Cheng P T
Calcif Tissue Int. 1985 Jan;37(1):91-4. doi: 10.1007/BF02557685.
Octacalcium phosphate (OCP, Ca8H2(PO4)6 . 5H2O) has been found in kidney stones and dental calculi and proposed by Brown et al. [1] to be a precursor of hydroxyapatite in bone and teeth formation. As saliva and urine often have acidic pH which favors OCP formation, the question remains then whether OCP can form in the more basic extracellular fluid in the bone milieu. This paper shows that calcium phosphate crystal phases obtained from neutral and pH 7.4 solution mixtures containing [CaCl2] = 0.1-10.0 mM and [Na2HPO4] = 0.1-90.0 mM with 100-300 mosM at 37 degrees C include brushite (CaHPO4 . 2H2O), OCP, and hydroxyapatite (Ca5(OH)(PO4)3). In some solutions OCP transforms into hydroxyapatite after 1 or more days, but brushite has not been observed to transform into OCP. Assuming the extracellular bone fluid has [Ca2+] = 1.0 mM, 300 mosM, and pH 7.4, this work suggests that hydroxyapatite crystals would only form when the ambient [Pi] greater than 5 mM, and that octacalcium phosphate may be a precursor.
磷酸八钙（OCP，Ca8H2(PO4)6·5H2O）已在肾结石和牙结石中被发现，并且布朗等人[1]提出它是骨骼和牙齿形成过程中羟基磷灰石的前体。由于唾液和尿液的pH值通常呈酸性，有利于磷酸八钙的形成，那么问题仍然存在，即在骨环境中碱性更强的细胞外液中磷酸八钙是否能够形成。本文表明，在37℃下，从含有[CaCl2]=0.1 - 10.0 mM、[Na2HPO4]=0.1 - 90.0 mM且渗透压为100 - 300 mosM的中性和pH 7.4溶液混合物中获得的磷酸钙晶相包括透钙磷石（CaHPO4·2H2O）、磷酸八钙和羟基磷灰石（Ca5(OH)(PO4)3）。在某些溶液中，磷酸八钙在1天或更长时间后会转化为羟基磷灰石，但未观察到透钙磷石转化为磷酸八钙。假设细胞外骨液的[Ca2+]=1.0 mM、渗透压为300 mosM且pH为7.4，这项研究表明，只有当环境中的[Pi]大于5 mM时才会形成羟基磷灰石晶体，并且磷酸八钙可能是前体。
