In vitro Experiments on Exchange of Phosphate by Enamel and Dentin

Abstract

Krogh, Hoist, and Hevesy, 1 and Manly and Bale 2 demonstrated the presence of radioactive phosphorus in the whole teeth of animals receiving a parenteral administration of compounds of this isotope. Hevesy and Armstrong 3 in an investigation in which the enamel and dentin of cats' teeth were separately studied found, using radioactive phosphorus as an indicator, the rate of exchange of phosphate by the enamel of erupted mature teeth to be about one-tenth that of the dentin. The rate of exchange was such as to make highly improbable an ability of enamel of mature teeth to undergo significant changes of composition as a result of nutritional alterations.

The experiments herewith reported were carried out by agitating 0.12-0.15 mg of the finely pulverized specimens in 10 cc volumetric flasks completely rilled with the solution of labeled phosphate. The active material was supplied by the Radiation Laboratory of the Department of Physics of this University. The specimens after the stated time of contact with the active solution were recovered and washed ten times with water by centrifugation.

These results obtained in vitro, especially in the case of dentin, show a surprisingly rapid rate of exchange of phosphate. However, the relative rates of exchange by enamel and dentin are very similar to those observed in vivo by Hevesy and Armstrong. 3

Since dentin contains∗ 22.2% protein and enamel less than 1% of protein, the larger amount of labeled phosphate acquired by dentin from solutions at pH 7.5 might have been due, if the protein of dentin has an isoelectric point somewhat above pH 7.5, to combination of phosphate anion with dentin protein. The acid combining power of dentin protein in relation to hydrogen ion concentration is not known but it appears very unlikely that this protein could combine with anions in solutions of pH 13.