Purification and characterization of calcium-binding protein containing gamma-carboxyglutamic acid from rat bone

A gamma-carboxyglutamic acid (Gla)-containing protein was purified from rat femur cortical bone. The presence of 4 Gla residues in the rat protein was shown by amino acid analysis on alkaline and acid hydrolysates. The protein was extracted from rat cortical bone with 0.5 M EDTA (pH 7.6) and purified from the EDTA extracts by gel filtration on Sephadex G-100 and ion-exchange chromatography on DEAE-Sephadex A-25. The protein has a molecular weight of about 6,000 on the basis of amino acid composition. The protein had 56 amino acid residues containing significant amounts of Asp, Glu, and Gla (acidic amino acids). The protein showed calcium-binding activity with Kd = 0.2 mM and calcium-dependent electrophoretic mobility.