Correlations between osteocalcin content, degree of mineralization, and mechanical properties of C-Carpio rib bone

Osteocalcin is one of the most abundant noncollagenous proteins in bone. It is strongly associated with the mineral phase of bone, and has long been a ssociated as a marker of bone turnover. However, its relationship to bone c omposition, strength, and structure is unclear. Carp rib bone is an excelle nt model for the study, because osteocalcin represents almost 60% of the to tal extractable noncollagenous proteins found in it. Because of the abundan ce of osteocalcin relative to other extractable proteins, any changes in th e properties of carp rib bone would be more likely influenced by the osteoc alcin concentration. To test the hypotheses that the concentration of osteo calcin is reflected in other properties of bone, the correlations between t he osteocalcin concentration and the mineral content, microstructural prope rties, and physical characteristics of the bone mineral crystals were deter mined utilizing radioimmunoassay (RIA), spectrophotometry, nanoindentation, and small-angle X-ray scattering (SAXS) techniques, respectively. Osteocal cin concentration was found to be correlated to the molar Ca/P ratio and in versely correlated to the elastic modulus and hardness in the longitudinal plane. This study provides evidence for a putative relationship between the concentration of osteocalcin and the microstructural mechanical properties of bone. Correlations were also found between the mechanical properties in the longitudinal plane and both the phosphate content and the molar Ca/P r atio. However, no relationships could be identified between osteocalcin con centration and several parameters of bone crystals, as determined by SAXS. (C) 2000 John Wiley & Sons, Inc.