Quantitative electrospray mass spectrometry for the rapid assay of enzyme inhibitors

20S proteasome hydrolysis of LLVY substrates to determine preferences for moieties in its primed substrate channel

The proteasome is an essential multi-catalytic enzyme in cells that is responsible for degrading proteins with a ubiquitin-dependent or -independent mechanism. Many activity-based probes, inhibitors, and stimulators have been developed to study or modulate the activity of the proteasome. The development of these proteasome probes or inhibitors have been based on their interaction with the amino acids of the β5 substrate channel proceeding the catalytically active threonine residue. There is potential for positive interactions with a substrate to increase selectivity or cleavage rate with the β5 substrate channel after the catalytic threonine as evidenced by the proteasome inhibitor belactosin. To study what moieties the proteasome could accept in its primed substrate channel, we developed a liquid chromatography- mass spectrometry (LC-MS) method to quantitate the cleavage of substrates by purified human proteasome. This method allowed us to rapidly evaluate proteasome substrates that contain a moiety that could interact with the S1' site of the β5 proteasome channel. We were able to determine a preference for a polar moiety at the S1' substrate position. We believe this information can be used in the design of future inhibitors or activity-based probes for the proteasome.

WITHDRAWN: Bioactive peptides derived from meat proteins

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Mass spectrometry approaches to monitor protein-drug interactions

Recent advances in mass spectrometry-based approaches have enabled the investigation of drug-protein interactions in various ways including the direct detection of drug-target complexes, the examination of drug-induced changes in the target protein structure, and the monitoring of enzymatic target activity. Mass spectrometry-based proteomics methods also permit the unbiased analysis of changes in protein abundance and post-translational modifications induced by drug action. Finally, chemoproteomic affinity enrichment studies enable the deconvolution of drug targets under close to physiological conditions. This review provides an overview of current methods for the characterization of drug-target interactions by mass spectrometry and describes a protocol for chemoproteomic target binding studies using immobilized bioactive molecules.

Quantitative mass spectrometry combined with separation and enrichment of phosphopeptides by titania coated magnetic mesoporous silica microspheres for screening of protein kinase inhibitors

We describe herein the development of a matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) approach for screening of protein kinase inhibitors (PKIs). MS quantification of phosphopeptides, the kinase-catalyzed products of nonphosphorylated substrates, is a great challenge due to the ion suppression effect of highly abundant nonphosphorylated peptides in enzymatic reaction mixtures. To address this issue, a novel type of titania coated magnetic hollow mesoporous silica spheres (TiO(2)/MHMSS) material was fabricated for capturing phosphopeptides from the enzymatic reaction mixtures prior to MS analysis. Under optimized conditions, even in the presence of 1000-fold of a substrate peptide of tyrosine kinase epidermal growth factor receptor (EGFR), the phosphorylated substrates at the femtomole level can be detected with high accuracy and reproducibility. With a synthetic nonisotopic labeled phosphopeptide, of which the sequence is similar to that of the phosphorylated substrate, as the internal standard, the MS signal ratio of the phosphorylated substrate to the standard is linearly correlated with the molar ratio of the two phosphopeptides in peptide mixtures over the range of 0.1 to 4 with r(2) being 0.99. The IC(50) values of three EGFR inhibitors synthesized in our laboratory were then determined, and the results are consistent with those determined by an enzyme-linked immunosorbent assay (ELISA). The developed method is sensitive, cost/time-effective, and operationally simple and does not require isotope/radioative-labeling, providing an ideal alterative for screening of PKIs as therapeutic agents.

