Immunoassay reagents for thyroid testing. 3. Determination of the solution binding affinities of a T4 monoclonal antibody Fab fragment for a library of thyroxine analogs using surface plasmon resonance

Quantitation of Thyroid Hormone Binding to Anti-Thyroxine Antibody Fab Fragment by Native Mass Spectrometry

Thyroid hormones are important regulatory hormones, acting on nearly every cell in the body. The two main thyroid hormones are l-thyroxine (tetraiodo-l-thyronine, T₄) and 3,3',5-triiodo-l-thyronine (T₃), which are produced in the thyroid gland and secreted into the blood stream. Other important thyroid hormone metabolites are 3,3'-diiodo-l-thyronine (T₂) and l-thyronine (T₀), which may show increased levels in circulation due to dietary iodine deficiency or other medical disorders. Owing to their central role in cellular functions, sensitive and specific detection methods for thyroid hormones are needed. In this work, native mass spectrometry (MS) was used to quantitate thyroid hormone binding to the anti-T₄ antibody Fab fragment. First, the binding affinity for T₂ was determined via direct ligand titration experiments. Then, the affinities for the other ligands were determined by competition experiments using T₂ as the "low-affinity" reference ligand. The highest affinity was measured for T₃, followed by T₄, T₂, and T₀ (K _d = 29, 3.4, and 260 nM and 130 μM, respectively). Thus, it is evident that the number and positions of the iodine substituents within the thyronine rings are important for the ligand binding affinity of anti-T₄ Fab. Surprisingly, structurally related tetrahalogen bisphenols were also able to bind to anti-T₄ Fab with nanomolar affinities.

Synthesis Coupled to Scintillation Proximity Affinity Screening

The rapidly changing developments in genomics and combinatorial chemistry, generating new drug targets and large numbers of compounds, are beginning to push the limits of screening efficiently. Thus, there is a need for novel tools and strategies to improve high throughput screening. A novel approach is to couple synthesis and screening on a common platform, rather than to increase the rate at which traditional screening methods can be implemented. We have developed a proprietary grafted polymer with special fluorescence characteristics referred to as Electronically Encoded Fluorescence matriX (EFX™), which has the sturdiness and required functionality for direct chemical synthesis as well as suitable surface characteristics for measuring interactions in aqueous solution. This matrix is fabricated into a MicroTube reactor, and each tube is associated with an electronically encoded tag. The system follows a homogenous assay protocol and is based on the scintillation proximity principle. Using solid-phase chemistry, a variety of small molecules may be synthesized onto the EFX. A simple binding assay can be conducted by combining a collection of MicroTubes with any radiolabeled acceptor molecule. The MicroTubes that carry active compounds are selected based on the photon mission or fluorescence characteristics. We validated this approach by evaluating the interactions of biotin with radiolabeled streptavidin.

Effective chemiluminogenic systems based on acridinium esters bearing substituents of various electronic and steric properties

A series of new acridinium esters, variously substituted in the benzene ring, have been investigated for the mechanism of light generation and ability to show chemiluminescence in various environments.

Gold nanoparticles in biomedical applications: recent advances and perspectives

Gold nanoparticles (GNPs) with controlled geometrical, optical, and surface chemical properties are the subject of intensive studies and applications in biology and medicine. To date, the ever increasing diversity of published examples has included genomics and biosensorics, immunoassays and clinical chemistry, photothermolysis of cancer cells and tumors, targeted delivery of drugs and antigens, and optical bioimaging of cells and tissues with state-of-the-art nanophotonic detection systems. This critical review is focused on the application of GNP conjugates to biomedical diagnostics and analytics, photothermal and photodynamic therapies, and delivery of target molecules. Distinct from other published reviews, we present a summary of the immunological properties of GNPs. For each of the above topics, the basic principles, recent advances, and current challenges are discussed (508 references).

Allosteric modulation of hormone release from thyroxine and corticosteroid-binding globulins

The release of hormones from thyroxine-binding globulin (TBG) and corticosteroid-binding globulin (CBG) is regulated by movement of the reactive center loop in and out of the β-sheet A of the molecule. To investigate how these changes are transmitted to the hormone-binding site, we developed a sensitive assay using a synthesized thyroxine fluorophore and solved the crystal structures of reactive loop cleaved TBG together with its complexes with thyroxine, the thyroxine fluorophores, furosemide, and mefenamic acid. Cleavage of the reactive loop results in its complete insertion into the β-sheet A and a substantial but incomplete decrease in binding affinity in both TBG and CBG. We show here that the direct interaction between residue Thr(342) of the reactive loop and Tyr(241) of the hormone binding site contributes to thyroxine binding and release following reactive loop insertion. However, a much larger effect occurs allosterically due to stretching of the connecting loop to the top of the D helix (hD), as confirmed in TBG with shortening of the loop by three residues, making it insensitive to the S-to-R transition. The transmission of the changes in the hD loop to the binding pocket is seen to involve coherent movements in the s2/3B loop linked to the hD loop by Lys(243), which is, in turn, linked to the s4/5B loop, flanking the thyroxine-binding site, by Arg(378). Overall, the coordinated movements of the reactive loop, hD, and the hormone binding site allow the allosteric regulation of hormone release, as with the modulation demonstrated here in response to changes in temperature.

