Specific inhibitor of papain family cysteine proteinases. Inhibits papain, chymopapain, bromelain, ficin, human cathepsins B, H and L, actinidain and house dustmite endopeptidase 1, but does not inhibit human bleomycin hydrolase. Inhibits papain with an IC(50) of 2.47 nM. Does not inhibit cysteine proteinases belonging to other families including clostripain, streptopain and calpain. Glycosylated. Causes an allergic reaction in human. Binds IgE. Induces basophil activation. Belongs to the cystatin family. Phytocystatin subfamily.