Timed appearance of a calcium-binding protein containing gamma-carboxyglutamic acid in developing chick bone

Bone contains a small protein, rich in the vitamin K-dependent calcium-binding amino acid γ-carboxyglutamate (Gla). This protein, named osteocalcin, is extractable by neutral EDTA demineralization and contains over 80% of the total peptide Gla found in bone. Osteocalcin binds Ca²⁺ ions with moderate affinity (2 moles of Ca²⁺/6500 g of protein; K_d = 0.83 mM). Osteocalcin appears in embryonic chick bones (mandible, calvaria, tibiotarsus, and femur) coincident with the first histologically observable deposition of bone mineral at 8 to 12 days after fertilization. The quantity of this protein increases dramatically during development with characteristic onset and kinetics for each type of bone. In the long bone diaphysis (midshaft), the fraction of noncollagen protein represented by osteocalcin increases 100- to 200-fold between the 8th and 20th day. Relative to total bone protein, the increase is about 35-fold. Osteocalcin may play a role in the development of mineralized tissues and may be a characteristic product of cells differentiated with respect to bone and/or cartilage formation.