Use of synthetic peptides as tracer antigens in fluorescence polarization immunoassays of high molecular weight analytes

Assays to Estimate the Binding Affinity of Aptamers

Aptamers have become coming-of-age molecular recognition elements in both diagnostic and therapeutic applications. Generated by SELEX, the 'quality control' of aptamers, which involves the validation of their binding affinity against their respective targets is pivotal to ascertain their potency prior to use in any downstream assays or applications. Several aptamers have been isolated thus far, however, the usage of inappropriate validation assays renders some of these aptamers dubitable in terms of their binding capabilities. Driven by this need, we provide an up-to-date critical review of the various strategies used to determine the aptamer-target binding affinity with the aim of providing researchers a better comprehension of the different analytical approaches in respect to the molecular properties of aptamers and their intended targets. The techniques reported have been classified as label-based techniques such as fluorescence intensity, fluorescence anisotropy, filter-binding assays, gel shift assays, ELISA; and label-free techniques such as UV-Vis spectroscopy, circular dichroism, isothermal titration calorimetry, native electrospray ionization-mass spectrometry, quartz crystal microbalance, surface plasmon resonance, NECEEM, backscattering interferometry, capillary electrophoresis, HPLC, and nanoparticle aggregation assays. Hybrid strategies combining the characteristics of both categories such as microscale thermophoresis have been also additionally emphasized. The fundamental principles, complexity, benefits, and challenges under each technique are elaborated in detail.

Self-Competitive Inhibition of the Bacteriophage P22 Tailspike Endorhamnosidase by O-Antigen Oligosaccharides

The P22 tailspike endorhamnosidase confers the high specificity of bacteriophage P22 for some serogroups of Salmonella differing only slightly in their O-antigen polysaccharide. We used several biophysical methods to study the binding and hydrolysis of O-antigen fragments of different lengths by P22 tailspike protein. O-Antigen saccharides of defined length labeled with fluorophors could be purified with higher resolution than previously possible. Small amounts of naturally occurring variations of O-antigen fragments missing the nonreducing terminal galactose could be used to determine the contribution of this part to the free energy of binding to be ∼7 kJ/mol. We were able to show via several independent lines of evidence that an unproductive binding mode is highly favored in binding over all other possible binding modes leading to hydrolysis. This is true even under circumstances under which the O-antigen fragment is long enough to be cleaved efficiently by the enzyme. The high-affinity unproductive binding mode results in a strong self-competitive inhibition in addition to product inhibition observed for this system. Self-competitive inhibition is observed for all substrates that have a free reducing end rhamnose. Naturally occurring O-antigen, while still attached to the bacterial outer membrane, does not have a free reducing end and therefore does not perform self-competitive inhibition.

Strategies to bioengineer aptamer-driven nanovehicles as exceptional molecular tools for targeted therapeutics: A review

Aptamers are a class of folded nucleic acid strands capable of binding to different target molecules with high affinity and selectivity. Over the years, they have gained a substantial amount of interest as promising molecular tools for numerous medical applications, particularly in targeted therapeutics. However, only the different treatment approaches and current developments of aptamer-drug therapies have been discussed so far, ignoring the crucial technical and functional aspects of constructing a therapeutically effective aptamer-driven drug delivery system that translates to improved in-vivo performance. Hence, this paper provides a comprehensive review of the strategies used to improve the therapeutic performance of aptamer-guided delivery systems. We focus on the different functional features such as drug deployment, payload capacity, in-vivo stability and targeting efficiency to further our knowledge in enhancing the cell-specific delivery of aptamer-drug conjugates. Each reported strategy is critically discussed to emphasize both the benefits provided in comparison with other similar techniques and to outline their potential drawbacks with respect to the molecular properties of the aptamers, the drug and the system to be designed. The molecular architecture and design considerations for an efficient aptamer-based delivery system are also briefly elaborated.

A Comparison of Optical, Electrochemical, Magnetic, and Colorimetric Point-of-Care Biosensors for Infectious Disease Diagnosis

Each year, infectious diseases are responsible for millions of deaths, most of which occur in the rural areas of developing countries. Many of the infectious disease diagnostic tools used today require a great deal of time, a laboratory setting, and trained personnel. Due to this, the need for effective point-of-care (POC) diagnostic tools is greatly increasing with an emphasis on affordability, portability, sensitivity, specificity, timeliness, and ease of use. In this Review, we discuss the various diagnostic modalities that have been utilized toward this end and are being further developed to create POC diagnostic technologies, and we focus on potential effectiveness in resource-limited settings. The main modalities discussed herein are optical-, electrochemical-, magnetic-, and colorimetric-based modalities utilized in diagnostic technologies for infectious diseases. Each of these modalities feature pros and cons when considering application in POC settings but, overall, reveal a promising outlook for the future of this field of technological development.

