Which substrate is used by alkaline phosphatase in enzyme-based detection methods?

• A. Naphthol-AS-phosphate
• B. Hydrogen peroxide
• C. Tetramethylbenzidine
• D. Diaminobenzene

Answer: (A)

Alkaline phosphatase detection hinges on a phosphate ester substrate that, after enzymatic hydrolysis, generates a reactive phenol which then forms a colored product at the site of the target. Naphthol-AS-phosphate is that substrate: the enzyme removes the phosphate group to produce Naphthol-AS, which immediately couples with a diazonium salt present in the detection reagents to yield a colored, insoluble precipitate at the antigen location. This visible signal is how the binding is visualized in immunohistochemistry. The other substances are typical for horseradish peroxidase-based detection, where hydrogen peroxide drives the oxidation of chromogens like tetramethylbenzidine or diaminobenzene to produce color; they are not substrates for alkaline phosphatase.