Background：

Human saliva appears to contain several cysteine proteinase inhibitors that are immunologically related to cystatin S but that differ in their specificity due to amino acid sequence differences. Cystatin SN， with a pI of 7.5， is a much better inhibitor of papain and dipeptidyl peptidase I than is cystatin S， although both inhibit ficin equally well./This protein strongly inhibits papain and ficin， partially inhibits stem bromelain and bovine cathepsin C， but does not inhibit porcine cathepsin B or clostripain. Papain is inhibited non-competitively./Thiol protease inhibitor.