p-Nitrophenyl Phosphate (PNPP) is a non-proteinaceous, non-specific substrate used to assay protein, alkaline and acid phosphatases. Can be used for quick analysis of the protein phosphatase activity under different conditions. 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 that absorbs maximally at 405 nm. However, since you mentioned lipase as your enzyme, I would believe that your pNpp refers to p-nitrophenyl palmitate. If so, then please refer to the attached document. Its simplified. Alternatively, please follow the link: https://www.sciencedirect.com/science/article/pii/S0141022999000770