Fluorescence Polarization Assays for the Detection of Proteases and Their Inhibitors

Fluorescence polarization (FP) is a powerful technology for the determination of the sizes of molecules in solution and has been extensively used in studies of the binding of small ligands to their receptors. Alternatively, it can be used to monitor the conversion of large molecules to small ones, for example, in studies of hydrolytic enzymes such as proteases, DNases, RNases, and carbohydratases. Proteases are becoming increasingly important targets of drug discovery in the fields of infectious diseases, inflammatory disorders, and apoptosis, among others. FP has been shown to be an extremely useful assay technology for the nonspecific and specific detection and measurement of proteases and their inhibitors. Although fluorescein is the fluorophore most often used for FP, it suffers from the disadvantages of being essentially nonfluorescent below pH 6, where many proteases of interest operate, and derivatives are not available in the red portion of the spectrum, where very few interferences are encountered. The development of highly sensitive, pH-independent assays using BODIPY(r)*-labeled a-caseins at fluorescein and Texas Red wavelengths are described. These assays can detect less than 1 ng/ml of pepsin, at pH 2.0, and less than 10 ng/ml trypsin, at pH 7.4, with a 5-min incubation time. Using 1 ttg/ml trypsin and BODIPY(r) TR-X-labeled cv-casein, soybean trypsin inhibitor (STI) can readily be detected at less than 50 ng/ml with a 5-min incubation time. Since these assays require a single, stable reagent in a homogeneous format, they readily lend themselves to the high throughput screening of compound libraries for protease inhibitors.

A Simple Theoretical Model for Fluorescence Polarization Binding Assay Development

Fluorescence polarization is a screening technology that is radioactivity free, homogeneous, and ratiometric. The signal measured with this technology is a weighted value of free and bound ligand. As a consequence, saturation curves are accessible only after calculation of the corresponding concentrations of free and bound ligand. To make this technology more accessible to assay development, the authors propose a simple mathematical model that predicts fluorescence polarization values from ligand and receptor total concentrations, depending on the corresponding dissociation constant. This model was validated using data of Bodipy-NDP-αMSH binding to MC5, obtained after either ligand saturation of a receptor preparation or, conversely, receptor saturation of a ligand solution. These experimental data were also used to calculate the actual concentration of free and bound ligand and receptor and to obtain pharmacological constants by Scatchard analysis. A general method is proposed, which facilitates the design of fluorescence polarization binding assays by relying on the representation of theoretical polarization values. This approach is illustrated by the application to 2 systems of very different affinities.

Rheostatic Regulation of the SERCA/Phospholamban Membrane Protein Complex Using Non-Coding RNA and Single-Stranded DNA oligonucleotides

The membrane protein complex between sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and phospholamban (PLN) is a prime therapeutic target for reversing cardiac contractile dysfunctions caused by calcium mishandling. So far, however, efforts to develop drugs specific for this protein complex have failed. Here, we show that non-coding RNAs and single-stranded DNAs (ssDNAs) interact with and regulate the function of the SERCA/PLN complex in a tunable manner. Both in HEK cells expressing the SERCA/PLN complex, as well as in cardiac sarcoplasmic reticulum preparations, these short oligonucleotides bind and reverse PLN's inhibitory effects on SERCA, increasing the ATPase's apparent Ca(2+) affinity. Solid-state NMR experiments revealed that ssDNA interacts with PLN specifically, shifting the conformational equilibrium of the SERCA/PLN complex from an inhibitory to a non-inhibitory state. Importantly, we achieved rheostatic control of SERCA function by modulating the length of ssDNAs. Since restoration of Ca(2+) flux to physiological levels represents a viable therapeutic avenue for cardiomyopathies, our results suggest that oligonucleotide-based drugs could be used to fine-tune SERCA function to counterbalance the extent of the pathological insults.

