Enzyme-labelled assay

Anderson J, Rowe LW. The use of an enzyme-labelled assay as an aid to reading micro virus-neutralisation tests. J. Immunol. Methods 1982 53 183-186. 

Recently, an enzyme-labelled assay for 2,4-D has been presented [19]. The label was 2,4-D conjugated to the enzyme tobacco peroxidase, which allowed for both colorimetric and chemiluminesence detection. In this instance, the 2,4-D MIP was synthesised in the form of microspheres [20]. In contrast, despite its higher binding... 

The primary use of EIA when it was first developed was for histological labeling and localization of specific cell macromolecules. Eor example, enzymes labeled with peroxidase were used to locate specific cellular compartments and stmctures for microscopic examination. The flexibiUty of EIA was recognized quickly and it was adapted for use as a laboratory assay. 

Chemiluminescence and bioluminescence are also used in immunoassays to detect conventional enzyme labels (eg, alkaline phosphatase, P-galactosidase, glucose oxidase, glucose 6-phosphate dehydrogenase, horseradish peroxidase, microperoxidase, xanthine oxidase). The enhanced chemiluminescence assay for horseradish peroxidase (luminol-peroxide-4-iodophenol detection reagent) and various chemiluminescence adamantyl 1,2-dioxetane aryl phosphate substrates, eg, (11) and (15) for alkaline phosphatase labels are in routine use in immunoassay analyzers and in Western blotting kits (261—266). 

Enzyme Immunosensors. Enzyme immunosensors are enzyme immunoassays coupled with electrochemical sensors. These sensors (qv) require multiple steps for analyte determination, and either sandwich assays or competitive binding assays maybe used. Both of these assays use antibodies for the analyte of interest attached to a membrane on the surface of an electrochemical sensor. In the sandwich assay type, the membrane-bound antibody binds the sample antigen, which in turn binds another antibody that is enzyme-labeled. This immunosensor is then placed in a solution containing the substrate for the labeling enzyme and the rate of product formation is measured electrochemically. The rate of the reaction is proportional to the amount of bound enzyme and thus to the amount of the analyte antigen. The sandwich assay can be used only with antigens capable of binding two different antibodies simultaneously (53). 

Our standard incorporation assays contained resuspended particulate enzyme, labelled UDP-Gal (0.1 mM) and (10 mM) in resuspension buffer (Tris, pH 7.5). After incubation, reaction mixtures were heated briefly to 100°C and soluble lupin galactan was added, to ensure the precipitation of small amounts of galactan formed in the en me reaction and dissolved during the heating step. Precipitation of macromolecular products was achieved by adding methanol to a final concentration of 70%. The pellet was freed of soluble labelled products, including residual UDP-Gal, by repeated extraction with hot 70% methanol and was then analysed for labelled (l- )-P-D-galactan. The supernatant was analysed for soluble labelled products. 

Another commonly used ELISA format is the immobilized antibody assay or direct competitive assay (Eigure 3). The primary anti-analyte antibody is immobilized on the solid phase and the analyte competes with a known amount of enzyme-labeled hapten for binding sites on the immobilized antibody. Eirst, the anti-analyte antibody is adsorbed on the microtiter plate wells. In the competition step, the analyte and enzyme-labeled hapten are added to microtiter plate wells and unbound materials are subsequently washed out. The enzyme substrate is then added for color production. Similarly to indirect competitive immunoassay, absorption is inversely proportional to the concentration of analyte. The direct competitive ELISA format is commonly used in commercial immunoassay test kits. 

In a direct immunoassay the immobilized antibody binds to the corresponding antigen. The competitive immunoassay relies upon the competition of the analyte with a labelled analyte for antibody binding. These formats are widely used for high throughput affinity arrays. A sandwich immunoassay is based on the trapping or capture of the analyte by another antibody. In ELISA (enzyme linked immunosorbent assays) the second antibody is conjugated with an enzyme. The bound enzyme labelled antibody is detected by its ability to break down its substrate to a colored product. 

