Which of the following is commonly used as a chromogenic substrate with horseradish peroxidase?

• A. Hydrogen peroxide
• B. Naphthol-AS-phosphate
• C. Tetramethylbenzidine
• D. 3,3'-diaminobenzidine

Answer: (D)

Horseradish peroxidase reveals the location of an antibody by oxidizing a chromogenic substrate to produce a visible color precipitate at the site of the antigen. The substrate must create a stable, insoluble color that can be seen under light microscopy and remain through tissue processing. The classic, widely used chromogenic substrate for HRP is 3,3'-diaminobenzidine, which yields a brown, durable precipitate right where the enzyme is active, providing clear contrast with a counterstain. Hydrogen peroxide acts as the oxidant for HRP, not the color-producing substrate. Naphthol-AS-phosphate is used with alkaline phosphatase, not HRP. Tetramethylbenzidine is another HRP substrate and can give a blue color (or brown under certain conditions), but it is less commonly used as the standard in routine immunohistochemistry.