Vitamin K as a Ligand of Steroid and Xenobiotic Receptor pp. 103-112
Authors: (Kotaro Azuma, Kuniko Horie-Inoue, Yasuyoshi Ouchi, Satoshi Inoue, University of Tokyo, and Saitama Medical University, Japan)
Abstract: Vitamin K is a fat-soluble vitamin essential for blood coagulation. Natural vitamin K includes vitamin K1 (phylloquinone), present mainly in vegetables, and vitamin K2 (menaquinone), which is synthesized by microorganisms and is present in food such as natto (fermented soy beans). Vitamin K1 is converted to vitamin K2, the functionally active form, in the body . Vitamin K was shown to play an essential role in the hepatocytes by maintaining the activity of coagulation factors II, VII, IX, and X and of anticoagulants, protein C, and protein S. Recently, extrahepatic actions of vitamin K have also been reported. The administration of vitamin K was shown to prevent bone fracture [2, 3], and this led to its clinical application in cases of osteoporosis in East Asian countries. Epidemiological studies have shown that the lack of vitamin K causes osteoarthritis  and imposes a risk of coronary artery disease . Some clinical studies have suggested its antitumor activity in cases of hepatocellular carcinoma [6-8] and other cancers .