Dentin matrix collagen: evidence for a covalently linked phosphoprotein attachment

A repeated triple lysine motif anchors complexes containing bone sialoprotein and the type XI collagen A1 chain involved in bone mineralization

While details remain unclear, initiation of woven bone mineralization is believed to be mediated by collagen and potentially nucleated by bone sialoprotein (BSP). Interestingly, our recent publication showed that BSP and type XI collagen form complexes in mineralizing osteoblastic cultures. To learn more, we examined the protein composition of extracellular sites of de novo hydroxyapatite deposition which were enriched in BSP and Col11a1 containing an alternatively spliced "6b" exonal sequence. An alternate splice variant "6a" sequence was not similarly co-localized. BSP and Col11a1 co-purify upon ion-exchange chromatography or immunoprecipitation. Binding of the Col11a1 "6b" exonal sequence to bone sialoprotein was demonstrated with overlapping peptides. Peptide 3, containing three unique lysine-triplet sequences, displayed the greatest binding to osteoblastic cultures; peptides containing fewer lysine triplet motifs or derived from the "6a" exon yielded dramatically lower binding. Similar results were obtained with 6-carboxyfluorescein (FAM)-conjugated peptides and western blots containing extracts from osteoblastic cultures. Mass spectroscopic mapping demonstrated that FAM-peptide 3 bound to 90 kDa BSP and its 18 to 60 kDa fragments, as well as to 110 kDa nucleolin. In osteoblastic cultures, FAM-peptide 3 localized to biomineralization foci (site of BSP) and to nucleoli (site of nucleolin). In bone sections, biotin-labeled peptide 3 bound to sites of new bone formation which were co-labeled with anti-BSP antibodies. These results establish the fluorescent peptide 3 conjugate as the first nonantibody-based method to identify BSP on western blots and in/on cells. Further examination of the "6b" splice variant interactions will likely reveal new insights into bone mineralization during development.

The effects of sequential and continuous chelation on dentin

OBJECTIVE

Proteolytic and demineralizing agents have a profound influence on the dentin ultrastructure, which plays a key role in the mechanical integrity of the tooth and integrity of dentin-biomaterial interfaces. In-depth characterization of dentin treated with a novel root canal irrigation protocol comprising sodium hypochlorite (NaOCl) and etidronate (HEDP) is lacking. This study comprehensively characterized and compared the effects of the continuous chelation (NaOCl/HEDP) and sequential chelation (NaOCl/EDTA) protocols on dentin.

METHODS

Dentin blocks, dentin powder and root canals of mandibular premolars were distributed into Group 1, Saline (control); Group 2, NaOCl/EDTA; and Group 3, NaOCl/HEDP. Ultrastructural characteristics of the treated dentin were investigated using electron microscopy and light microscopy, while the surface roughness was analyzed using atomic force microscopy. Chemical compositional changes were characterized using Fourier transform infrared spectroscopy (FTIR) and energy-dispersive-X-ray spectroscopy (EDS), while collagen degradation was determined using ninhydrin assay. Data were statistically analyzed using multiple-factor one-way ANOVA and Tukey HSD tests (P = 0.05).

RESULTS

NaOCl/HEDP resulted in partially degraded, yet mineralized collagen fibers, with minimal alteration to the subsurface matrix. Conversely, NaOCl/EDTA dissolved the hydroxyapaptite encapsulation, exposing collagen fibre bundles. There was no significant difference in the surface roughness between the two protocols (P > 0.05). NaOCl/HEDP resulted in homogenous distribution of organic and inorganic components on the treated surface.

SIGNIFICANCE

This study highlighted that continuous chelation (NaOCl/HEDP) resulted in a frail surface collagen layer while sequential chelation (NaOCl/EDTA) exposed bare collagen fibres. These surface and sub-surface effects potentially contribute to structural failures of dentin and/or dentin-biomaterial interfacial failures.

The unique biomineralization transcriptome and proteome of Lytechinus variegatus teeth

BACKGROUND

Matrix-regulated biomineralization involves the specific nucleation and growth of mineral phases within or upon preformed structured organic matrices. We hypothesized that there might be a general mechanism whereby anionic, phosphorylated mineral ion-binding proteins assist in specifically locating the mineral ions with respect to the mineralizing structural organic matrix. Here we extended these studies to invertebrate mineralization in Lytechinus variegatus (Lv) teeth.

