Effect of pyrophosphate on the exchangeable calcium pool of hydroxyapatite crystals

Abstract

Pyrophosphate added to an equilibrated suspension of hydroxyapatite crystals caused a loss of calcium from the bulk solution without a loss of exchangeable calcium from the system. The evidence suggests that this loss was due to an entry of calcium into the hydration layer around the crystals. This redistribution of calcium was probably the result of the binding of pyrophosphate to the crystal surface. The changes showed that the binding of 2 μmoles of pyrophosphate was associated with the uptake of 1 μmole of calcium into the hydration layer from the bulk solution and a loss of 1 μmole of phosphate from the hydration layer into the bulk solution. The change in the distribution of exchangeable calcium after the addition of pyrophosphate was largely reversed by inorganic pyrophosphatase. These findings suggest that changes in the amount of pyrophosphate in bone in vivo might lead to changes in the distribution of the exchangeable calcium and thus might form a basis for a simple system for the minimization of fluctuations in the plasma calcium.