Composition of saliva

Introduction to Composition of Saliva

Saliva is a dilute fluid, over 99% being made up of water. The concentrations of dissolved solids(organic and inorganic) are characterized by wide variation, both between individuals and within a single individual.

The relative proportion contributed to the whole saliva by each of the gland pairs depends on the degree of stimulation. Thus, under resting conditions, the submandibular glands contribute 69%, the parotid glands 26%, and the sublingual glands 5% (mean values) of the total secretion derived from these three major gland pairs. Under conditions of increased exogenous stimulation, the submandibular glands again account for the largest and the sublingual glands for the smallest fraction of the total secretion from the major glands, but the relative proportion of the total secretion contributed by the parotid glands increases. Thus, the submandibular glands contribute 63.7%, the parotid glands 34%, and the sublingual glands only 2.8%. It has also been established that the mucosal and labial glands in the oral cavity make some contribution to the total volume of saliva under stimulated conditions.

(Youngt and Schneyer, 1981, P.1 )


Organic composition of saliva proteins

Proteins comprise the bulk of the organic content of saliva but identification of each of them has been more difficult. In studies on human parotid saliva, the presence of at least 20 separate protein fractions has been reported, while in submandibular saliva 21 fractions have been found.



(Youngt and Schneyer, 1981, P27)


Salivary proteins: protective properties saliva proteins & functions

(Amerongen and Bolscher andVeerman, Page 4, Figure 1)






alpha-Amylase (Dental Caries. Page 38)
  • Major digestive enzyme of saliva.
  • 80% is synthesized in the parotid glands, 20% in the sub-mandibular glands.
  • most abundant, accounting for as much as 40-50% of the total salivary gland-produced protein
  • 60-120mg/100ml in parotid saliva, (30% of parotic saliva)
  • 25mg/100ml in submandibular saliva
  • Hydrolyzes alpha-1,4 bonds of staches such as amylase and amylopection
  • Maltose is the major end-product (20% is glucose)
  • Several isoenzymes present, both glycosylated and non-glycosylated,
  • Molecular weights ranging from 54 to 57 kDa, depending on the degree of glycosylation


Antimicrobial proteins


antimicrobial proteins 1
( Amerongen and Bolscher and Veerman, Page 2, Table 1)


Most of the proteins in saliva exhibit antimicrobial functions and they are thus grouped under one category. Summarized below is the quality, quantity, origin and brief discription of functions of each individual protein component of saliva. For more detailed elaboration on antimicrobial functions of these proteins, please refer to Function of saliva.



composition of saliva - Cariology
composition of saliva - Cariology
composition of saliva - Cariology

composition of saliva - Cariology

composition of saliva - Cariology

composition of saliva - Cariology



  • OTHER ORGANIC COMPOUNDS

    Free amino acids
    • low concentrations (below 0.1mg/100ml).

    Urea
    • about 12-20mg/100ml.
    • Hydrolysed by many bacteria with the release of ammonia, leading to a rise in pH.

    Carbohydrates (Dental Caries. Page 40)
    • Normally only traces of free carbohydrates exist in saliva.
    • Glucose present at concentrations 0.5-1mg/100ml, but may be raised in diabetics.

    Lipids
    • Secreted by major salivary glands.
    • Concentration 80-100mg/l.
    • Comprised of neutral lipids (about 75%), glycolipids (20-30%) and phospholipids (2-5%)
    • Neutral lipids are mainly free fatty acids, cholesterol, monoglycerides, diglycerides and triglycerides.
    • Minor salivary glands contain more lipids (about 400mg/l), glycolipids forming the major group.
    • Majority of lipids in all saliva secretions are associated with proteins

    Inorganic Composition of Saliva

    Hydrogen Ions (Ole Fejerskov & Edwina Kidd, 2008, Pg 34)
    Hydrogen ions in saliva have several sources of origin:
    • secretion via glands as inorganic or organic acids
    • produced by oral microbiota
    • taken into oral cavity in acidic drinks
    • The concentration of H+ in saliva has the greatest influence on the chemical reactions in the oral cavity:
      • equilibria between calcium phosphate in dental hard tissue and surrounding liquid phase
      • solubility, as well as activity, of important salivary enzymes
    The variable sources of hydrogen ions as well as its ability to complex with many substances makes the acid-base balance a complex and mutable process
    For more information refer to Buffering capacity of saliva.

