Alkaline phosphatase is an enzyme found in human tissues such as liver, bone, intestine, kidney, and placenta. Methods determine the level of activity of alkaline phosphatase by measuring the rate of conversion of substrate catalyzed per unit time. In the alkaline phosphatase rate method, a substrate called p-nitrophenyl phosphate (pNPP) is used. Alkaline phosphatase catalyzes the hydrolysis reaction of pNPP to produce p-nitrophenol (pNP) and inorganic phosphate. The generated pNP can be used to determine alkaline phosphatase activity by measuring the change in its absorbance.