Systematic purification of free and matrix-bound phosphophoryns of bovine dentin: presence of matrix-bound phosphophoryn as a distinct molecular entity

Association of bovine dentine phosphophoryn with collagen fragments

Bovine dentine phosphophoryn (BDP), a protein rich in aspartyl (Asp) and O-phosphoseryl (Ser(P)) residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentine. Phosphophoryn was purified from bovine dentine using EDTA extraction, Ca(2+) precipitation, anion exchange and size exclusion chromatography. The purified protein migrated on SDS-PAGGE as a single band. The protein was dephosphorylated using a chelex alkaline dialysis procedure, repurified using anion exchange and size exclusion chromatography and then subjected to cleavage with trypsin. The digest was subjected to reversed-phase HPLC and analysed by Q-TOF mass spectrometry. The only non-trypsin peptides that could be identified were two collagen Type I alpha2 peptides whose sequence was determined by fragmentation analysis. The association of collagen fragments with highly purified phosphophoryn suggests that the EDTA extraction method yields BDP that is strongly bound to collagen fragments. This association now helps explain discrepancies in molecular weight and amino acid composition data for various phosphophoryn preparations compared with the same data calculated from the C-terminal extension of mouse, rat and human dentine sialophosphoprotein (DSPP) gene products. Analysis of the mutation pattern of the clinical disorder Osteogenesis Imperfecta within the region enclosed by the identified collagen fragments reveals that phosphophoryn associates with a segment of collagen that is crucial for structure and/or function.

Protein dynamics of bovine dentin phosphophoryn

Bovine dentin phosphophoryn (BDP), a protein rich in aspartyl (Asp) and o-phosphoseryl [Ser(P)] residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentin. The elucidation of the structure-function relationship of phosphophoryn has been a challenge because of its high-molecular weight, high negative charge, repetitive sequence, and lability. We have used the dynamic behavior of the (1)H NMR signal at 600 MHz to provide insight into the molecular dynamics of phosphophoryn. Our results indicate that phosphophoryn is a molecule of uniformly high mobility, thus belonging to a recently identified class of intrinsically disordered proteins that are characterized by sequences of low complexity and rich in polar and charged residues. The significance of our results is that phosphophoryn, because of its uniform nature has the potential to be replaced by biomimetic synthetic peptide analogs that together with amorphous calcium phosphate may lead to the development of novel, nontoxic, apatite-based dental restorative materials.

Crystallinity and solubility characteristics of hydroxyapatite adsorbed amino acid

Hydroxyapatite (HAp) was synthesized in the presence of a variety of amino acids in order to investigate the effect of amino acid on the crystallinity and the solubility characteristics of HAp in the HAp-synthesizing condition. In the results of X-ray diffraction analysis, HAp synthesized in the presence of glycine (HAp-Gly), serine (HAp-Ser), aspartic acid (HAp-Asp) and glutamic acid (HAp-Glu) showed poor crystallinity compared with HAp synthesized in the absence of amino acid (HAp-con). The results of Fourier transform infrared spectroscopy suggested the adsorption of these amino acids on HAp. Scanning electron microphotographs showed that the size and morphology of HAp adsorbed amino acids changed significantly. Furthermore, the solubility of these HAps increased significantly compared to HAp-con, each differing in four amino acids. However, other amino acids did not affect the crystallinity and morphology of HAp and had no significant change in their solubility. Collectively, this study suggests that the crystallinity and the solubility of synthesized HAp are different owing to the variation of amino acids in the HAp synthesizing condition. It is expected that digestion-regulated HAp materials could be synthesized by using amino acid in their synthesizing condition.

