Thiol protease inhibitor. Has high papain inhibitory activity and inhibits to a lesser extent fish cathepsins L, S, K, F, X and bovine cathepsin B in vitro. Optimum pH is 5-6 and pH 6-8 for papain and bovine cathepsin B, respectively. Stable between 20-40 degrees Celsius. Ubiquitously expressed in normal tissues including brain, eye, gill, heart, gullet, liver, spleen, stomach, pyloric ceca, intestine, kidney and muscle. Expressed, but not up-regulated, in lipopolysaccharide (LPS)-stimulated tissues including kidney, spleen, muscle and gill. Belongs to the cystatin family.