Enzyme-linked sandwich immunoassay of macromolecular antigens using the rabbit antibody-coupled glass rod as a solid phase

DNA-Redox Cation Interaction Improves the Sensitivity of an Electrochemical Immunosensor for Protein Detection

A simple DNA-redox cation interaction enhancement strategy has been developed to improve the sensitivity of electrochemical immunosensors for protein detection. Instead of labeling with fluorophores or redox-active groups, the detection antibodies were tethered with DNA single strands. Based on the electrostatic interaction between redox cations ([Ru(NH₃)₆]³⁺) and negatively charged DNA backbone, enhanced electrochemical signals were obtained. Human chorionic gonadotropin (hCG) detection has been performed as a trial analysis. A linear response range up to the concentration of 25 mIU/mL and a detection limit of 1.25 mIU/mL have been achieved, both are comparable with the ultrasensitive enzyme-linked immunosorbent assay (ELISA) tests. The method also shows great selectivity towards hCG over other hormones such as thyroid stimulating hormone (TSH) and follicle stimulating hormone (FSH). By and large, our approach bears the merits of cost effectiveness and simplicity of instrumentation in comparison with conventional optical detection methods.

Rational conversion of affinity reagents into label-free sensors for Peptide motifs by designed allostery

Optical biosensors for short peptide motifs, an important class of biomarkers, have been developed based on "affinity clamps", a new class of recombinant affinity reagents. Affinity clamps are engineered by linking a peptide-binding domain and an antibody mimic domain based on the fibronectin type III scaffold, followed by optimization of the interface between the two. This two-domain architecture allows for the design of allosteric coupling of peptide binding to fluorescence energy transfer between two fluorescent proteins attached to the affinity clamp. Coupled with high affinity and specificity of the underlying affinity clamps and rationally designed mutants with different sensitivity, peptide concentrations in crude cell lysate were determined with a low nanomolar detection limit and over 3 orders of magnitude. Because diverse affinity clamps can be engineered, our strategy provides a general platform to generate a repertoire of genetically encoded, label-free sensors for peptide motifs.

Development and clinical application of sensitive enzyme immunoassay for macromolecular antigens--a review

Radioimmunoassay has been a powerful tool to measure haptens and antigens which are important for the investigation and diagnosis of diseases, especially endocrine disorders. However, the use of radioisotopes in radioimmunoassay suffers from serious disadvantages. Radioisotope-labeled reagents are unstable and hazardous to health. The disposal of radioactive wastes is not easy. Furthermore, the sensitivity of radioimmunoassay is limited by the detection limit of radioisotope that depends upon the half-life. The detection limit of the most widely used radioisotope, 125I, with a half-life of 60 days is 5 to 10 amol, when it is carrier-free. By contrast, the use of enzymes has obvious advantages. Some enzymes are very stable and cause no health hazards or waste disposal problems, provided that appropriate substrates are chosen. The detection limits of some enzymes are lower than that of 125I and will be further improved in the future. Therefore, enzyme immunoassay is potentially more sensitive than radioimmunoassay. This article reviews the development and clinical application of sensitive enzyme immunoassay for macromolecular antigens, which has been replacing radioimmunoassay.

A rapid quantitative capillary tube enzyme immunoassay for human chorionic gonadotropin in urine

A quantitative capillary tube enzyme immunoassay (CTEIA) method for the determination of human urinary chorionic gonadotropin (hCG) has been developed. The method utilizes an antibody-coated capillary tube through which the test fluid is passed and a urease-labelled second antibody in an immunometric format. Any hCG in the test solution is 'captured' by the immobilized antibody which is hybridoma derived and specific for the beta-subunit of hCG. The second hCG-specific antibody, conjugated to the enzyme urease, is used to detect the captured hCG on the internal surface of the capillary tube. The amount of urease bound to the surface is determined by the introduction of a substrate solution containing urea and the pH indicator bromothymol blue. The rate of colour change, from yellow to blue, caused by the release of ammonia from urea by urease, is determined in a spectrophotometer using a cell holder adapted to accommodate capillary tubes. The initial rate of absorbance change is directly proportional to the concentration of hCG in the sample in the range 0-100 mIU/ml. The test can detect concentrations of hCG as low as 10 mIU/ml in a total elapsed time of 5 min.

