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Structure, Activity, and Distribution of Fish Osteocalcin*

Osteocalcin (bone Gla protein) is an extracellular matrix protein synthesized by osteoblasts that is a marker of bone. Osteocalcin probably originated in the ancestors of Teleostei or bony fish and of the Tetrapoda or amphibians, reptiles, birds, and mammals. We have characterized theCyprinus carpio (carp) osteocalcin for mineral binding to hydroxyapatite, amino acid sequence, and extent of secondary structure. Hydroxyapatite binding is enhanced in the presence of calcium. The α-helical content of teleost osteocalcin increases and β-sheet structure decreases upon calcium binding, similar to findings in calf osteocalcin. The gene structure and primary sequence of prepro-osteocalcin from 2 pufferfish compared with carp shows that there are many conserved features in teleost osteocalcin genes. Using an immunoassay for carp osteocalcin, we determined that the relative content of osteocalcin is highest in dorsal fin spines and other bones and lowest in scales. The carp osteocalcin antibodies, cross-reactive to other species of fish, were used to study the role of osteocalcin in teleost model systems.