MATERIALS AND METHODS

The tooth proteins were extracted and the phosphoproteins occluded in the mineral were enriched by passage through a ProQ Diamond phosphoprotein enrichment column, and subjected to MS/MS analysis. A Lv RNA-seq derived transcriptome database was generated. The MS/MS data found 25 proteins previously classified as "Predicted uncharacterized proteins" and many of the spicule matrix proteins. As these 25 proteins were also identified with the transcriptome analysis, and were thus no longer "hypothetical" but real proteins in the Lv tooth. Each protein was analyzed for the presence of a signal peptide, an acidic pI≤4, and the ability to be phosphorylated.

RESULTS

Four new Lv tooth specific Pro-Ala-rich proteins were found, representing a new class of proteins.

CONCLUSION

The tooth is different from the spicules and other urchin skeletal elements in that only the tooth contains both "high" and "very high" magnesium calcite, [Ca(1-X) Mg(X) CO3], where X is the mole fraction of Mg. We speculate that our newly discovered proline-alanine rich proteins, also containing sequences of acidic amino acids, may be involved in the formation of high magnesium and very high magnesium calcite.

A transmission electron microscope study using vitrified ice sections of predentin: structural changes in the dentin collagenous matrix prior to mineralization

The assembly of the collagenous organic matrix prior to mineralization is a key step in the formation of bones and teeth. This process was studied in the predentin of continuously forming rat incisors, using unstained vitrified ice sections examined in the transmission electron microscope. Progressing from the odontoblast surface to the mineralization front, the collagen fibrils thicken to ultimately form a dense network, and their repeat D-spacings and banding patterns vary. Using immunolocalization, the most abundant noncollagenous protein in dentin, phosphophoryn, was mapped to the boundaries between the gap and overlap zones along the fibrils nearest the mineralization front. It thus appears that the premineralized collagen matrix undergoes dynamic changes in its structure. These may be mediated by the addition and interaction with the highly anionic noncollagenous proteins associated with collagen. These changes presumably create a collagenous framework that is able to mineralize.

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.

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.

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.

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.

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.

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

Free and matrix-bound phosphophoryns, both highly phosphorylated proteins in dentin, were prepared from EDTA extract and CNBr-digests of bovine dentin. The two components were purified by DEAE-cellulose, SP-Sephadex, and gel filtration chromatography. The matrix-bound component was eluted as a distinct peak from the free component in the above chromatographic systems. Amino acid composition of the purified matrix-bound component indicated that this component consisted of phosphophoryn and collagen in the ratio of 2:3 based on the number of the residues. The matrix-bound component could not be reconstituted by mixing phosphophoryn with collagen CNBr peptides. Artificial crosslink products of free phosphophoryn and collagen CNBr-peptides by the carbodiimide method showed similar properties to the physiological matrix-bound phosphophoryn. The bond between phosphophoryn and collagen of the matrix-bound component is assumed to be a covalent crosslink.

The amino-acid composition of the non-collagenous organic matrix of human cementum

Human cementum was demineralized and exhaustively extracted with EDTA and then digested with collagenase. The insoluble residue after digestion was extracted successively with 8M urea and with 8M urea containing 0.1M mercaptoethanol. The non-collagenous fraction accounted for a larger proportion of the total organic matrix than previously found in bone and dentine, largely due to the presence of more collagenase-insoluble material. Fractionation of the EDTA-soluble material resulted in less-acidic fractions, showing similarities to the corresponding fractions of bone and dentine, and anionic fractions with lower levels of acidic amino acids than those from other hard tissues. Fractions obtained from the soluble collagenase-released material after ion-exchange chromatography and gel filtration, although more heterogeneous than those of bone and dentine, showed many similarities, thus confirming the close homology within this fraction from the various hard tissues. The insoluble residue after collagenase digestion appeared to be of the acid-structural protein type found also in bone, dentine and a wide range of connective tissues.

