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Horseradish peroxidase is a 44,173.9-dalton glycoprotein with six lysine residues which can be conjugated to a labeled molecule. It produces a coloured, fluorimetric [6] or luminescent derivative of the labeled molecule when incubated with a proper substrate, allowing it to be detected and quantified.
3,3′,5,5′-Tetramethylbenzidine or TMB is a chromogenic substrate used in staining procedures in immunohistochemistry as well as being a visualising reagent used in enzyme-linked immunosorbent assays . [1] TMB is a white solid that forms a pale blue-green liquid in solution with ethyl acetate.
Horseradish peroxidase has an accessible active site, and many compounds can reach the site of the reaction. On the other hand, for an enzyme such as cytochrome c peroxidase , the compounds that donate electrons are very specific, due to a very narrow active site.
A suitable alternative to radioimmunoassay would substitute a nonradioactive signal in place of the radioactive signal. When enzymes (such as horseradish peroxidase) react with appropriate substrates (such as ABTS or TMB), a change in color occurs, which is used as a signal. However, the signal has to be associated with the presence of antibody ...
Some examples of substrates used (also known as chromogens) are AEC (3-Amino-9-EthylCarbazole), or DAB (3,3'-Diaminobenzidine). Use of one of these reagents after exposure to the necessary enzyme (e.g., horseradish peroxidase conjugated to an antibody reagent) produces a positive immunoreaction product.
It is commonly used as a substrate with hydrogen peroxide for a peroxidase enzyme (such as horseradish peroxidase) or alone with blue multicopper oxidase enzymes (such as laccase or bilirubin oxidase). Its use allows the reaction kinetics of peroxidases themselves to be followed.
In chromogenic immunohistochemistry an antibody is conjugated to an enzyme, such as alkaline phosphate and horseradish peroxidase, that can catalyze a color-producing reaction in the presence of a chromogenic substrate like diaminobenzidine. [5] The colored product can be analyzed with an ordinary light microscope. [13]
The detection of horseradish peroxidase by enzymatic chemiluminescence (ECL) is a common method of detecting antibodies in western blotting. Another example is the enzyme luciferase, this is found in fireflies and naturally produces light from its substrate luciferin.