Recent studies of the mineral phase in bone and its possible linkage to the organic matrix by protein-bound phosphate bonds

Abstract

The most widely accepted hypothesis to account for maturational changes in the X-ray diffraction characteristics of bone mineral has been the `amorphous calcium phosphate theory', which postulates that an initial amorphous calcium phosphate solid phase is deposited that gradually converts to poorly crystalline hydroxyapatite. Our studies of bone mineral of different ages by X-ray radial distribution function analysis and $^{31}$P n.m.r. have conclusively demonstrated that a solid phase of amorphous calcium phosphate does not exist in bone in any significant amount. $^{31}$P n.m.r. studies have detected the presence of acid phosphate groups in a brushite-like configuration. Phosphoproteins containing O-phosphoserine and O-phosphothreonine have been isolated from bone matrix and characterized. Tissue and cell culture have established that they are synthesized in bone, most likely by the osteoblasts. Physicochemical and pathophysiological studies support the thesis that the mineral and organic phases of bone and other vertebrate mineralized tissues are linked by the phosphomonester bonds of O-phosphoserine and O-phosphothreonine, which are constituents of both the structural organic matrix and the inorganic calcium phosphate crystals.