Tracking the emergence of high affinity aptamers for rhVEGF165 during capillary electrophoresis-systematic evolution of ligands by exponential enrichment using high throughput sequencing

Capillary electrophoresis-systematic evolution of ligands by exponential enrichment (CE-SELEX) is a powerful technique for isolating aptamers for various targets, from large proteins to small peptides with molecular weights of several kilodaltons. One of the unique characteristics of CE-SELEX is the relatively high heterogeneity of the ssDNA pools that remains even after multiple rounds of selection. Enriched sequences or highly abundant oligonucleotide motifs are rarely reported in CE-SELEX studies. In this work, we employed 454 pyrosequencing to profile the evolution of an oligonucleotide pool through multiple rounds of CE-SELEX selection against the target recombinant human vascular endothelial growth factor 165 (rhVEGF165). High throughput sequencing allowed up to 3 × 10(4) sequences to be obtained from each selected pool and compared to the unselected library. Remarkably, the highest abundance contiguous sequence (contig) was only present in 0.8% of sequences even after four rounds of selection. Closer analyses of the most abundant contigs, the top 1000 oligonucleotide fragments, and even the eight original FASTA files showed no evidence of prevailing motifs in the selected pools. The sequencing results also provided insight into why many CE-SELEX selections obtain pools with reduced affinities after many rounds of selection (typically >4). Preferential amplification of a particular short polymerase chain reaction (PCR) product allowed this nonbinding sequence to overtake the pool in later rounds of selection suggesting that further refinement of primer design or amplification optimization is necessary. High affinity aptamers with 10(-8) M dissociation constants for rhVEGF165 were identified. The affinities of the higher abundance contigs were compared with aptamers randomly chosen from the final selection pool using affinity capillary electrophoresis (ACE) and fluorescence polarization (FP). No statistical difference in affinity between the higher abundance contigs and the randomly chosen aptamers was observed, supporting the premise that CE-SELEX selects a uniquely heterogeneous pool of high affinity aptamers.

Rapid screening and identification of dominant B cell epitopes of HBV surface antigen by quantum dot-based fluorescence polarization assay

A method for quickly screening and identifying dominant B cell epitopes was developed using hepatitis B virus (HBV) surface antigen as a target. Eleven amino acid fragments from HBV surface antigen were synthesized by 9-fluorenylmethoxy carbonyl solid-phase peptide synthesis strategy, and then CdTe quantum dots were used to label the N-terminals of all peptides. After optimizing the factors for fluorescence polarization (FP) immunoassay, the antigenicities of synthetic peptides were determined by analyzing the recognition and combination of peptides and standard antibody samples. The results of FP assays confirmed that 10 of 11 synthetic peptides have distinct antigenicities. In order to screen dominant antigenic peptides, the FP assays were carried out to investigate the antibodies against the 10 synthetic peptides of HBV surface antigen respectively in 159 samples of anti-HBV surface antigen-positive antiserum. The results showed that 3 of the 10 antigenic peptides may be immunodominant because the antibodies against them existed more widely among the samples and their antibody titers were higher than those of other peptides. Using three dominant antigenic peptides, 293 serum samples were detected for HBV infection by FP assays; the results showed that the antibody-positive ratio was 51.9% and the sensitivity and specificity were 84.3% and 98.2%, respectively. In conclusion, a quantum dot-based FP assay is a very simple, rapid, and convenient method for determining immunodominant antigenic peptides and has great potential in applications such as epitope mapping, vaccine designing, or clinical disease diagnosis in the future.

Screening interaction between ochratoxin A and aptamers by fluorescence anisotropy approach

By taking advantage of the intrinsic fluorescence of ochratoxin A (OTA), we present a fluorescence anisotropy approach for rapid analysis of the interactions between OTA and aptamers. The specific binding of OTA with a 36-mer aptamer can induce increased fluorescence anisotropy (FA) of OTA as the result of the freedom restriction of OTA and the increase of molecular volume, and the maximum FA change is about 0.160. This FA approach enables an easy way to investigate the effects of buffer compositions like metal ions on the affinity binding. FA analysis shows the interaction between OTA and aptamer is greatly enhanced by the simultaneous presence of Ca(2+) and Na(+), while the binding affinity of aptamer decreases more than 18-fold when only Ca(2+) exists, and the binding is completely lost when Ca(2+) is absent. Crucial region of the aptamer for binding can be mapped through FA analysis and aptamer mutation. The demonstrated FA approach maintains the advantages of FA in simplicity, rapidity, and robustness. This investigation will help the development of aptamer-based assays for OTA detection in optimizing the binding conditions, modification of aptamers, and rational design.

