A novel immunoassay with direct relevance to protection against organophosphate poisoning

Development of Immunochromatographic Assay for Identification of Organophosphate Pesticides in Environmental Samples

Microtiter plate enzyme linked immunoassay (ELISA) experiments in competitive format were performed utilizing polyclonal antibody for establishing a detection system for organophosphate pesticides. IC50 value of and limit of detection (LOD) value was determined by standard inhibition curve and value obtained were 0.05 μgmL(-1) and 0.001 μgmL(-1), respectively. Specificity of antibody was investigated with different organophosphate pesticides. Immunochromatographic assay (ICA) experiments were also designed in competitive format by making use of immunochromatographic strip which was assembly of three main components: conjugate pad, membrane and adsorbent pad. Membrane was coated with hapten-OVA conjugate (test line) and antirabbit IgG (control line). ICA experiments were performed by employing gold-labeled antibody as a detector reagent which was applied over conjugate pad. Visual detection limit obtained from ICA was 0.5 μgmL(-1). Major advantage of strip assay was rapid result, i.e., less than 10 min. which makes it suitable for onsite applications.

Towards the fabrication of a label-free amperometric immunosensor using SWNTs for direct detection of paraoxon

A label-free immunosensor based on SWNTs modified GC electrodes has been developed for the direct detection of paraoxon. Based on aryldiazonium salt chemistry, forest of SWNTs can be vertically aligned on mixed monolayers of aryldiazonium salt modified GC electrodes by C-C bonding, which provides an interface showing efficient electron transfer between biomolecules. PEG molecules were introduced to the interface to resist non-specific protein adsorption. Ferrocenedimethylamine (FDMA) was subsequently attached to the ends of SWNTs through the amide bonding followed by the attachment of epitope i.e., paraoxon hapten to which a paraoxon antibody would bind. This immunosensor shows good selectivity and high specificity to paraoxon, and is functional for the detection of paraoxon in both laboratory and field by a displacement assay. There is a linear relationship between electrochemical signal of FDMA and the concentration of paraoxon over the range of 2-2500 ppb with a lowest detected limit of 2 ppb in 0.1 M phosphate buffer at pH 7.0. The SWNTs based amperometric immunosensor provides an opportunity to develop the sensing system for on-site sensitive detection of a spectrum of insecticides.

Development of IC-ELISA for detection of organophosphorus pesticides in water

Diethyl (carboxymethyl) phosphonate (DECP) was used as the hapten to develop an indirect competitive enzyme-linked immunosorbent assay (IC-ELISA) for detecting organophosphorus pesticides (OPs). Conjugator of DECP with bovin serum albumin (BSA) was used as the immunogen for producing the polyclonal antibodies (PcAbs). Three antisera were obtained after the immune procedure. Characterization studies of the PcAbs indicated that the titer of antiserum-1 was highest in 3 antisera, and antiserum-1 had high affinity and specificity to the parathion, dichlorvos and pirimiphos. The IC-ELISA showed an IC50 of 0.428 micro g/mL with a detection limit of 0.0125 micro g/mL to parathion. The assay also indicated that the IC50 values of pirimiphos and dichlorvos were 0.331 micro g/mL and 1.25 micro g/mL respectively, and the detection limits of pirimiphos and dichlorvos were 0.0116 micro g/mL and 0.048 micro g/mL respectively. Recoveries of parathion, pirimiphos and dichlorvos spiked into water samples ranged from 90% to 160%. The results indicated that the ELISA could be a convenient and supplemental analytical tool for monitoring OPs residues in environmental water samples.

Development of an enzyme-linked immunosorbent assay for the organophosphorus insecticide bromophos-ethyl

A competitive enzyme-linked immunosorbent assay (ELISA) was developed for the quantitative detection of the organophosphorus insecticide bromophos-ethyl. Four bromophos-ethyl derivatives (haptens) were synthesized and were coupled to carrier proteins through the pesticide thiophosphate group to use as an immunogen or as a coating antigen. Rabbits were immunized with either one of two haptens coupled to bovine serum albumin for production of polyclonal antibodies, and the sera were screened against one of the haptens coupled to ovalbumin. Using the serum with highest titer, an antigen-coated ELISA was developed, which showed an IC(50) of 3.9 ng/mL with a detection limit of 0.3 ng/mL (20% inhibition). An antibody-coated ELISA using an enzyme tracer was also developed, which showed an IC(50) of 6.5 ng/mL with a detection limit of 1.0 ng/mL (20% inhibition). The antibodies showed negligible cross-reactivity with other organophosphorus pesticides except with the insecticides bromophos-methyl and chlorpyrifos in the antibody-coated assay only. Recoveries of bromophos-ethyl from fortified crop and water samples ranged from 82 to 128% and from 95 to 127%, respectively.