Non-collagenous proteins of rat dentin. Evidence that phosphoprotein is not covalently bound to collagen

The non-collagenous proteins of rat dentin that remain firmly bound to the matrix after demineralization were studied in order to ascertain if they are covalently linked to insoluble dentin collagen. After solubilization with CNBr or with bacterial collagenase, unusually small amounts of dentin phosphoprotein were detected in the matrix. The phosphoprotein obtained by CNBr digestion of the matrix was separated from collagen peptides using two chromatographic steps. Thus even this small quantity of phosphoprotein found in decalcified rat dentin matrix was not covalently bound to collagen.

Comparative studies of phosphoprotein preparations from rat incisor dentin

Phosphoprotein was obtained from rat incisor dentin either by extraction at elevated ionic strength after acetic acid demineralization, or by extraction simultaneous with demineralization in neutral EDTA solution. Purification of solubilized proteins was achieved by Sepharose 4B and DEAE-cellulose chromatography. Polyacrylamide gel electrophoresis of the material from the two preparations resulted in one single band. Except for the amino acid analyses, no evidence for a difference between the two phosphoprotein preparations could be found. After additional purification by iso-electric focusing the amino acid analyses demonstrated a similar composition. It is concluded that the two methods for phosphoprotein extraction yield the same product when purified properly. The study did not give any unequivocal answer as to if any phosphoprotein component exists in rat incisor dentin which is covalently linked to the collagen matrix.

Phosphoprotein modulation of apatite crystallization

Several phosphoprotein preparations (phosvitin, rat incisor and fetal calf molar dentin phosphoproteins) all inhibit apatite growth/replication from pre-existing crystal seeds in metastable solutions. Two stages of the crystal growth process were inhibited by these phosphoproteins. First an initial lag period was induced, probably associated with seed surface phenomena. This period was prolonged indefinitely when a combination of phosphoprotein precoated seeds was used together with soluble phosphoproteins in the crystal growth reaction. Second, the phosphoproteins prolonged that stage of the reaction where octacalcium phosphate is the predominant mineral phase present prior to its conversion to the final apatite product. Pre-treatment of the phosphoproteins with calcium diminished their inhibitory activity to seeded crystal growth as well as towards de novo apatite formation in synthetic extracellular fluids. The presence of collagen diminished the inhibitory activity of the phosphoproteins towards de novo precipitation but had no effect on phosphoprotein-modulated apatite crystal growth in the seeded systems. These results suggest a potential regulatory role for phosphoproteins in dentin mineralization.

The nature of covalent complexes of phosphoproteins with collagen in the bovine dentin matrix

The bovine dentin matrix still contains some noncollagenous proteins after thorough extraction and decalcification. These have been obtained following digestion of the matrix by cyanogen bromide. Peptides containing non-collagenous portions were isolated by chromatography on diethylaminoethyl cellulose columns and fractionated on hydroxyapatite columns. Several fractions were obtained. The principal component was a complex between a highly-phosphorylated serine-aspartic acid-rich protein and a collagen peptide. These collagenous and non-collagenous moieties could not be separated from each other even under highly dissociative solvent conditions. After digestion with collagenase, the resulting phosphoprotein fraction still contained a few residues of hydroxyproline and hydroxylysine, and an enhanced content of proline, compared to the equivalent directly extractable phosphophoryn of the matrix. These data were interpreted as indicating that the phosphophoryn which is not extractable in 0.5M ethylenediaminetetraacetic acid is in fact covalently bound to some specific section of the matrix collagen. The covalent modification of the collagen matrix with highly acidic phosphoproteins may have an important role in the mineralization process.

Phosphopeptides of enamel matrix

Although the tripeptides Glu-O-Phosphoserine-Tyr and Glu-O-Phosphoserine-Leu have been identified in embryonic bovine enamel proteins, 1, 2 the issue of whether both sequences occur in each of the phosphopeptides, or whether certain sequences occur in specific peptides only, has recently been resolved by isolating homogeneous samples of E33 and E44. All three of the Ser residues of both peptides are phosphorylated. All three in E3 are in the sequence Glu-O-Phosphoserine-Leu, and all three in E4 are in the sequence Glu-O-Phosphoserine-Tyr. It was not possible to sequence either of the polypeptide chains directly by automatic peptide sequencing. However, a partial sequence of E4 was constructed from data derived from peptides isolated after cyanogen bromide, trypsin and chymotrypsin digestions. The presence of Glu, Tyr and Leu adjacent to and near the O-Phosphoserine [Ser(L)] residues and the 2 degrees, 3 degrees and higher ordered structures of the enamel phosphopeptides may be important in calcium binding and mineralization.

