Calcium phosphate phase transformations in serum

Summary

A better knowledge of the pathological calcification mechanisms should provide a rational basis for their control. In the present study, dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) was used as a source of calcium and phosphate ions to investigate the mechanism of formation of more basic and more insoluble calcium phosphates in ultrafiltered serum (u.f.s.). DCPD crystals were suspended in u.f.s. at 37°C by constant stirring; samples were removed periodically for calcium and phosphate analysis and pH measurement. Occasionally, samples of solids were removed for X-ray diffraction. The experiments were carried out both with and without a 5.5% CO₂ atmosphere. After initially becoming saturated with DCPD, the u.f.s. composition changed and became saturated with respect to octacalcium phosphate (OCP, Ca₈H₂ (PO₄)₆·5H₂O). At this point OCP crystals were detected in the solid phase by X-ray diffraction. Further stirring changed the composition so that it became undersaturated with both DCPD and OCP and shifted toward, but did not reach, a value so low as to be saturated with hydroxyapatite (OHAp, (Ca₅(PO₄)₃OH). The presence of CO₂ in the atmosphere slowed down, but did not prevent, the above sequence of events. The above results strongly suggest that calcifications, beneficial and pathological, that take place in serum may involve OCP as a precursor, which hydrolyzesin situ to a more basic apatitic product. The results also indicate that direct formation of OHAp in u.f.s. is a very slow process and may occur only rarely. The process appears to be similar in whole serum.