Horseradish peroxidase is a 44kDa glycoprotein with 6 lysine residues which can be conjugated to labeled biomolecules using Lightning-Link
The enzyme label can be visualized by chromogenic reactions; for example diaminobenzidine (DAB) in the presence of hydrogen peroxide (H202) is converted in to a water insoluble brown pigment
Other substrates which can be used to measure horseradish peroxidase activity include ABTS, TMB and TMBUS.