Blood coagulation cascade.
Distribution in Nature
Found in green leafy vegetables; very little in fruits and grains. Meats and dairy products
have low to moderate levels of Vit K
Phylloquinones: synthesized in plants (component of chloroplasts).
Menaquinones: synthesized by bacteria
Menadione: synthetic form from lab
Microbial synthesis in the rumen / intestine (with animals that practice coprophagy)
seems to be the most significant in most animal species.
Absorption and Metabolism
Vitamin K synthesized by microbial populations of the rumen and lower GI (obtained by
coprophagy in monogastrics).
Absorption depends upon the form of the vitamin…
Phylloquinone can be absorbed by an energy dependant process in the SI.
Menaquinones and menadione absorbed via passive diffusion in the distal part of
the small intestine as well as the colon!!
After absorption into the IMC Vit K is incorporated into the chylomicron where it is
transported to the liver (as part of the chylomicron remnant) and exported as part of the
Menadione is rapidly metabolized and excreted in the urine.
Catabolized Phylloquinone and Menaquinones are excreted in the feces via bile Tissue distribution:
While “Natural sources” are readily taken up by the liver and other tissues, it has a
relatively short half-life. Consequently there is little long-term storage.
Little Vit K crosses the placenta and there is little in human milk. Human infants are
more susceptible to hemorrhage if they only consume human milk.
Vit K is a cofactor for several carboxylation reactions which occur on the zymogens for
several of the blood clotting factors in the clotting cascade (carboxylates many glutamyl
residues). This carboxylation confers calcium binding capacities to these proteins. This
facilitates the formation of Ca +2 bridges between the clotting factors abad the
phospholipids on membrane surface of the blood platelets and the exposed “Collegen
fibers” and a “Tissue Factor”.
Four clotting factors are involved:
Prothrombin (Factor II)