Vitamin Ko, Menaquinones
Fig. 4.9 Structures of compounds with vitamin K activity.
about their bioavailability and hence their significance as a source of vitamin K.
Free phylloquinone is absorbed from the small intestine with about 80% efficiency, but only about a tenth of the vitamin K1 present in dark green leafy vegetables is absorbed. Vitamin K is carried in chylomicrons (Section 5.2.3) and delivered mainly to the liver, in chylomicron remnants (Section 5.2.5). About two-thirds of the vitamin K intake is soon excreted from the body (into the faeces via the bile) and as there is no evidence of an entero-hepatic circulation, it is presumed that a constant intake is essential.
Like many water-soluble vitamins, but in contrast to other fat-soluble vitamins, vitamin K has a specific function as an enzyme cofactor. The enzyme, present in the endoplasmic reticulum of liver cells and in some other tissues, is y-glutamyl- (or vitamin K-dependent-) carboxylase. It requires both molecular oxygen and carbon dioxide and inserts a carboxyl group into the glutamyl residue of several specific calcium-binding proteins. The reaction is a post-translational modification of the protein. The active form of vitamin K in this reaction is the reduced quinol form, which donates hydrogen to glutamic acid, being transformed in the process to an epoxide. The 2,3-epoxide is then transformed to the quinone form by vitamin K epoxide reductase in a dithiol-dependent reaction and the quinone can be further reduced to the quinol, thus completing the so-called vitamin K epoxide cycle.
The best-studied role of vitamin K has been in relation to the blood coagulation cascade. Four of the procoagulant proteins of the cascade depend on the presence of vitamin K for their formation (factor II, or prothrombin, and factors VII, IX and X). The formation of y-carboxyglutamate residues in these protein factors provides efficient chelating sites for Ca ions, enabling ion bridges to be made between the factor and the surface phospholipids of platelets and endothelial cells. Anticoagulants, such as
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