Isolation of DNA aptamers using micro free flow electrophoresis

A micro free flow electrophoresis (μFFE) device was used to select DNA aptamers for human immunoglobulin E (IgE). The continuous nature of μFFE allowed 1.8 × 10(14) sequences to be introduced over a period of 30 min, a 300-fold improvement in library size over capillary electrophoresis based selections (CE-SELEX). Four rounds of selection were performed within four days. Aptamers with low nM dissociation constants for IgE were identified after a single round of μFFE selection.

Methods for measuring aptamer-protein equilibria: a review

Aptamers are single stranded DNA or RNA molecules that have been selected using in vitro techniques to bind target molecules with high affinity and selectivity, rivaling antibodies in many ways. In order to use aptamers in research and clinical applications, a thorough understanding of aptamer-target binding is necessary. In this article, we review methods for assessing aptamer-protein binding using separation based techniques such as dialysis, ultrafiltration, gel and capillary electrophoresis, and HPLC; as well as mixture based techniques such as fluorescence intensity and anisotropy, UV-vis absorption and circular dichroism, surface plasmon resonance, and isothermal titration calorimetry. For each method the principle, range of application and important features, such as sample consumption, experimental time and complexity, are summarized and compared.

Peptides that mimic glycosaminoglycans: high-affinity ligands for a hyaluronan binding domain

BACKGROUND

Hyaluronan (HA) is a non-sulfated glycosaminoglycan (GAG) that promotes motility, adhesion, and proliferation in mammalian cells, as mediated by cell-surface HA receptors. We sought to identify non-carbohydrate ligands that would bind to and activate cell-surface HA receptors. Such analogs could have important therapeutic uses in the treatment of cancer, wound healing, and arthritis, since such ligands would be resistant to degradation by hyaluronidase (HAse).

RESULTS

Peptide ligands that bind specifically to the recombinant HA binding domain (BD) of the receptor for hyaluronan-mediated motility (RHAMM) were obtained by screening two peptide libraries: (i) random 8-mers and (ii) biased 8-mers with alternating acidic side chains, i.e. XZXZXZXZ (X=all-L-amino acids except Cys, Lys, or Arg; Z=D-Asp, L-Asp, D-Glu, or L-Glu). Selectivity of the peptide ligands for the HABD was established by (i) detection of binding of biotin- or fluorescein-labeled peptides to immobilized proteins and (ii) fluorescence polarization of FITC-labeled peptides with the HABD in solution. HA competitively displaced binding of peptides to the HABD, while other GAGs were less effective competitors. The stereochemistry of four biased octapeptides was established by synthesis of the 16 stereoisomers of each peptide. Binding assays demonstrated a strong preference for alternating D and L configurations for the acidic residues, consistent with the calculated orientation of glucuronic acid moieties of HA.

CONCLUSIONS

Two classes of HAse-resistant peptide mimetics of HA were identified with high affinity, HA-compatible binding to the RHAMM HABD. This demonstrated that non-HA ligands specific to a given HA binding protein could be engineered, permitting receptor-specific targeting.

Determination of equilibrium dissociation constants in fluorescence polarization

A simple mathematical model (the FP K(d) model) is used to generate the dissociation equilibrium constant (K(d)) for G protein-coupled receptor-ligand binding measured using fluorescence polarization (FP) saturation curve analysis. The model generates data that may be analyzed by the method of Scatchard. The validity of the FP K(d) model is proven in six model systems in which the modeled K(d) values are within a factor of 5 of inhibitory equilibrium constant values obtained from radioligand competition assays.

Fluorescence polarization and anisotropy in high throughput screening: perspectives and primer

Fluorescence polarization and anisotropy are two nearly equivalent techniques that have together, over the past 5 years, achieved wide use in high throughput screening in drug discovery. These are single-label methods that can be used to construct homogeneous assays that are fast, sensitive, and resistant to some significant interferences. Moreover, the assays are relatively inexpensive. This review surveys the peer-reviewed literature on the subject and explores some of the fundamental issues that bear on assay performance.