Phosphophoryns-major noncollagenous proteins of rat incisor dentin

Freshly excised rat incisors were immediately cleaned and demineralized in 0.5 M ethylene diaminetetracetic acid at pH 7.5. The extracts were freed of calcium, diffusible phosphate and low molecular weight polypeptide components by dialysis in membranes with cut-off of 3500 molecular weight. The extract was resolved into at least 7 protein components by chromatography on DEAE-cellulose at pH 8.2. The composition of each protein component was determined. Two proteins, rich in serine, phosphorous and aspartic acid were unlike any proteins attributed to enamel, and hence were considered to be components of incisor dentin. These were the principal non-collagenous components of the teeth. Further purification was carried out under dissociative conditions on Sepharose CL-6B gel filtration columns in 3.0 M guanidine hydrochloride. The two phosphoproteins have mol wts, by this method, of 71,000 and 65,000, respectively, and differ in content of apolar amino acids, although both contain greater than 70 residue % of seryl (or phosphoseryl) and aspartyl residues. The name "phosphophoryns" is proposed to describe these dentinal proteins. The insoluble collagenous matrix remaining after the original demineralizing extraction was degraded with cyanogen bromide. Several non-collagenous protein components were released as well as the typical collagen derived peptides. Two collagen phosphoprotein complex peptides were also isolated, demonstrating as in bovine dentin, the probable direct covalent interaction of a dentin phosphoprotein with hte collagen of the mineralized matrix.

Isolation, characterization and partial amino acid sequence of a phosphorylated polypeptide (E4) from bovine embryonic dental enamel

A phosphorylated polypeptide (E4) of molecular weight 5000-6000, has been isolated from bovine embryonic enamel by Bio-Gel P-10 gel filtration and DE-52 ion-exchange chromatography. The peptide contains three serine residues all of which are phosphorylated. All three O-phosphoserine residues are in glutamic acid-O-phosphoserine-tyrosine sequences that are distributed relatively evenly along the polypeptide chain. Although it was not possible to sequence the entire polypeptide chain directly by automatic peptide sequencing, a partial sequence and peptide map was constructed on the basis of the sequence and composition of peptides derived by cyanogen bromide, trypsin and chymotrypsin digestion. The presence of glutamic acid, tyrosine and leucine adjacent to and near the O-phosphoserine residues may be important in calcium binding and in mineralization.

Noncollagenous proteins of a rat dentin matrix possessing bone morphogenetic activity

An insoluble preparation of rat dentin matrix was shown to possess bone morphogenetic protein (BMP) activity, i.e. the capacity to induce the formation of catilage and bone when implanted intramuscularly. Since BMP activity was previously attributed to noncollagenous proteins (NCP) of bone and dentin, the nature of NCP of the rat dentin was examined. After treatment of the matrix with purified bacterial collagenase, three NCP were solubilized concomitantly with digestion of the dentin collagen to smaller peptides. The three proteins were separated by anion-exchange chromatography on DEAE-cellulose. Two of the NCP were rich in asparate, glutamate, glycine, serine, and alanine, and thus displayed compositions similar to acidic proteins of other connective tissues. The third NCP was shown by amino acid composition to be the aspartate, serine-rich phosphoprotein, which occurs mostly in a soluble form in rat dentin. This observation supports the view that a portion of dentin phosphotprotein is firmly bound.

