Vitamin K activity is associated with at least two distinct natural substances, designated as vitamin K1 and vitamin K2. Vitamin Kv or phylloquinone (phytonadione), is 2-methyl-3-phytyl-1, 4-naphthoquinone; it is found in plants and is the only natural vitamin K available for therapeutic use. Vitamin K2 is actually a series of compounds (the menaquinones) in which the phytyl side chain of phylloquinone has been replaced by a side chain built up of 2-13 prenyl units. Considerable synthesis of menaquinones occurs in gram-positive bacteria; indeed, intestinal flora synthesize the large amounts of vitamin K contained in human and animal feces. In animals, menaquinone-4 can be synthesized from the vitamin precursor menadione (2-methyl-1,4-naphthoquinone), or vitamin K3. Depending on the bioassay system used, menadione is at least as active on a molar basis as phylloquinone.
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