Evidence that free gamma carboxyglutamic acid circulates in serum

We report a rapid, sensitive and reproducible method to measure free gamma-carboxyglutamic acid (GLA) in serum using precolumn derivatization with O-phthalaldehyde, reversed-phase chromatography and deproteinization of serum by ultrafiltration. Serum free GLA level in 62 healthy adults was 167 +/- 46 pmol/ml and was increased (302 +/- 195 pmol/ml) in 14 patients with primary hyper-parathyroidism, a disease characterized by an increased bone turnover. Peptide bound GLA averaged 413 pmol/ml. In rabbits receiving massive doses of warfarin during 6 days there was a time- and dose-dependent decrease of serum free GLA but a significant fraction was still measurable. These data indicate that free GLA circulates and originates both from the metabolism of the vitamin K-dependent clotting factors and from bone metabolism.