Direct demonstration that the vitamin k-dependent bone gla protein is incompletely γ-carboxylated in humans

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Abstract
Incomplete vitamin K-dependent γ-carboxylation has been found in bone Gla protein (BGP) isolated from each of 20 different human bone samples. Using N-terminal protein sequencing of the methyl-esterified protein (Anal Biochem 1991;199:93-97), a method that directly measures the percentage of γ-carboxylation at each target glutamate residue, the extent of incomplete BGP γ-carboxylation was found to depend strongly on sequence position, with (x̄ + SD) 67 + 14% γ-carboxylation at residue 17, 88 + 9% γ-carboxylation at residue 21, and 93 + 4% γ-carboxylation at residue 24. There is a strong correlation between the incomplete γ-carboxylation at glutamate residues 17 and 21 for BGP purified from the 20 bone samples (p < 0.001), which suggests that individual differences in the efficiency of BGP γ-carboxylation during synthesis probably cause the observed differences in percentage BGP γ-carboxylation between bone samples. These results have been interpreted using a kinetic treatment of γ-carboxylation. This treatment predicts the existence of differences in the extent of γ-carboxylation between glutamate residues in BGP, as well as the correlation between percentage carboxylation at Glu17 and Glu21. Although the molecular basis of incomplete BGP γ-carboxylation is at present unknown, if incomplete BGP γ-carboxylation were caused only by differences in the availability of vitamin K in bone cells, this kinetic treatment predicts that the range in BGP γ-carboxylation observed in the 20 human bone samples studied here could be explained by a relatively modest fivefold range in the vitamin K levels of these individuals.
Funding Information
  • U.S. Public Health Services (AR27029)