Functional aspects of the human salivary cystatins in the oral environment

OBJECTIVES: The aim of this study was to investigate the cysteine protease inhibitory properties of the human salivary cystatins S, SA and SN in order to identify potential in vivo target cysteine proteases which may include those involved in periodontal tissue destruction. In addition, the potentia l role of the salivary cystatins with respect to the tooth mineral balance and pellicle formation was also investigated. METHODS: Salivary cystatins S major, S minor, SA, SA, (a truncated form of SA) and SN were purified from human submandibular sublingual saliva. Sensit ive fluorometric assays were used to test the inhibitory action of each pur ified form of salivary cystatin against a variety of cysteine proteases and to determine whether pH affected their inhibitory activity towards the wel l-characterized cysteine protease papain. Their potential role in the miner al balance of the tooth was assessed by the measurement of calcium binding and the rate of binding to carbonated apatite (CAP). RESULTS AND CONCLUSIONS: Salivary cystatin SN was found to inhibit the huma n lysosomal cathepsins B, H and L and salivary cystatin SA was found to inh ibit human lysosomal cathepsin L in vitro. These proteases are involved in periodontal tissue destruction and these data suggest that salivary cystati ns SA and SN are involved in the control of the proteolytic events in vivo. Salivary cystatin S was not an inhibitor of the cysteine proteases tested suggesting that its primary role is not as a cysteine protease inhibitor. H owever, S was able to bind more calcium and bind more rapidly to CAP than S A or SN, suggesting that its primary role in the oral environment is likely to be the involvement with the mineral balance of the tooth.