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Vitamin K2 (the menaquinones) is a group name for a family of related compounds, generally subdivided into short-chain menaquinones (with MK-4 as the most important member) and the long-chain menaquinones, of which MK-7, MK-8 and MK-9 are nutritionally the most recognized. == Description == Vitamin K2, the main storage form in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K2 homologues are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues. For example, menaquinone-4 (abbreviated MK-4) has four isoprene residues in its side chain. Menaquinone-4 (also known as menatetrenone from its four isoprene residues) is the most common type of vitamin K2 in animal products since MK-4 is normally synthesized from vitamin K1 in certain animal tissues (arterial walls, pancreas, and testes) by replacement of the phytyl tail with an unsaturated geranylgeranyl tail containing four isoprene units, thus yielding menaquinone-4. This homolog of vitamin K2 may have enzyme functions distinct from those of vitamin K1. Menaquinone-7 is different from MK-4 in that it is not produced by human tissue. MK-7 may be converted from phylloquinone (K1) in the colon by ''E. coli'' bacteria. However, bacterially derived menaquinones (MK-7) appear to contribute minimally to overall vitamin K status. MK-4 and MK-7 are both found in the United States in dietary supplements for bone health. The U.S. Food and Drug Administration (FDA) has not approved any form of vitamin K for the prevention or treatment of osteoporosis; however, MK-4 has been shown to decrease the incidence of fractures up to 87%. MK-4 (45 mg daily) has been approved by the Ministry of Health in Japan since 1995 for the prevention and treatment of osteoporosis. All K vitamins are similar in structure: they share a “quinone” ring, but differ in the length and degree of saturation of the carbon tail and the number of “side chains”.〔Shearer MJ.2003 in Physiology. Elsevier Sciences LTD. 6039-45.〕 The number of side chains is indicated in the name of the particular menaquinone (e.g., MK-4 means that four molecular units - called isoprene units - are attached to the carbon tail) and this influences the transport to different target tissues. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vitamin K2」の詳細全文を読む スポンサード リンク
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