Vitamin K Vitamers

As shown in Figure 5.1, compounds with vitamin K activity have a 2-methyl-1,4-naphthoquinone ring. There are two naturally occurring vitamers: phylloquinone (from plants) has a phytyl side chain, whereas the menaquinones (from bacteria) have a polyisoprenyl side chain, with up to 15 isoprenyl units (most commonly 6 to 10), shown by menaquinone-w. Bacteria also form a variety of

Figure 5.1. Vitamin K vitamers and the vitamin K antagonists dicoumarol and warfarin. Relative molecular masses (Mr): phylloquinone, 450.7; menaquinone-4, 447.4; menaquinone-5, 512.8; menaquinone-6, 580.0; menaquinone-7, 649.0; menaquinone-8,717.1; menadione, 172.2; menadiol diacetate, 258.3; dicoumarol, 336.3; and warfarin 308.3.

Figure 5.1. Vitamin K vitamers and the vitamin K antagonists dicoumarol and warfarin. Relative molecular masses (Mr): phylloquinone, 450.7; menaquinone-4, 447.4; menaquinone-5, 512.8; menaquinone-6, 580.0; menaquinone-7, 649.0; menaquinone-8,717.1; menadione, 172.2; menadiol diacetate, 258.3; dicoumarol, 336.3; and warfarin 308.3.

menaquinones with differing degrees of saturation of the side chain, variations in the positions of the double bonds, and sometimes additional methylation of the naphthoquinone ring. The pattern of menaquinones synthesized is useful in the taxonomy and identification of bacteria.

Phylloquinone is vitamin K1, menaquinones are vitamin K2, and the synthetic compounds menadione and menadiol are vitamin K3. In addition to menadione itself, menadiol diacetate (acetomenaphthone) is used in pharmaceutical preparations, and two water-soluble derivatives, menadione sodium bisulfite and menadiol sodium phosphate, have been used for administration of the vitamin by injection and in patients with malabsorption syndromes that would impair the absorption of menadione, phylloquinone, and menaquinones, which are lipid soluble.

Menaquinones are synthesized by intestinal bacteria, but it is unclear how much they contribute to vitamin K nutrition, because they are extremely hy-drophobic, and will only be absorbed from regions of the gastrointestinal tract where bile salts are present - mainly the terminal ileum. However, prolonged use of antibiotics can lead to vitamin K deficiency and the development of vitamin K-responsive hypoprothrombinemia (Section 5.4), as can dietary deprivation of phylloquinone. In vitro, menaquinones 2 to 6 have the same activity as phylloquinone as coenzyme for the solubilized liver microsomal vitamin K-dependent carboxylase (Section 5.3.1), whereas menaquinones with a side chain longer than seven have lower activity (Suttie, 1995). In extrahepatic tissues, the principal active vitamer is menaquinone-4 (Thijssen and Drittij-Reijnders, 1996; Thijssen et al., 1996).

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