Protein Z-dependent Protease Inhibitor (ZPI), also known as SerpinA10 (SERine Proteinase INhibitor-clade A10) is a monomeric, secreted member of the A (or extracellular) clade within the serpin superfamily of protease inhibitors. In general, members of this superfamily regulate multiple proteolytic cascades, and are particularly effective due to the fact that their inhibitory activities can be fine-tuned through the participation of discrete, non-serpin co-factors. Serpins are unusual in that they are one-time use, non-recyclable proteins whose native state is thermodynamically unstable. The activity of recombinant human SERPINA10 was measured by its ability to inhibit Coagulation Factor?X cleavage of a fluorogenic peptide substrate Mca-RPKPVE-Nval-WRK(Dnp)-NH2 in the assay buffer 50 mM Tris, 10 mM CaCl2, 150 mM NaCl, 0.05% (w/v) Brij-35, pH 7.5. Coagulation Factor?X was diluted to 5 ug/ml in the assay buffer and 25 ul different concentrations of recombinant human SERPINA10 (MW: 78.26 KD) was incubated with 25 ul diluted Coagulation Factor?X at 37 ℃ for 30?minutes. Loading 50?μL 20?μM substrate to start the reaction including a substrate blank containing 50?μL of assay buffer and 50?μL of 20?μM substrate. Then read at excitiation and emission wavelengths of 320 nm and 405 nm, respectively, in kinetic mode for 5 minutes. The result was shown in Figure 1 and it was obvious that recombinant human SERPINA10 significantly decreased Coagulation Factor?X activity. The inhibition IC50 was <75 nM.