Vitamin K-dependent carboxylase: requirements for carboxylation of soluble peptide and substrate specificity.

Rat liver microsomes contain a triton X-100 solubilizable vitamin K-dependent carboxylase activity that converts specific glutamyl residues of precursor proteins to γ-carboxyglutamyl residues. This activity has been studied utilizing synthetic peptides as substrates for the enzyme. When compared to the carboxylation of the endogenous microsomal precursors, the peptide carboxylase activity is more sensitive to the action of various inhibitors, and requires a higher concentration of vitamin K for maximal activity. The apparent K_m for the peptide Phe-Leu-Glu-Glu-Leu was found to be 4 mM. Substrate specificity depends on residues adjacent to the carboxylated Glu residues and macromolecular recognition sites.