Simultaneous screening for two different antibodies in ELISA by combining two solid phases: microtiter plate and Falcon assay screening test system lid

An enzyme-linked immunosorbent assay (ELISA) is described that used a combination of 2 solid phases: a microtiter plate covered with a Falcon assay screening test (F.A.S.T.) system lid. By coating each solid phase with a different antigen, it was possible to simultaneously detect 2 different antibodies. The results of the combined test were compared and found similar to those obtained in separate assays on the same solid phases by the usual ELISA method. Further, the combined test was more economical in time, work and materials than 2 separate tests for the 2 antibodies. This method was used in a serological survey for 2 turkey viral infections: hemorrhagic enteritis and paramyxovirus type 3.

A new rapid semi-quantitative enzyme immunoassay suitable for determining immunity to tetanus

A sensitive enzyme immunoassay for rapidly assessing a patient's state of immunity to tetanus is described. The test, which uses 50 microliter sample of blood, plasma or serum, is done in a capillary tube and, by comparison with two adjacent reference tubes containing standardised sera, places immunity to tetanus in one of three categories--low-negative (less than 0.01 IU/ml), intermediate (0.01-1.28 IU/ml) or high (greater than 1.28 IU/ml). In a study of 90 clinical specimens assayed both by toxin neutralisation bioassay and capillary enzyme immunoassay the enzyme immunoassay accurately assessed the state of immunity to tetanus of the patients concerned.

Phenotype characteristics of human B cells studied by Epstein-Barr virus infection. I. Immunoglobulin expression on human lymphoblastoid cells established from various lymphoid cells

Forty-seven lymphoblastoid cell lines were established from human fetal lymphoid tissues, cord blood lymphocytes (CBL) and adult peripheral blood lymphocytes (PBL) by Epstein-Barr virus (EBV) infection. Their surface immunoglobulin (sIg), intracytoplasmic immunoglobulin (cIg) expression, and immunoglobulin (Ig) content in the culture supernatant were tested. Expression of sIgM, sIgG and sIgA were predominant on fetus-derived cell lines, while sIgD was the most prominent on CBL-derived cells. Though cIg expression did not vary between cell lines of different origin, Ig content in the culture supernatant differed greatly. Fetus- and CBL-derived cells secreted IgM exclusively, but PBL-derived cells secreted not only IgM, but also IgG and IgA abundantly. These results indicate that the lymphoblastoid cells established by EBV infection reflect the Ig phenotype of the cell from which they originated.

A rapid semi quantitative capillary enzyme immunoassay for digoxin

A rapid and sensitive enzyme immunoassay (EIA) which does not require highly trained personnel or specialised instrumentation is described for the estimation of digoxin in serum, plasma or whole blood samples. The method is based on the ability of digoxin in a clinical sample to inhibit the binding of urease-conjugated sheep-antidigoxin immunoglobulin to a glass capillary tube coated internally with a human serum albumin-digoxin conjugate. The bound enzyme activity can then be measured using a substrate solution containing urea and a pH indicator, most suitably bromocresol purple. The enzymic hydrolysis of urea produces ammonia which causes a vivid yellow to purple colour change in the pH indicator. Plasma samples from 92 patients receiving digoxin were screened in parallel with reference plasma containing 1.3 or 3.8 nmol/l digoxin. The results were available within a total test time of 30 min, and showed excellent correlation with those obtained by radioimmunoassay.

A new enzyme immunoassay system suitable for field use and its application in a snake venom detection kit

A new rapid enzyme immunoassay (EIA) field kit for the detection of antigen in clinical specimens is reported. A kit for the detection and identification of snake venom in specimens collected from envenomated patients is described as an example of such a field kit. Tabulated data were obtained from a pilot study in which kits were issued to hospitals and laboratories reporting regular admission of snake bitten patients or presentation of specimens from such patients. Questionnaires returned from participants in the trial indicated that the test configuration was highly acceptable and an excellent correlation was obtained between clinical diagnosis and the detection and identification of snake venom in specimens of blood, urine or bite-site swab washings.

Coated tube enzyme immunoassay: factors affecting sensitivity and effects of reversible protein binding to polystyrene

Coated tube enzyme immunoassay using alkaline phosphatase conjugated to rabbit (anti-human IgG) antiserum was studied to determine conditions of maximum sensitivity. The competitive binding assay utilized showed a large increase in sensitivity with immobilized antigen levels below the levels giving rise to the maximum in the coating-antigen dilution series. The effects of reversible antigen binding to the solid phase were investigated by comparison of untreated polystyrene tubes, polystyrene tubes treated with glutaraldehyde and glass tubes activated with an aminosilane. The use of glutaraldehyde treated tubes reduced, and the use of activated glass tubes prevented the time dependent release of immobilized antigen seen with the untreated polystyrene tubes. By comparison of these solid phases, it is shown that reversible antigen immobilized in a competitive binding assay gives rise to poorer conjugate binding (three-fold), and poorer sensitivity (six-fold). A noncompetitive response was found to occur at high free antibody levels and low competing antigen concentrations. This binding behavior is moderated by the minimization of the reversible antigen immobilization.

