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VITAMIN K
Vitamin K is the only fat soluble vitamin with a specific coenzyme function. It is required for the production of blood clotting factors, essential for coagulation (in German–Koagulation; hence the name K for this vitamin).
Chemistry
- Vitamin K exists in different forms.
- Vitamin K1 (phylloquinone) is present in plants. Vitamin K2 (menaquinone) is produced by the intestinal bacteria and also found in animals.
- Vitamin K3 (menadione) is a synthetic form. All the three vitamins (K1, K2, K3) are naphthoquinone derivatives.
- Isoprenoid side chain is present in vitamins K1 and K2.
- The three vitamins are stable to heat. Their activity is, however, lost by oxidizing agents, irradiation, strong acids and alkalies.
Absorption, transport and storage
- Vitamin K is taken in the diet or synthesized by the intestinal bacteria.
- Its absorption takes place along with fat (chylomicrons) and is dependent on bile salts.
- Vitamin K is transported along with LDL and is stored mainly in liver and, to a lesser extent, in other tissues.
Biochemical functions
- The functions of vitamin K are concerned with blood clotting process.
- It brings about the posttranslational (after protein biosynthesis in the cell) modification of certain blood clotting factors.
- The clotting factors II (prothrombin), VII, IX and X are synthesized as inactive precursors (zymogens) in the liver.
- Vitamin K acts as a coenzyme for the carboxylation of glutamic acid residues present in the proteins and this reaction is catalysed by a carboxylase (microsomal).
- It involves the conversion of glutamate (Glu) to J-carboxyglutamate (Gla) and requires vitamin K, O2 and CO2
- The formation of J-carboxyglutamate is inhibited by dicumarol, an anticoagulant found in spoilt sweet clover.
- Warfarin is a synthetic analogue that can inhibit vitamin K action.
- Vitamin K is also required for the carboxylation of glutamic acid residues of osteocalcin, a calcium binding protein present in the bone.
- The mechanism of carboxylation is not fully understood. It is known that a 2,3-epoxide derivative of vitamin K is formed as an intermediate during the course of the reaction.
- Dicumarol inhibits the enzyme (reductase) that converts epoxide to active vitamin K.
Recommended dietary allowance (RDA)
- Strictly speaking, there is no RDA for vitamin K, since it can be adequately synthesized in the gut.
- It is however, recommended that half of the body requirement is provided in the diet, while the other half is met from the bacterial synthesis.
- Accordingly, the suggested RDA for an adult is 70-140 Pg/day.
Dietary sources
- Cabbage, cauliflower, tomatoes, alfa alfa, spinach and other green vegetables are good sources. It is also present in egg yolk, meat, liver, cheese and dairy products.
