Noncollagenous proteins of rat compact bone

Abstract

In order to obtain a comprehensive overview of the noncollagenous proteins (NCPs) of bone matrix, the NCPs were extracted from rat compact bone and fractionated using methods aiming to prevent artifactual degradation and losses of protein. The NCP content of rat bone was found to be similar to that of rat dentin in several respects but different in others. The soluble NCPs of bone fell into four categories: acidic glycoproteins, gamma-carboxyglutamate-containing proteins, phosphoproteins, and proteoglycans. With the exception of the gamma-carboxyglutamate-containing proteins, the majority of NCPs had apparent molecular weights exceeding 50,000. As in rat dentin, several gamma-carboxyglutamate-containing proteins could be demonstrated in rat bone. Earlier studies have only taken one molecular species into consideration. No highly phosphorylated phosphoprotein could be demonstrated in bone. However, at least two phosphoproteins with a low degree of phosphorylation were found to be present. No plasma proteins could be demonstrated in any of the chromatographic fractions from the EDTA extracted NCPs by means of double diffusion. The NCPs, remaining firmly associated with the collagenous matrix after thorough demineralization and extraction, were analyzed after CNBr and collagenase degradation of the matrix. Much smaller amounts of phosphoprotein were recovered after CNBr digestion than reported earlier. Collagenase digestion released small amounts of acidic glycoprotein, phosphoprotein, and proteoglycan. The results give additional evidence that this small remainder might be explained, not by any covalent linkage to collagen, but by an inefficient extraction.