Fluorescence polarization immunoassay: detection of antibody to Brucella abortus

Fluorescence polarization immunoassay (FPA) is a homogeneous immunoassay useful for rapid and accurate detection of antibody or antigen. The principle of the assay is that a fluorescent dye (attached to an antigen or an antibody fragment) can be excited by plane-polarized light at the appropriate wavelength. As a rule, a small molecule rotates faster when in solution than a larger molecule. The rotation rate may be assessed by measuring light intensity in the vertical and horizontal planes. Generally, the time it takes a molecule to rotate through a given angle is an indication of its size. When a small molecule that rotates rapidly is bound to a larger molecule, the rotation rate is decreased and this decrease is measured. Because it is a primary antigen-antibody interaction, the rate of reaction is very rapid and usually a result may be obtained in minutes. This technology was applied to the detection of antibody to Brucella abortus in serum and milk, providing for the first time a rapid primary binding assay that is cost effective for use in the field.

Ligand binding and structural properties of segments of GABAA receptor alpha 1 subunit overexpressed in Escherichia coli

The gamma-aminobutyric acid, type A (GABA(A)), receptor is the target for numerous therapeutic compounds. In the present study, the Gln(28)-Leu(296), Gln(28)-Arg(276), Gln(28)-Arg(248), and Gln(28)-Glu(165) (numbering of bovine precursor protein) segments of its alpha(1) subunit were overexpressed in Escherichia coli, along with Cys(166)-Leu(296) produced previously, for structural analysis by circular dichroism and ligand binding studies by fluorescence spectroscopy. Results showed that the protein segments were rich in beta-sheet structures. Binding of the fluorescent benzodiazepine Bodipy-FL Ro-1986 was evident from fluorescence resonance energy transfer and fluorescence anisotropy measurements. The binding affinity was in the micromolar range. The binding was attributable more to Cys(166)-Leu(296) than to Gln(28)-Glu(165) and was inhibited by known central benzodiazepine site ligands. Three point mutations, Y187A, T234A, and Y237A, were found to perturb protein secondary structures. Studies with the single Trp mutants W198Y and W273Y indicated that Trp(273) was closer to the binding site than Trp(198).

Capillary electrophoretic immunoassays for digoxin and gentamicin with laser-induced fluorescence polarization detection

New immunoassays for therapeutic drugs digoxin and gentamicin have been described, which involved the separation of free and antibody-bound drug by capillary electrophoresis (CE) and the detection by laser-induced fluorescence polarization (LIFP). While the fluorescein-labeled digoxin and gentamicin (tracers) displayed negligible fluorescence polarization in solution, the complex formation between these small molecules and their antibodies resulted in substantial increases in fluorescence polarization due to the increase in molecular size. The LIFP detection, capable of measuring vertically and horizontally polarized fluorescence components simultaneously, provides enhanced capability for the identification of complex in capillary electrophoretic immunoassays. Proper adjustments of the running buffer pH and the ratio of antibody to tracer are essential for optimization of the performance of these assays. The digoxin-antibody complex remained stable during CE separation with running buffer pH ranging from 9.3 to 12. Calibration curves covering a concentration range of 0.05 to 0.5 ng/ml were obtained with a running buffer of pH 12. The concentration and mass detection limits were 0.02 ng/ml and 26 zmol, respectively. For gentamicin assay, the running buffer pH 10 was used to reduce the adsorption of the tracer while minimizing the dissociation of the antibody-tracer complex during the separation. The calibration curves covered a concentration range 0.05-1.0 microg/ml, with a concentration detection limit of 25 ng/ml and a mass detection limit of 52 amol of gentamicin.

New assay technologies for high-throughput screening

The use of high-throughput screening for early stage drug discovery imposes several constraints on the format of assays for therapeutic targets of interest. Homogeneous cell-free assays based on energy transfer, fluorescence polarization spectroscopy or fluorescence correlation spectroscopy provide the sensitivity, ease, speed and resistance to interference from test compounds needed to function in a high-throughput screening mode. Similarly, novel cell-based assays are now being adapted for high-throughput screening, providing for in situ analysis of a variety of biological targets. Finally, recent advances in assay miniaturization mark a transition to ultra high-throughput screening, ensuring that identification of lead compounds will not be the rate-limiting step in finding new drugs.

