Formation and transformation of octacalcium phosphate, OCP: a preliminary report

Octacalcium phosphate, Ca8H2(PO4)6 X 5H2O (OCP), occurs in pathological calcifications frequently as one of the crystalline components of human dental calculi. OCP has also been presumed a necessary precursor of biological apatites in both normal (enamel, dentine, cementum, bones) and pathological (e.g., phosphatic renal stones) calcifications. This study investigated the optimum conditions for the direct in vitro formation of OCP in solutions and in gel systems and the factors affecting its formation and transformation or hydrolysis to apatite. It was observed in both solution and gel systems that the formation of OCP was dependent on definite conditions of pH and temperature (the higher the temperature the lower the pH at which OCP forms, and vice versa), and on the presence of other ions. The presence of pyrophosphate inhibited OCP formation favoring instead the formation of amorphous calcium phosphate while the presence of citrate or carbonate favored the formation of "apatitic" calcium phosphate at the expense of OCP. The presence of oxalate ions caused the formation of mixed OCP/calcium oxalate phases. Hydrolysis of OCP to apatite was suppressed in the presence of magnesium or pyrophosphate, and promoted in the presence of carbonate or fluoride ions. In the presence of oxalate ions, partial hydrolysis of OCP to calcium oxalate and not to apatite was observed. Results from this study give insights on the factors (e.g., pH, temperature, presence of ions besides calcium and phosphate) which influence the formation of OCP and its transformation to apatite and/or calcium oxalate. Ions which demonstrated significant effect on the formation and/or transformation of OCP were magnesium, pyrophosphate, carbonate, citrate, fluoride and oxalate.