    Calcium Ions
    The concentration of calcium in saliva is influenced by salivary flow rate:
    • calcium is exreted by the glands together with proteins into the lumina of the acini by active transport
    • concentration strongly affected by circadian rhythm
    • either circulating as Ca++ ion or as bound calcium (to phosphate, bicarbonate, small organic ions and macromolecules) depending on salivary pH value
    • special roles of macromolecules such as statherin and histidine in oral calcium homeostasis
    • normal ratio of Ca++ to bound calcium is 1:1, with Ca++ concentration increasing with decreasing pH; most salivary calcium is in ionised form when pH is less than 4
    Inorganic Phosphate
    Inorganic phosphate in saliva found as phosphoric acid H3PO4 and its conjugates: H2PO4-, HPO4,2- and PO4,3-
    Its concentration is affected by salivary flow rate as well as salivary pH, and also (to a smaller extent compared to calcium) circadian rhythm:
    • concenrations of each type dependent on salivary pH; decreased pH leads to decreased concentration of the tertiary ion
    • with increased flow rate decreases total inorganic phosphate concentration
    Depending on pH, inorganic phosphate can be complexed to inorganic ions or proteins. Less than 10% forms dimer form pyrophosphoric acid - an inhibitor of calcium phosphate precipitation; and influences calculus formation.
    Functions of inorganic phosphate include
    • contributes to solubility product of calcium phosphate, which is crucial in maintaining tooth structure
    • important as a buffer (refer to Buffering capacity of saliva.)
    • an essential nutrient for oral microflora for metabolic pathways

    Fluoride
  • The concentration of fluoride in saliva depends on fluoride in the environment, such as fluoridated drinking water and dental products used for caries prophylaxis. Basal concentration of fluoride is less than 1 micromol per litre, but can be much higher in places where levels of fluoride in drinking water are high. Fluoride also enters saliva via facilitated transport over membranes of salivary gland tissue. Clearance rate of inorganic phosphate is dependent on salivary flow rate (Ole Fejerskov & Edwina Kidd, 2008, Pg 38).



    Variations in salivary composition
    (Effects of Diet on Salivary Secretion and Composition, by Colin Dawes)
    Factors that influence salivary composition
    1. Non-Dietary
    SOURCE OF SALIVA. (Page 3)
    -The main electrolytes in human saliva are sodium, potassium, calcium, chloride, bicarbonate, and inorganic phosphate; -There are quantitative differences in the relative proportions of these electrolytes in the major salivary gland secretions. For instance, parotid saliva is relatively low in calcium and high in phosphate as compared with submandibular and sublingual secretions. -Types of proteins and their concentrations are different in the different secretions, eg, most of the salivary amylase is derived from the parotid glands.


    FLOW RATE. (Page 3)

    - Flow rate has a decided influence on salivary composition. In general, as the flow rate is increased slightly above the unstimulated rate, sodium and bicarbonate concentrations and pH increase, whereas potassium, calcium, phosphate, chloride, urea, and protein concentrations decrease. At higher flow rates, sodium, calcium, chloride, bicarbonate, and protein concentrations and pH increase, whereas the phosphate concentration decreases and the potassium concentration shows little further change. The effect of flow rate on the composition of saliva has been studied in greatest detail with parotid saliva, but limited studies suggest that these findings also apply to the other major salivary gland secretions. Thus, flow rate is a critical variable in any study of the effects of diet on saliva.


    DURATION OF STIMULATION (Page 3) .