Apatite induction by insoluble dentin collagen

Phosphoproteins are thought to play a role in mineral formation in dentin. A portion of this phosphoprotein is bound to collagen. We have investigated the requirement for bound phosphate in mineral induction by isolated dentin collagen. Insoluble bovine dentin collagen obtained by ethylene-diamino-tetra-acetic acid (EDTA) demineralization had 19.5 mol of P/mol of collagen that could not be extracted with 0.5 M EDTA in 4 M guanidine HCl. When this collagen was incubated in supersaturated solutions that did not spontaneously precipitate, apatite was induced. With progressive enzymatic dephosphorylation, induction times for mineral formation became progressively longer. The dentin did not induce mineral formation when 90% of the ester phosphate was removed. Insoluble bone collagen, which had even less phosphate, also did not induce mineral formation. Mineral induction times by dentin collagen increased with decreasing solution saturations. Using these data, the interfacial tension for mineral induction was determined to be 90 ergs/cm2. This value approximated that of phosphatidic acid liposomes and of phosvitin cross-linked to agarose beads, and it might reflect the energetics of heterogeneous nucleation on a highly phosphorylated surface. Sequestering of calcium-phosphate clusters on the phosphoprotein probably accounts for the observed calcium binding by dentin collagen in excess of that required to neutralize the phosphate esters of the collagen. Because the phosphoprotein is immobilized at a low density on the collagen, it cannot self-associate in calcium-phosphate solutions as it does when it is free in solution. This immobilized phosphoprotein allows the mineral clusters formed on its surface to grow into a crystalline order.

Gene expression of TGF-beta 1 and elaboration of extracellular matrix using in situ hybridization and EM radioautography during dentinogenesis

BACKGROUND AND METHODS

The expressions of TGF-beta 1 and Type I collagen mRNA were studied by in situ hybridization and immunohistochemistry then the secretory pathway of dentin phosphoprotein was investigated electron microscopic radioautography in rat incisors.

RESULTS AND CONCLUSIONS

Expression of TGF-beta 1 mRNA was observed in dental papilla cells before dentin formation. The signals were most intense in pre- and postodontoblasts and during dentinogenesis, but became weaker in the secretory region during the dentin formation. Type I collagen mRNA was expressed in essentially the same as that of TGF-beta 1. These results suggest that TGF-beta 1 plays an important role in the differentiation of, and collagen synthesis by odontoblasts. Radioautography showed radioactivity in the rough endoplasmic reticulum 5 min after injection of 3H-serine. Silver grains were observed over the cylindrical portions of the cis-face of the Golgi apparatus at 10 min and over the cylindrical portions of the transface at 20 min. The secretory granules showed the strongest reaction between 20 min and 1 h after injection. At 45 min, a significant labeled band appeared at the mineralization front. The pathway of 3H-proline was essentially the same as that of 3H-serine, but 3H-proline moved more slowly. Secretory granules were heavily labeled from 30 min; no labeling was found at the mineralization front at 45 min. The labeling pattern with 3H-serine appears to be closely related to the localization of phosphoproteins. Dentin phosphoproteins are related to secretory granules and are secreted by odontoblasts as the mineralization front, being involved in the process of dentin mineralization.

Electrophoretic gels of dentin matrix proteins as diffusion media for in vitro mineralization

Non-collagenous proteins of dentin and bone have important effects on mineralization which have been studied by various in vitro systems. We developed an in vitro mineralization system using electrophoretic gels as diffusion media of calcium and phosphate ions. Calcium and phosphate ions were diffused naturally or propelled by electric potential. Calcium phosphate was precipitated in the gel, and the precipitation was affected by proteins in the gel which had been separated by electrophoresis. We applied this system to analysis of non-collagenous proteins of dentin. Among the proteins, phosphophoryns promoted calcium phosphate precipitation in the natural-diffusion system. A non-collagenous protein having a molecular mass of 60 kDa inhibited precipitation. The results were different, however, in the electric-diffusion system, in which phosphophoryns had a negative effect. The present system enabled us to compare the effects of plural proteins rapidly, even using unpurified material.

