Radioautographic visualization and biochemical identification of O-phosphoserine- and O-phosphothreonine-containing phosphoproteins in mineralizing embryonic chick bone

Disordered Conformation with Low Pii Helix in Phosphoproteins Orchestrates Biomimetic Apatite Formation

The interplay between noncollagenous proteins and biomineralization is widely accepted, yet the contribution of their secondary structure in mineral formation remains to be clarified. This study demonstrates a role for phosvitin, an intrinsically disordered phosphoprotein, in chick embryo skeletal development, and using circular dichroism and matrix least-squares Henderson-Hasselbalch global fitting, unravels three distinct pH-dependent secondary structures in phosvitin. By sequestering phosvitin on a biomimetic 3D insoluble cationic framework at defined pHs, access is gained to phosvitin in various conformational states. Induction of biomimetic mineralization at near physiological conditions reveals that a disordered secondary structure with a low content of P_II helix is remarkably efficient at promoting calcium adsorption, and results in the formation of biomimetic hydroxyapatite through an amorphous calcium phosphate precursor. By extending this finding to phosphorylated full-length human recombinant dentin matrix protein-1 (17-513 AA), this bioinspired approach provides compelling evidence for the role of a disordered secondary structure in phosphoproteins in bone-like apatite formation.

c-Myc and Mxi1 immunoreactivities in the calcifying areas of the epiphyseal-plate cartilage matrix of growing rats

We looked for the protooncogene protein, c-Myc, its dimerization partner, Max, and the repressors of its transactivation activity, Mad1 and Mxi1, in the epiphyseal-plate cartilage matrix of growing rats by immunocytochemistry in the electron microscope. c-Myc and Mxi1 immunoreactivities were found in the calcifying areas of the cartilage matrix only. There was no immunolabeling in response to anti-Max or anti-Mad1 antibodies. Mxi1 immunoreactivity was mainly in the early calcifying areas, in the calcification front and ahead of it, whereas c-Myc immunoreactivity was essentially in the incompletely calcified regions of the matrix. The two immunolabelings occurred mainly over the large type II collagen fibrils of the cartilage matrix and over the thin filaments connecting them. c-Myc and Mxi1 immunoreactivities were rarely found along the dark cristallites. There was no immunolabeling associated with the matrix vesicles, or in their immediate surroundings. The data suggest that the protooncogene proteins, c-Myc and Mxi1, could be implicated in the calcification involving type II collagen fibrils of the epiphyseal-plate cartilage. The absence of Max immunoreactivity from the calcifying cartilage matrix raises the question of whether there are other c-Myc- and Mxi1-dimerization partners.

Identification of the phosphorylated sites of metabolically 32P-labeled osteopontin from cultured chicken osteoblasts

Osteopontin (OPN) is one of the major secretory phosphoproteins in both calcifying and non-calcifying tissues. Evidence has accumulated for the biological importance of the phosphoproteins and, in particular, the phosphate groups in bone formation, resorption, and calcification. The precise locations of the phosphate groups in the OPN molecule were determined by metabolically labeling OPN with 32P in cultured chicken osteoblasts, followed by purification to homogeneity. N-terminal sequencing showed a single sequence of WPVSKRQHAISA, consistent with that deduced from both cDNA, and previous amino acid sequencing of the protein isolated from chicken bone. Three 32P-labeled peptides were isolated by reverse-phase high performance liquid chromatography of thrombin-digested, 32P-labeled OPN. The N-terminal sequencing of each of these thrombin fragments gave single sequences as follows: WPVSKSRQHAIS, SHHTHRYHQDHVD, and ASKLRKAARKL, with approximate molecular masses of 5, 30, and 20 kDa. These data demonstrate that 32P was incorporated throughout the N- to C-terminal sequence of the protein. Thrombin specifically cleaved chicken OPN at two sites: between Arg-22 and Ser-23, which generated the 5-kDa N-terminal end fragment, and another between Lys-138 and Ala-139, which generated the 30- and 20-kDa fragments. To further define the exact locations of the phosphorylated amino acids and the surrounding amino acid sequences, OPN was digested with trypsin, which generated seven major 32P-labeled peptides whose amino acid sequences were determined. The phosphorylated peptide regions of osteopontin were identified as amino acids 8-18 (QHAIS*AS*S*EEK), 39-54 (LASQQTHYS*S*EENAD), 150-171 (LIEDDAT*AEVGDSQLAGLWLPK), 179-191 (ELAQHQSVENDSR), 194-205 (FDS*PEVGGDSK), 214-219 (ES*LASR), and 239-248 (HSIENNEVTR). The phosphorylated amino acid sites are followed by an asterisk (*). Of the seven identified phosphorylated peptide regions, three were localized on the N-terminal end of the osteopontin molecule (with five phosphorylated serines) and contained the sequence motifs that were phosphorylated by casein kinase II type(s), whereas the remaining four peptides are concentrated toward the C-terminal half of the molecule (with five phosphorylated residues) and contained recognition motifs for other kinases as well as casein kinase II.

