Store protected from light.
Protein Phosphatase Assay: The PNPP phosphatase activity assay is very simple, non-expensive, and routinely used for the unit determination of all NEB protein phosphatases. PNPP has apparent Km values for protein phosphatases in the range of 0.5 -10 mM (2-5).
The PNPP phosphatase activity is assayed in a reaction mixture (50 μl) containing 50 mM PNPP and a protein phosphatase buffer supplemented with additional components when required. The reaction is initiated by addition of enzyme and quenched after 5-10 minutes by addition of 1 ml of 1 N NaOH (or 1 ml of 0.5 M EDTA for Mn2+-dependent protein phosphatases, Lambda PP and PP1). The amount of product, p-nitrophenol, is determined by reading the absorbance at 405 nm and using a molar extinction coefficient of 18,000 M-1 cm-1 (16,000 M-1cm-1 for 0.5 M EDTA) (1, 3).
One unit of the protein phosphatase activity is defined as the amount of enzyme that hydrolyzes 1 nanomole of PNPP in one minute at 30°C in a total reaction volume of 50 μl under standard reaction condition.
To estimate the protein phosphatase activity accurately it is essential to ensure linear kinetics of dephosphorylation.