Saturation of human salivary secretions with respect to calcite and inhibition of calcium carbonate precipitation by salivary constituents.

The state of saturation of human salivary secretions with respect to calcite has been investigated. This property cannot be calculated exactly because of uncertainties in the values of the solubility product constant of calcite, the dissociation constants of carbonic acid, and PCO2 values of saliva. Minimum and maximum limits for this saturation, however, can be established using appropriate values for the constants and salivary PCO2. Values that give the minimum degree of saturation show that 8 of the 70 samples of human saliva investigated would be supersaturated with respect to calcite, while 64 of the 70 samples appeared to be supersaturated when values giving the maximum degree of saturation were used. In the latter case, the ratio of ionic activity products to solubility product was above 10 for several samples and over 18 for the most supersaturated sample. Since these results show that supersaturation of saliva with respect to calcite may be a common condition, human salivary secretions were investigated for the presence of inhibitors of calcite precipitation. Inorganic phosphate and the acidic proline-rich proteins, known to be inhibitors of calcite precipitation, and human salivary statherin, now shown to have a similar activity, are present in saliva at concentrations considerably higher than those required to inhibit calcite precipitation under salivary conditions. Quantitatively, phosphate is by far the most important inhibitor of calcite precipitation present in saliva, suggesting that inhibition of calcite precipitation by the macromolecules may be of secondary significance. It seems more likely that the function of these molecules is to inhibit precipitation of calcium phosphate salts, as previously proposed. These different inhibitory activities, however, are likely to be factors in the differences in composition of oral and dental calculi in different species, and may need to be considered in the formation of calcite stones in the pancreas.