Vitamin K-dependent carboxylase. Carboxylation of aspartyl residues to beta-carboxyaspartyl residues in synthetic substrates.

The rat liver microsomal vitamin K-dependent carboxylase has been shown to carboxylate the aspartyl residue in N alpha-t-butyloxycarbonylaspartic acid alpha-benzyl ester to a beta-carboxyaspartyl residue. The enzyme also carboxylates aspartyl residues in low molecular weight peptides to beta-carboxyaspartyl residues, but much less effectively than Glu residues in homologous peptides are carboxylated to gamma-carboxyglutamyl residues. The carboxylated product was identified by comparison to authentic beta-carboxyaspartate, by loss of 50% radioactivity incorporated from 14CO2 upon acid hydrolysis, and by identification of [14C]aspartic acid following acid hydrolysis. When peptides containing both aspartyl and glutamyl residues were used as substrates for the enzyme, the only carboxylated product detected was gamma-carboxyglutamic acid.