Inhibits trypsin and chymotrypsin with a 1:1 stoichiometry, with dissociation constants of 1.56 nM and 120 nM respectively. Inhibits plasma kallikrein, factor XIIa and plasmin with dissociation constants of 5.0 nM, 150 nM and 18 nM respectively. Does not inhibit factor Xa, thrombin, tissue kallikrein or cysteine proteinases such as papain and bromelain. Retains activity after incubation for 10 minutes at pH 2.0 to 12.0. Activity is retained when heated for 10 minutes up to 60 degrees Celsius. Inactive at 100 degrees Celsius. Heterodimer of an alpha and a beta chain linked by a disulfide bond. Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.