p-Nitrophenyl Phosphate (PNPP) is a non-proteinaceous, non-specific substrate used to assay protein, alkaline and acid phosphatases. The PNPP phosphatase activity is measured using a continuous or single-point spectrophotometric assay based on the ability of phosphatases to catalyze the hydrolysis of PNPP to p-nitrophenol, a chromogenic product with absorbance at 405 nm (1). The assay can be used for the quick analysis of the protein phosphatase activity under any non-standard conditions.
The advantage of the PNPP phosphatase activity assay is that unlike radioactive assays the substrate concentration can be much higher than the Km. The initial velocity can be recorded in the continuous assay, but the assay volume is larger than in a radioactive assay (about 1 ml to fill a 1 ml spetrophotometer cuvette) (1,2). The reaction volume in a single-point assay can be very small because the reaction is stopped with the amount of NaOH enough to fill the cuvette (1,3,4).