Wikipedia
Para-Nitrophenylphosphate
para-Nitrophenylphosphate (pNPP) is a chromogenic substrate for acid and alkaline phosphatase in ELISA and conventional spectrophotometric assays. These enzymes catalyse the hydrolysis of pNPP to yellow para-nitrophenol. This product absorbs strongly at 405 nm and can be measured with a visible light spectrophotometer.
{PNPP}+H2O ->[_{AP}] {PNP} + P_i