The binding of pyrophosphate and two diphosphonates by hydroxyapatite crystals

The binding on hydroxyapatite has been studied of inorganic pyrophosphate (PP_j) and two diphosphonates, disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) and disodium dichloromethylene diphosphonate (Cl₂MDP). The binding is greatest for PP_i, less for EHDP and least for Cl₂MDP. The binding of all three is accompanied by a release of orthophosphate into the solution and an uptake of calcium by the crystals. A competition exists between EHDP and PP_i, EHDP being more potent in displacing PP_i than the reverse. An analysis with Scatchard plots suggests that the binding is not restricted to one class of binding sites with equal affinity and that the affinity of the first site is in the order EHDP>PP_i>Cl₂MDP. The results correlate well with previous data on the effect of these compounds on apatite crystal formation and dissolutionin vitro and on bone formation and resorptionin vivo To Prof. F. Leuthardt to his 70th birthday.