Publisher Summary This chapter discusses the role of vitamin K in the post-translational modification of proteins. It also reviews action of vitamin K and vitamin K antagonists. Protein biosynthesis starts with the attachment of an mRNA to a ribosome. Proteins destined to remain in the cell are synthesized on free cytoplasmic ribosomes, whereas secretory proteins are exclusively formed on ribosomes bound to the rough endoplasmic reticulum (RER). Vitamin K-dependent carboxylase is involved in the conversion of glutamic acid (Glu) residues into γ-carboxyglutamic acid (Gla) residues. When carboxylase is blocked in vivo by the administration of vitamin K antagonists, precursors of the Gla-containing proteins accumulate in the rough endoplasmic reticulum (RER) and when the blockade is prolonged, the non-carboxylated proteins may be excreted into the extracellular environment. Since the discovery of carboxylase, many attempts have been undertaken to purify the enzyme. The methods employed include the use of various detergents, chaotropic agents, ion exchange and size exclusion columns, sulfhydryl binding, hydrophobic columns as well as affinity chromatography on carrier-bound heparin or lectins.