The small molecular weight, gamma-carboxyglutamic acid-containing protein of rabbit bone tissue

The major gamma-carboxyglutamic acid-containing protein of rabbit cortical bone isolated and purified from near-neutral (pH 7.5) EDTA-extracts by DEAE-cellulose and Sephadex G 75 column chromatography had a molecular weight of about 5600 based on integral amino-acid composition; this was confirmed by high-performance liquid chromatography. The purified protein had high glutamic acid and aspartic-acid contents, two to three residues of gamma-carboxyglutamic acid per molecule and zero levels of serine, threonine, methionine, histidine and tryptophan. Equilibrium dialysis indicated that the protein has a weak affinity for calcium ions with a formation constant of 1515 M-1 at I 0.15, pH 7.5, 25 degrees C with a binding capacity of 2 mol calcium per mole protein.