a tunable wavelength dye laser pumped by a pico-second pulse nitrogen laser for excitation and a spectrograph with ICCD for detection had developed to demonstrate the enhancement in detection sensitivity by the following three approaches: direct detection of native fluorescence, improvement of signal-to-noise ratio by pulse laser excitation and time delay detection, and selective spectral acquisition by multi-channel detection. Riboflavin, flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) have been selected as they are dietetically important and microchip CE provides a promising onsite detection method. The results indicate a strong effect of wavelength on detection sensitivity and the need to tune wavelength for direct detection. Under optimized conditions (excitation 450 nm, emission 520 nm, gate delay time 45 ns, 20 mM phosphate buffer at pH 7.1), the following results were obtained under static condition: Working ranges (0.6-350 microg/l, r > 0.99), detection limits (0.15-1.0 microg/l) and peak height repeatability (1.8-2.2% R.S.D.), all within the applicability range for body fluids or beverages such as human urine and cow milk. Baseline separation of three flavins was obtained under dynamic condition and the fluorescence spectra acquired assist the identification of alkaline-degraded products of riboflavin. Thus, the capability to check peak purity and identify unknown peaks has been demonstrated.

Determination of endogenous extracellular signal-regulated protein kinase by microchip capillary electrophoresis

The application of microchip capillary electrophoresis (CE) to the assay of extracellular signal-regulated protein kinase (ERK) is presented. In this assay, ERK catalyzes the transfer of gamma-phosphate from adenosine 5(')-triphosphate to the threonine residue of a fluorescently labeled nonapeptide (APRTPGGRR), and the phosphorylated and nonphosphorylated peptides were detected by fluorescence. The phosphorylated and nonphosphorylated peptides and the internal standard were separated within 20s, and the increase in magnitude of the phosphorylated peptide peak was monitored to assess ERK activity. ERK reactions were prepared off-chip and analyzed on a single-lane glass microchip fabricated by standard methods. It was demonstrated that microchip CE could be used to measure endogenous amounts of ERK by spiking known concentrations of recombinant ERK2 into the lysates of serum-starved human umbilical vein endothelial cells (HUVEC) and recovering between 90 and 100% for all samples. Endogenous ERK activity was determined by microchip where HUVEC were stimulated with 500pM vascular endothelial growth factor (VEGF) at different times before cell lysis. The results showed a transient VEGF-mediated ERK activation that peaked at 10min, which was consistent with previous reports using conventional techniques. The microchip assay provided a rapid, accurate, and precise alternative to conventional methods of determining endogenous ERK activity.

Stopped-flow enzyme assays on a chip using a microfabricated mixer

This paper describes a microfabricated enzyme assay system including a micromixer that can be used to perform stopped-flow reactions. Samples and reagents were transported into the system by electroosmotic flow (EOF). Streams of reagents were merged and passed through the 100-pL micromixer in < 1 s. The objective of the work was to perform kinetically based enzyme assays in the stopped-flow mode using a system of roughly 6 nL volume. Beta-galactosidase (beta-Gal) was chosen as a model enzyme for these studies and was used to convert the substrate fluorescein mono-beta-D-galactopyranoside (FMG) into fluorescein. Results obtained with microfabricated systems using the micromixer compared well to those obtained with an external T mixing device. In contrast, assays performed in a microfabricated device by merging two streams and allowing mixing to occur by lateral diffusion did not compare well. Using the microfabricated mixer, Km and kcat values of 75 +/- 13 microM and 44 +/- 3 s(-1) were determined. These values compare well to those obtained with the conventional stopped-flow apparatus for which Km was determined to be 60 +/- 6 microM and kcat was 47 +/- 4 s(-1). Enzyme inhibition assays with phenylethyl-beta-D-thiogalactoside (PETG) were also comparable. It was concluded that kinetically based, stopped-flow enzyme assays can be performed in 60 s or less with a miniaturized system of roughly 6 nL liquid volume when mixing is assisted with the described device.

Protein proteolysis and the multi-dimensional electrochromatographic separation of histidine-containing peptide fragments on a chip

This paper reports a system for three-dimensional electrochromatography in a chip format. The steps involved included trypsin digestion, copper(II)-immobilized metal affinity chromatography [Cu(II)-IMAC] selection of histidine-containing peptides, and reversed-phase capillary electrochromatography of the selected peptides. Trypsin digestion and affinity chromatography were achieved in particle-based columns with a microfabricated frit whereas reversed-phase separations were executed on a column of collocated monolithic support structures. Column frits were designed to maintain constant cross sectional area and path length in all channels and to retain particles down to a size of 3 microm. Cu(II)-IMAC selection of histidine-containing peptides from standard peptide mixtures and protein digests followed by reversed-phase chromatography of the selected peptides was demonstrated in the electrochromatography mode. The possibility to run a comprehensive proteomic analysis by combining trypsin digestion, affinity selection, and a reversed-phase separation on chips was shown using fluorescein isothiocyanate-labeled bovine serum albumin as an example.

