Comparison between hepatic and nonhepatic vitamin K-dependent carboxylase

Vitamin K-dependent carboxylase is a microsomal enzyme system involved in the carboxylation of protein-bound glutamic acid residues. In mammals, the enzyme is found in many different types of tissue. Hence carboxylated ('Gla-containing') proteins are widely distributed in nature. Neither in vitro nor in vivo differences have been observed with respect to the vitamin K-binding sites of the various carboxylases. Differences between the substrate-binding sites could only be compared after suitable substrates became available. These substrates were prepared from descarboxyprothrombin, osteocalcin and a sperm Gla protein. Substantial differences were detected between the Michaelis constants of various carboxylases for the three substrates mentioned above. It is concluded that vitamin K-dependent carboxylase is a group name for a family of isoenzymes.