Protein kinases of cultured osteoblasts: selectivity for the extracellular matrix proteins of bone and their catalytic competence for osteopontin

The enzyme activities of the major kinases found within the cytosolic and microsomal fractions of embryonic avian calvaria osteoblasts were assayed for their specificity for various noncollagenous extracellular matrix (ECM) proteins of bone. At least 6 proteins with M(r)'s of 66, 58, 50, 36, 30, and 22 kD out of more than 30 of the noncollagenous proteins of the bone ECM were phosphorylated by the kinase(s) found in both osteoblast cellular fractions. The purification and N-terminal sequence analysis of three of the above proteins, M(r)'s 66 and 58 kD (+50 kD), identified them as chicken bone sialoprotein (BSP) and osteopontin (OPN), respectively. Heparin, a specific inhibitor of factor-independent protein kinase (FIPK) activity, blocked the phosphorylation of all six ECM proteins by the microsomal kinase(s) but only inhibited the phosphorylation of the 66, 50, and 36 kD by the cytosolic enzyme(s). Casein kinase II (a known FIPK) showed a similar phosphorylation pattern of the same bone ECM proteins as the FIPK(s) found in osteoblast cell extracts, while purified cyclic adenosine monophosphate (cAMP)-dependent protein kinase did not phosphorylate any of the ECM proteins. Use of dephosphorylated casein showed that in comparison with casein kinase II, casein was a poor substrate for the FIPK found in the osteoblast cellular extracts. Further studies, using FIPK(s) of osteoblasts and purified chicken OPN or bacterially produced recombinant murine OPN as a substrate, showed that both species of OPN were excellent substrates for the FIPK(s) found in osteoblasts. The phosphorylation of the purified chicken and recombinant mouse OPNs were evaluated by quantitative analysis using commercially available protein kinases. cAMP-dependent kinase showed no phosphorylation of either protein, and cyclic guanodine monophosphate (cGMP)-dependent kinase and protein kinase C incorporated 1.2 and 0.5 mol phosphate/mol OPN, respectively. However, both chicken and mouse OPNs were significantly phosphorylated by casein kinase II (9.3 and 9.0 mol of phosphate/mol of OPN, respectively). These results demonstrate that the noncollagenous proteins of the bone ECM, and in particular OPN, are predominantly phosphorylated by FIPK(s), and this class of kinase is the major enzyme found within the microsomal fraction of osteoblasts.

Characterization and partial purification of microsomal casein kinase II from osteoblast-like cells: an enzyme that phosphorylates osteopontin and phosphophoryn

Microsomal casein kinase II (mCKII) is a membrane-bound enzyme present in the microsomal fractions of ROS 17/2.8 osteoblast-like cells. It phosphorylates acidic matrix phosphoproteins such as phosphophoryn and osteopontin. Addition of 1.0% Nonidet P-40 facilitates extraction of the optimum amount of detergent-solubilized and -activated enzyme from microsomal fractions. mCKII was partially purified over 3000-fold by sequential chromatography over DEAE-cellulose and heparin-agarose. SDS-polyacrylamide gels, showed that mCKII contained 43 kDa and 31 kDa polypeptides, corresponding to the alpha- and beta-subunits of the enzyme, respectively. The alpha subunit was identified by anti-CKII antiserum and the beta subunit, by its ability to undergo autophosphorylation. The enzyme was inhibited by 50% with 0.4 micrograms/ml heparin and stimulated by 100% with 1.0 mM spermine when casein was used as a substrate. The phosphorylation of phosphophoryn was reduced to 50% by 0.8 micrograms/ml heparin, but was increased to 2-2.5 fold by 5 to 15 mM spermine, which may be due to substrate-directed effects. Kinetic analysis showed that the apparent Km values for phosphophoryn (0.39 microM) and for osteopontin (2.1 microM) were lower than that for casein (21.3 microM). Vmax values of phosphophoryn and osteopontin were 2.2-fold and 4.6-fold higher than that of casein. Using the ratio Vmax/Km as a measure of kinetic specificity, osteopontin and phosphophoryn appear to be the more specific substrates than casein for mCKII. Thus, both proteins can be considered as physiological substrates for mCKII.

