Identification of organic phosphorus covalently bound to collagen and non-collagenous proteins of chicken-bone matrix. The presence of O-phosphoserine and O-phosphothreonine in non-collagenous proteins, and their absence from phosphorylated collagen

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Abstract
Non-collagenous phosphoproteins, almost all of which can be extracted in EDTA at neutral pH with proteinase inhibitors, are identified in the matrix of chicken bone, and are not covalently bound to collagen. All the peptides containing .gamma.-carboxyglutamic acid are present in the EDTA extract and none in the insoluble residue, confirming that none is covalently linked to chicken bone collagen. Organic P is also found in chicken bone collagen, principally in the .alpha.2-chains. Of the total protein-bound organic P present in chicken bone matrix, approximately 80% is associated with the non-collagenous proteins and 20% with collagen. The soluble non-collagenous proteins contain O-phosphoserine and O-phosphothreonine and these account for all of their organic P content. Collagen contains neither O-phosphoserine nor O-phosphothreonine. No phosphorylated hydroxy amino acid, phosphoamidated amino acid or phosphorylated sugar could be identified in purified components of collagen, which contain approximately 4-5 atoms of organic P/molecule of collagen. Peptides containing organic P 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 P moiety in intact collagen chains strongly suggest that at least part of the organic P in collagen is present as phosphorylated glutamic acid. The 2 major chemically different protein fractions in chicken bone matrix that contain organic P may represent 2 distinct metabolic pools of organic P under separate biological control.