Hydrolysis of Diphosphate Ion to Orthophosphate Ion and Formation of Hydroxyapatite in the Aqueous Phase

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The formation of mixed precipitates of calcium diphosphate (Ca_2PPi) and calcium orthophosphate (hydroxyapatite, HAP, mainly) from alkaline solutions of CaCl_2,K_2HPO_4,K_4P_2O_7 and NH_4OH was studied. The chemical composition of the precipitate determined by direct chemical analysis of the precipitate was different from that obtained by calculation from the filtrate composition after incubation (72 h, 37℃). It was concluded that, although diphosphate (PPi) is usually hydrolyzed only in acidic solution, PPi was hydrolyzed to orthophosphate (Pi) on the surface of the precipitate in alkaline solution during the incubation, followed by further precipitate formation (CaPi) with excess Ca^<2+> in the solution. The extent of hydrolysis of PPi depended on the amounts of Ca^<2+> added and precipitate formed. It was confirmed that PPi is hydrolyzed in an alkaline solution of K_4P_2O_7 depending on the amount of synthesized HAP or Ca_2PPi, but is not hydrolyzed in the absence of any solid. HAP seems to catalyze the hydrolysis reaction of PPi more effectively than Ca_2PPi. When the amount of Ca^<2+> was less than that of total phosphate, another counter cation in the precipitate was K^+. X-Ray powder diffractometry of the precipitates revealed diffraction peaks of HAP, but those of Ca_2PPi were not found. The precipitates formed in solutions containing enough Ca^<2+> were crystalline HAP, although the initial solution did not contain Pi but just PPi. The significance of this hydrolysis is discussed in relation to the formation of biological hard tissues.
公益社団法人日本薬学会の論文
1982-11-25

Hydrolysis of Diphosphate Ion to Orthophosphate Ion and Formation of Hydroxyapatite in the Aqueous Phase

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