Temporal studies on the tissue compartmentalization of bone sialoprotein (BSP), osteopontin (OPN), and SPARC protein during bone formation in vitro

To study the role of noncollagenous proteins in bone formation, the synthesis and tissue distribution of BSP (bone sialoprotein), OPN (osteopontin) and SPARC (secreted protein acidic and rich in cysteine) were analyzed using pulse-chase and continuous labeling protocols during bone formation by cultures of rat calvarial cells. Following a 1 h labeling period with [35S]methionine or [35SO4], radiolabeled BSP was rapidly lost from the cells and appeared transiently in the culture medium and in a 4 M GuHCl extract (G1) of the mineralized tissue. Coinciding with the loss of BSP from these compartments, radiolabeled BSP increased in demineralizing, 0.5 M EDTA extracts (E) of the bone, in a subsequent GuHCl extract (G2), and in a bacterial collagenase digest (CD fraction) of the extracted tissue, over a 24 h chase period. In comparison, the 55 kDa form of OPN, with a small amount of the 44 kDa OPN, was secreted almost entirely into the culture medium. Most of the 44 kDa OPN, together with some 55 kDa OPN, accumulated rapidly in the E extract but could not be detected in either G extract or in the CD fraction. SPARC appeared transiently in the G1 extract, but was otherwise quantitatively secreted into the culture medium from where it was lost by complexing and/or degradation. When cultures were continuously labeled over a 12 day period with [35S]methionine, radiolabeled BSP and 44 kDa OPN accumulated in the E extract together with a small amount of SPARC. Some radiolabeled BSP also accumulated in the G2 extract. From the relative incorporation of [35SO4] over the same time period, a time-dependent loss in sulphate from the BSP was evident. Using a 24 h pulse-labeling protocol, the amount of radiolabeled BSP and OPN in the E extract and the BSP in the G2 extract were not altered significantly over a 12-day chase period. These studies demonstrate that the 44 kDa OPN and most of the BSP are rapidly bound to the hydroxyapatite crystals where they may regulate crystal formation and growth during bone formation. Some BSP is deposited in the osteoid and appears to become masked by the formation of hydroxyapatite, indicating a potential role for this protein in epitactic nucleation of hydroxyapatite crystal formation.

The role of extracellular matrix components in dentin mineralization

The extracellular matrix of dentin consists of mineral (hydroxyapatite), collagen, and several noncollagenous matrix proteins. These noncollagenous matrix proteins may be mediators of cell-matrix interactions, matrix maturation, and mineralization. This review describes the current knowledge of the chemistry of mineral crystal formation in dentin with special emphasis on the roles of the dentin matrix proteins. The functions of some of these matrix proteins in the mineralization process have been deduced based on in vitro studies. Functions for others have been postulated based on analogy with some of the bone matrix proteins. Evidence suggests that several of these matrix proteins may have multiple effects on nucleation, crystal growth, and orientation of dentin hydroxyapatite.

Immunohistochemical localization of a approximately 66 kD glycosylated phosphoprotein during development of the embryonic chick tibia

Localization of a approximately 66 kD glycosylated phosphoprotein during morphogenesis of the embryonic chick tibia has been accomplished using immunohistochemistry. Although initial expression of the tibial osteoblast phenotype is detected as early as stage 28.5, with the deposition of osteoid matrix beginning at stage 30, little or no immunoreactivity against the approximately 66 kD glycosylated phosphoprotein is observed in pre-osteoblasts, osteoblasts, osteocytes, or in the uncalcified osteoid matrix during the early events of tibia development. Immunoreactivity was first observed at stage 32 when mineralization of the osteoid matrix is initiated. At this and all later stages, the phosphoprotein is located almost exclusively in the extracellular matrix at the mineralization front with essentially no detectable staining in the adjacent unmineralized osteoid matrix. Similarly, no cellular staining is observed when even the lightly mineralized extracellular matrix is strongly immunoreactive. Only scant immunostaining is present over the heavily mineralized regions, although demineralization of these areas with EDTA exposes a low intensity, punctate staining pattern. Additionally, cryosections of developing calvaria stained with this antiserum only display reactivity in regions of bone matrix undergoing mineralization. These localization studies support the hypothesis that this phosphoprotein is intimately associated with the process of bone matrix mineralization in the developing chick long bone.