Ironically, AP is the enzyme of choice for some applications due to its stability. Since it can withstand the moderately high temperatures associated with hybridization assays better than HRP, AP often is the enzyme of choice for labeling oligonucleotide probes. AP also is capable of maintaining enzymatic activity for extended periods of substrate development. Increased sensitivity can be realized in ELISA procedures by extending the substrate incubation time to hours and sometimes even days. These properties make AP the second most popular choice for antibody-enzyme conjugates (behind HRP), being used in almost 20 percent of all commercial enzyme-linked assays. 

Enzymes useful for detection purposes in ELISA techniques (Chapter 26) also can be employed in the creation of highly sensitive DNA probes for hybridization assays. The attached enzyme molecule provides detectability for the oligonucleotide through turnover of substrates that can produce chromogenic or fluorescent products. Enzyme-based hybridization assays are perhaps the most common method of nonradioactive detection used in nucleic acid chemistry today. The sensitivity of enzyme-labeled probes can approach or equal that of radiolabeled nucleic acids, thus eliminating the need for radioactivity in most assay systems. 

Urdea, M.S., Warner, B.D., Running, J.A., Stempien, M., Clyne, J., and Horn, T. (1988) A comparison of non-radioactive hybridization assay methods using fluorescent, chemiluminescent and enzyme-labeled synthetic oligodeoxyribonucleotide probes. Nucleic Acids Res. 16, 4937-4956. 

A CL ISH assay for the detection of human papillomavirus (HPV) DNA was developed, in which the hybridization reaction was performed using either digoxigenin-, biotin-, or fluorescein-labeled probes [64], The hybrids were visualized using AP as the enzyme label and a highly sensitive 1,2-dioxetane phosphate as chemiluminescent substrate. This assay was applied to biopsy specimens from different pathologies associated with HPV, which had previously proved positive for HPV DNA by polymerase chain reaction (PCR). The analytical sensitivity was assessed using samples of HeLa and CaSki cell lines, whose content in HPV DNA is known (10-50 copies of HPV 18 DNA in HeLa cells and 400-600 copies... 

D. Enzyme-labeled antigen is used at a constant (and excess) concentration of approximately 10 pg/tnL (such that it is not the limiting factor in the assay), allowed to react, and excess is washed away (PBST). 

Enzyme-linked immunosorbent assay (ELISA) is comparable to the immuno-radiometric assay except that an enzyme tag is attached to the antibody instead of a radioactive label. ELISAs have the advantage of nonradioactive materials and produce an end product that can be assessed with a spectrophotometer. The molecule of interest is bound to the enzyme-labeled antibody, and the excess antibody is removed for immunoradiometric assays. After excess antibody has been removed or the second antibody containing the enzyme has been added (two-site assay), the substrate and cofactors necessary are added in order to visualize and record enzyme activity. The level of molecule of interest present is directly related to the level of enzymatic activity. The sensitivity of the ELISAs can be enhanced by increasing the incubation time for producing substrate. 

Enzyme labels are usually associated with solid-phase antibodies in the technique known as enzyme-linked immunosorbent assay (ELISA). There are several variants of this technique employing both competitive and non-competitive systems. However it is best used in combination with two monoclonal antibodies in the two-site format in which an excess of antibody is bound to a solid phase such as a test-tube or microtitre plate the test antigen is then added and is largely sequestered by the antibody (Figure 7.12). After washing... 

Figure 7.15 Enzyme-multiplied immunoassay (EMIT). The three reactants, test (or standard) antigen, enzyme-labelled antigen and a limited amount of antibody are allowed to react and reach an equilibrium position. The unbound labelled antigen which remains is the only source of enzyme activity, the bound enzyme being inactivated. This free enzyme can be quantitated using a direct kinetic assay method and is proportional to the amount of unlabelled antigen originally present.

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