Use of a long-lifetime Re(I) complex in fluorescence polarization immunoassays of high-molecular-weight analytes

We describe a new class of fluorescence polarization immunoassays based on the luminescence from a Re(I) metal-ligand complex. Re(I) complexes are extremely photostable and possess useful photophysical properties including long lifetimes, high quantum yields, and high emission polarization in the absence of rotational diffusion. In the present study, a conjugatable, highly luminescent Re(I) metal-ligand complex, [Re(bcp)(CO)3(4-COOHPy)](ClO4), where bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 4-COOHPy is isonicotinic acid, has been evaluated for use in fluorescence polarization immunoassays (FPIs) with high-molecular-weight antigens. This Re(I) complex (Re) displays highly polarized emission (with a maximum anisotropy near 0.3) in the absence of rotational diffusion and a long average lifetime (2.7 microseconds) when bound to human serum albumin (HSA) in oxygenated aqueous solution. The emission polarization of the Re-HSA conjugate is sensitive to the binding of anti-HSA, resulting in a significant increase in anisotropy. The labeled HSA was also used in a competition immunoassay where unlabeled HSA was also used as an antigen. These experimental results, combined with theoretical predictions, demonstrate the potential of this Re(I) metal-ligand complex as a luminescence probe in FPIs of high-molecular-weight analytes (10(5)-10(8) Da).

Involvement of water molecules in the association of monoclonal antibody HyHEL-5 with bobwhite quail lysozyme

Fluorescence polarization spectroscopy and isothermal titration calorimetry were used to study the influence of osmolytes on the association of the anti-hen egg lysozyme (HEL) monoclonal antibody HyHEL-5 with bobwhite quail lysozyme (BWQL). BWQL is an avian species variant with an Arg-->Lys mutation in the HyHEL-5 epitope, as well as three other mutations outside the HyHEL-5 structural epitope. This mutation decreases the equilibrium association constant of HyHEL-5 for BWQL by over 1000-fold as compared to HEL. The three-dimensional structure of this complex has been obtained recently. Fluorescein-labeled BWQL, obtained by labeling at pH 7.5 and purified by hydrophobic interaction chromatograpy, bound HyHEL-5 with an equilibrium association constant close to that determined for unlabeled BWQL by isothermal titration calorimetry. Fluorescence titration, stopped-flow kinetics, and isothermal titration calorimetry experiments using various concentrations of the osmolytes glycerol, ethylene glycol, and betaine to perturb binding gave a lower limit of the uptake of approximately 6-12 water molecules upon formation of the HyHEL-5/BWQL complex.

On the interaction between a bactericidal antibody and a PorA epitope of Neisseria meningitidis in outer membrane vesicles: a competitive fluorescence polarization immunoassay

This paper describes a method for determining the affinity constant (Ka) of the binding between an antibody Fab fragment and a membrane-embedded protein epitope under equilibrium conditions. Monoclonal antibody MN12H2, directed against outer membrane protein PorA of Neisseria meningitidis, is used in a competitive fluorescence polarization assay with a cyclic peptide-fluorescein conjugate as a tracer antigen. Displacement experiments with PorA-containing and PorA-deficient meningococcal outer membrane vesicles revealed highly specific binding of MN12H2 Fab to the membrane-embedded PorA P1.16 epitope with Ka of 1.5 x 10(8) M-1.

A fluorescence polarization based Src-SH2 binding assay

The tyrosine kinase pp60c.src has been implicated as being a potential therapeutic target in several human diseases including cancer and osteoporosis. An important region within this kinase is the SH2 domain (Src homology 2) which binds to phosphorylated tyrosine residues contained within specific peptide sequences. Homologous domains are found in a variety of cytoplasmic proteins and have been shown to be essential for controlling many important signaling pathways. Developing specific inhibitors of SH2 interactions would therefore be extremely useful for modulating a variety of signaling pathways and potentially be useful for the treatment of human disease. Current methodology for the development of organic molecules as drug leads requires the ability to test thousands of individual compounds or natural product extracts in biochemical assays. Such tests must be reproducible, simple, and versatile. This paper describes an assay based on fluorescence polarization for measuring the binding of compounds to the Src-SH2 domain. The assay is insensitive to changes in fluorescence intensity working even in solutions with moderate optical density and functions in the presence of up to 20% dimethyl sulfoxide. These features make it especially useful for high-throughput screening of both natural and synthetic compound libraries.

