Protective Functions

Mucin is a glycoprotein that interacts with water. This complex is essential for the hydration of the oral mucosa, for the lubrication of oral tissues, and to protect mucosal surfaces from trauma associated with normal masticatory function. Reduced water results in a relative increase in the concentration of mucin, making the saliva more viscous and sticky. Proline-rich glycoproteins promote the lubrication of oral tissues and sulpho-mucin assists in clearing bacteria from the oral cavity by binding to and aggregating microorganisms.

Amylase inhibits the growth of certain bacterial species. Lysozymes break down peptidoglycan in the cell wall of some Gram-positive bacteria, e.g., S. mutans. Lactoferrin binds ferric iron and inhibits bacteria requiring iron for their survival, e.g., S mutans; and appears to be associated with antifungal, antiviral, antiinflammatory, and immunomodulatory activity. Lactoperoxidases catalyze the oxidation of salivary thiocyanite by hydrogen peroxide to hypothiocyanite, which inactivates bacterial enzymes essential for bacterial survival.

Histatins inhibit the growth of C. albicans, S. mutans, and P. gingivalis: neutralize lipopolysaccharides of the outer plasma membrane of Gram-negative bacteria; and modulate inflammation by blocking histamine release. Secretory immunoglobulin A (IgA) aggregates bacteria and, along with proline-rich glycoproteins, prevent bacterial adhesion to oral tissues. Secretory IgA can also neutralize viruses, bacteria, and bacterial toxins.

Another important function of saliva is to maintain the health and integrity of oral soft tissues. Cystatins inhibit destructive enzymes, i.e., cysteine proteases; while epithelial growth factor (EGF), transforming growth factor-alpha (TGF-α), transforming growth factor-beta (TGF-β), fibroblast growth factor (FBF), insulin-like growth factors (IGF-I and IGF-II), and nerve growth factor (NGF) promote wound healing.

Bicarbonate (HCO3-) ions play a major role in the salivary buffering system and mastication is a powerful stimulus of the secretion of sodium bicarbonate into parotid saliva. The resting pH of saliva (range 5.3 to 7.8) is lowest during sleep and it increases during waking hours. As bicarbonate ion concentrations increase, so does the pH and buffering capacity of saliva. This suppresses the growth of acidophilic microorganisms such as S. mutans and C. albicans and facilitates remineralization.

Phosphate ions also contribute to the buffering capacity of saliva. The mechanism for the buffering action of inorganic phosphates is due to the ability of secondary inorganic phosphate ions (HPO42-) to bind to hydrogen ions (H2PO4-). This is particularly important in the resting salivary buffering system. In addition, a minor role is played in the salivary buffering system by a number of proteins, i.e., macromolecules containing H-binding sites; peptides, e.g., sialin; and urea, which is metabolized by oral bacteria to ammonia and CO resulting in an increase in the pH.

Saliva plays a pivotal role in maintaining the integrity of enamel. While saliva contains many inorganic ions (e.g., calcium, phosphates, fluoride, magnesium, sodium, potassium, and chloride), several salivary components promote supersaturation of saliva with calcium ions and phosphate ions. High concentrations of calcium and phosphates promote post-eruptive maturation of enamel and remineralization of carious teeth before cavitation occurs. Another critical factor to stabilize enamel hydroxyapatite is fluoride.

Statherin, a phospho-protein with strong affinity for both calcium and enamel, inhibits the precipitation of and crystal growth of calcium phosphate. Similarly, proline-rich glycoproteins bind to calcium phosphate crystals and prevent their growth. Together, statherin and proline-rich glycoproteins prevent the formation of calculus. In addition, proline-rich glycoproteins are a key component of the pellicle, bind strongly to enamel, and by binding a considerable portion of the total salivary calcium help maintain an optimal calcium-phosphate ionic ratio.