Antimicrobial and other protective properties
The saliva contains many different proteins and some other small organic proteins that together protect the oral cavity (the soft tissues as well as the teeth) from frictional wear, dryness, erosion, pathogenic bacteria, and so on (see Box 12).
Lubrication and other protective properties. Almost all salivary proteins are glycoproteins; that is, they contain variable amounts of carbohydrates linked to the protein core. Glycoproteins are often classified according to their cellular origin and subclassified on the basis of their biochemical properties. A characteristic feature is that many occur in multiple forms, constituting families; these families, may, however, exhibit remarkable functional differences.
Mucous glycoproteins, the mucins, are of acinar cell origin, have a high molecular weight, and contain more than 40% carbohydrate. The mucins are produced by the minor salivary glands in the palate and provide a nonfrictional, lubricant layer that protects the soft tissues from wear and tear and facilitates swallowing of food.
Because the mucins have a strongly negative charge, other negatively charged molecules, such as those contained in the cell walls of many oral bacteria, are repelled from the mucin-coated oral mucosa. Among other properties, the mucins also bind water and thereby protect the oral mucosa from drying out.
Serous glycoproteins have a much lower molecular weight than mucins and contain less than 50% carbohydrate: Many belong to a group called proline-rich glycoproteins (PRPs), of which several are phosphorylated. These proteins are secreted from the parotid and submandibular glands.
The collective name glycoprotein refers to all carbohydrate-linked proteins, making this group very heterogenous and large. Most salivary proteins, such as secretory IgA, lactoferrin, peroxidases, and agglutinins, belong to this group. Because human saliva is supersaturated with respect to most calcium phosphate salts, some proteins are necessary to inhibit their spontaneous precipitation in the salivary glands and their secretions. Such proteins include statherin and PRPs. The resulting stable but supersaturated state of the saliva with respect to calcium phosphate salts constitutes a protective and reparative environment of importance for the integrity of the teeth.
Statherin is present in both submandibular and parotid salivas. Proline-rich proteins form a complex group of proteins with large numbers of genetic variants, some of which also have the ability to inhibit spontaneous precipitation of calcium phosphate salts. The molecular size of PRPs ranges from 106 to 150 amino acid residues. Like statherin, PRPs are remarkable for their high degree of compositional and charge asymmetry. Proline-rich proteins are readily adsorbed from saliva to hydroxyapatite surfaces and it is most likely that these adsorbed PRPs inhibit the crystal growth of calcium phosphate salts. Although present in whole saliva, PRPs are also susceptible to proteolytic degradation by oral microorganisms.
a-Amylase is one of the most important salivary enzymes, accounting for as much as 40% to 50% of the total salivary gland-produced protein. Most (80%) is synthesized in the parotid glands and the remainder in the submandibular glands. The biologic role of salivary amylase is to split starch into maltose, maltotriose, and dextrins. Maltose can be further fermented by oral bacteria. Therefore, although amylase in saliva clears starch-containing food debris from the mouth, acids are formed in this process. In this way, starch may have some cariogenic potential. Salivary a-amylase is inactivated in the acidic parts of the gastrointestinal tract, and therefore its action is limited to the oral area.
Antimicrobial properties. As described earlier, saliva plays a significant role in maintaining an appropriate balance within the ecosystem associated with tooth surfaces. This balance is of great significance in the control of dental caries, because saliva will enhance the ability of some bacteria to survive and will reduce the competitiveness of others. Saliva achieves this control over the oral flora through its components, which can be constantly present or activated by a specific host response.
The major antimicrobial proteins are listed in Box 13. Many studies have shown that
most of these proteins can inhibit the metabolism, adherence, or even the viability of
cariogenic microorganisms in vitro (for review see Tenovuo, 1997). However, their
role in vivo is largely unknown: It seems that they are important for the control of
microbial overgrowth in the mouth, but their selectivity against pathogens has not
been determined.
