Salivary Buffering, Bicarbonate & pH
   
Saliva and buffering

Saliva contains protein and phosphate both of which are capable of buffering the pH. Protein, however, is not a very effective buffer because it does not have a preponderance of ionised groups, nearly all of the charged groups of amino acids are involved in the peptide bonds. Phosphate, however, is a good buffer and phosphate salts are widely used in science to prepare buffer solution. However, although we think of saliva as containing a lot of phosphate it just does not contain enough to allow it to be really effective in vivo.

This leaves us with bicarbonate which is often referred to as the major buffer of saliva. Although it is true that bicarbonate in solution can act as a pH buffer this is not the whole story and in an open system such as the mouth, bicarbonate acts mainly to neutralise acid.

 
Bicarbonate does not buffer

The ability to deal with the acid produced within dental plaque is one of the major mechanisms by which saliva protects teeth from decay. It is usual to refer to this ability as the "buffering" power of saliva but actually this is not correct

What is buffering?

buffer is an ionic compound which resists change in the concentration of an ion in solution. Usually we use the word to refer to the ability of a compound to resist change in pH but, actually, it could be change in calcium ions (Calcium buffer). They work by temporarily removing the ion of interest from solution by binding it. So long as it is bound and not free in solution it is not "active" in the chemical sense. When conditions change, buffers can release their bound ions which then become active again.

pH buffers resist change in the acidity or alkalinity of a solution by mopping up or releasing hydrogen ions to counter the effect of added acid or base but they only work best over a fairly narrow pH range. The important point is that the hydrogen ions are not permanently removed from the system.

Carbonic acid in solution

In aqueous solution, carbonic acid dissociates into a bicarbonate ion and a proton or into carbon dioxide and water depending on the conditions such as pH and the relative concentrations of each of the products ie carbon dioxide and bicarbonate (we can forget about water here and the concentration of hydrogen is equivalent to the pH, of course).

In a closed system an equilibrium would be set up.

However, in an open system any carbon dioxide formed stands a chance of being lost as a gas and this is what happens in an open beaker left on a bench. If the concentration of carbon dioxide above the liquid was kept at zero, eventually all the carbonic acid and bicarbonate would be lost from the solution but this would take quite a long time.

 
 
What happens in the mouth?

In the mouth the concentration of carbonic acid stays remarkably constant at about 1.3mMol/L. Two things do change, however, these are the pH and the bicarbonate concentration. Both are very important and central to how saliva protects teeth.

 

1. A drop in pH. When acid is produced within dental plaque the increase in hydrogen ion concentration will drive the dissociation equation to the left, producing more carbonic acid which, in turn, produces more carbon dioxide and water. Since the mouth is an open system the carbon dioxide is lost to the atmosphere. The acid has, thus, been removed from the system, it has been neutralised, not buffered.

This will only happen if there is enough bicarbonate ion present to interact with the hydrogen ions which brings us neatly onto the next important event.

2. A rise in bicarbonate concentration. The concentration of bicarbonate in saliva is linked to the flow rate. As the rate of saliva production increases the more bicarbonate ion is produced as a by-product of cell metabolism. So stimulated saliva contains more bicarbonate than resting saliva which is convenient because it is during eating when saliva flow is raised that plaque acid is produced in highest quantities. This ensures there is enough bicarbonate present to mop up surplus hydrogen ions. However, there is a little more to it that this because the concentration of bicarbonate is largely responsible for determining the actual pH of saliva in the first place.

 
 
In saliva the reaction is driven by Carbonic Anhydrase

Saliva is unique in that it contains a form of carbonic anhydrase called Carbonic Anhydrase VI which is secreted by serous acinar cells of the parotid and submandibular glands. No other secreted fluid contains such an enzyme.

Carbonic anhydrase drives the reaction converting carbonic acid to carbon dioxide and water (see above) which effectively mops up available protons. Some very recent work has shown that this carbonic anhydrase forms part of the tooth pellicle where it is available to convert any protons produced by overlying dental plaque to carbon dioxide and water so long as bicarbonate is available.

In other words the enzyme is situated right beneath dental plaque where it is likely to do most good.

Importance of saliva pH

The events so far described are happening within dental plaque and the bicarbonate must diffuse into plaque from saliva in order to be effective at neutralising any acid. However, teeth are bathed by saliva and if the pH was not kept sufficiently high they would run the risk of erosion. In fact, the normal, resting, pH of the mouth does not fall much below about pH 6.3 and the reason for this is the bicarbonate present.

The following uses a simple equation relating pH and ion concentrations with the dissociation constant pK of carbonic acid to demonstrate this and help you to better understand the connection.

 
 
Bicarbonate determines saliva pH

he relationship between pH, pK and the ratio of bicarbonate and carbonic acid present in solution is known as the Henderson-Hasselbach equation and is shown on the right with bicarbonate and carbonic acid concentrations substituted for the anion and acid concentrations respectively. The derivation of this equation is beyond the scope of this tutorial but will be found in most elementary chemistry and biochemistry texts.

The important reaction is the dissociation of carbonic acid into bicarbonate and a proton. The pK of this reaction is 6.1.

 

 

Henderson-Hasselbach Equation

   

Substituting this value for pK into the Henderson-Hasselbach equation we get...............

 
 

Conventionally, anything inside square brackets is a concentration. They must always be in the same units.

Carbonic acid in saliva

The carbonic acid concentration of saliva is remarkably constant at about 1.3mMol/L but the bicarbonate concentration varies with the flow rate. This is possible because of bicarbonate pumps situated in the secretory units of salivary glands. For the purposes of this demonstration we will consider 3 values of bicarbonate concentration

2mMol/L (Low flow)
30mMol/L (Intermediate flow)
60mMol/L (High flow)

   

Substituting the first of these values into the Henderson-Hasselbach equation together with the value for the carbonic acid concentration (1.3 mMol/L) we get.......

This value of pH 6.29 is close to the measured pH of saliva taken at the resting flow rate.

 

and 2 divided by 1.3 is 1.54, therefore

the log to base 10 of 1.54 is 0.187, so

therefore the pH of the solution is

   

Substituting the other 2 values of bicarbonate concentration into the equation we get.

 

at 30mMol/L and

 

at 60mMol/L

 

If we plot a range of values of bicarbonate concentrations against pH we discover that the greatest effect on pH is found in the region of 1-15 mMol/L. The graph opposite shows the effect, note that bicarbonate values greater than about 60 mMol/L are at the upper end of the physiological range. Plotting higher values would have little meaning since they would not normally be attained.

Note the similarity of this graph to a graph showing the effect of flow rate on saliva pH.

     

 

 

 

 

 

 

 

 

 

SUMMARY

1.

The pH of the mouth must be maintained near neutral for normal tooth maintenance.

2.

Oral pH is buffered to a small extent by saliva proteins and phosphate.The major influence on saliva pH is bicarbonate ion which is a by-product of cell metabolism.

3.

Bicarbonate concentration increases in saliva as the flow rate rises and is due to the increased metabolic rate. This, in turn, raises the pH (more alkaline) of saliva.

4.

Bicarbonate ions diffuse into dental plaque and neutralise acid produced by plaque bacteria when carbohydrate is fermented.

5.

This reaction is driven by a unique type of carbonic anhydrase which is secreted into saliva by serous acinar cells of the parotid and submandibular glands.

6.

Bicarbonate ions maintain the pH of saliva above 6.3.

   

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