Phosphoproteins dentine

Phosphoproteins can be extracted from bone and dentine with EDTA. The phosphoproteins in dentine form about 10% of the total protein present, and have a very high serine and aspartine content with about half of the serine residues phosphorylated. Isolated dentine phosphoprotein has been shown to catalyse the formation of apatite from amorphous tricalcium phosphate, and it may act in this way in teeth (Chapter 11.1) [13]. 

Important molecules of this class are dentin phosphoryns or sialoproteins containing high aspartic acid (Asp) and extremely high serine (Ser) amoimts, which can be phosphorylated by the enzymes casein kinases I and II. Examples include rat dentin phosphoprotein with 43% Ser and 31% Asp [155], rat phosphoryn with 56% Ser and 32% Asp [ 156], hiunan dentin phosphophoryn with 58% Ser and 26% Asp [157] or a moUuscan shell glycoprotein with 32% Ser and 20% Asp [158]. It seems that phosphoserin (P-Ser) is of especial importance for polymer-crystal interactions, very Ukely in blocked sequences. In addition, the so-called Asprich proteins were recently identified in nacre and can contain up to 53 mol % Asp and 11% Glu [ 159,160]. 

MacDougall M. et aL 1997. Dentin phosphoprotein and dentin sialoprotein are cleavage products expressed from a single transcript coded by a gene on human chromosome 4. Dentin phosphoprotein DNA sequence determination. / Bio/ Chem 272(2) 835-842. 

Modifications of the organic matrix other than degradation have been studied for their effect on dentin de- or remineralization. Glutardialdehyde cross-linking of matrix in dentin lesions inhibits progressive demineralization (Boonstra et al., 1993). Removal of soluble phosphoproteins promotes calcification of demineralized dentin (Clarkson et al., 1991). 

In addition, peak VI (fig. 1) contained two compounds, one identified as lysinoalanine (table 1). Lysinoalanine is a well-known artefact of alkaline protein treatment but is supposed to be formed in dentin by the reaction between a collagen lysine- and a phosphoprotein phosphoserine residue (Fujimoto et al., 1981). Both compounds were not detected by HPLC after FMOC-derivatization, most likely because of fluorescence quenching inherent to the close vicinity of several FMOC groups attached to one molecule. Thus the unknown compound seems rather similar to lysinoalanine. We suggest the unknown compound is histidinoalanine, which is present in dentin (Fujimoto et al., 1982) and likely shows fluo-rence quenching in its FMOC derivate. 

Carmichael, D. J., Veis, A., Wang, E. T. Dentin matrix collagen evidence for a covalently linked phosphoprotein attachment. Calc. Tiss. Res. 7, 331 (1971)... 

A phosphoprotein from teeth assists the nucleation of dentine. This protein has a number of phosphorylated serine residues.460... 

Unlike bone, dentine is not regenerated and it does not contain osteoclasts for resorption. Phosphoproteins calledphosphoryns are present at the mineralisation front, and these contain about 50% of serine residues of which about 90% are phosphorylated (Chapter 10.2). 

A second category of NCPs with Ca-binding properties is classified as mineraUzed tissue specific, since they are found in all the calcified connective tissues, namely, dentin, bone, and cementum. These proteins include osteocalcin (OC) and bone sialoprotein (BSP). A serine-rich phosphoprotein called dentin matrix protein 1 (Dmp-1), whose expression was first described as being restricted to odontoblasts [5], was later shown to be expressed by osteoblasts and cementoblasts [6]. Other NCPs include osteopontin and osteonectin [secreted protein acidic and rich in cysteine (SPARC)]. The fourth category of dentin NCPs is not expressed in odontoblasts but is primarily synthesized in the liver and released... 

George A. et aL 1993. Characterization of a novel dentin matrix acidic phosphoprotein. ImpUcations for induction ofbiomineralization. /Bio/ Chem 268(17) 12624-12630. 

Phosphoamino acids that are part of proteins known to bind metal ions are posttranslational modifications introduced by specific protein kinases (Meggio et al, 1981 Vogel and Biidger, 1982c). The bovine milk protein casein and the hen egg-white protein ovalbumin, as well as possibly the human saliva acidic proline-iich proteins share sequence homology of their phosphorylated sites. Dephosphorylation of such sites by enzymatic phosphatase treatment usually reduces the affinity of such proteins for metal-ion binding (Bennick et al., 1981). Hence it is likely that dianionic phosphoryl moieties are directly involved in the complexation of metal ions. This seems particularly important for the two polyelectrolyte proteins that contain large amounts of phosphoserine residues, phosvitin purified from egg yolk (Ta-borsky, 1974), and the phosphoprotein purified from dentine (Linde et al, 1980). 

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