Thiol protease inhibitor. Has high papain, bovine cathepsin B and fish cathepsins F and X inhibitory activity and inhibits fish cathepsins L, S and K to a lesser extent in vitro. May be involved in innate immunity. Greatly decreased inhibitory activity against papain protease by metal ions including ZnSO(4), CuSO(4), HgCl(2) and CoCl(2). Decreased inhibitory activity against papain protease by detergents including Tween 20, SDS and Brij 35. Optimum pH is 6-7.5 and pH 6.5-8 for papain and bovine cathepsin B, respectively. Stable between 20 and 40 degrees Celsius. Activity for papain drops rapidly over 60 degrees Celsius. Retains inhibitory activity against papain for 10 days at 37 and 30 degrees Celsius. Ubiquitously expressed in normal and lipopolysaccharide (LPS)-stimulated tissues including brain, eye, gullet, heart, liver, muscle, stomach, kidney, spleen, pyloric ceca, intestine and gill. By LPS. Slightly increased expression 24 hours post-injection in spleen and muscle. Belongs to the cystatin family.