Application of a sol particle immunoassay to the determination of affinity constants of monoclonal antibodies

Preparation, characterization, and chemical stability of gold nanoparticles coated with mono-, bis-, and tris-chelating alkanethiols

A systematically varying series of monolayer-protected clusters (MPCs) was prepared by exposing small gold nanoparticles ( approximately 2 nm in diameter) to the following four adsorbates: n-octadecanethiol ( n - C18), 2-hexadecylpropane-1,3-dithiol ( C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol ( C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane ( t - C18). The resultant MPCs were characterized by solubility studies, UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). All of the MPCs were soluble in common organic solvents; moreover, analysis by TEM showed that the core dimensions were unaffected by exposure to any of the adsorbates. Separate studies by XPS revealed that the sulfur atoms in all MPCs were predominantly bound to the surface of gold (i.e., approximately 85% or better). Analysis by FT-IR showed that MPCs functionalized with n - C18 possessed alkyl chains having the greatest conformational order in both the solid-state and dispersed in solution; in contrast, those generated from the other three adsorbates were more liquid-like with reduced conformational order (or crystallinity). The rate of nanoparticle decomposition induced by cyanide ions was monitored by UV-vis spectroscopy. While MPCs functionalized with n - C18 showed the fastest rate of decomposition, those functionalized with C18C3 were the most resistant to decomposition. Overall, the following trend in chemical stability was observed, C18C3 >> C18C2 > t - C18 >> n - C18.

A protein assay based on colloidal gold conjugates with trypsin

The standard sol particle immunoassay (SPIA) is based on a biospecific aggregation of gold nanoparticle conjugates, followed by conventional spectrophotometry. Here we propose a novel SPIA format that uses microtitration immunological plates and an enzyme-linked immunosorbent assay reader. The novel and standard assays are exemplified by determination of immunoglobulin G by using 15-nm colloidal gold-protein A conjugates. We also describe a novel sol particle-trypsin assay using conjugates of gold nanoparticles with trypsin. The method is based on measuring spectral extinction changes caused by the addition of protein to a conjugate solution. The changes in the extinction spectra are presumed to be related to aggregation of gold nanoparticles caused by polyvalent binding of protein molecules to the trypsin molecules of the conjugates.

Assessment of the functional affinity constant of monoclonal antibodies using an improved enzyme-linked immunosorbent assay

The use of an enzyme-linked immunosorbent assay (ELISA) for the determination of affinity constants implies heterogeneous measurements. Therefore, despite their simplicity, direct solid-phase binding assays are not common. Many investigators have serious, and mostly justified, reservations about the application of solid-phase affinity methods. They refer to problems such as diffusion effects and difficulties in reaching equilibrium due to heterogeneous binding and co-operativity. Accordingly, functional affinity determinations are often described as meaningless. These objections apply to the measurement of the affinity of a monoclonal antibody using the enzyme-linked immunosorbent assay of Beatty et al. (J. Immunol. Methods (1987) 100, 173), which is based on the effect of antibody affinity on the sigmoidal dose response curve. The affinity constant is calculated by mathematical equations, based on the Law of Mass Action and the authors made a number of important assumptions--avoiding the above mentioned problems--in order to justify the use of the Law of Mass Action. By carefully examining these assumptions we have developed an improved ELISA procedure for functional affinity determinations on the basis of a primary coating with the antigen only. the coating conditions were validated by employing gold labelled colloidal particles and physical counting of the bound particles under the scanning electron microscope. Since monovalent binding between human chorionic gonadotropin and its monoclonal antibody could be achieved under equilibrium conditions, the application of the Law of Mass Action and hence of the Beatty formula became possible. We conclude that under these conditions functional affinity determinations are appropriate.

A reliable method to estimate the association constant for a monoclonal antibody and a protein antigen by enzyme linked immunosorbent assay, ELISA

The association constant Ka for mouse monoclonal antibody raised against human angiotensinogen was calculated using a mathematical model, SAM I. K1 represents the equilibrium constant for the binding of antibody to the solid phase with antigen previously absorbed. K2 represents the interaction between antibody and antigen in solution (Ag + Ab = AgAb). K3 represents binding to the antigen absorbed on the solid phase by an antigen-antibody complex. K4 represents the second binding of the antigen to the antigen-antibody complex (AgAb + AgAb = (Ag)2Ab). The model unveils cooperativity for the first (K1 and K2) and second (K3 and K4) binding of antigen to antibody. The model gives the association constant in a high affinity interaction between antigen and antibody.

Characterization of monoclonal antibodies physically adsorbed onto polystyrene latex particles

The physicochemical and immunochemical properties of monoclonal antibodies (MAbs), adsorbed onto polystyrene latex particles, have been investigated. Both native and pH 2 pretreated MAbs were compared before and after immobilization. It was found that the antigen binding capacity of the immobilized, acidic pretreated MAbs was significantly higher than for the immobilized, native IgG molecules. This enhanced antigen binding capacity appeared to be due to an improved molecular orientation following adsorption of the monomeric, pH 2 treated IgG fraction. Additionally, experiments using F(ab')2 fragments demonstrated that the Fc portion of the MAb molecule is of major importance for achieving the enhanced binding capacity. Binding studies showed that the (apparent) affinity of native and pH 2 pretreated MAbs were similar; the K(a) values of the immobilized MAbs were higher than those of MAbs in solution.