A newly proposed biologic function for PRPs is the ability of adsorbed acidic PRPs to
selectively mediate bacterial adhesion on tooth surfaces. Recently it was shown that
the negative charge of these acidic PRPs binds electrostatically to calcium on the
tooth surfaces, while the outer ends, consisting of proline and glutamine amino acids,
attract and bind very strongly to the harmless and protective normal microflora of the
teeth (Streptococcus oralis, Streptococcus sangius, and Streptococcus mitis). This
may explain early scanning electron micrographs obtained by Lie (1978), showing
how a gram-positive “pioneer colonizer” attaches to the pellicle-covered tooth surface,
in contrast to a gram-negative bacterium and the pellicle (Figs 104 and 105).
This primary colonization of the protective normal microflora occurs during the first
24 hours after cleaning. However, recent research has shown that the so-called
secondary colonization by other, more pathogenic microorganisms (gram-positive as
well as gram-negative) is strongly related to the binding between galactose amine
structures on the surfaces of the normal microflora as well as the secondary
colonizers. The production and the individual structures of acidic PRPs and galactose
amines are genetically related and may partly explain individual variations in plaque
formation rates. This is a field of ongoing research (Stromberg, 1996).
The lysozyme in whole saliva is derived from the major and minor salivary glands,
gingival crevicular fluid, and salivary leukocytes (polymorphonuclear neutrophil
leukocytes). Salivary lysozyme is present in newborn babies at levels equal to those of
adults, suggesting a preeruptive antimicrobial function. The classic concept of the
antimicrobial action of lysozyme is based on its muramidase activity, ie, the ability to
hydrolyze the bond between N-acetylmuramic acid and N-acetylglucosamine in the
peptidoglycan layer of the bacterial cell wall. Gram-negative bacteria are more
resistant to lysozyme because of the protective function of the outer
lipopolysaccharide layer. In addition to its muramidase activity, lysozyme is strongly
cationic, and can activate bacterial “autolysins,” which can destroy the cell wall
components.
Lactoferrin is an iron-binding glycoprotein secreted by the serous cells of the major
and minor salivary glands. Polymorphonuclear leukocytes are also rich in lactoferrin
and release it into gingival fluid and whole saliva. The biologic function of lactoferrin
is attributed to its high affinity for iron and its consequent expropriation of this
essential metal from pathogenic microorganisms. This bacteriostatic effect is lost if
the lactoferrin molecule is saturated with iron, a factor that should be taken into
account in areas where the drinking water is rich in iron. In its iron-free state
(apolactoferrin), it has a bactericidal, irreversible effect against a variety of
microorganisms, including mutans streptococci. Apolactoferrin can also agglutinate
Streptococcus mutans cells.
Salivary peroxidase is produced in the acinar cells of the parotid and submandibular
glands but not in the minor salivary glands. Salivary peroxidase systems have two
major biologic functions: (1) antimicrobial activity and (2) protection of host proteins
and cells from hydrogen peroxide toxicity.
Salivary agglutinins are glycoproteins that have the capacity to interact with
unattached bacteria, resulting in clumping of bacteria into large aggregates that are
more easily flushed away by saliva and swallowed: the term aggregation is therefore
often used synonymously with agglutination. Listed in Box 13 are salivary proteins
with agglutinating capacity. The most potent agglutinin is a high-molecular weight
glycoprotein that has been isolated from human parotid saliva. Despite a
concentration in parotid saliva of only 0.001%, it is very effective. Mucins are also
able to agglutinate bacteria. In high-molecular weight glycoproteins, sugar residues
and sialic acid are important for the interaction with bacteria.
The secretory immunoglobulins, most notably secretory IgA, act by aggregating
bacteria. They target specific bacterial molecules, such as adhesins, or enzymes, such
as glucosyl transferase. Studies of the correlation between secretory IgA levels and
caries prevalence have reported conflicting results (Riviere and Papaginnoulis, 1987).
The saliva also contains IgG and IgM from serum and local production in the gingival
tissues.
The conflicting results of recent longitudinal clinical studies of the relative predictive
values of antimicrobial salivary components for caries incidence (Tenovuo et al,
1997) may be attributed to the fact that dental caries is a multifactorial disease.
