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

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

Answer: (A)

In enzyme-based detection for immunohistochemistry, alkaline phosphatase needs a substrate that, once dephosphorylated, forms a colored product right at the site of the antigen. Naphthol-AS-phosphate fits this role because AP removes the phosphate to release naphthol, which then couples with a diazonium salt to produce a visible, insoluble dye at the target location. This localized color makes the antigen detectable under a light microscope. The other options don’t fit this histochemical use: hydrogen peroxide and tetramethylbenzidine are associated with horseradish peroxidase detection, producing color through a peroxidase reaction, not alkaline phosphatase. p-nitrophenyl phosphate is a substrate used mainly in spectrophotometric assays to measure AP activity by yielding p-nitrophenol, but it does not form the characteristic tissue-localized precipitate used in immunohistochemical staining.