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.
[1]Confident assignment of intact mass tags to human salivary cystatins using top-down Fourier-transform ion cyclotron resonance mass spectrometry." Ryan C.M., Souda P., Halgand F., Wong D.T., Loo J.A., Faull K.F., Whitelegge J.P.J. Am. Soc. Mass Spectrom. 21:908-917(2010). [2]Signal peptide prediction based on analysis of experimentally verified cleavage sites." Zhang Z., Henzel W.J.Protein Sci. 13:2819-2824(2004). [3]Newly identified proteins in human nasal lavage fluid from non-smokers and smokers using two-dimensional gel electrophoresis and peptide mass fingerprinting." Ghafouri B., Stahlbom B., Tagesson C., Lindahl M. Proteomics 2:112-120(2002).
