Neprilysin/CD10, also known as NEP and neutral endopeptidase, is a zinc metallopeptidase expressed at the cell surface of a variety of cells. The enzyme functions both as an endopeptidase with a thermolysin-like specificity and as a dipeptidylcarboxypeptidase. NEP has been shown to be involved in the degradation of enkephalins in the mammalian brain and the inactivation of circulating atrial natriuretic peptide. NEP has also been identified as the common acute lymphocytic leukemia antigen (CALLA), and is expressed on the surface of lymphocytes in some disease states. The activity of recombinant human NEP is measured by its ability to cleave a fluorogenic peptide substrate MCA-Arg-Pro-Pro-Gly-Phe-Ser-Ala-Phe-Lys(DNP)-OH in the assay buffer 50 mM Tris, 0.05% Brij-35, pH 9.0. The rhNEP is diluted to 0.1 ug/ml in assay buffer, then add 50 µL protein into a black well plate and start the reaction by adding 50 µL of 20 µM substrate (diluted by assay buffer), with a substrate blank containing 50 µL assay buffer, 50 µL substrate, and no rhNEP. Then read at excitiation and emission wavelengths of 320 nm and 405 nm, respectively, in kinetic mode for 5 minutes. The specific activity of recombinant human NEP is > 3500 pmol/min/µg.