Identify the substrate associated with alkaline phosphatase from the following options.

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

Answer: (C)

Alkaline phosphatase detection in immunohistochemistry relies on a phosphate ester substrate that, when dephosphorylated, yields a product capable of forming a colored precipitate with a diazonium salt at the site of enzyme activity. Naphthol-AS-phosphate is a classic substrate for this purpose; after ALP removes the phosphate group, the resulting naphthol-AS couples with a diazonium salt to produce a localized, insoluble colored product that clearly marks the enzyme’s location in tissue.

Hydrogen peroxide and tetramethylbenzidine are associated with peroxidase enzymes rather than alkaline phosphatase, used in colorimetric reactions driven by HRP. While p-nitrophenyl phosphate can be dephosphorylated by ALP in enzymatic assays, it is more typical in simple spectrophotometric measurements than in histochemical staining, where the precipitating chromogen method with naphthol-AS-phosphate is preferred.

Thus, the substrate linked to alkaline phosphatase in this context is Naphthol-AS-phosphate.