Purification, composition, and31P NMR spectroscopic properties of a noncollagenous phosphoprotein isolated from chicken bone matrix

Abstract

Fractionation of the EDTA-soluble, noncollagenous proteins of the organic matrix of chicken bone by Sephadex G-100 molecular sieving has revealed that the majority of the organic phosphorus is present in two fractions, from one of which a homogeneous phosphoprotein has been isolated. The purified phosphoprotein has an apparent molecular weight of 12,000 and contains bothO-phosphoserine andO-phosphothreonine.³¹P-NMR spectroscopy demonstrates that all of the organic phosphorus exists in the form of phosphomonoesters which have an average pK₂ of 6.8. The phosphoprotein is highly acidic due to its high content of dicarboxylic acids in addition to the presence of organic phosphorus. The characteristic amino acid composition of the phosphoprotein establishes its noncollagenous nature and highlights the differences among bone, dentin, and enamel phosphoproteins. The absence ofγ-carboxyglutamic acid distinguishes it from osteocalcin, the noncollagenousγ-carboxyglutamic acid-containing peptide of bone matrix.