Chemically, all vitamin Ks are 2-methyl 1,4-naphtho-quinone derivatives containing variable aliphatic side chains at C3. Phylloquinone (Fig. 28.16) invariably contains a phytyl side chain. Menaquinones (Fig. 28.17) are a series of compounds that have a longer side chain with more unsaturation. This side chain may be composed of 1 to 13 prenyl (isoprenyl) units.
Many other closely related compounds possess vitamin K activity. Of particular historical note are the water-soluble menadione (2-methyl-1,4-naphthoquinone; vitamin K3), which is no longer marketed because of toxicities, and mena-diol (2-methylnaphthalene-1,4-diol; vitamin K4). Menadione contains the naphthoquinone ring without any attached prenyls and can be thought of as MK-0 (see Fig. 28.17). Menadiol is menadione with the two keto groups reduced to hydroxyls. Both compounds are bioactivated to the active form through alkylation. Phylloquinone naturally occurs as a trans-isomer and has an R,R,E configuration. The synthetic, commercially available form is a mixture of cis- and trans-isomers, with no more than 20% cis.
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