Principles of Biochemistry

10.2.4 Vitamin K

The substances (natural and synthetic) with vitamin K activity are napthoquinones. Two naturally occurring vitamin K have been identified. Vitamin K₁, Phylloquinone occurs in plants while Vitamin K₂, multiprenylmenaquinone is present in animal tissues. Menadione (Vitamin K₃) is the most important synthetic compound. Vitamins K is called coagulation factor.

Functions: Vitamin K is required for the biosynthesis of blood clotting factors. Vitamin K has been shown to be involved in the maintenance of normal levels of blood clotting factors II, VII, IX, and X, all which are synthesized in the liver initially as inactive precursor proteins. Vitamin K acts as cofactor of the carboxylase that forms γ-carboxyglutamate in precursor proteins.

Deficiency: Its deficiency interferes with the formation of prothrombin and thus reduces the clotting tendency of blood. Hence the clotting time is reduced. Internal and external hemorrhages may ensue either spontaneously or following injury or surgery.

Sources : Vitamin K is widely distributed in plant and animal tissues used as food, and production of the vitamin by the micro flora of the intestine virtually ensures that dietary deficiency does not occur in adults. However, newborn infants are vulnerable to the deficiency, because the placenta does not pass the vitamin to the fetus efficiently and the gut is sterile immediately after birth. In normal infants, the plasma concentration decreases immediately after birth but recovers after food is absorbed. If the prothrombin level drops too low, the hemorrhagic syndrome may appear.