Acidic amino acid-rich sequences as binding sites of osteonectin to hydroxyapatite crystals

Osteonectin, an acidic noncollagenous protein of bone and dentin, has affinity to hydroxyapatite crystals. Binding sites to hydroxyapatite of this protein were determined by a proteolytic experiment and an in vitro binding experiment using synthetic peptide analogues. Osteonectin was adsorbed on hydroxyapatite crystals and digested with trypsin. A peptide was left adsorbed on the crystal even after the digestion. The peptide was identified as an amino terminal peptide containing glutamic acid-rich sequences, which have been assumed to be possible hydroxyapatite-binding sites. Poly glutamic acid sequences were synthesized as models of the binding sites. Glu6 peptide was bound to the hydroxyapatite with a dissociation constant of 2.4 microM. Peptides containing fewer glutamic acids had lower affinity to the crystal. Effects of these peptides on in vitro mineralization were examined by a gel system in microtiter plates. The Glu6 peptide had a positive effect on the mineralization in this system, whereas Asp6 peptide had a negative effect. These effects indicate the presence of an interaction between these peptides and mineral crystals.

Extracellular processing of dentin matrix protein in the mineralizing odontoblast culture

Odontoblasts that we prepared from bovine incisors produced a dentin-specific protein, phosphophoryn, and accumulated it in mineralized nodules. The time course of mineralization was detected by measuring osteocalcin and mineral in the nodules. The sequence of developmental expression of proteins in this mineralizing dentin cell culture is very similar to that in bone cells, suggesting a common mechanism for matrix mineralization in bone and dentin. Casein kinase II, which phosphorylates bone phosphoproteins and dentin phosphorylates bone phosphoproteins and dentin phosphophoryn, also emerges coinciding with the initiation of mineralization. Furthermore, we have detected extracellular phosphorylation by casein kinase II of a dentin protein of M(r) 60,000, which we recovered from the phosphophoryn fraction in CaCl2 precipitate.

A new method for in vitro calcification using acrylamide gel and bovine serum

To investigate the mechanism of biological calcification in vitro, a model system consisting of an acrylamide gel block (1 x 3 x 3 mm) and fetal bovine serum was developed. Mineral deposition was induced in gel blocks which were immersed in 300 microliters of fetal bovine serum at 37 degrees C for 7 days in a CO2 incubator. X-ray diffraction indicated that the mineral was hydroxyapatite with low crystallinity. Effects of the concentration of acrylamide gel, the partial pressure of CO2 and matrix proteins within the gel on the mineral formation were investigated. In the gel concentration range of 10-60%, the largest amount of crystal grew in 40% acrylamide gel, where the serum protein did not penetrate. With an increase in the partial pressure of CO2 the Ca content in the gel block increased, reached the highest level at about 3.5% CO2 and then began to decrease. In 40% gel and at 5% CO2, the mineral formation was enhanced by phosvitin, phosphophoryn, demineralized dentin powder and alkaline phosphatase. Mineral deposition occurred around the collagen fibers immobilized in 40% acrylamide gel. These results indicate that 1) a putatively serum-derived inhibitor of calcification with high-molecular weight was prevented from penetrating into the 40% acrylamide gels, 2) immobilized polyanionic proteins and alkaline phosphatase were able to increase mineral deposition and 3) the partial pressure of CO2 greatly influenced the mineral deposition. It was concluded that this gel system is useful to investigate the mechanism of biological calcification in vitro.

In vitro and in vivo association of dentin phosphophoryn with alpha1CB6 peptide of type I collagen

