Protein Composition of Submandibular Salivary Stones

Objectives: Salivary stones occur as one or more calcified structures in the salivary glands or ducts. They are mainly composed of inorganic material such as hydroxyapatite, with smaller amounts of organic material such as proteins and lipids. The protein composition of salivary stones is poorly examined. The purpose of this study is to explore the protein composition as well as the location of proteins in salivary stones.
Methods: A total of 35 surgically removed salivary stones were used for this study. Four stones were mechanically divided into different layers to investigate the location of proteins. The remaining stones (n=31) were freeze-dried and pulverized, after which the protein composition of the salivary stones was investigated with biochemical techniques (gel electrophoresis, Western blotting and ELISA).
Results: With gel electrophoresis and Western blotting, the following proteins were identified in submandibular salivary stones: α-amylase (in 100% of the stones), lysozyme (95%), lactoferrin (85%), secretory-IgA (85%), MUC7 (60%), complement C4 (60%) and C-reactive protein (35%). The presence of lactoferrin, secretory-IgA, albumin, lysozyme and α-amylase was confirmed with ELISA. There was a near-significant negative correlation between the dry weight of the salivary stone and total protein concentration (r = -0,456, p = 0.066). Gel electrophoresis gave no clear information about the distribution of proteins over the different layers of salivary stones.
Conclusions: Salivary stones contain a variety of proteins. Furthermore, they exhibit a considerable variation in protein composition. It is hypothesized that proteins secreted in saliva as well as the inflammatory reaction in the tissue surrounding the stone may influence the type and amount of proteins that are encapsulated in salivary stones.