The binding of pyrophosphate and two diphosphonates by hydroxyapatite crystals

Abstract

The binding on hydroxyapatite has been studied of inorganic pyrophosphate (PPj) and two diphosphonates, disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) and disodium dichloromethylene diphosphonate (Cl2MDP). The binding is greatest for PPi, 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 PPi, EHDP being more potent in displacing PPi 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>PPi>Cl2MDP. The results correlate well with previous data on the effect of these compounds on apatite crystal formation and dissolution in vitro and on bone formation and resorption in vivo © 1973 Springer-Verlag.