A small number of molecules of phosphophoryns, dentin phosphoproteins, are associated to collagen tightly with the maturation of dentin. As an in vitro model of the process, we examined non-covalent association of phosphophoryns and collagen CNBr peptides. Cyanogen bromide peptides of type I collagen were separated with electrophoresis and transferred to a membrane, which was probed with labeled phosphophoryns. Phosphophoryns were bound preferentially to alpha1CB6 peptide. Another experiment using a cleavable crosslinking agent also demonstrated the affinity between phosphophoryns and the alpha1CB6 region of collagen. The matrix-bound fraction of phosphophoryns was solubilized by CNBr digestion of bovine dentin, and was partially purified. Compositional analysis revealed that the fraction was composed of association products of phosphophoryns and collagen at the ratio of 2:3. Considering the high molecular weight of the fraction, one phosphophoryn molecule should be associated with several collagen peptides. The fraction reacted with the antibody against alphalCB6 peptide. When the fraction was digested with lysyl endopeptidase, several peptides that coincided with peptides from alpha1CB6 were released. One of the peptides was sequenced and identified to be a peptide with Asp 975 of the alpha1(I) chain as an amino terminal residue. The alpha1CB6 peptide may be involved in the matrix-bound fraction, although involvement of other collagen peptides cannot be ruled out. Phosphophoryns may associate with collagen at the alpha1CB6 region at first, and then be immobilized on it, possibly by covalent crosslinking, with maturation of dentin. Binding of several alpha1CB6 peptides to a single phosphophoryn molecule is possible in current 3-dimensional models of collagen packing in mineralized tissue, which allows alignment of several hole zones in register.

Differences in composition of cell-attachment sialoproteins between dentin and bone

Matrices of dentin and bone were compared with respect to the content of cell-attachment sialoproteins. The levels of two sialoproteins, osteopontin (OPN) and bone sialoprotein (BSP), were determined in dentin and bone by immunochemical procedures. Polyclonal antibodies against bovine BSP and an antibody against the amino-terminal decapeptide of rat OPN were used. The relative levels of OPN and BSP in dentin were less than one-tenth of the levels in bone. The differences between dentin and bone levels of OPN and BSP were thus larger than those for osteonectin or bone Gla protein in the two tissues. The scarcity of the cell-attachment proteins in dentin may reflect the metabolic inactivity of dentin.

Affinity of bone sialoprotein and several other bone and dentin acidic proteins to collagen fibrils

Bone and dentin contain several kinds of mineral-binding proteins and cell-attachment proteins. The authors examined the affinity of these proteins to type I collagen, a major matrix protein of the tissue. Bone sialoprotein (BSP), bone Gla protein (BGP), bone small proteoglycan II (PG II), osteonectin (ON), and dentin phosphophoryn (DPP) were labeled with fluorescein isothiocyanate and incubated with reconstituted type I collagen fibril. DPP, BGP, BSP, and PG II were absorbed significantly to the collagen fibril at physiological ionic strength with dissociation constants of 10(-6)-10(-7) M. BSP and PG II enhanced the fibrillogenesis of collagen. These acidic proteins can affect the surface properties of collagen fibril, and BSP, having the cell-attachment sequence Arg-Gly-Asp, possibly mediates interaction between collagen fibril and cells.

Preferential adsorption of dentin and bone acidic proteins on the (100) face of hydroxyapatite crystals

Interaction of bone and dentin proteins with minerals is an elementary step in the regulation of mineralization in these tissues. Adsorption of acidic non-collagenous proteins on hydroxyapatite was examined using fluorescence-labeled protein and synthetic hydroxyapatite. Phosphophoryn, bone Gla protein, osteonectin and bone small proteoglycan II were prepared and labeled with fluorescein. All of these proteins were adsorbed on hydroxyapatite with a dissociation constant on the order of 10(-7) M. The more acidic proteins had lesser binding capacities. Hydroxyapatite single crystals were incubated with labeled proteins and observed with a fluorescence microscope. Phosphophoryn and other acidic proteins were adsorbed preferentially on the (100) face of the crystal. This preferential adsorption of the acidic proteins may be responsible for the morphogenesis of biological hydroxyapatite.

