Gregory A. Wemhoff

A continuous flow immunoassay for rapid and sensitive detection of small molecules

An immunosensor operating in continuous flow and capable of detecting low molecular weight antigens is described. The approach differs from previously described continuous flow assays by not requiring incubation steps or the introduction of additional reagents following the loading of the sample into the system. Detection of the antigen is rapid, occurring within 3 min in the system described. The assay is based on the binding of labeled antigen to an immobilized antibody, with subsequent displacement of the labeled antigen when antigen is present in the buffer flow. Signal detection occurs downstream of the antigen recognition event. In this study, the hapten 2,4-dinitrophenol (DNP) as DNP-lysine was used as model antigen. To generate a labeled antigen, DNP was coupled to the terminal amino group of insulin A chain (tetra S-sulfonate form) which provides two tyrosine residues for the introduction of an 125I-label (DNP-Ins-125I) or three carboxyl groups for the attachment of three fluorescein residues (DNP-Ins-Fl). The radiolabeled antigen was used to establish assay conditions. Subsequently, fluorescein was substituted for the radioisotope label in order to develop an assay independent of the restrictions associated with isotopes. Using this flow immunoassay, we were able to detect DNP-lysine down to a detection limit of 143 nM (29 pmol/200 microliters) using DNP-Ins-125I or DNP-Ins-Fl as labeled antigen. The density of immobilized antibody and the flow rate were identified to be critical parameters for the sensitivity of the assay.

Detection of Cocaine Using the Flow Immunosensor

Abstract A continuous flow immunosensor has been designed for the detection of cocaine in aqueous samples. The continuous flow immunosensor relies on the displacement of fluorophore-labeled antigen from immobilized monoclonal antibody. The sensitivity and accuracy of the flow immunosensor were investigated while varying the parameters of immobilized antibody density, flow rate, amount of antibody-coated Sepharose used in each column, and the saturation of antibody binding sites with fluorophore-labeled antigen. Using a low density of immobilized anti-benzoylecgonine antibody, as little as 5 ng/ml cocaine could be detected. Small amounts of antibody-coated Sepharose could be used repeatedly and the lifetime of the column was proportional to the amount of Sepharose used. Results were obtained in less than a minute and cross-reactivity against various other drugs was negligible.

Kinetics of antibody binding at solid-liquid interfaces in flow

We have developed the theoretical framework for a displacement immunoassay conducted in flow under nonequilibrium conditions. Using a repetitive displacement technique, we determined the displacement rate and apparent dissociation rate constant at different flow rates. Our data suggest that the kinetics are best described by a first-order function. The displacement efficiency, the displacement rate, and therefore the apparent dissociation rate constant were calculated and demonstrated to be flow rate dependent. The theoretical framework developed in this study was successful in predicting the behavior of antigen displacement in flow.

Novel trifunctional carrier molecule for the fluorescent labeling of haptens

We developed a novel trifunctional carrier molecule for the synthesis of hapten-fluorophore conjugates as reporter molecules in immunoassays. This carrier eliminates some of the disadvantages associated with currently used fluorophore-labeling procedures including high nonspecific binding. The backbone of the carrier consists of the 21 amino acid residues of the insulin A-chain molecule. This polypeptide provides a single site (terminal amino group) for covalent coupling of the hapten, three carboxyl groups for the attachment of fluorophores, and four sulfhydryl groups for derivatization with hydrophilic residues to compensate for the hydrophobic effect of the attached fluorophores. The sites for fluorophore attachment are 4, 17, and 21 amino acids away from the hapten attachment site. This spatial separation minimizes quenching of the fluorescence signal due to interaction of the fluorophores with each other and with the attached hapten. In this study, 2,4-dinitrophenol (DNP) was selected as model hapten, fluorescein as label, and S-sulfonate groups as hydrophilic residues. The properties of the DNP-insulin A-chain-fluorescein conjugate (DNP-Ins-Fl) were compared to those of a DNP derivative labeled with a single fluorescein moiety via a small lysine spacer (DNP-Lys-Fl). The DNP-Ins-Fl conjugate exhibited a 3-fold lower nonspecific adsorption to immobilized non-immune IgG contributing to an approximately 3-fold more efficient displacement from the binding sites of an immobilized monoclonal anti-DNP antibody by the antigen DNP-lysine. Furthermore, at equimolar concentrations the DNP-Ins-Fl generated a 2.6-fold higher fluorescent signal than DNP-Lys-Fl.(ABSTRACT TRUNCATED AT 250 WORDS)

hSMG-1 is a granzyme B-associated stress-responsive protein kinase.

Granzyme B plays a key role in cell-mediated programmed cell death. We previously demonstrated that p53 is a functional determinant in the granzyme B-induced cytotoxic T-lymphocyte response. However, the pathways leading to activation of p53 by granzyme B remain incompletely understood. We now demonstrate that granzyme B-induced DNA damage signaling as revealed by histone H2AX phosphorylation and subsequent activation of the stress kinase CHK2. Confocal microscopy analysis indicates that granzyme B treatment of tumor cells induced an early translocation of endonuclease caspase-activated DNase. DNA microarray-based global transcriptional profiling and RT-PCR indeed revealed genes related to DNA damage. Among these genes, hSMG-1, a genotoxic stress-activated protein, was constantly upregulated in tumor cells following granzyme B treatment. Knockdown of the hSMG-1 gene in T1 tumor target cell line resulted in a significant inhibition of granzyme B- and CTL-induced killing. Our data suggest that granzyme B may exert cell death through DNA damage signaling and uncover a novel molecular link between the DNA damage pathway and granzyme B-induced cell death.

Development Of A Continuous Flow Immunosensor

The biosensor described below capitalizes on the specificity and reversibility of antibody-analyte interactions, and is designed to operate continuously. Detection of the analyte of interest relies on the specific displacement of labelled-analyte from the antibody binding site. When an aqueous sample containing the analyte is introduced into the buffer flow, the analyte competes for the analyte binding sites with the labelledanalyte already bound by the antibody. An amount of the labelled analyte proportional to the amount of the analyte present is displaced, enters the buffer flow, and is subsequently detected downstream. In the model system, radioisotopes and fluorophores were used as analyte labels in the development of a sensor capable of detecting small molecular weight analytes. Dinitrophenol @NP) was used as the analyte to construct a model system and could be detected at the nanomolar level. The assay does not require the introduction of exogenous reagents throughout the analysis of a sample. In addition, incubation of the sample in the detection column was not required since significant displacement of the labelled DNP-analyte occurred in continuous flow. Due to the wide range of specificities that antibodies can express, this system has the potential of being designed to detect many different analytes.