Recombinant AsCystatin inhibits various C1 cysteine proteases including cathepsin L (Ki is 0.89 pM), papain (Ki is 1.74 pM) and cathepsin B (Ki is 0.69 nM). This activity has also been observed in the crude venom. This protein has no toxic activity and its function in the venom is unknown. It may play a role as a housekeeping or regulatory protein. Expressed at a low level by the venom gland (at protein level). The recombinant protein does not inhibit calpain-1 (CAPN1), a C2 family cysteine protease and legumain (LGMN), a C13 family cysteine protease. Does not provoke cell death (PC3 prostate cancer cells) following continuous exposure to 6 uM for 24 hours (PubMed:21172403). Belongs to the cystatin family.