Development of an immunoassay for diagnosis of exposure to toxic organophosphorus compounds

Currently, diagnosis of exposure to toxic low-molecular-weight compounds is effected by the use of chromatographic techniques. Such an approach is limited by the need for expensive equipment and sample clean-up before carrying out the analysis. To overcome those drawbacks, we have been involved in the development of an immunoassay for diagnosis of exposure to toxic organophosphorus compounds such as pinacolylmethyl phosphonofluoridate (soman), which is a chemical warfare agent. Prior estimates suggested that it is necessary to be able to detect soman at a concentration below 2.5 x 10(-7) M. Using four previously developed monoclonal antibodies, an enzyme-linked immunosorbant assay (ELISA) was used to optimize assay conditions and identify the antibody with the highest apparent affinity. The minimum required assay time was 2.0-2.5 h with no loss in sensitivity. To determine the specificity of the highest affinity antibody, a competitive inhibition enzyme immunoassay (CIEIA) was performed with six structural analogs of soman. The IC50 values for these analogues were 5 x 10(-7) M for 4-nitrophenylpinacolylmethylphosphonate, 8 x 10(-7) M for dipinacolylmethylphosphonate, 2 x 10(-6) M for diisopropylmethylphosphonate, 3 x 10(-5) M for 4-nitrophenylmethyl(phenylphosphinate) and 6.5 x 10(-5) M for 4-nitrophenylethyl(phenyl)phosphinate. 4-Nitrophenyl-di(n-butyl)phosphinate did not inhibit binding. Those inhibitors with branched alkyl side-chains, similar to the soman molecule, were effective inhibitors. Compounds, which contained predominately aromatic groups, were poor inhibitors. We are continuing to probe the binding specificity of the monoclonal antibody to determine its utility in further assay development. Our present results suggest that the antibody chosen may have the appropriate specificity and affinity for immunodiagnosis of exposure to soman.

Monoclonal anti-idiotype antibody mimicking the pesticide binding site of cutinase: potential for broad specificity organophosphate recognition

An anti-idiotype monoclonal antibody (Mab) able to bind the organophosphate pesticides, chlorfenvinphos (CFV), ethyl paraoxon, tetrachlorfenvinphos and demeton-s-methyl, has been produced using as immunogen a Mab which binds to the active site of cutinase. The principle of using an anti-idiotype antibody as the mimic of a site on a protein able to bind a group of ligands has, therefore, been demonstrated, and may have implications for future research on broad specificity immunoanalysis of groups of compounds.

Syntheses of haptens containing dioxaphosphorinan methoxyacetic acid linker arms for the production of antibodies to organophosphate pesticides

Four generic heterobifunctional reagents, namely 2-(2-chloro-5-methyl-1,3,2-dioxaphosphorinan-5-yl)methoxyacetic acid methyl ester, p-sulfide, 2-(2-chloro-5-methyl-1,3,2-dioxaphosphorinan-5-yl)-methoxyacetic acid methyl ester, p-oxide, 2-(2-mercapto-5-methyl-1,3,2-dioxaphosphorinan-5-yl)-methoxyacetic acid bispotassium salt, p-sulfide-, and (2-methoxy-5-methyl-1,3,2-dioxaphosphorinan-5-yl)methoxyacetic acid, methyl ester, have been synthesized and used to prepare organophosphate, thiophosphate, and dithiophosphate haptens containing a functional carboxyl group which can be used to conjugate the haptens to proteins. These hapten-protein conjugates have been used as antigens for preparing polyclonal sera against all classes of organophosphate pesticides. The eight examples used protein-hapten conjugates of chlorpyrifos, parathion, diazinon, paraoxon, azinphos, dimethoate, demeton, and dichlorvos. These were all immunogenic and resulted in sera containing antibodies that recognized the corresponding parent pesticide with high specificity.

Production and characterization of antibodies directed against organophosphorus nerve agent VX

A strategy is described to raise high-affinity antibodies directed against the organophosphorus nerve agent VX [O-ethyl S-(2-diisopropylamino)ethyl)methyl phosponothionate]. Ten chemical derivatives of VX (haptens) have been synthesized. Their structures differ principally from VX structure by substitution of S-atom by an O-atom or CH2-group and by introduction of a reactive group (carboxylic acid, arylamine or primary amine) on the O-ethyl side chain. None of these haptens, except one, exhibit potential toxicity as tested by their inhability to inhibit acetylcholinesterase (E.C. 3.1.1.7.). After coupling with a protein carrier, they were injected intradermally to rabbits. Nine of these immunogenic conjugates led to the appearance of antibodies able to bind VX in a competitive solid phase immunoassay. The apparent titer and affinity of the antisera differed greatly depending on the hapten used. The highest affinity (9 nM) was observed with the VX derivative bearing O-S substitution and O-ethyl-carboxylic side chains. The antibodies appear specific for VX, since cross-reactivity with other nerve agents (Soman, Sarin or Tabun) was low. However, two haptens elicited antibodies with affinity to Soman or Sarin in the micromolar range. Antibodies were able to neutralize VX inhibition of acetylcholinesterase in vitro but not in vivo.