Extending matrix-assisted laser desorption/ionization triple quadrupole mass spectrometry enzyme screening assays to targets with small molecule substrates

Mass spectrometry (MS)-based high-throughput screening (HTS) has tremendous potential as an alternative to current screening methods due to its speed, sensitivity, reproducibility and label-free readout. We recently reported that a new generation matrix-assisted laser desorption/ionization triple quadrupole (MALDI-QqQ) mass spectrometer is ideally suited for a variety of enzyme assays and screening protocols. However, all the targets measured to date had peptide substrates that were easily monitored by selected ion monitoring (SIM) without interference from the MALDI matrix. To further extend the application to enzymes with small molecule, non-peptide substrates, we evaluated this method for measuring enzyme activity and inhibition of acetylcholinesterase (AChE). Due to the potential of MALDI matrix interference, multiple reaction monitoring (MRM) was investigated for selective MS/MS transitions and to accurately measure the conversion of acetylcholine into choline. Importantly, ionization, detection and MRM transition efficiency differences between the substrate and product can be overcome by pre-balancing the MRM transitions during method development, thus allowing for a direct readout of the enzyme activity using the ratio of the substrate and product signals. Further validation of the assay showed accurate concentration-dependent inhibition measurements of AChE with several known inhibitors. Finally, a small library of 1008 drug-like compounds was screened at a single dose (10 microM) and the top 10 inhibitors from this primary screen were validated in a secondary screen to determine the rank order of inhibitory potency for each compound. Collectively, these data demonstrate that a MALDI-QqQMS-based readout platform is amenable to measuring small molecule substrates and products and offers significant advantages over current HTS methods in terms of speed, sensitivity, reproducibility and reagent costs.

Direct and efficient monitoring of glycosyltransferase reactions on gold colloidal nanoparticles by using mass spectrometry

A simple and efficient assay for glycosyltransferase activity on gold colloidal nanoparticles (GCNPs) by using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) is demonstrated by the enzymatic synthesis of the Lewis X trisaccharide on GCNPs containing GlcNAc residues. GCNPs containing multivalent sugars were well dispersed in aqueous solution and proved to be excellent acceptor substrates for the glycosyltransferase reaction. Direct LDI-TOF MS analysis of these GCNPs provided the ion peaks of the sugar derivatives, chemisorbed through S--Au linkages onto the GCNPs, even in the presence of contaminants such as proteins and salts. Thus, it enabled the rapid and direct detection of the enzymatic reaction on the GCNPs by subjecting a small amount (0.15 muL) of the reaction mixture to MS analysis without purification. Subsequent MS/MS analyses (LDI-LIFT-TOF/TOF method) of the product-carrying GCNPs enabled the structures of the sugar derivatives that had been constructed on the GCNPs by enzymatic glycosylation to be determined. A quantitative inhibition assay for glycosyltransferase by using LDI-TOF MS analysis on the GCNPs was demonstrated by using uridine 5'-diphosphate (UDP) as the inhibitor. This simple assay was then applied to the detection of the enzymatic activity of a crude cell extract of Escherichia coli, which produces Neisseria meningitidis beta-1,4-galactosyltransferase (beta-1,4-GalT). In this case, the GCNPs were roughly purified by means of ultrafiltration to remove the buffer and detergents before MS analysis. That the GCNPs are dissolved in solution in the reaction medium but are solid in the purification process is greatly advantageous for the simple and efficient detection of enzymatic activity in crude biological samples. Thus, GCNPs containing a variety of biomolecules may become a versatile and efficient tool for the rapid and direct monitoring of metabolism (metabolomics) in living cells when combined with LDI-TOF MS analysis.

Mass spectrometry for enzyme assays and inhibitor screening: an emerging application in pharmaceutical research

Robust methods that monitor enzyme activity and inhibitor potency are crucial to drug discovery and development. Over the past 20 years, mass spectrometric methods have increasingly been used to measure enzyme activity and kinetics. However, for rapid screening of inhibitory compounds, various forms of fluorescence and chemiluminscence readout have continued to dominate the market. As the sensitivity, speed, and miniaturization of mass spectrometry methods continue to advance, opportunities to couple mass spectrometry with screening will continue to come to the forefront. To appreciate the tremendous potential for MS-based screening assays, it becomes necessary to understand the current state of capabilities in this arena. Thus, this review is intended to capture how mass spectrometry for studying enzymes activity has progressed from simple qualitative questions (i.e., is the product detected?) to quantitative measures of enzyme activity and kinetics and then as a tool for rapidly screening inhibitory compounds as an alternative to current methods of high throughput drug screening.

