Calcium phosphate saturation levels in ultrafiltered serum

Summary

Calcifications occurring in arteriosclerotic plaque and other pathological deposits are important health concerns, and the nature of these deposits and their mechanisms of formation warrant investigation. Crystals of the relevant calcium phosphates were equilibrated with the undiluted ultrafiltered human serum (u.f.s.) at 37°C by constant stirring and periodically removing samples for calcium and phosphate analysis and for pH measurement. The solubility measurements were carried out both with and without a 5.5% CO₂ atmosphere, the physiological partial pressure of CO₂. The apparent ion activity products of well-crystallized dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), and hydroxyapatite (OHAp) equilibrated, in u.f.s. were calculated from the calcium and phosphate concentrations and pH in each case for comparison with their known, solubility products. In this way the well-crystallized calcium phosphates serve as fiducial solubility standards, thereby minimizing errors due to complexing of calcium and phosphate ions by u.f.s. constituents. Under 5.5%, CO₂ native u.f.s. was found to be substantially undersaturated with respect to DCPD, slightly supersaturated with respect to OCP, and highly supersaturated with respect to OHAp. The ion activity product of DCPD in DCPD-saturated u.f.s. was 2.4×10⁻⁷, and the ion activity product of OCP in OCP-saturated u.f.s. was 4×10⁻⁴⁹, slightly above their solubility products (K_sp(DCPD)=2.3×10⁻⁷, K_sp(OCP)=2.5×10⁻⁴⁹). The ion activity products of DCPD and OCP in u.f.s. under CO₂ indicate that the concentrations of calcium and phosphate complexing agents (except bicarbonate) are quite low. The u.f.s. remained supersaturated with respect to OHAp even after 2 months of equilibration. This is attributed to the presence of crystal growth inhibitors in u.f.s.