Developmental appearance and ultrastructural immunolocalization of a major 66 kDa phosphoprotein in embryonic and post-natal chicken bone

Biochemical analyses and immunocytochemistry were used to examine the developmental appearance of a major approximately 66 kDa bone phosphoprotein (66 kDa BPP) in the mid-diaphyseal region of embryonic and post-natal chicken tibiae in vivo. Total protein and O-phosphoserine (Ser-P) and O-phosphothreonine (Thr-P) content of 8-, 12-, and 18-day embryonic, and 4-wk post-natal chicken tibiae were determined by amino acid analysis. Similar bone samples were carried through a wide variety of tissue-processing regimes including different protocols for fixation, decalcification, dehydration, and embedding prior to electron microscopy. For immunocytochemistry, tissue sections were incubated with a polyclonal antibody raised in rabbits against 66 kDa BPP, and the antigen was revealed by the high-resolution protein A-gold technique. Amino acid analysis, Western blotting, and immunocytochemistry all showed the presence and increasing concentration of bone phosphoprotein with advancing developmental age. Immunogold labeling was observed over osteoblasts and mineral deposits throughout the bone with the most intense reaction occurring at the mineralization front in embryonic tibiae. Electron probe X-ray microanalysis confirmed the association of 66 kDa BPP with mineral. The levels of phosphoprotein in the tissue were directly correlated with increasing degrees of mineralization. These observations are consistent with previous proposals suggesting that phosphoproteins may play a significant role in the calcification of bone matrix.

Expression and ultrastructural immunolocalization of a major 66 kDa phosphoprotein synthesized by chicken osteoblasts during mineralization in vitro

Embryonic chicken osteoblasts cultured over a 30 day period were used as a model system for studying the expression of bone phosphoproteins during cellular differentiation and the possible role of these proteins in extracellular matrix mineralization. Accumulation of total phosphoprotein in the cultures, as determined by O-phosphoserine (Ser-P) and O-phosphothreonine (Thr-P) amino acid analysis, revealed a greater than 10-fold increase over the 30 day period. Total phosphoprotein synthesis, as assessed by (32P)-, (3H)-Ser-P, and (14C)-Thr-P protein labeling, showed the highest levels concurrent with initial mineral deposition within the matrix. The major phosphoprotein present in chicken bones and synthesized by the cultured osteoblasts had a molecular weight of approximately 66 kDa. This 66 kDa bone phosphoprotein (66 kDa BPP) was purified to homogeneity and was used for antibody production. Application of this antibody in Western blot analysis revealed that 66 kDa BPP was present only in protein extracts of mineralizing cultured osteoblasts and was absent in cultures of non-mineralizing chondrocytes, myoblasts, and tendon fibroblasts. The 66 kDa BPP in vitro accumulated continuously in the extracellular matrix in a manner that paralleled both phosphoprotein synthesis and total phospho-amino acid production. A comparison of the results obtained in vitro to those from developing embryonic tibiae in vivo demonstrated a similar qualitative and temporal expression of phosphoprotein and a continual accumulation of 66 kDa BPP in the matrix with advancing mineralization and developmental age. Ultrastructural immunocytochemistry using the 66 kDa BPP antibody and the protein A-gold technique revealed specific immunolabeling over electron-dense regions of mineralization in the cultures that appeared identical to the distribution of labeling observed in vivo (McKee et al.: Connect. Tissue Res., 21:21-29, 1989; Anat. Rec., 228:77-92, 1990). These results demonstrate that this major 66 kDa BPP was expressed concurrently with other differentiated osteoblast functions and suggests that it may play a role in the initiation or regulation of mineralization.

