Serine protease inhibitor (PubMed:16839309, PubMed:23075397). Inhibits porcine pancreatic elastase with a Ki of 58.3 nM, human neutrophil elastase with a Ki of 3.6 nM, cathepsin G with a Ki of 153.5 nM, chymotrypsin with a Ki of 26.7 nM and subtilisin with a Ki of 0.68 nM. Does not inhibit neutrophil protease 3 or pancreatic trypsin (PubMed:16839309). No decrease in activity observed after incubating at pH 2.5, pH 7.4 and at pH 11.0 for 1 hour to overnight (PubMed:16839309, PubMed:23075397). Thermostable (PubMed:16839309, PubMed:23075397). No decrease in activity was observed after heating for 1 hour at up to 95 degrees Celsius (PubMed:16839309, PubMed:23075397). Mono-phosphorylated form. Di-phosphorylated form. Tri-phosphorylated form.