Formation of octacalcium phosphate and subsequent transformation to hydroxyapatite at low supersaturation: A model for cartilage calcification

Abstract

By means of X-ray powder diffraction (XRD), octacalcium phosphate (OCP, Ca₈H₂(PO₄)₆·5H₂O, Ca/P=1.33) has been shown to be a metastable precursor of hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂, Ca/P=1.67) duringde novo HA formation in aqueous solutions containing [CaCl₂]=1.0–2.0 mM and [Na₂HPO₄]≤3 mM, kept at 37°C, 300 mosM and stirred gently at 150 rpm. At the precipitation boundary with initial pH=7.4, OCP is stable for at least 24 hours when [Ca]=1.0 mM, and is less stable when [Ca]=2.0 mM, transforming into a mixture of OCP and HA within 24 hours. Almost complete transformation to HA within 24 hours takes place with high [Ca] and high [Pi]. Statistical analysis of the pH 7.4 precipitation boundary data supports the XRD findings: although [Ca][Pi] values vary significantly (P2 for [Ca]=1.0 mM and 2.00±0.10 mM² for [Ca]=2.0 mM), [Ca]^1.33[Pi] values do not vary with [Ca] suggesting that the initial precipitation process is 6(1.33 Ca+Pi)→OCP. With initial pH=7.6, a different precipitation boundary with lower [Ca][Pi] values has been determined. These findings strongly support the fact that rat epiphyseal cartilage fluid which has [Ca_UF][Pi_UF]=2.6 mM² (UF=ultrafiltrate) and pH 7.6 [2] should be able to supportde novo calcification.