Screening of acetylcholinesterase inhibitors in snake venom by electrospray mass spectrometry

An electrospray ionization/mass spectrometry (ESI/MS)-based assay for the determination of acetylcholinesterase (AChE)-inhibiting activity in snake venom was developed. It allows the direct monitoring of the natural AChE substrate acetylcholine (AC) and the respective product choline. The assay scheme was employed in the screening for neurotoxic activity in fractions of the venom of Bothrops moojeni. AChE inhibition was assessed in two fractions. As a positive control, the established AChE inhibitor 1,5-bis(4-allyl-dimethylammoniumphenyl)pentan-3-one dibromide (BW284c51) was used, a dose-response curve for this compound was generated and the IC50 value for the inhibitor was determined to be 1.60 ± 0.09 × 10-9 mol L-1. The dose-response curve was used as "calibration function" for the venom inhibition activity, resulting in BW284c51-equivalent concentrations of 1.76 × 10-9 mol L-1 and 1.07 × 10-9 mol L-1 for the two fractions containing activity. The ESI/MS-based assay scheme was validated using the established Ellman reaction. The data obtained using both methods were found to be in good agreement. The ESI/MS-based assay scheme is therefore an attractive alternative to the standard colorimetric assay.

Determination of angiotensin converting enzyme inhibitory activity by high-performance liquid chromatography/electrospray-mass spectrometry

A sensitive and rapid method for determination of angiotensin converting enzyme (ACE) inhibitory activity was developed based on a combination of enzymatic reaction followed by high performance liquid chromatography/electrospray-mass spectrometry (HPLC-ESI-MS) determination of its product. The most commonly used substrate hippuryl-histidyl-leucine (HHL) or hippuryl-glycyl-glycine (HGG) hydrolysis catalyzed by purified rabbit lung ACE or human plasma ACE was investigated in the presence of benazeprilat. The incubation time was 8 min for purified lung ACE, and 16 min for human plasma ACE. The produced hippuric acid (HA) was separated from substrate HHL or HGG by HPLC on a C(18) column with isocratic elution within 6.5 min, and quantified by electrospray ionization mass spectrometry (ESI-MS) with p-phthalic acid as an internal standard (IS). The limit of detection of HA was 6.0 ng/ml. HHL or HGG hydrolysis catalyzed by purified lung ACE displayed excellent accuracy and reproducibility. The small total reaction volume, the low concentration of substrate, and the simple treating procedures present the advantages of the new method. Furthermore, the total time of the whole procedure for one sample with the novel method is less than 1/2 of that of the conventional HPLC or spectrophotometry method, while the accuracy and the precision of the new method are almost the same as the conventional HPLC method with UV detection.

Time-Resolved Micro Liquid Desorption Mass Spectrometry: Mechanism, Features, and Kinetic Applications

Liquid water beam desorption mass spectrometry is an intriguing technique to isolate charged molecular aggregates directly from the liquid phase and to analyze them employing sensitive mass spectrometry. The liquid phase in this approach consists of a 10 µm diameter free liquid filament in vacuum which is irradiated by a focussed infrared laser pulse resonant with the OH-stretch vibration of bulk water. Depending upon the laser wavelength, charged (e.g. protonated) macromolecules are isolated from solution through a still poorly characterized mechanism. After the gentle liquid-to-vacuum transfer the low-charge-state aggregates are analyzed using time-of-flight mass spectrometry. A recent variant of the technique uses high performance liquid chromatography valves for local liquid injections of samples in the liquid carrier beam, which enables very low sample consumption and high speed sample analysis. In this review we summarize recent work to characterize the ‘desorption’ or ion isolation mechanism in this type of experiment. A decisive and interesting feature of micro liquid beam desorption mass spectrometry is that — under certain conditions — the gas-phase mass signal for a large number of small as well as supramolecular systems displays a surprisingly linear response on the solution concentration over many orders of magnitude, even for mixtures and complex body fluids. This feature and the all-liquid state nature of the technique makes this technique a solution-type spectroscopy that enables real kinetic studies involving (bio)polymers in solution without the need for internal standards. Two applications of the technique monitoring enzyme digestion of proteins and protein aggregation of an amyloid model system are highlighted, both displaying its potential for monitoring biokinetics in solution.

