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

Identification of lysyl oxidase and TRAMP as the major proteins in dissociative extracts of the demineralized collagen matrix of porcine dentine

Carbonated apatite (dahllite) is formed within and between collagen fibrils in the mineralization of connective tissues. However, the mechanism of crystal nucleation at these sites has not been resolved. To identify non-collagenous proteins that may be involved in the nucleation process we have utilized a dissociative extraction procedure to isolate proteins associated non-covalently with the de-mineralized collagen matrix of dentine isolated from tooth roots of adult porcine incisors. Following extraction of dentine fragments with 4M GuHCl (G1-extract) and 0.5M EDTA (E-extract), de-mineralized collagen matrix-associated proteins were isolated with a second series of extractions with 4M GuHCl (G2-extract). Analysis of the G2-extracts on SDS-PAGE revealed two major 32 kDa and 24 kDa protein bands, comprising > 80% of the extracted non-collagenous proteins. The 32 kDa protein was purified by FPLC on hydroxyapatite and Mono Q resins, followed by HPLC reverse-phase chromatography. Small amounts of 26 kDa and 6 kDa proteins, which appear to represent proteolytically processed, disulphide-linked fragments of the 32 kDa protein, co-eluted with the major protein. The 32 kDa protein was identified as lysyl oxidase from amino acid sequence analysis of a 13 kDa CNBr peptide obtained from protein purified by preparative electrophoresis on SDS-PAGE. Fractionation of the 24 kDa protein on FPLC Mono Q resin generated < 5 closely eluting protein peaks. The proteins from these peaks were similar in size, staining properties, amino acid composition and CNBr digestion patterns. Each protein was immunoreactive with antibodies raised against a tyrosine-rich acidic matrix protein (TRAMP), reported previously to co-purify with lysyl oxidase. These studies, therefore, show that lysyl oxidase, which is important in collagen cross-link formation, and proteins with properties of TRAMP, a protein that can modulate collagen fibrillogenesis, are the major proteins in dissociative extracts of de-mineralized porcine dentine.

The overwet phenomenon in two-component acetone-based primers containing aryl amine and carboxylic acid monomers

OBJECTIVES

The overwet phenomenon was first reported when a moist bonding technique was used with an earlier commercial version of All-Bond 2 (Bisco) that contained BPDM in primer B. This study investigated whether ultrastructural features of the overwet phenomenon could also be detected in other commercially available two-component acetone-based primers containing BPDM, PMDM and PMGDM, as well as an experimental two-component primer containing DSDM.

METHODS

Thirty 1 mm dentin discs prepared from third molars were each conditioned with 10% H3PO4 for 20 s and rinsed for 20 s. They were randomly divided into 5 groups: Group I (Bond-It, Jeneric/Pentron:PMGDM); Group II (Wet Bond, Chameleon Dental Products:PMGDM); Group III (Tenure S, Den-Mat:PMDM); Group IV (present commercial version of All-Bond 2, Bisco:BPDM) and Group V (Experimental two-component primer system containing DSDM in primer B). Following a moist bonding technique using the respective system, discs from each group were further bonded together to form three disc pairs using a chemical-cured resin. Bonded disc pairs were demineralized in EDTA and processed for TEM examination. For this ultramicroscopical study, results such as the features of the overwet phenomenon were analyzed by visual inspection of the specimens in each group (n = 12).

RESULTS

Isolated blister-like spaces of variable dimensions were observed within the primer layer in all groups and possessed the following characteristics: 1) a layer of resin-impregnated dentin was always present along the base of the primary blister; 2) surface primer globules, sometimes containing secondary blisters, were identified within these primary blisters; 3) dentinal tubules within the blister-like spaces were not completely sealed; 4) primer globules were circumscribed by a halo of fine kinked strands of material.

SIGNIFICANCE

Although the technique of moist bonding is based on valid biological principles, incorporation of resin monomers that are immiscible with water rendered the application of current two-component, acetone-based primers very technique-sensitive in terms of tubular seal, when used on moist, acid-conditioned dentin. Further studies should be directed at elimination of this type of oil-in-water (O/W) "macroemulsion" formation through optimal micellar solubilization of these resin monomers in water.

EDTA-insoluble, calcium-binding proteoglycan in bovine bone

A calcium ion precipitable, trypsin-generated proteoglycan fragment has been isolated from the demineralized, EDTA-insoluble matrices of bone. The demineralized matrix was completely digested with trypsin, increasing concentrations of CaCl2 were added to the supernatant, and the resulting precipitates were analyzed. The amount of precipitate gradually increased with higher concentrations of calcium and was reversibly solubilized by EDTA. After molecular sieve and anion exchange chromatography, a proteoglycan-containing peak was obtained. Immunochemical analysis showed that this peak contained chondroitin 4-sulfate and possibly keratan sulfate. Amino acid analysis showed that this proteoglycan contained high amounts of aspartic acid/asparagine (Asx), serine (Ser), glutamic acid/glutamine (Glx), proline (Pro), and glycine (Gly); however, it contained little leucine (Leu) which suggests that it is not a member of the leucine-rich small proteoglycan family. In addition, significant amounts of phosphoserine (P-Ser) and hydroxyproline (Hyp) were identified in hydrolysates of this fraction. A single band (M(r) 59 kDa) was obtained on SDS-PAGE that stained with Stains-all but not with Coomassie Brilliant Blue R-250. If bone powder was trypsinized prior to demineralization, this proteoglycan-containing fraction was not liberated. Collectively, these results indicate that a proteoglycan occurs in the demineralized matrix that is precipitated with CaCl2 and is closely associated with both mineral and collagen matrices. Such a molecule might facilitate the structural network for the induction of mineralization in bone.

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.