Alkaline phosphatase (AP) is the most widely used alternative labeling enzyme to HRP. It is usually isolated from calf intestine. The function of the AP enzyme is to hydrolyze phosphates that exist in proteins and nucleotides. These phosphates are hydrolyzed to intermediate phenolic compounds. Those compounds then react with the diazonium salts present within the applied chromogen. This reaction produces a color specific to the chromogen used, which can be visualized via light microscopy.
Chromogens that are commonly used with alkaline phosphatase are:
- Fast red TR/naphthol
- Fast blue BB
- Nitroblue tetrazolium (NBT)