Monitoring enzymatic conversions by mass spectrometry: a critical review

This review highlights recent advances in the application of electrospray ionisation and matrix-assisted laser desorption/ionisation mass spectrometry (MS) to study enzymatic reactions. Several assay schemes for different fields of application are presented. The employment of MS as a means of detection in pre-steady-state kinetic studies by rapid-mixing direct analysis and rapid-mixing quench flow techniques is discussed. Several steady-state kinetic studies of a broad range of different enzymatic systems are presented as well as enzyme inhibition studies for various target enzymes. As a promising new development multiplex assays, which monitor the conversion of several substrates simultaneously in one experiment, are described. This assay type has been used for competition studies, enzymatic activity screenings and for diagnostic purposes in clinical chemistry. Generally, it can be concluded that mass spectrometry offers an intriguing alternative as detection methodology in enzymatic bioassays. Its applicability for the monitoring the conversion of naturally occurring substrates and its overall versatility make MS an especially promising tool for the study of enzyme-catalysed processes.

Mass spectrometric approaches for the investigation of dynamic processes in condensed phase

Mass spectrometry (MS) offers many advantages over other established spectroscopic techniques employed for the investigation of processes in condensed phase. The sensitivity, specificity, and speed afforded by MS-based methods enable to obtain very valuable insights into the mechanism of complex dynamic processes. Off-line methods rely on quenching to halt the progress of the reaction of interest and allow for the implementation of a broad range of analytical procedures for sample fractionation, isolation, or desalting. On the contrary, on-line methods are designed to carry out the real-time monitoring of dynamic processes through a continuous uninterrupted analysis of reaction mixtures, with the only caveat that the sample solutions be directly amenable to the available ionization technique. The utilization of rapid mixing devices in direct connection with a mass spectrometer or included in off-line schemes provides access to the initial moments of a reaction, which can offer very important information about the reaction mechanism. This report summarizes the different off- and on-line strategies developed to study chemical and biochemical reactions in solution and obtain kinetic/mechanistic information. The merits of the various experimental designs, the characteristics of the different instrumental setups, and the factors affecting time resolution are discussed with the aid of specific examples, which highlight the contributions of MS to the different facets of the investigation of dynamic processes in condensed phase.

A Mass Spectrometry Plate Reader: Monitoring Enzyme Activity and Inhibition with a Desorption/Ionization on Silicon (DIOS) Platform

A surface-based laser desorption/ionization mass spectrometry assay that makes use of Desorption/Ionization on Silicon Mass Spectrometry (DIOS-MS) has been developed to monitor enzyme activity and enzyme inhibition. DIOS-MS has been used to characterize inhibitors from a library and then to monitor their activity against selected enzyme targets, including proteases, glycotransferase, and acetylcholinesterase. An automated DIOS-MS system was also used as a high-throughput screen for the activity of novel enzymes and enzyme inhibitors. On two different commercially available instruments, a sampling rate of up to 38 inhibitors per minute was accomplished, with thousands of inhibitors being monitored. The ease of applying mass spectrometry toward developing enzyme assays and the speed of surface-based assays such as DIOS for monitoring inhibitor effectiveness and enzyme activity makes it attractive for a broad range of screening applications.

