Horseradish peroxidase (HRP)-conjugated anti-monkey antibodies are enzyme-linked immunoreagents designed for sensitive detection of non-human primate (NHP) immunoglobulins and antigens in immunoassays. As with other enzyme-conjugated anti-monkey reagents, these products are secondary antibodies—anti-monkey immunoglobulins raised in a different host species (most commonly goat, rabbit, or mouse) and covalently linked to HRP. They bind to monkey primary antibodies or to monkey immunoglobulins present in a sample and provide an enzymatic signal for detection. HRP remains the most widely used enzyme label in immunoassays due to its rapid kinetics, high turnover rate, and compatibility with a broad range of chromogenic and chemiluminescent substrates.

The HRP Enzyme

Horseradish peroxidase (EC 1.11.1.7) is a heme-containing oxidoreductase isolated from the root of Armoracia rusticana (horseradish). The enzyme catalyzes the oxidation of a wide variety of organic and inorganic substrates using hydrogen peroxide (H₂O₂) as the electron acceptor. HRP is a glycoprotein with a molecular weight of approximately 44 kDa and contains a protoporphyrin IX heme prosthetic group at its active site. One unit of activity is defined as the amount that decomposes one micromole of hydrogen peroxide per minute at 25°C and pH 7.0.

The catalytic mechanism of HRP involves a two-step cycle. In the first step, the ferric enzyme reacts with H₂O₂ to form Compound I, an oxyferryl porphyrin π-cation radical. In the second step, Compound I oxidizes a reducing substrate to form Compound II, which is then reduced back to the resting ferric state by a second substrate molecule. The rate constant for Compound I formation is affected by an ionizing group with a pK of approximately 4, and kinetic parameters vary among HRP isoenzymes.