Highly purified (>95% by SDS-PAGE). Osteocalcin, also known as Bone-Gla-Protein (BGP), is a single chain (Mr=5800) vitamin K dependent protein produced by osteoblasts. It is found in high concentrations in bone (1-4). Post-translational modification by a vitamin K dependent carboxylase produces three g-carboxyglutamic acid residues at positions 17, 21 and 24. The mature protein contains 49 amino acids and a single intrachain disulfide bond joining Cys23 to Cys29. The secondary structure is highly calcium dependent and contains 14% alpha-helix, 20% beta-sheet and 67% random form in the presence of calcium, and 1% alpha-helix, 20% beta-sheet and 79% random form in the absence of calcium (4). BGP binds to phospholipid vesicles in the presence of calcium ions (Kd=6 x 10e-6 M) (5). BGP also binds hydroxyapatite and is an efficient inhibitor of hydroxyapatite seeded crystal growth, suggesting a regulatory role in bone mineralization (6). Although the fundamental role of BGP in vivo remains e