The monovalent binding affinity of high binding site density nanoparticle-antibody bioconjugates is shown to exceed the intrinsic affinity of the original, monoclonal antibody. The nanoparticle-antibody bioconjugates were prepared by covalent coupling of antibodies to long-lifetime fluorescent, europium(III) chelate nanoparticles, 107 nm in diameter. Experiments were carried out in standard microtitration wells to determine solid-phase association and dissociation rate constants, nonspecific binding, and affinity constants of the various binding site density nanoparticle-antibody bioconjugates and the conventionally labeled monoclonal antibody. The affinity constant for monovalent binding of a high binding site density bioconjugate (5.4 x 10(10) M(-1)) was 8-fold higher than the intrinsic affinity of the antibody (6.6 x 10(9) M(-1)). The separately measured association (2.5 x 10(6) M(-1) s(-1)) and dissociation (3.7 x 10(-5) s(-1)) rate constants of the bioconjugate were 2-fold higher and 4-fold lower, respectively, compared to the antibody. The dependence of the association rate constant of the density of the binding sites enhanced the kinetics and the affinity of the high binding site density bioconjugates. The nanoparticle labels with high specific activity, low nonspecific binding, and enhanced binding affinity of the nanoparticle-antibody bioconjugates contribute to the design of the next generation immunoassays with extreme sensitivity.
Leucine zippers (coiled coils) are dimerization motifs found in several DNA-binding transcription factors. A parallel leucine zipper composed of the acidic chain X1-EYQALEKEVAQLEAENX2-ALEKEVAQLEHEG-amide and the basic chain X1-EYQALKKKVAQLKAKNX2ALKKKVAQLKHKG-amide was designed to study the kinetics of folding of a heterodimeric leucine zipper and to investigate the role of electrostatic attraction between oppositely charged peptide chains to the folding reaction. Each peptide alone did not form a leucine zipper at ionic strength (mu) < 1 M because of electrostatic repulsion between like charges in a homodimer. Therefore, the formation of the heterodimeric leucine zipper could be investigated by simple mixing of acidic and basic chains. To monitor folding, a fluorescent label was located either at the N-terminus (X1 = fluorescein-GGG, X2 = Q) or in the center of the coiled coil (X1 = acetyl, X2 = W). Folding could be described by a simple two-state reaction involving the disordered monomers and the folded heterodimer. The same bimolecular rate constant (k(on)) was observed independent of the location of the fluorescent label, indicating that both fluorescence probes monitored the same reaction. Lowering of the ionic strength increased k(on) from 4 x 10(6) M-1 s-1 (mu = 525 mM) to about 5 x 10(7) M-1 s-1 (mu = 74 mM). When extrapolated to mu = O, k(on) was approximately 10(9) M-1 s-1, which is near the diffusion limit. In contrast, the rate of dissociation depended very weakly on ionic strength; k(off) decreased only by about 2-fold when mu was lowered from 525 to 74 mM. Equilibrium association constants (Ka) of the heterodimeric zippers measured directly and calculated from kinetic constants (Ka = k(on)/k(off) were in good agreement. The observed two-state mechanism, the strong dependence on ionic strength of k(on) but not of k(off), and the nearly diffusion-limited association rate at very low ionic strength point to a folding pathway in which the formation of an electrostatically stabilized dimeric intermediate may be rate-limiting and the subsequent folding to the final dimer is very rapid and follows a "down-hill" free energy landscape. The small increase of k(off) at increasing ionic strength indicates a minor contribution of electrostatics to the stability of the folded leucine zipper.
We developed a europium nanoparticle-based immunoassay (ENIA) for the sensitive detection of anthrax protective antigen (PA). The ENIA exhibited a linear dose-dependent pattern within the detection range of 0.01 to 100 ng/ml and was approximately 100-fold more sensitive than enzyme-linked immunosorbent assay (ELISA). False-positive results were not observed with serum samples from healthy adults, mouse plasma without PA, or plasma samples collected from mice injected with anthrax lethal factor or edema factor alone. For the detection of plasma samples spiked with PA, the detection sensitivities for ENIA and ELISA were 100% (11/11 samples) and 36.4% (4/11 samples), respectively. The assay exhibited a linear but qualitative correlation between the PA injected and the PA detected in murine blood (r ؍ 0.97731; P < 0.0001). Anthrax PA was also detected in the circulation of mice infected with spores from a toxigenic Sterne-like strain of Bacillus anthracis, but only in the later stages of infection. These results indicate that the universal labeling technology based on europium nanoparticles and its application may provide a rapid and sensitive testing platform for clinical diagnosis and laboratory research.
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