Bone Formation Markers : Osteocalcin
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p511
2025-12-18
60
Osteocalcin (OC) is a small protein of 5.8 kDa synthesized by osteoblasts, odontoblasts, and hypertrophic chondrocytes. The protein originates from a 75 amino acid propeptide which undergoes carboxylation of three glutamic acid residues through a vitamin K-dependent process. After carboxylation, a sequence of 25 amino acids is detached from the propeptide, and the mature protein is released into the cell. The high affinity of γ-glutamic residues for hydroxyapatite suggests an involvement of this protein in the process of bone mineralization. The newly synthesized OC is minimally released from the cell into the bloodstream while a greater amount is incorporated into the bone matrix, representing about 15% of the non-collagenous proteins of bone. Following bone resorption, osteocalcin is released into the bloodstream. By interacting with cell surface proteins and receptors, OC plays an active role in the organization of the extracellular matrix. However, although known for more than 20 years, its exact role is still to be defined. It has long been considered a marker of bone formation, but recent studies have re-evaluated its function, attributing it also an endocrine role and a regulator of energy metabolism. However, the results of clinical studies show a good correlation between serum OC levels and the rate of bone formation, verified by histomorphometry. Therefore, this biomarker is mainly used to evaluate the bone formation process. However, it is a protein subject to rapid degradation; there fore, the intact form and fragments of various sizes are found in the circulation. The intact molecule, consisting of 49 amino acids (aa), is present in serum for about 36% while circulating proteolytic enzymes can give rise to different fragments: an N-terminal fragment (1–19 aa), a fragment including the 20–43 aa tract, a C-terminal fragment (44–49 aa), a mid-N-terminal fragment (1–43 aa), and a mid-C- terminal fragment (20–49 aa).
Commercially available methods are immunometric using polyclonal and monoclonal antibodies directed against different epitopes of the marker. Consequently, numerous works in the literature that have compared the results of OC obtained by different methods demonstrate the impossibility of comparing the data obtained. The high heterogeneity of OC fragments in serum, associated with the instability of the molecule and the undefined role in the processes of bone turnover, determine significant limitations to using this marker in clinical practice. However, the methods that measure both the intact molecule and the fragment 1–43 seem to reflect the real synthesis of osteocalcin and, for this reason, would be preferred.
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