Use of a phosphophoryn-Ca(+2)-collagen composition that mimics a mineralization front in unicortical defects in long bones

The present study was designed to ascertain if dynamic ionic matrices that mimic the mineralization front could be used as active scaffolds for bone repair. Dentinal phosphophoryn calcium salts were extracted from unerupted bovine dentine using chatopic buffers and EDTA. The phosphophoryns were subsequently isolated following precipitation with CaCl2. The phosphophoryn-Ca+2 salts were then mixed with pepsin solubilized bovine skin collagen and lyophilized into hardened sponges. Three groups of 4 beagle dogs were employed such that one leg served as an experimental test site for a mechanical wound, while the contralateral leg served as a control. Animals were sacrificed at 1,3, and 6 month intervals. The test specimens were harvested, fixed, and processed for routine histology, examined with image histomorphometric analysis, and scored. Tabulation of these data indicated that phosphophoryn-Ca(+2)-collagen enhances the repair of mechanically formed osseous defects in the distal femur of beagle dogs. This enhanced rate of bone repair was manifest by earlier filling of bony voids with osteoid and trabecular bone. Eventually, this process was followed by recortification of the surface defects. These data suggest that components derived from a mineralization front may influence bone formation in unicortical defects within long bones.

An analysis of the biochemical and biosynthetic properties of dentin phosphoprotein

Dentin phosphoprotein (DPP) and dephosphorylated DPP (deP-DPP) were isolated from developing bovine incisor and purified by DEAE-Sephacel and Sepharose CL-6B chromatography. Intact DPP showed partial resistance to proteolytic cleavage; while deP-DPP was completely susceptible to protease digestion. This result suggested that phosphate residues within DPP molecule may protect its structural integrity. DPP was also very sensitive to heat. When DPP was heated at 100 degrees C in the presence of sodium dodecyl sulfate, increasing concentrations of SDS resulted in a protective effect from degradation. An analysis of DPP biosynthesis was carried out by isolation of the odontoblast tissue/cells attached to forming dentin or scraped from dentin surface followed by incubation with radioactive amino acids and phosphate. DPPs produced in vitro were larger in size than those DPPs extracted from mature dentin matrix. To determine the size of the unprocessed form of DPP, Xenopus laevis oocytes were incubated in [32P]-phosphate-containing medium after microinjection of poly(A)+ mRNA extracted from the bovine odontoblasts. Radioactive DPP analyzed by two-dimensional polyacrylamide gel electrophoresis and immunoprecipitation with an anti-DPP monoclonal antibody was shown to have an apparent Mr of 150-160 k. These data imply that changes in molecular size of DPP may occur by post-translational cleavage or other modification such as degradation within dentin matrix in vivo.

High-resolution solid-state nuclear magnetic resonance spectra of dentin collagen

Insoluble collagen of bovine dentin was characterized by high-resolution solid-state 13C nuclear magnetic resonance (NMR) spectroscopy using a cross-polarization magic angle spinning procedure. A downfield shift was observed in the signal of hydroxyproline C beta compared with that in skin collagen, indicating a distortion in the hydroxyproline structure. A signal of 31P NMR was detected in dentin collagen that was compatible with the presence of matrix-associated phosphoprotein.

Soluble glycosylated phosphoproteins of cementum

Ethylenediaminetetraacetate and hydrochloric acid (EDTA) (HCl) extracts of cementum were fractionated by molecular sieving, ion exchange chromotography, and reverse phase high precision liquid chromatography (HPLC). Nine fractions were isolated, all of which contained serine phosphate, threonine phosphate, and high concentrations of aspartic acid (asp) and glutamic acid (glu). Five of the fractions obtained by repeated HPLC consisted of a single band by SDS-PAGE; the others contained at least one other minor component. All of the protein bands stained with both Rhodamine B and alcian blue, the latter consistent with analytical determinations that demonstrated that the phosphoprotein component contained a significant amount of carbohydrate, including neuraminic acid.

