Direct demonstration that the vitamin K-dependent bone Gla protein is incompletely gamma-carboxylated in humans

Incomplete vitamin K-dependent gamma-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 gamma-carboxylation at each target glutamate residue, the extent of incomplete BGP gamma-carboxylation was found to depend strongly on sequence position, with (chi +/- SD) 67 +/- 14% gamma-carboxylation at residue 17.88 +/- 9% gamma-carboxylation at residue 21, and 93 +/- 4% gamma-carboxylation at residue 24. There is a strong correlation between the incomplete gamma-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 gamma-carboxylation during synthesis probably cause the observed differences in percentage BGP gamma-carboxylation between bone samples. These results have been interpreted using a kinetic treatment of gamma-carboxylation. This treatment predicts the existence of differences in the extent of gamma-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 gamma-carboxylation is at present unknown, if incomplete BGP gamma-carboxylation were caused only by differences in the availability of vitamin K in bone cells, this kinetic treatment predicts that the range in BGP gamma-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.