pNP

Esters of 4-Nitrophenol (also called p-nitrophenol or 4-hydroxynitrobenzene) which is a phenolic compound that has a nitro group at the opposite position of the hydroxyl group on the benzene ring. Amounts of 4-nitrophenol produced by a particular enzyme in the presence of its corresponding substrate can be measured with a spectrophotometer at or around 405 nm and used as a proxy measurement for the amount of the enzyme activity in the sample.