Phosphoprotein synthesis and secretion by odontoblasts in rat incisors as revealed by electron microscopic radioautography

The secretory pathway of dentin phosphoproteins in rat incisors was studied by electron microscopic radioautography after the injection of 3H-serine, and the results were compared with those using 3H-proline as a tracer. Five min after injection of 3H-serine, radioactivity was found in the rough endoplasmic reticulum. At 10 min, silver grains were observed over the spherical portions of the cisface of the Golgi apparatus. At 20 min after injection, silver grains were seen over the cylindrical portions of the transface of the Golgi apparatus. The secretory granules showed the strongest reaction from 20 min to 1 hr. At 45 min, a significant labeled band appeared at the mineralization front. At 1 hr, the labeling at the mineralization front began to appear in the mineralized dentin, and after 12 hr this labeled band was located within the mineralized dentin. The pathway of 3H-proline was essentially the same as that of 3H-serine, but 3H-proline moved more slowly than 3H-serine, especially in transit from the rough endoplasmic reticulum to the Golgi apparatus. Secretory granules were heavily labeled from 30 min to 1 hr after injection of 3H-proline; no labeling was found at the mineralization front at 45 min. The labeling seen initially over the predentin was over the mineralized dentin no earlier than 6 hr after injection. The labeling pattern with 3H-serine is closely related to the localization of phosphoproteins, whereas the pattern with 3H-proline reflects the production of collagen rather than of phosphoproteins. The present radioautographic results indicate that dentin phosphoproteins are related to secretory granules and are secreted by odontoblasts at the mineralization front and also that phosphoproteins are involved in the process of mineralization of the circumpulpal dentin.

Two classes of dentin phosphophoryns, from a wide range of species, contain immunologically cross-reactive epitope regions

An immunological species comparison, using a monospecific rabbit polyclonal antibody directed against rat incisor alpha-phosphophoryn, has been undertaken to assess the similarity in epitope regions among various dentin phosphophoryns (PP) that were prepared from human, monkey, bovine, ovine, and echinoderm teeth. Dentin extracellular matrix proteins were extracted with a standard method using 0.5 M EDTA in the presence of enzyme inhibitors. Final phosphophoryn purification was performed on DEAE ion exchange HPLC. Cross-reactivity of the polyclonal antibody was examined by enzyme-linked immunosorbant assay (ELISA) and dot-blot. The results of this investigation demonstrate a cross-reactivity of the rat-alpha-phosphophoryn antibody (anti-RIPP) with at least one phosphophoryn component in each dentin studied, indicating the existence of similar antigenic determinants among these proteins. It would seem that these epitope regions have been strongly conserved since the epitope region is also present in the phosphoprotein of echinoderm teeth. No cross-reactivity was found with phosvitin (a phosphoserine-rich phosphoprotein), rat serum albumin, bovine serum albumin, or collagen type IV. However, a new and distinct second cross-reactive phosphophoryn, not calcium ion-precipitable, was found in the EDTA insoluble fraction from the teeth. These results indicate that dentin phosphophoryns are specific phenotypic markers for odontoblast expression. Because of the species cross-reactivity, the polyclonal anti-RIPP antibody may be a useful probe in studying the distribution of phosphophoryns in other species, such as human teeth.

Increase of dentin phosphophoryn with dentin formation

Dentin phosphophoryn was quantified on bovine and rabbit dentin at three developmental stages. Phosphophoryn was extracted from teeth with 0.6M HCl, and quantified as optical density on DEAE-cellulose chromatogram or as phosphoserine content. Bovine phosphophoryn showed progressive increase with formation of dentin. Matrix-associated phosphophoryn was also quantified as phosphoserine content in insoluble dentin residue which was extracted with 6 M urea after decalcification. This fraction increased with formation of dentin both in bovine and rabbit dentin. Phosphophoryn is thought to be related to the later stage of dentin formation.

Effects of dentin phosphophoryn on precipitation of calcium phosphate in gel in vitro

In vitro precipitation of calcium phosphate was carried out using a one-dimensional double diffusion system in agar gel. Bovine dentin phosphophoryn enhanced the sharpness of the precipitation bands, although it reduced the total amount of the precipitates. Dephosphorylated phosphophoryn had no effect on the pattern of precipitates. Therefore, phosphophoryn is thought to raise the local density of nucleation in spite of its inhibitory activity on apatite formation.