Development of an LC–MS based enzyme activity assay for MurC: application to evaluation of inhibitors and kinetic analysis

An enzyme activity assay, based on mass spectrometric (MS) detection of specific reaction product following HPLC separation, has been developed to evaluate pharmaceutical hits identified from primary high throughput screening (HTS) against target enzyme Escherichia coli UDP-N-acetyl-muramyl-L-alanine ligase (MurC), an essential enzyme in the bacterial peptidoglycan biosynthetic pathway, and to study the kinetics of the enzyme. A comparative analysis of this new liquid chromatographic-MS (LC-MS) based assay with a conventional spectrophotometric Malachite Green (MG) assay, which detects phosphate produced in the reaction, was performed. The results demonstrated that the LC-MS assay, which determines specific ligase activity of MurC, offers several advantages including a lower background (0.2% versus 26%), higher sensitivity (> or = 10 fold), lower limit of quantitation (LOQ) (0.02 microM versus 1 microM) and wider linear dynamic range (> or = 4 fold) than the MG assay. Good precision for the LC-MS assay was demonstrated by the low intraday and interday coefficient of variation (CV) values (3 and 6%, respectively). The LC-MS assay, free of the artifacts often seen in the Malachite Green assay, offers a valuable secondary assay for hit evaluation in which the false positives from the primary high throughput screening can be eliminated. In addition, the applicability of this assay to the study of enzyme kinetics has also been demonstrated.

Multiplex inhibitor screening and kinetic constant determinations for yeast hexokinase using mass spectrometry based assays

An electrospray ionization mass spectrometry based assay was developed for kinetic measurements and inhibitor screening of yeast hexokinase. There is considerable discrepancy in the literature as to the accuracy of kinetic data obtained for hexokinase. In the assay described herein, the product, glucose 6-phosphate was directly monitored by ion trap mass spectrometry and quantified using an internal standard, 2 deoxy-glucose 6-phosphate. The kinetic parameters, K(M) and V(max) for the two substrates were determined without using a coupling enzyme as is normally employed in the traditional spectrophotometric assay for systems lacking a chromophore. In addition, hexokinase was successfully immobilized onto an amino-link gel, and a mock library was screened against the immobilized enzyme for the identification of possible inhibitors. After comparing the mass spectra of the library before and after incubation, trehalose 6-phosphate, ADP, and oxidized glutathione were differentiated from other weak or non-inhibitors. Inhibition behavior of ADP with respect to ATP was further evaluated with the ESI-MS assay and the value of K(i) was determined. This ESI-MS assay was demonstrated to be both accurate and precise for determining kinetic constants and for identifying enzyme inhibitors.

Biomedical and biological mass spectrometry

This review focuses on biological and biomedical mass spectrometry, and covers a selection of publications in this area included in the MEDLINE database for the period 1987-2001. Over the last 15 years, biological and biomedical mass spectrometry has progressed out of all recognition. The development of soft ionization methods, such as electrospray ionization and matrix-assisted laser desorption ionization, has mainly contributed to the remarkable progress, because they can easily produce gas-phase ions of large, polar, and thermally labile biomolecules, such as proteins, peptides, nucleic acids and others. The innovations of ionization methods have led to remarkable progress in mass spectrometric technology and in biochemistry, biotechnology and molecular biology research. In addition, mass spectrometry is one of the powerful and effective technologies for drug discovery and development. It is applicable to studies on structural determination, drug metabolism, including pharmacokinetics and toxicokinetics, and de novo drug discovery by applying post-genomic approarches. In the present review, the innovative soft ionization methods are first discussed along with their features. Also, the characteristics of the mass spectrometers which are active in the biological and biomedical research fields are also described. In addition, examples of the applications of biological and biomedical mass spectrometry are provided.