Binding of the Brucella abortus lipopolysaccharide O-chain fragment to a monoclonal antibody. Quantitative analysis by fluorescence quenching and polarization

An antigenic O-chain polysaccharide fragment derived from Brucella abortus lipopolysaccharide was labeled with 14.8 +/- 1.8 (n = 5) and 52.3 +/- 2.4 (n = 3) micromol of fluorescein/g of polysaccharide (designated FL1 and FL2, respectively) for use in investigating the binding of O-chain to a specific murine antibody YsT9 under equilibrium conditions. Upon binding to YsT9, the fluorescence of FL1 and FL2 was quenched 45-57% with no shift in the excitation and emission spectra, and polarization of fluorescence increased by 300-335%. With fluorescence quenching and polarization as sensitive signals for antibody-bound labeled O-chains, the equilibrium constants for binding of FL1, FL2, and unlabeled O-chain to YsT9 were determined to be within a similar order (1.5 x 10(7) to 2.0 x 10(7) M-1) using a nonlinear curve fitting approach rather than Scatchard analysis. These results indicated that covalent attachment of fluorescein groups to the O-chain did not influence the recognition of the YsT9-defined epitope by the antibody. The reversibility of the O-chain-antibody reaction was also demonstrated by showing a rapid depolarization of the labeled O-chain-antibody complex in the presence of unlabeled O-chain, suggesting that this displacement experiment could be exploited to quantify the Brucella polysaccharide antigen. The study described here provides a useful model for characterization of the complex formation between a carbohydrate-binding protein and a carbohydrate ligand and also for the design of a homogeneous assay system to quantitate antigens or antibodies of clinical interest.

Synthesis and evaluation of Ru-complexes as anisotropy probes for protein hydrodynamics and immunoassays of high-molecular-weight antigens

We investigated three unsymmetrical Ru-complexes, namely [Ru(bpy)2 (phen-ITC)]2+, [Ru(bpy)2(dcbpy)] and [Ru(bpy)2(mcbpy)]+ for use as probes for rotational diffusion and in immunoassays of high-molecular-weight antigens. For this purpose we synthesized reactive forms of these metal-ligand complexes and conjugated them to human serum albumin (HSA). The maximal anisotropies (r0) for the HSA-bound forms in frozen solution are 0.23, 0.17 and 0.14 for the (dcbpy), (mcbpy) and (phen-ITC) derivatives, respectively. The activated Ru metal-ligand complexes have either one or two NHS-esters or an isothiocyanate group as the reactive moiety. The usefulness of these complexes in immunoassays was determined by titration of the labeled HSA with polyclonal anti-HSA. The highest steady state anisotropy (r) values (0.190) were observed for the [Ru(bpy)2(dcbpy)]-labeled HSA on titration with polyclonal antibody. However, a relative increase in the steady state anisotropy (r/r0) on titration with polyclonal antibody was found for the phen-ITC probe (96%), as compared to the dcbpy (83%) or mcbpy (79%) derivatives. These findings were confirmed by time-resolved frequency-domain measurements. In particular the higher mean correlation times calculated for the phen-ITC derivative suggests reduced local probe motion for this probe when bound to HSA as compared to the (mcbpy) and (dcbpy) conjugates.

Fluorescence polarization--a new tool for cell and molecular biology

Fluorescence polarization (FP) equilibrium binding assays differ from other types of binding studies in one important regard: they require no steps to separate free from bound tracer and are therefore fast, simple and accurate.

Fluorescence polarization immunoassay of a high-molecular-weight antigen based on a long-lifetime Ru-ligand complex

We describe a new class of fluorescence polarization immunoassays based on the luminescence from an asymmetrical Ru-ligand complex. We found that such a complex displays larger polarization values than those of comparable symmetrical complexes and appear to be highly photostable in aqueous solution. We synthesized a conjugatable Ru-ligand complex, which was used to label human serum albumin (HSA) as the antigen. The Ru-ligand complex displays a long decay time near 400 ns when covalently linked to proteins. We found that the steady-state polarization of labeled HSA was sensitive to binding of anti-HSA, resulting in a 200% increase in polarization. The labeled HSA was also used in a competitive format using unlabeled HSA as the antigen. The time-resolved anisotropy decays demonstrate increased correlation times for labeled HSA in the presence of anti-HSA, an effect which was partially reversed in the presence of unlabeled HSA. These results demonstrate the potential of the metal-ligand complexes to be used in the fluorescence polarization immunoassay of high-molecular-weight analytes. The use of such metal-ligand complexes enable fluorescence polarization immunoassays which bypass the usual limitation to low-molecular-weight antigens, which is a consequence of the 2-5 ns decay time of the previously used fluorophores.