Transphosphorylation from nucleoside Di- and triphosphates by apatite crystals.

In a previous study (l), the authors showed that mineralized bone from a wide variety of sources possessed adenosine triphosphatase-like activity at neutral pH which was stable to heating at 100” for 30 minutes. A part of the activity was associated with the collagen matrix, since residual nucleotide phosphatase was present in bone particles which had been heated prior to complete demineralization, as well as in native unmineralized collagenous tissues and in highly purified reconstituted collagens. However, a significant part was unaccounted for unless it was assumed that the mineral portion of the bone also possessed this catalytic activity. Since it was not possible to rule out contamination of the collagen preparations by adsorption or coprecipitation of cellular ATPases, we have examined the acellular bone (operculum) of codfish. Fresh, sterile, and fully mineralized codfish, operculi were also shown to possess ATPase-like activity which was resistant to heating at 100” for 30 minutes (1). The characteristics of the reaction were atypical compared to other ATPases since it was not possible to inhibit the reaction by reagents such as fluoride, p-chloromercuribenzoate, and pentachlorophenol. A possible explanation for these findings was that apatite crystals per se were participating in the reaction. This reasoning was based in part on the reports of Lowenstein, which described a nonenzymatic transfer of the terminal phosphorus of ATP to orthophosphate to form inorganic pyrophosphate in the presence of bivalent metal ions at alkaline pH (2, 3). This reasoning was confirmed when residual ATPase-like activity was observed in the apatite crystals of bone particles which had been ashed at 600” for 12 to 18 hours. In the present study, synthetic apatite crystals were also found to form inorganic pyrophosphate by the transfer of the terminal phosphorus of nucleoside triphosphates and diphosphates to crystal surface phosphate groups at physiological temperature and pH. The reaction was similar to that described by Lowenstein (2, 3), but differed in several important respects. Furthermore, the transphosphorylation proceeded more readily with those inorganic crystals possessing the general