Horseradish peroxidase (HRP) is isolated from horseradish roots and belongs to the ferroprotoporphyrin group of peroxidases. It is a single chain polypeptide containing four disulfide bridges. It is a glycoprotein containing 18% carbohydrate. The carbohydrate composition consists of galactose, arabinose, xylose, fucose, mannose, mannosamine, and galactosamine depending upon the specific isozyme.

• Enzyme Commission Number: E.C.1.11.1.7
• Extinction coeficient: EmM = 100 (403 nm)
• Isoelectric point (pI): 3,0 to 9,0

It is used for production of peroxidase conjugated antisera. It is a widely used label for immunoglobulins in many different immunochemistry applications including ELISA, immunoblotting, and immunohistochemistry. It can be conjugated to antibodies by several different methods including glutaraldehyde, periodate oxidation, through disulfide bonds, and also via amino and thiol directed crosslinkers. It is the most desired label for antibodies, since it is the smallest and most stable of the three most popular enzyme labels (HRP, β-galactosidase, and alkaline phosphatase) and its glycosylation leads to lower non-specific binding.

When incubated with a substrate, horseradish peroxidase produces a coloured, fluorimetric, or luminescent derivative of the labeled molecule, allowing quantification. It has been shown to slightly reduce the level of inhibition in a cydAB mutant. Inhibitors are sodium azide, cyanide, L-cystine, dichromate, ethylenethiourea, hydroxylamine, sulfide, vanadate, p-aminobenzoic acid, and Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mn²⁺, Ni²⁺, and Pb²⁺ ions.

Unit definition: The amount of enzyme causing the production of one milligram of purpurgallin in 20 seconds at 25 °C under the given assay conditions.