Methods for biocatalyst screening

Biocatalysts are now widely accepted as useful alternative tools to classic organic synthetic techniques for the regio- and enantioselective synthesis under mild reaction conditions in many fields of chemistry. The development of techniques for the rational or evolutionary design of novel or modified enzymes has increased the need for fast and reliable methods for the identification of the most powerful catalysts. We present a short overview on screening techniques in this area. Beside classical methods such as spectrophotometry and fluorimetry, a number of new approaches like methods based on the measurement of pH changes or IR-thermography have been recently developed. Additionally the use of electrospray and matrix-assisted laser desorption/ionization mass spectrometry has gained increasing influence in this field of biotechnology.

Monitoring enzyme catalysis with mass spectrometry

Mass spectrometry is a rapid, sensitive, and accurate quantitative approach for the direct monitoring of enzyme-catalyzed reactions that does not require a chromophore or radiolabeling and thus provides a viable alternative to existing analytical techniques. In this study the proteolysis of intact viral capsid proteins, the alpha-glucosidase-catalyzed hydrolysis of p-nitrophenyl-alpha-glucopyranoside and the lipoprotein lipase-catalyzed ester hydrolysis of resorufin were examined. Matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry were used to examine the proteolysis of viral protein capsids, providing information about capsid dynamics and the stabilizing force of viral protein/RNA interactions. In addition, k(cat) and K(m) values of enzyme-catalyzed hydrolysis were obtained (without the use of a chromophore). These results also demonstrate the effect an unnatural substrate can have on enzyme activity. Overall, mass spectrometry provides for efficient and quantitative analysis of enzyme-catalyzed reactions, as well as the direct observation of reaction dynamics.

Mass spectrometry in combinatorial chemistry

In the fast expanding field of combinatorial chemistry, profiling libraries has always been a matter of concern--as illustrated by the buoyant literature over the past seven years. Spectroscopic methods, including especially mass spectrometry and to a lesser extent IR and NMR, have been applied at different levels of combinatorial library synthesis: in the rehearsal phase to optimize the chemistry prior to library generation, to confirm library composition, and to characterize after screening each structure that exhibits positive response. Most of the efforts have been concentrated on library composition assessment. The difficulties of such analyses have evolved from the infancy of the combinatorial concept, where large mixtures were prepared, to the recent parallel syntheses of collections of discrete compounds. Whereas the complexity of the analyses has diminished, an increased degree of automation was simultaneously required to achieve efficient library component identification and quantification. In this respect, mass spectrometry has been found to be the method of choice, providing rapid, sensitive, and informative analyses, especially when coupled to chromatographic separation. Fully automated workstations able to cope with several hundreds of compounds per day have been designed. After a brief introduction to describe the combinatorial approach, library characterization will be discussed in detail, considering first the solution-based methodologies and secondly the support-bound material analyses.

Electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry. Emerging technologies in biomedical sciences

Tremendous progress in biomedical sciences has been made possible in part by recent advances in bioanalytical methods, in particular biological mass spectrometry. Since the introduction of electrospray ionization mass spectrometry (ESI-MS) in 1984 and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) in 1988, the field of bioanalytical mass spectrometry has seen rapid growth. In concert with separation techniques such as capillary electrophoresis and high performance liquid chromatography, mass spectrometry allows characterization of a large array of small organic molecules, peptides, proteins, oligonucleotides, and RNA fragments. Thus, substantially more expedient and definitive determination of molecular weight is now possible by mass spectrometric analysis. In this commentary, general descriptions of ESI- and MALDI-MS are presented. Furthermore, several recent developments and applications in addressing difficult biological problems are discussed.

Mass spectrometric analysis in combinatorial chemistry

The continuing success of the combinatorial approach is heavily reliant on analytical methodologies, which allow for the rapid and accurate characterisation of medicinally relevant molecules from compound libraries. Mass spectrometry has recently been touted as the most suitable tool for a range of combinatorial applications such as structural elucidation and screening. The refinement of conventional methods, developments of techniques such as Fourier transform ion cyclotron resonance and new screening methodologies have allowed the medicinal chemist to tackle the growing analytical challenges posed by combinatorial chemistry.

