What does pNPP stand for in the alkaline phosphatase assay?

What does pNPP stand for in the alkaline phosphatase assay?

• A. Para-nitrophenylphosphate
• B. Phenyl phosphate
• C. Diphenyl phosphate
• D. Para-nitrophenylphosphonate

Answer: (A)

In the alkaline phosphatase assay, the substrate name is tied to the chemical that releases a colored product when dephosphorylated. pNPP stands for para-nitrophenyl phosphate. The “p” signals a para substitution on the phenyl ring, and the nitro group on that ring (nitrophenyl) becomes para-nitrophenol when the enzyme removes the phosphate. para-nitrophenol has strong absorbance at around 405 nm, producing the yellow color that is measured to quantify enzyme activity. The other options don’t fit because they lack the nitro group, use different linkages, or have two phenyl groups, none of which are the standard chromogenic substrate used here.

In the alkaline phosphatase assay, the substrate name is tied to the chemical that releases a colored product when dephosphorylated. pNPP stands for para-nitrophenyl phosphate. The “p” signals a para substitution on the phenyl ring, and the nitro group on that ring (nitrophenyl) becomes para-nitrophenol when the enzyme removes the phosphate. para-nitrophenol has strong absorbance at around 405 nm, producing the yellow color that is measured to quantify enzyme activity. The other options don’t fit because they lack the nitro group, use different linkages, or have two phenyl groups, none of which are the standard chromogenic substrate used here.