Identification and characterization of the major chicken bone phosphoprotein. Analysis of its synthesis by cultured embryonic chick osteoblasts

The major phosphoprotein synthesized by cultured chicken embryo osteoblasts had a molecular mass of approximately 66 kDa. The 32P label on the protein was cleaved by acid phosphatase treatment and O-[32P]phosphoserine and O-[32P]phosphothreonine could be identified after partial acid hydrolysis. The phosphoprotein contributed approximately 2.0% of the total protein synthesized by osteoblasts and was shown to be secreted, as shown by its presence in the culture media. Glycosylation was demonstrated by the fact that it could be labelled with [3H]galactosamine. The major approximately 66-kDa phosphoprotein was resolved by isoelectric focusing into three major variants with pI values ranging over 3.7 - 3.9; all three forms appear to be the result of variation in the extent of protein phosphorylation. An identical approximately 66-kDa phosphoprotein could be extracted from chicken bones which had both the same range of pI values and an identical elution position following DEAE-Sephacel chromatography. Analysis of the protein isolated from bone demonstrated the presence of sialic acid and, while amino-terminal sequence analysis and internal tryptic fragment sequence analysis of about 25% of the protein revealed little similarity to the rat phosphoprotein osteopontin, a conserved nine-residue sequence spanning the Arg-Gly-Asp cell-binding site of the rat protein osteopontin, was identified in the approximately 66-kDa chicken protein. Peptide mapping with Staphylococcus aureus V8 protease of the in vivo protein compared to the in vitro synthesized protein demonstrated identical peptide fingerprints. The two proteins also had comparable amino acid compositions. Several smaller-molecular-mass phosphoproteins ranging in size over about 55 - 29 kDa were also observed in the HCl extracts of bone. Peptide mapping of these species demonstrated that the approximately 66-kDa, approximately 55-kDa, and approximately 45-kDa species had a common core of peptide fragments. Pulse/chase experiments in culture revealed no evidence for a defined pathway of intracellular proteolysis associated with the approximately 66-kDa species since this phosphoprotein remained the prevalent species after a 24-h chase. Because of the predominant association of all the smaller-molecular-mass forms with the cell layer and an absence of a quantitative conversion to any of the smaller forms over a 24-h chase, these results suggested that the lower-molecular-mass species were not the result of proteolytic processing during synthesis or secretion, but rather represent proteolysis of the approximately 66-kDa component in the extracellular matrix.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of calcification: role of collagen fibrils and collagen-phosphoprotein complexes in vitro and in vivo

Samples of decalcified chicken bone together with varying concentrations of phosphoproteins from bone or egg yolk (phosvitin) were used in vitro as heterogenous nucleators for the induction of Ca-P apatite crystals. The lag time between exposure of the collagen-phosphoprotein complexes and the time nucleation of crystals occurred decreased as the concentration of Ser(P) and Thr(P) increased. Enzymatic cleavage of the phosphate groups by wheat germ and phosphatase reversed this effort, indicating that the phosphate group per se principally facilitated the nucleation of Ca-P crystals by the phosphoprotein complex and collagen.

Calcification of dentinal collagen by cultured rabbit periosteum: the role of alkaline phosphatase

Periostea were dissected from 1-2 weeks old rabbit calvaria and folded around decalcified and extracted bovine dentin matrix slices (DMS). The cocultures were grown in serum-containing medium supplemented with beta-glycerophosphate or other organic phosphate esters. [45Ca]-uptake measurements indicated that the DMS calcified. Initiation of the calcification process was associated with alkaline phosphatase activity and could be prevented by adding the inhibitor L-levamisole to the culture medium. Using [32P]-adenosine-monophosphate as a substrate for phosphatase activity it was demonstrated that very little, if any, phosphate was utilized for the phosphorylation of higher molecular weight substances. The results suggest that over 99% of the phosphate produced was laid down in inorganic form. Further, it was noted that mineral deposition in the DMS was accompanied by the simultaneous inclusion of methylene blue and PAS-positive substances whose nature, origin and function remain to be determined. The results lend support to the theory that alkaline phosphatase is involved in the initiation of calcification processes by raising the local concentration of phosphate ions.