Impact of mass spectrometry on combinatorial chemistry

In the past few years, the emergence of combinatorial chemistry has drawn increasing attention and a great deal of analytical research has been centered around this new methodology. These new methods capable of producing vast numbers of samples, which are in many cases highly complex, demand fast and reliable analytical techniques able to provide high quality information concerning sample compositions. Mass spectrometry (MS) is the method of choice to face these analytical challenges. In particular, the introduction of electrospray ionization (ESI and matrix assisted laser desorption/ionization (MALDI) have been the driving forces for many of the recent innovations, not only within the fields of the biosciences, but also in combinatorial chemistry. These ionization techniques are extremely versatile for the characterization of both single compound collections and compound mixture collections. The high-throughput capabilities, as well as many possible couplings with separation techniques (HPLC, CE) have been thus facilitated. However, mass spectrometry is not only limited to use as an instrument for synthesis control, but also plays an increasing role in the identification of active compounds from complex libraries. Recently, new initiatives for library analysis and screening have arisen from the application of the latest developments in mass spectrometry, Fourier transform ion cyclotron resonance (FTICR).

Selective inhibition of beta-1,4- and alpha-1,3-galactosyltransferases: donor sugar-nucleotide based approach

A combined rational and library approach was used to identify bisphosphonates (IC50 = 20 microM) and galactose type 1-N-iminosugar (IC50=45 microM) as novel motifs for selective inhibition of beta-1,4-galactosyltransferase (beta-1,4-GalT) and alpha-1,3-galactosyltransferase (alpha-1,3-GalT), respectively. Our results demonstrate that, though these two galactosyltransferases both utilize the same donor sugar-nucleotide (UDP-Gal), the difference in their mechanisms can be utilized to design donor sugar or nucleotide analogues with inhibitory activities selective for only one of the galactosyltransferases. Investigation of beta-1,4-GalT inhibition using UDP-2-deoxy-2-fluorogalactose (UDP-2-F-Gal), UDP, and bisphosphonates, also led to the observation of metal dependent inhibition of beta-1,4-GalT. These observations and the novel inhibitor motifs identified in this study pave the way for the design and identification of even more potent and selective galactosyltransferase inhibitors.

Enzymatic assay of galactosyltransferase by capillary electrophoresis

The kinetic parameters of a galactosyltransferase-catalyzed reaction were determined for the first time using capillary zone electrophoresis (CZE) using the methylumbelliferyl (MU) glycoside of N-acetylglucosamine as the acceptor molecule. The CZE was performed using borate buffer and the enzymatic transformations were monitored at 214 nm. The kinetic parameters obtained for MU-GlcNAc were Km = 35.9 microM and Vmax = 7.5 micromol/min/mg, and those for UDP-Gal were Km = 115.3 microM and Vmax = 12.4 micromol/min/mg. A representative inhibition assay was also carried out using UDP as an inhibitor to give the Ki value of 83.9 microM against MU-GlcNAc. The structure of the synthetic product was also confirmed using 1H NMR spectroscopies after isolation by simple chromatography.

Protein phosphorylation: technologies for the identification of phosphoamino acids

Protein phosphorylation plays a central role in many biological and biomedical phenomena. In this review, while a brief overview of the occurrence and function of protein phosphorylation is given, the primary focus is on studies related to the detection and analysis of phosphorylation both in vivo and in vitro. We focus on phosphorylation of serine, threonine and tyrosine, the most commonly phosphorylated amino acids in eukaryotes. Technologies such as radiolabelling, antibody recognition, chromatographic methods (HPLC, TLC), electrophoresis, Edman sequencing and mass spectrometry are reviewed. We consider the speed, simplicity and sensitivity of tools for detection and identification of protein phosphorylation, as well as quantitation and site characterisation. The limitations of currently available methods are summarised.