On the problem of covalent linkages between phosphoproteins and collagen in bovine dentin and bone

The majority of phosphoproteins in bovine bone and dentin are insoluble in EDTA and guanidine hydrochloride (Gu.HCl) at 2 degrees C. After removal of EDTA and Gu.HCl-soluble proteins at 2 degrees C, collagen alpha-chains and alpha-chain polymers were extracted from bovine bone and dentin in Gu.HCl at elevated temperatures and purified by several chromatographic techniques and SDS-PAGE. Small amounts of O-phosphoserine were found in all collagen components. In contrast, O-phosphoserine was not detected in the purified collagen components soluble in EDTA or Gu.HCl at 2 degrees C nor was hydroxyproline detected in the EDTA-soluble phosphoproteins. In contrast, although the vast majority of EDTA-insoluble collagen and phosphoprotein molecules can be readily dissociated by a variety of molecular sieving and ion-exchange chromatographic procedures, a small number are very strongly associated or covalently cross-linked. These results are consistent with the findings that both hydroxyproline and hydroxylysine are present in purified phosphoprotein components released from the EDTA-insoluble tissue by bacterial collagenase. The hydroxylysine/100 hydroxyproline ratios in the phosphoprotein-collagen complexes are much higher than those in dentin or bone collagens.

Changes in interaction of bovine dentin phosphophoryn with calcium and hydroxyapatite by chemical modifications

Dentin phosphophoryn was found to lose its precipitability with calcium ion when dephosphorylated or carboxylate-blocked, whereas precipitation of the intact protein was induced at calcium concentration as low as 2 mM. On the other hand, the adsorption capacity of phosphophoryn onto the hydroxyapatite surface increased when acidic groups were diminished by the above modifications.

Identification of dentin phosphophoryn localization by histochemical stainings

Phosphophoryn, the most abundant of the dentin non-collagenous proteins, has been considered to be related in function to the mineralization process. In the present study, identification of dentin phosphophoryn localization was attempted using newly developed, precautionary histological methods by which phosphophoryn was retained in the sections during the specimen preparation and stained selectively in situ. Phosphophoryn was found to be present widely in all of the calcified dentin except the mantle dentin, the external, first-formed portion of dentin, but was not found in the predentin, the inner, uncalcified layer of dentin. These results indicate that phosphophoryn is apparently related to the mineral phase of calcified dentin and that the mineralization process of mantle dentin, which is formed before the odontoblasts are fully differentiated, may be different from that of circumpulpal dentin.

In vivo cleavage of dentin phosphophoryn following beta elimination of its phosphoserine residues

Phosphophoryn, a unique phosphoprotein in dentin, has a polydisperse character. Keto acid was found to be present in this protein by condensation reaction with dinitrophenyl (DNP)-hydrazine and was identified to be pyruvate from the result that [3H]lactate was formed by the reduction of phosphophoryn with [3H]NaBH4. In the case of phosvitin, incorporation of DNP-hydrazine was much less than in phosphophoryn, even after the same treatment as the extraction procedure of phosphophoryn. DNP-hydrazine incorporation was much higher in the phosphophoryn of mature dentin than in dentin of unerupted teeth. From these results, pyruvate was though to be formed time dependently in vivo by beta elimination of phosphoserine residues and subsequent cleavage of the peptide chain, which may contribute to the polydispersity of phosphophoryn.

Immunohistochemical studies with a monoclonal antibody on the distribution of phosphophoryn in predentin and dentin

A monoclonal antibody was raised against phosphophoryn, a unique noncollagenous phosphoprotein in dentin. Mouse myeloma NS-I cells were fused with spleen cells obtained from BALB/c mice immunized with phosphophoryn from fetal calf tooth germs. Mice inoculated with the hybridoma produced ascites fluid containing the antibody and this reacted only with a band of phosphophoryn transblotted from polyacrylamide gel. Immunohistochemical studies with the antibody showed that phosphophoryn was present in odontoblasts, odontoblastic processes and dentin, but not in the matrix of predentin, and that the phosphophoryn content of the dentin layer was high at and around the predentin-dentin junction and gradually decreased toward the enamel layer. The area corresponding to mantle dentin was not stained with the antibody.