pNPP

pNPP (para-Nitrophenylphosphate) is a chromogenic substrate for alkaline phosphatase (AP), a label commonly conjugated to antibodies. Upon catalysis by AP, the phosphate group of pNPP is hydrolyzed to yield a product of yellow color. The resulting color change exhibits an absorbance maximum at 405 nm. pNPP is often used to measure proteins and antigens in ELISA applications. It can also be used to detect the activity of various phosphatases, including protein tyrosine phosphatases.