Sampling BIAS at channel junctions in gated flow injection on chips

The commonly used gated injection scheme was examined and found to suffer from multiple levels of electrokinetic sampling bias, including a new type based on transradial electrokinetic selection (TREKS). TREKS occurs as analytes of differing electrophoretic mobilities migrate around the corner at a channel junction in a microchip. The overall sample bias in gated injection was shown to be time-dependent and resulted in a larger sample bias against components of negative electrophoretic mobility. A new injection procedure for microchip devices based on interstream diffusion at zero potential is proposed. Diffusion of molecules into the separation channel is the main driving force for this type of injection. The new scheme is shown to be useful for injection of complex samples with multiply charged components, such as peptide mixtures. This procedure allows sampling of volumes from 12 to 45 pL, reproducible retention times (RSD < 1.5%), and reproducible peak areas (RSD < 2.3%).

Studies of reversible inhibition, irreversible inhibition, and activation of alkaline phosphatase by capillary electrophoresis

Reversible inhibition, irreversible inhibition, and activation of calf intestinal alkaline phosphatase (EC 3.1.3.1) have been studied by capillary electrophoresis. The capillary electrophoretic enzyme-inhibitor assays were based on electrophoretic mixing of inhibitor and enzyme zones in a substrate-filled capillary. Enzyme inhibition was indicated by a decrease in product formation detected in the capillary by laser-induced fluorescence. Reversible enzyme inhibitors could be quantified by Michaelis-Menten treatment of the electrophoretic data. Reversible, competitive inhibition of alkaline phosphatase by sodium vanadate and sodium arsenate has been examined, and reversible, noncompetitive inhibition by theophylline has been studied. The K(i) values determined for these reversible inhibitors using capillary electrophoresis are within the range of values reported in the literature for the same enzyme-inhibitor combinations. Irreversible inhibition of alkaline phosphatase by EDTA at concentrations of 1.0mM and above has been observed. Activation of alkaline phosphatase has also been observed for EDTA at concentrations from 20 to 400 microM.

Detection technologies in proteome analysis

Common strategies employed for general protein detection include organic dye, silver stain, radiolabeling, reverse stain, fluorescent stain, chemiluminescent stain and mass spectrometry-based approaches. Fluorescence-based protein detection methods have recently surpassed conventional technologies such as colloidal Coomassie blue and silver staining in terms of quantitative accuracy, detection sensitivity, and compatibility with modern downstream protein identification and characterization procedures, such as mass spectrometry. Additionally, specific detection methods suitable for revealing protein post-translational modifications have been devised over the years. These include methods for the detection of glycoproteins, phosphoproteins, proteolytic modifications, S-nitrosylation, arginine methylation and ADP-ribosylation. Methods for the detection of a range of reporter enzymes and epitope tags are now available as well, including those for visualizing beta-glucuronidase, beta-galactosidase, oligohistidine tags and green fluorescent protein. Fluorescence-based and mass spectrometry-based methodologies are just beginning to offer unparalleled new capabilities in the field of proteomics through the performance of multiplexed quantitative analysis. The primary objective of differential display proteomics is to increase the information content and throughput of proteomics studies through multiplexed analysis. Currently, three principal approaches to differential display proteomics are being actively pursued, difference gel electrophoresis (DIGE), multiplexed proteomics (MP) and isotope-coded affinity tagging (ICAT). New multiplexing capabilities should greatly enhance the applicability of the two-dimensional gel electrophoresis technique with respect to addressing fundamental questions related to proteome-wide changes in protein expression and post-translational modification.

Microfluidic chips for clinical and forensic analysis

This review gives an overview of developments in the field of microchip analysis for clinical diagnostic and forensic applications. The approach chosen to review the literature is different from that in most microchip reviews to date, in that the information is presented in terms of analytes tested rather than microchip method. Analyte categories for which examples are presented include (i) drugs (quality control, seizures) and explosives residues, (ii) drugs and endogenous small molecules and ions in biofluids, (iii) proteins and peptides, and (iv) analysis of nucleic acids and oligonucleotides. Few cases of microchip analysis of physiological samples or other "real-world" matrices were found. However, many of the examples presented have potential application for these samples, especially with ongoing parallel developments involving integration of sample pretreatment onto chips and the use of fluid propulsion mechanisms other than electrokinetic pumping.

Use of bioaffinity interactions in electrokinetically controlled assays on microfabricated devices

In this contribution, the role of bioaffinity interactions on electrokinetically controlled microfabricated devices is reviewed. Interesting applications reported in the literature include enzymatic assays, where enzyme and enzyme inhibition kinetics were studied, often in combination with electrophoretic separation. Attention is paid towards developments that could lead to implementation of electrokinetically controlled microdevices in high-throughput screening. Furthermore, enzyme-facilitated detection in combination with electrophoretic separation on microdevices is discussed. Various types of immunoassays have been implemented on the microchip format. The selectivity of antibody-antigen interaction has been exploited for the detection of analytes in complex sample matrices as required, for example, in clinical chemistry. Binding kinetics as well as stoichiometry were studied in chip-based assays. Automated mixing protocols as well as the demonstration of a parallel immunoassay allow implementation of microdevices in high-throughput screening. Furthermore, demonstration of immunoassays on cheap polymeric microdevices opens the way towards the fabrication of disposable devices, a requirement for commercialization and therefore for application in routine analyses.