An improved preparation of antibody-coated polystyrene beads for sandwich enzyme immunoassay

An improved preparation of antibody-coated polystyrene beads for sandwich enzyme immunoassay of human thyroid-stimulating hormone (TSH) was described. Rabbit anti-TSH IgG was purified by eluting at pH 2.5 from a TSH-Sepharose column, diluted 3 or 9 fold with normal rabbit IgG and used for coating polystyrene beads by physical adsorption. In a sandwich enzyme immunoassay of TSH using rabbit (anti-TSH) Fab'-beta-D-galactosidase conjugate, beta-D-galactosidase activities specifically bound to thus prepared polystyrene beads in the presence of TSH was 2.8-6.3 fold higher than those bound to polystyrene beads coated with anti-TSH IgG before purification. A similar effect was observed when guinea pig anti-pork insulin IgG, rabbit (anti-human IgE) IgG and goat (anti-human IgE) IgG were treated at pH 2.5. This improvement may be based on a conformational change of Fc in IgG molecule which was caused by the treatment at pH 2.5. Other sandwich immunoassays such as fluoro- and radio-immunoassays may also be improved in the same way.

Enzyme immunoassay: a review

In the last few years, the use of enzyme labels in immunoassays has been investigated. The aim of this review is to evaluate critically the role of such labels in clinical biochemistry. Special attention has been given to the problems involved in preparing enzyme labels and the ways in which such labels can be used in a variety of heterogeneous and homogeneous assay systems.

An enzyme immunoassay for the measurement of thyroglobulin in human serum

An enzyme-linked sandwich immunoassay using silicone rods coated with rabbit (anti-human thyroglobulin) immunoglobulin G and rabbit (anti-human thyroglobulin) monovalent fragment of immunoglobulin F (Fab') conjugated with beta-D-galactosidase was developed for the measurement of thyroglobulin in human serum. The volume of serum needed for the assay was as little as 2 microliters. The sensitivity of the assay was 3.5 ng/ml, which is equal to or rather higher than that of radioimmunoassay. The specificity of the assay was demonstrated by the following observations: (1) The absence of crossreaction of thyroxine and triiodothyronine, (2) non-detectability of thyroglobulin in the sera of patients who underwent total thyroidectomy, (3) parallelism of the standard curve with dilutions of reference serum. The precision of the assay was proven by the demonstration of the sufficient recovery of human thyroglobulin added to sera (92--99%) and coefficients of variance in within and between assays were 6.2--9.3 and 2.5--5.3%, respectively. Furthermore, a highly significant correlation was observed between thyroglobulin concentrations measured by our enzyme immunoassay and those by radioimmunoassay (r = 0.99, p less than 0.001, n = 63). Human thyroglobulin in serum was detectable in 90% of 146 normal subjects, the concentration (mean +/- S.D.) being 13.3 +/- 10.3 ng/ml.

A comparison of different enzyme-antibody conjugates for enzyme-linked immunosorbent assay

Calf intestinal alkaline phosphatase (CIAP)- and horseradish peroxidase (HRP)-linked goat antibody (Ab) conjugates have been prepared by two procedures. One procedure is by glutaraldehyde coupling of the proteins; the other is by intermolecular disulfide bond formation of the appropriately modified proteins. The conjugates, specific for rabbit IgG, were tested for their effectiveness as reagents in enzyme-linked immunosorbent assays for ragweed antigen E specific rabbit antibody. The CIAP-Ab conjugate prepared by glutaraldehyde coupling and the HRP-Ab conjugate made by disulfide bond formation were equally effective reagents for immunoassay, but the HRP-Ab conjugate obtained by glutaraldehyde coupling was definitely less useful than the other two conjugates. The assay procedure utilized an antigen E coupled paper disc as the immunosorbent and it was sensitive in the range of 0.5--100 ng per test. Inhibition of the enzyme-linked immunosorbent assay was used as a sensitive technique for measuring antigenic activity of antigen E or its derivatives.

Enzyme immunoassays with special reference to ELISA techniques

In this review outlines are given on various types of enzyme immunoassay. The applications to such enzyme immunoassays, especially ELISA, are dealth with in detail. It is concluded that these techniques have high sensitivity and will be suitable in due course as routine laboratory tests.