Detection of cortisol in saliva with a flow-filtered, portable surface plasmon resonance biosensor system

Saliva provides a useful and noninvasive alternative to blood for many biomedical diagnostic assays. The level of the hormone cortisol in blood and saliva is related to the level of stress. We present here the development of a portable surface plasmon resonance (SPR) biosensor system for detection of cortisol in saliva. Cortisol-specific monoclonal antibodies were used to develop a competition assay with a six-channel portable SPR biosensor designed in our laboratory. The detection limit of cortisol in laboratory buffers was 0.36 ng/mL (1.0 nM). An in-line filter based on diffusion through a hollow fiber hydrophilic membrane served to separate small molecules from the complex macromolecular matrix of saliva prior to introduction to the sensor surface. The filtering flow cell provided in-line separation of small molecules from salivary mucins and other large molecules with only a 29% reduction of signal compared with direct flow of the same concentration of analyte over the sensor surface. A standard curve for detection of cortisol in saliva was generated with a detection limit of 1.0 ng/mL (3.6 nM), sufficiently sensitive for clinical use. The system will also be useful for a wide range of applications where small molecular weight analytes are found in complex matrixes.

Surface plasmon resonance-based immunoassays

Surface plasmon resonance (SPR) has been successfully incorporated into an immunosensor format for the simple, rapid, and nonlabeled assay of various biochemical analytes. Proteins, complex conjugates, toxins, allergens, drugs, and pesticides can be determined directly using either natural antibodies or synthetic receptors with high sensitivity and selectivity as the sensing element. Immunosensors are capable of real-time monitoring of the antigen-antibody reaction. A wide range of molecules can be detected with lower limits ranging between 10(-9) and 10(-13) mol/L. Several successful commercial developments of SPR immunosensors are available and their web pages are rich in technical information. This review highlights many recent developments in SPR-based immunoassay, functionalizations of the gold surface, novel receptors in molecular recognition, and advanced techniques for sensitivity enhancement. Furthermore, it describes the challenge of current problems and provides some insights toward the future technologies.

A molecular basis for functional peptide mimicry of a carbohydrate antigen

Peptides may substitute for carbohydrate antigens in carbohydrate-specific immunological reactions. Using the recognition properties of an anti-Lewis Y (LeY) antibody, BR55-2, as a model system, we establish a molecular perspective for peptide mimicry by comparing the three-dimensional basis of BR55-2 binding to LeY with the binding of the same antibody to peptides. The peptides compete with LeY, as demonstrated by enzyme-linked immunosorbent assay and Biacore analysis. The computer program LUDI was used to epitope map the antibody-combining site, correlating peptide reactivity patterns. This approach identified amino acids interacting with the same BR55-2 functional residue groups that recognize the Fucalpha(1-3) moiety of LeY. Molecular modeling indicates that the peptides adopt an extended turn conformation within the BR55-2 combining site, serving to overlap the peptides with the LeY spatial position. Peptide binding is associated with only minor changes in BR55-2, relative to the BR55-2-LeY complex. Anti-peptide serum distinguishes the Fucalpha(1-3) from the Fucalpha(1-4) linkage, therefore differentiating difucosylated neolactoseries antigens. These results further confirm that peptides and carbohydrates can bind to the same antibody-binding site and that peptides can structurally and functionally mimic salient features of carbohydrate epitopes.

Application of surface plasmon resonance toward studies of low-molecular-weight antigen-antibody binding interactions

Methods for studying low-molecular-weight antigen-antibody binding interactions using surface plasmon resonance detection are presented. The experimental parameters most relevant to studies of low-molecular-weight antigen-antibody binding interactions are discussed. Direct kinetic analysis of the binding interactions is most informative, providing both apparent association and dissociation rate constants from which equilibrium constants can be calculated. Equilibrium analysis, including steady-state and solution affinity studies, offers an alternative approach to direct kinetic analysis when knowledge of the individual kinetic rate constants is not required or difficult to determine. The various methods are illustrated by studies of an anti-T(4) Fab fragment binding interaction with several thyroxine analogs. The methods utilized were dependent on the affinity of the interaction. The high-affinity anti-T(4) Fab fragment/l-T(4) binding interaction was evaluated using direct kinetic analysis. An intermediate affinity anti-T(4) Fab fragment/l-T(3) binding interaction was evaluated using a combination of direct kinetic analysis, steady-state analysis, and solution affinity analysis. The relatively weak anti-T(4) Fab fragment/l-T(2) binding interaction was evaluated using steady-state and solution affinity analysis protocols. Several thyroxine tracers that could not be immobilized to a biosensor surface were also evaluated via the solution affinity format. In cases where a given binding interaction was examined using multiple methods the results were comparable.

Synthesis of immunoreagents for detection of deoxypyrrololine, a cross-link of bone collagen

Two immunogens (5,6) and two probes (fluorescent 7 and chemiluminescent 8) were prepared from benzyl ester (-)-10. These immunoreagents (5,6 and 7,8) are useful for detection of collagen cross-link (+)-deoxypyrrololine (Dpl, 4), and for development of assays for osteoporosis.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

Resin-supported labeling reagents

Resin-supported fluorescein, coumarin, acridinium, and biotin active esters were prepared from a new N-hydroxysuccinimidyl resin in high yield. The active esters were used to prepare representative conjugates with estriol, thyroxine, phenytoin, and desipramine haptens without need for purification beyond removal of the spent resin.