On the state of anionic groups of demineralized matrices of bone and dentine

Calcium-binding and biochemical studies have been applied to characterize the state of the carboxylate and protein-bound phosphate groups in the EDTA-demineralized matrices of rat bone and dentine. The organic phosphate and carboxylate content of demineralized bone is virtually identical to that of purified steer skin collagen whereas demineralized dentine has a significantly higher phosphate and carboxylate content, presumably due to the presence of an acidic non-collagenous phosphoprotein. Two classes of calcium-binding sites can be detected in demineralized bone, demineralized dentine, and purified, reconstituted collagen. The number of strong calcium-binding sites correlates with the number of protein-bound phosphate groups. Depending on the preparative procedure, seven to nine such sites (per collagen molecule) are present in dentine, and one to two in the purified reconstituted collagen and in bone. The binding constant for the dentinal sites (1.1 X 10(4) M-1), however, is 20 times greater than that for bone or reconstituted collagen fibrils from skin. We tentatively conclude that the strong calcium-binding site in bone and reconstituted collagen is of the form protein-PO-4Ca++ whereas in dentine it is of the form protein (formula: see text); the weak binding sites in bone and dentine are of the form protein-COO-Ca++; and that approximately 160 of the 217 carboxylate groups of the collagen molecules of dentine or bone are present as electrostatic linkages of the form protein-COO-+H3N-protein.

Glycosaminoglycans of EDTA soluble and insoluble dentin in dentinogenesis imperfecta type I

A study of glycoasminoglycans (GAGs) in dentinogenesis imperfecta Type I (DI I) revealed increased GAG in DI I EDTA soluble dentin as compared to controls. EDTA insoluble GAG contained less GAG than age-matched controls. The role of GAG in dentin pathosis is discussed.

The extraction of phosphoproteins from bovine dentin

The phosphoprotein obtained by the neutral pH tris buffer extraction of acetic acid demineralized bovine dentin has been compared with the phosphoprotein extracted directly during the neutral pH EDTA demineralization process. The phosphoproteins isolated by DEAE-cellulose chromatography from the neutral pH EDTA demineralization extract are not identical to those isolated by the same procedure from the dentin which had been subjected to acid demineralization. The two demineralization procedures yield phosphoproteins different in amino acid content and in presence of 260 nm UV absorbing moiety. Even after sequential acid demineralization, trisbuffer extraction and EDTA extraction, the residual dentin contains phosphoprotein. A peptide fragment containing both collagen and phosphoprotein moieties has been isolated following digestion and cleavage of the insoluble dentin collagen with cyanogen bromide. The acid demineralization process appears to be accompanied by degradation which removes both protein and non-protein components from the phosphoprotein.

Direct identification of the calcium-binding amino acid, gamma-carboxyglutamate, in mineralized tissue

A direct approach has been developed for quantitative identification of the calcium-binding amino acid, gamma-carboxyglutamate, in proteins. This should be advantageous for the study of numerous systems where specific roles for the binding of calcium or other divalent cations are suspected. Investigation of mineralized tissue, where calcium-binding proteins are implicated in the mineralization process, revealed that gamma-carboxyglutamate was present in proteins solubilized from chicken bone with neutral aqueous ethylenediamine tetraacetic acid. This was established by direct isolation of the amino acid from alkaline hydrolysates and its quantitative conversion to glutamic acid by decarboxylation in 0.05 M HCl at 100 degrees. The kinetics of decarboxylation and chromatographic behavior are identical to those of gamma-carboxyglutamate from human prothrombin. After resolution of the soluble bone proteins by phosphate gradient elution from hydroxyapatite, gamma-carboxyglutamate was found to be concentrated primarily in one BaSO4-adsorbable anionic protein species; bone collagen was devoid of the amino acid. In view of the recently discovered requirement of vitamin K for generation of calcium binding sites (gamma-carboxyglutamate) by gamma-carboxylation of specific glutamic acid residues in prothrombin, our findings may implicate vitamin K metabolism in normal bone development and suggest a role for the gamma-carboxyglutamate-rich protein in regulation of calcium salt deposition in mineralized tissues.

The composition of the insoluble collagenous matrix of bovine predentine

Predentine obtained from bovine teeth by microdissection was extracted with EDTA and tris-NaCl solutions. The insoluble residue consisted mainly of collagen and resembled dentine collagen in overall amino acid composition. The residue differed in containing no detectable phosphoprotein, a much larger amount of collagen hexose and more non-collagenous glycoprotein, the neutral sugar composition of which was determined. Differences were also observed in the contents of reducible collagen cross-links. Half of the total phosphorus found in the predentine could not be accounted for solely by hydroxyapatite. The remainder was partly soluble and dialysable.