Which chromogens are listed for alkaline phosphatase (AP) substrate?

• A. BCIP/NBT, Fast Red TR salt
• B. DAB
• C. ACE
• D. AEC

Answer: (C)

Alkaline phosphatase uses chromogenic substrates that form visible solid colors when the enzyme removes phosphate groups. The two most common AP chromogens are BCIP/NBT and Fast Red TR salt. BCIP serves as the phosphate donor and, when dephosphorylated by AP, couples with NBT to give a blue‑purple precipitate. Fast Red TR salt yields a red precipitate after dephosphorylation, providing a bright contrast.

DAB and AEC are substrates designed for horseradish peroxidase, producing brown and red/orange signals, respectively, not for alkaline phosphatase. ACE isn’t a standard, widely used AP chromogen in typical practice, so the canonical AP chromogens to recognize are BCIP/NBT and Fast Red TR salt.