Concentrations of osteocalcin and phosphoprotein as a function of mineral content and age in cortical bone

The contents of two classes of calcium binding proteins of bone matrix, osteocalcin and phosphoprotein, were measured as a function of age of the animal and of the mineral content of chicken bone powder, fractionated according to particle density by differential centrifugation in bromoform-toluene solutions. Whole bone and separated fractions were analyzed for gamma-carboxyglutamic acid (Gla) and for O-phosphoserine [Ser(P)] as indications of their osteocalcin and phosphoprotein contents, respectively. Except for the Ser(P)/collagen ratio of 2-year-old chicken bone, the concentrations of Gla and Ser(P) in the organic protein matrix of the bone increased with increasing mineral content of the tissue, calculated on the basis of the total protein content, the noncollagenous protein content, and the collagen content of the tissue sample. However, a significant difference was found between the concentrations of Gla and Ser(P) with respect to the calcium content of the tissue. The Gla concentration remained relatively constant, whereas the Ser(P) concentration decreased markedly with increasing calcium content. This may reflect the fact that the two classes of proteins have different biological functions and are concentrated in the tissue by different physical chemical mechanisms.