Isotopic Exchanges and the Existence of Tricalcium Phosphate in Bone

Abstract

THE problem of the physico-chemical constitution of the skeleton has received considerable attention in recent years^1–3. Tracer experiments indicate that phosphorus-32 and calcium-45 appear promptly and in considerable proportions in the bony tissues, whence they are removed slowly^4–6. These observations have led to an assumption that deposition of phosphorus-32 and calcium-45 in bony tissues is an exchange between bone and blood inorganic phosphate or calcium. Phosphorus-32 and calcium-45 have been used to study phosphate or calcium exchange between powdered bone and aqueous disodium hydrogen phosphate and calcium chloride solutions^7–9. Different authors^8,10,11 have concluded that the principal bone component is not tricalcium phosphate but rather hydroxylapatite to which phosphoric ions are adsorbed in such a quantity that the ratio of calcium to phosphorus is equal to 1 .94, which is the characteristic value for tricalcium phosphate. On the other hand, the findings of some workers^1,2 support the idea of the existence of tricalcium phosphate in bony tissues. The experiments reported here are an attempt to study the exchange mechanism of both phosphate and calcium between pure tricalcium phosphate and radioactive phosphate or calcium solutions. These experiments were also performed in order to show to what extent the isotopic exchange method can help to elucidate a particular problem of structural chemistry. (Calcium-45 was prepared by neutron bombardment in the pile at the Clinton Laboratories, Oak Ridge, Terin., and phosphorus-32 at the pile at Harwell, Berks.)