Non-collagenous phosphoproteins, almost all of which can be extracted in EDTA at neutral pH in the presence of proteinase inhibitors, are identified in the matrix of chicken bone, and are therefore not covalently bound to collagen. Similarly, all the peptides containing y-carboxyglutamic acid are present in the EDTA extract and none in the insoluble residue, confirming that none is covalently linked to chicken bone collagen. However, organic phosphorus is also found to be present in chicken bone collagen, principally in the a2-chains. Of the total protein-bound organic phosphorus present in chicken bone matrix, approx. 80 % is associated with the non-collagenous proteins and 20 % with collagen. The soluble non-collagenous proteins contain both O-phosphoserine and 0-phosphothreonine and these account for essentially all of their organic phosphorus content. In contrast, collagen contains neither 0-phosphoserine nor 0-phosphothreonine. Indeed, no phosphorylated hydroxy amino acid, phosphoamidated amino acid or phosphorylated sugar could be identified in purified components of collagen, which contain approximately four to five atoms of organic phosphorus per molecule of collagen. Peptides containing organic phosphorus were isolated from partial acid hydrolysates and enzymic digests of purified collagen components, which contain an as-yet-unidentified cationic amino acid. These data, the very high concentrations of glutamic acid in the phosphorylated peptides, and the pH-stability of the organic phosphorus moiety in intact collagen chains strongly suggest that at least part of the organic phosphorus in collagen is present as phosphorylated glutamic acid. This would indicate that the two major chemically different protein fractions in chicken bone matrix that contain organic phosphorus may represent two distinct metabolic pools of organic phosphorus under separate biological control.Phosphoproteins have been identified as part of the non-collagenous components of the organic matrices of mineralized vertebrate tissues such as enamel (Seyer & Glimcher, 1971;Papas et al., 1977;Seyer & Glimcher, 1977a), dentine Veis & Perry, 1967;Butler, 1972;Butler et al., 1972;Veis et al., 1972;Carmichael & Dodd, 1973;Pieri et al., 1975;Menanteau et al., 1977) and bone (Spector & Glimcher, 1972;Shuttleworth & Veis, 1972) [see Leaver et al. (1975b) for a review]. There still exists, however, at least two major points of controversy with regard to bone and dentine: (1) how much, if any, of the phosphorylated protein(s) is covalently bound to the collagen, and (2), if present, Abbreviation used: P,, organic phosphate. * Present address: I.N.S.E.R.M. U12 and U18, Hc6pital des Enfants malades, Paris, France. t To whom requests for reprints should be addressed. Vol. 177 the nature of the organic phosphate moiety covalently bound to the collagen.Although evidence has been presented that none of the phosphoprotein is covalently bound to the collagen of rat dentine (Butler, 1972;Butler et al., 1972Butler et al., , 1976, equally substantia...
O-phosphoserine [Ser(P)], O-phosphothreonine [Thr(P)], and gamma-carboxyglutamic acid (Gla) have been identified in native, calcified cementum and in non-collagenous proteins which can be extracted from the tissue in EDTA at neutral pH. The concentrations of Ser(P) and Thr(P) and the amino acid composition of the EDTA extractable proteins are more similar to those found in bone than in dentin or enamel. The concentration of Gla in cementum is lower than it is in bone and higher than it is in enamel, which contains essentially no Gla. Based on the contents of Gla in these mineralized tissues and the distribution of alkaline and acid phosphatases in these tissues, it is speculated that Gla may be part of these or other proenzymes rather than being involved directly and structurally with the deposition of the mineral phase.
Amino acid analyses of undecalcified samples of fossil crocodile and rhinoceros enamel and dentin from mature teeth revealed that the total protein content of these mineralized fossil tissues varied from ~0.01–0.007% by weight. Except in one instance, amino acid analyses of the enamel proteins revealed them to be free of collagen and to have an amino acid composition similar to the proteins obtained from the enamel of mature modern vertebrates. Molecular sieving of the acid soluble enamel proteins demonstrated that the components consisted principally of small peptides and free amino acids, as in the enamel of modern vertebrates.Based on the presence of hydroxyproline (hyp) and hydroxylysine (hyl), collagen was detected in undecalcified mature dentin of fossil rhinoceros, but not in undecalcified crocodile dentin. It was only by sequential extraction procedures that the presence of collagen in the dentin of fossil crocodile was established, emphasizing the utility and importance of analyzing the soluble components in specific extracts of the fossil. Based on these data and the concentrations of hyp and hyl in the material solubilized by the various solvents, the dentin of fossil rhinoceros contained considerably more collagen per weight and as a percentage of the total protein in the dentin than did the dentin of fossil crocodile.As with modern dentin, EDTA and dilute acid solubilize the noncollagenous proteins and peptides found in fossil enamel and dentin, some of which contain O-phosphoserine [Ser(P)], an amino acid unique to mineralized connective tissues. Similar to recent reported findings from fossil bone, less of the original content of the noncollagenous proteins, including those phosphorylated, is degraded and removed from the enamel and dentin during fossilization than the percentage of dentinal collagen and the nonphosphorylated domains of the enamel proteins which are removed. This selective resistance to degradation and removal of the noncollagenous proteins, including phosphoproteins, may reflect the strong interaction of these proteins with the mineral phase of fossilized tissues. The amount and close packing of the inorganic crystals may also inhibit the interaction of the proteins in the interior of the enamel and dentin with the geochemical environment.
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