Phylloquinone, biological activity

The double bond at C-3 of tbe side chedn is essentieil to activity and has trans-configuration cts-phylloquinone has no biological activity. This double bond seems to be involved in tbe deprotonation of the gluteunate substrate. 

The mechanism of intestinal absorption of compounds with vitamin K activity varies with their solubility. In the presence of bile salts, phylloquinone and the menaquinones are adequately absorbed from the intestine, almost entirely by way of the lymph. Phylloquinone is absorbed by an energy-dependent, saturable process in proximal portions of the small intestine menaquinones are absorbed by diffusion in the distal portions of the small intestine and in the colon. Following absorption, phylloquinone is incorporated into chylomicrons in close association with triglycerides and lipoproteins. The extremely low phylloquinone levels in newborns may be partly related to very low plasma lipoprotein concentrations at birth and may lead to an underestimation of vitamin K tissue stores. Absorbed phylloquinone and menaquinones are concentrated in the liver, but the concentration of phylloquinone declines rapidly. Menaquinones, produced in the lower bowel, are less biologically active than phylloquinone due to their long side chain. Very little vitamin K accumulates in other tissues. 

The fat-soluble vitamins can be divided into four different groups, based on biological activity vitamin A (retinol and carotenoids), vitamin D (chole-calciferol and ergocalciferol), vitamin E (tocopherols and tocotrienols), and vitamin K (phylloquinone and menaquinone), all of which may be present in a munber of closely related forms. 

Intestinal bacteria synthesize a variety of menaquinones, which are absorbed to a limited extent from the large intestine, again into the lymphatic system, cleared by the liver and released in VLDL. It is often suggested that about half the requirement for vitamin K is met by intestinal bacterial synthesis, but there is little evidence for this, other than the fact that about half the vitamin K in liver is phylloquinone and the remainder a variety of menaquinones. It is not clear to what extent the menaquinones are biologically active — it is possible to induce signs of vitamin K deficiency simply be feeding a phylloquinone-deficient diet without inhibiting intestinal bacterial action. 

Vitamin Kp also known as vitamin Kk20)> phylloquinone or phytylmenaquinone, contains a hexahydrotetraprenyl chain (2-methyl-3-phytyl-l,4-naphthoquinone, 5-44). Phylloquinone occurs in foods of plant origin. The side chain derived from phytol has 20 carbon atoms (four isoprenoid units, of which three are reduced). The isomer with a cis-configuration in the side chain is not biologically active. The systematic name of vitamin Kj is (2 ,7 R,ll R)-2-methyl-3-(3, 7Ml, 15 -tetramethyUiexadecyl)naphtho-l,4-quinone. The second compound with the activity of vitamin K is vitamin K2 (5-45), also known as vitamin K2( ), menaquinone, menaquinone-n, MK-n or 2-methyl-3-multiprenyl-l,4-naphthoquinone (n = 0-13). The most common menaquinones contain 4-10 isoprene units, such as MK-7, additionally called pharnoquinone. Vitamin K2 is produced by many bacteria and actinomycetes. A multiprenyl side chain... 

Biological samples may contain any or all of three forms of vitamin K vitamin Ki(20) (phylloquinone) which is synthesized by green plants and found in chloroplasts of photosynthetic plants and is the major form of vitamin K in plant and most animal foods vitamin K2, a group of vitamers (menaqui-nones) synthesized by bacteria and ranging from MK-4 to MK-13 according to the number of isoprene units in the side chain, and finally synthetic vitamin K3 (menadione), which exhibits vitamin K activity by virtue of its in vivo conversion to menaquinones, chiefly MK-4. The latter is a pharmaceutical and water-soluble form which does not occur in food and unlike the other forms is toxic at high intakes. Such water-soluble derivatives of menadione are used as animal feed supplement but owing to toxicity to humans menadione cannot be used in human medicine or as a food supplement. 

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