Ultrastructural immunolocalization of a major phosphoprotein in embryonic chick bone

Immunocytochemistry utilizing the protein A-gold technique was used to examine the ultrastructural cellular and extracellular distribution of a major phosphoprotein in chick bone. HCl-extracts of embryonic and neo-natal chick bones contain a major 66kD phosphoprotein (BPP) which was purified and used to raise polyclonal antibodies in rabbits. The mid-diaphyseal regions of 8-, 12- and 18-day embryonic chick tibiae were fixed with 1% glutaraldehyde and embedded in Epon or Lowicryl. Electron microscopy following incubation of tissue sections with the antibody and the protein A-gold complex revealed specific immunolabeling over the rER and Golgi apparatus of osteoblasts and over those areas of bone matrix containing Ca and P as determined by electron probe x-ray microanalysis. These included extracellular areas in the matrix undergoing early mineralization and electron dense patches occurring at the mineralization front and extending throughout the more mature bone regions. Biochemical analyses of bone tissue processed similarly to that used for immunocytochemistry confirmed the retention of phosphoprotein in the tissue. The spatial correlation of phosphoprotein in the extracellular matrix with Ca-P mineral deposits confirms an earlier report using 33Pi and radioautography and may indicate a role for phosphoproteins in calcification.

The effect of complexing phosphoproteins to decalcified collagen on in vitro calcification

Decalcified samples of chicken bone containing phosphoproteins of varying concentrations were used to assess the effect of phosphoproteins and of protein-bound Ser(P) and Thr(P) in the in vitro nucleation of a Ca-P solid phase from metastable solutions of Ca and P. Phosphoproteins of bone as well as the phosphoproteins from egg yolk (phosvitin) were used. Increasing concentrations of phosphoprotein [as measured by the amount of protein bound Ser(P) and Thr(P)] in the decalcified bone particles significantly reduced the time required for nucleation to occur after exposure to metastable solutions of Ca and P (decreased operational lag times). Treatment with wheat germ acid phosphatase markedly reduced the concentration of Ser(P) and Thr(P) in the decalcified bone samples and in the decalcified bone collagen samples complexed with phosphoproteins (almost to zero). The loss of the organic phosphate groups significantly increased the operational lag time, but did not abolish nucleation of apatite crystals by the bone collagen fibrils essentially devoid of Ser(P) and Thr(P). Bone phosphoproteins were not specific; substitution of phosvitin for bone phosphoproteins as complexes with bone collagen also proved to be effective facilitators of nucleation, which was interesting since both types of phosphoproteins have certain common chemical and structural characteristics. Noncollagenous components other than phosphoproteins were present in the decalcified bone samples. However, the marked dependence of the lag time on the Ser(P) and Thr(P) concentrations and the very marked diminution in the efficacy of the nucleation phenomenon as a result of treatment with wheat germ acid phosphatase, clearly suggests that the organic phosphate residues of the phosphoproteins play a direct and significant role in the process of in vitro nucleation of a solid phase of Ca and P (apatite) by bone collagen, and by implication, possibly in in vivo mineralization as well.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Casein phosphopeptides: their effect on calcification of in vitro cultured embryonic rat bone

The effect of casein phosphopeptides (CPP) and enzymatically dephosphorylated, otherwise identical peptides (DPP) on Ca solubility and mineralization was studied in an in vitro model. The model consisted of explanted embryonic rat bone rudiments (femora, tibiae, and metatarsals), cultured in Bigger's medium at two different concentrations of total calcium (1.78 mM and 3.24 mM). The medium enabled growth of the explants, but minor or no mineralization took place during a culture period of 11 days. The medium was unstable with respect to Ca solubility, and measurements with a selective electrode showed that during most of the time the actual concentration of soluble, ionic Ca2+ was not higher than about 0.7 mM. CPP added at a concentration of 10 mg/100 ml of the medium vastly improved Ca-solubility under culture conditions. The mean concentration of soluble, ionic Ca2+ in the presence of the peptides was 1.4 mM at total calcium = 1.78 mM and 2.3 mM at the higher total calcium level (3.24 mM). Rudiments cultured in the presence of CPP showed impressive increases in calcification of their diaphyseal area. This progress in mineralization was quantitatively assayed by measuring the projected dark diaphyseal area at the beginning and at the end of the culture period. It was also verified by histological sections of the rudiments. DDP did not enhance Ca solubility of the culture medium, nor did explants cultured in the presence of 10 mg/100 ml of DPP show any significant progress in mineralization. This indicated that the activity of CPP was dependent on the presence of serine-linked phosphate groups.

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

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 31P n.m.r. have conclusively demonstrated that a solid phase of amorphous calcium phosphate does not exist in bone in any significant amount. 31P 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. Physiochemical 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.