Metastable Equilibrium Solubility Behavior of Bone Mineral |
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| Authors: | A A Baig J L Fox Z Wang W I Higuchi S C Miller A M Barry M Otsuka |
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| Institution: | (1) Department of Pharmaceutics and Pharmaceutical Chemistry, 301 Skaggs Hall, University of Utah, Salt Lake City, Utah 84112, USA, US;(2) Department of Radiobiology, University of Utah, Salt Lake City, Utah 84112, USA, US;(3) Kobe Pharmaceutical University, Kobe, Japan, JP |
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| Abstract: | Previous studies have shown that carbonated apatites with a range of carbonate contents and crystallinities exhibit the phenomenon
of metastable equilibrium solubility (MES) distributions. The purpose of the present study was to investigate the solubility
behavior of bone mineral using the concepts of MES and MES distributions and, together with crystallinity and chemical composition
data, examine the similarity of bone mineral to carbonated apatite (CAP). Bone samples were harvested from 1-, 5-, and 8-month-old
rats. The organic components of the bone samples were removed by hydrazine deproteination. Carbonated apatite was synthesized
by the hydrolysis of dicalcium phosphate dihydrate (DCPD) in a NaHCO3-containing media at 50°C. The MES distributions of bone mineral and CAP were determined by equilibrating predetermined amounts
of CAP or bone mineral in a series of 0.1 M acetate buffers containing calculated levels of calcium and phosphate and maintained
at essentially constant pHs of 5.0, 5.3, 5.7, and 6.5. From the compositions of the equilibrating buffer solutions, ion activity
products based upon the stoichiometries of octacalcium phosphate, hydroxyapatite, and carbonated apatite were calculated in
an attempt to determine the function governing the dissolution of CAP and bone mineral. The results of this study demonstrated
that the MES distribution phenomenon appeared to hold for bone mineral and that the changes in crystallinity of bone mineral
with age correlated well with changes in the MES values. A CAP sample was prepared that was found to be an excellent synthetic
prototype closely mimicking the physicochemical behavior of bone mineral from an 8-month-old rat. Another finding of this
study was that the ion activity product function based upon the hydroxyapatite stoichiometry well described the MES results
obtained with both CAP and bone mineral. The interpretation that a surface complex with hydroxyapatite stoichiometry governs
the solubility behavior of bone mineral is, therefore, consistent with the experimental data. Other calcium phosphate stoichiometries
for the surface complex showed systematic variations in the MES profiles when the pH of the equilibrating solution was varied.
Received: 30 October 1997 / Accepted: 1 October 1998 |
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| Keywords: | : Carbonated apatites — Bone mineral — Solubility — Crystallinity — Calcium phosphates |
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