Formation of hydroxyapatite in serum

The kinetics of the formation of calcium-deficient and carbonated hydroxyapatite at 38°C were investigated by isothermal calorimetry. Hydroxyapatite (HAp) was formed by reaction of the particulate calcium phosphates CaHPO₄ and Ca₄ (PO₄)₂O. Compared with its rate of formation in DI water, the formation of calcium-deficient HAp is significantly inhibited in serum. When serum is diluted with DI water, the extent of inhibition varies with the extent of dilution. When collagen or HAp seeds are present the extent of inhibition in serum is reduced. The kinetics of HAp formation were also examined in various concentrations of albumin to establish the extent to which inhibition is associated with the presence of this plasma protein. While HAp formation is inhibited in albumin, the extent of inhibition is not as great as in serum. The formation of carbonated HAp is also inhibited in serum and albumin. However, the extent of inhibition is significantly reduced. The variations in sodium and carbonate in solution during HAp formation indicate that these species are incorporated at different rates, with carbonate incorporation being more rapid. Elevated sodium concentrations in solution result in solution pH values near 12. The reduction in the inhibition of HAp formation may be associated with the reaction to carbonated HAp occurring at elevated pH or with the influence of pH on protein adsorption.