    -When the flow rate of stimulated parotid saliva is maintained constant for several minutes, the composition of the saliva changes considerably with duration of stimulation. -Although almost all components of saliva show a change in concentration during the first one or two minutes of stimulation, the concentrations of total protein, calcium, bicarbonate, and chloride and the pH, are particularly affected and may not achieve steady state levels even after 15 minutes of stimulation at constant flow rate. -Total protein, calcium, and bicarbonate concentrations and pH increase with duration of stimulation, whereas the chloride concentration decreases in proportion to the rise in bicarbonate concentration. -Hence, in any study of the effect of diet on the composition of stimulated saliva, the duration of stimulation must be standardized.
    NATURE OF THE STIMULUS. (Page 3)

    -Recent studies have shown that the nature of the stimulus decidedly influenced the protein concentration in parotid saliva in some subjects. -14 Similar results have been reported by Caldwell and Pigman15 for submandibular saliva. -Other studies have shown that the salivary glands respond differently to electric, pharmacologic, and gustatory stimuli.
    PLASMA COMPOSITION. (Page 3)
    -For many components of saliva collected under standardized conditions, a positive correlation exists between the concentrations in plasma and saliva for calcium, urea, and potassium, whereas the phosphate concentration in human saliva is relatively independent of the concentration in plasma. -At a constant flow rate, the bicarbonate concentration in human saliva has been found to be directly related to plasma pCO2.
    TIME OF DAY. (Page 4)

    -The time of day at which saliva samples are collected can have a decided influence on both salivary flow rate and composition. -Thus in any study of the effect of diet on the composition of saliva it is always necessary to collect samples at one particular time of day.
    2. Dietary (Page 4-8)
    local reflex effects and the systemic effects on salivary flow rate and composition.
    -- local reflex effect: -> Foods that require vigorous mastication or that are highly flavored will cause a decided increase in salivary flow rate. The increase in flow rate alone will affect salivary composition.
    --systemic effects of diet: -> consumption of a particular diet causes either an immediate specific effect on salivary composition that is not attributable to a change in flow rate, or a long term change in salivary flow rate, or composition that may take some time to develop.


    FLOW RATE.
    - size and activity of the salivary glands is related to the degree of functional stimulation. -experiment showed that moderate reductions in food intake caused hypertrophy of the rat parotid gland, which was attributable to increased storage of secretory proteins. It was also found that salivary glands increased in size when the diet of rats was supplemented by non-nutritive cellulose. The feeding of pancreatin to rats is known to cause salivary gland enlargement which may be associated with an increased flow rate.


    FLUORIDE.

    -The concentration of fluoride in saliva is only about 0.01 ppm. -Although it has been shown that oral ingestion of 1 to 10 mg of fluoride can cause an increase in salivary fluoride levels over about the following two hours, there appears to be little data on the relation of salivary fluoride concentration to the level of dietary fluoride intake at more normal levels of intake.


    UREA.-
    -plaque microorganisms readily produce ammonia from urea to cause a rise in plaque pH. -the pH of plaque from fasting subjects is always higher than that of the adjacent unstimulated saliva, which suggests that in the absence of an external source of carbohydrate, the physiologic levels of urea in saliva are sufficient to allow the plaque microorganisms to maintain a positive pH differential between plaque and saliva. -the level of urea in saliva is directly related to the level in blood, and that the level of urea in blood is directly related to the level of protein intake. Thus diets high in protein might be expected to maintain relatively high salivary urea levels. Most of the experiments on the effect of diet on saliva have been confounded because of inadequacies in experimental design such as too few subjects, lack of control subjects, and inadequate standardization of the saliva collection technique.






    Reference:
    1. Young, J. A., & Schneyer, C. A. (1981). Composition of saliva in mammalia. Aust J Exp Biol Med Sci, 59(1), 1-53.
    2. Van Nieuw Amerongen, A., Bolscher, J. G., & Veerman, E. C. (2004). Salivary proteins: protective and diagnostic value in cariology? Caries Res, 38(3), 247-253.
    3. Virella, G., & Goudswaard, J. (1978). Measurement of salivary lysozyme. J Dent Res, 57(2), 326-328.
    4. Fejerskov,O., & Kidd, E. (2008). Dental Caries: The Disease and Its Clinical Management (2nd Ed.). Oxford: Blackwell Munksgaard Ltd
    5. Dawes, C. (1970). Effects of diet on salivary secretion and composition. J Dent Res, 49(6), 1263-1273
    6. Lenander-Lumikari, M., & Loimaranta, V. (2000). Saliva and dental caries. Adv Dent Res, 14, 40-47.
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