Vitamin K exist as phylloquinone or vitamin K 1 in plants and as menaquinone or vitamin K 2 produced by the intestinal bacteria. Vitamin K 3 or menadione is a synthetic form. All these three are napthoquinone derivatives.
Isoprenoid side chains are present in vitamins K1 and K2. All 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 diet or synthesized by intestinal bacteria. Its absorption takes place along with fat and is dependant on bile salts. Vitamin K is incorporated into chylomicrons and transported along with LDL and is stored in liver and to a lesser extent in other tissues.
· The functions of vitamin K are concerned with blood clotting process. It brings about post- translational modification of certain blood clotting factors.
· The clotting factors II, VI and X are synthesized as inactive precursors called zymogens in liver.
· Vitamin K acts as coenzyme for the carboxylation of glutamic acid residues present in the proteins and this reaction is catalysed by a carboxylare.
· It involves the conversion of glutamate to gamma carboxyglutamate and requires vitamin K and oxygen and carbondioxide.
· The formation of gamma carboxy glutamate is inhibited by dicoumarol, an anti coagulant found in spoilt sweet clover. Warfarin is the 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.
Recommended dietary allowance
Since this vitamin can be adequately synthesized by the intestinal bacteria there is no strict RDA. But it is suggested 70 to 140 micro grams of vitamin K per day should be supplemented in diet.
Cabbage, cauliflower, tomatoes, spinach, and other green vegetables are good sources. It is also present in egg, meat, liver, cheese, and dairy products.
Deficiency of vitamin K adversely affects blood coagulation. The individual bleeds profusely even for minor injuries. The blood clotting time is increased. Administration of large doses of vitamin K produces haemolytic anemia and juandice, particularly in infants. The toxic effect is due to increased break down of erythrocytes. Antagonists of vitamin K. The compounds namely heparin, bishydroxy coumarin act as anti coagulants and antagonists to vitamin K.