Vitamin K

VITAMIN K

Discovered in 1920s when chicks fed low fat cholesterol free diet had GI bleeding.

FORMS

Compounds with Vit K activity have a 2-methyl 1,4-naphthoquinone ring with a substitution at position 3

Naturally occuring forms of vit K:

• PHYLLOQUINONE (K1)
• phytyl group at position 3 
• MENAQUINONE or MK-n (K2)
• Unsaturated multiprenyl group at position 3 
• n indicates number of isoprenoid units in the side chain

SOURCES

ABSORPTION

No digestion, jejunum
MK's synthesized by bacteria
.
TRANSPORT/STORAGE

Chylomyicrons > tissue > liver
K1 (5-25%) can be metabolized to MK-4
Vit K leaves liver bound to VLDL
K1 in serum is 0.2 to 3.2 ng.mL (0.5 - 6.4 mmol/l).
Most taken up by liver within 24 hours.
Stored in cell membranes

FUNCTIONS

Main function: post-translational carboxylation of specific glutamic acid residues in protein to form o-carboxyglutamic acid (GLA) residues.

• Blood clotting
• bone remineralization
• apoptosis, growth, signal transduction

(1) CLOTTING

Proteins involved in carboxylation:

• factors II, VII, IX, X
• Protein C, S, Z, M

For blood to clot, fibrinogen (a soluble protein) must be converted to fibrin (an insoluble protein)
2 major pathways to clot formation: intrinsic and extrinsic.

Warfarin interferes with vti K production:

(2) Bone
Vit K dependant proteins, when carboxylated, bind to hydroxyapetite

• Osteocalcin
• Matrix GIa protein (MGP)

INTERACTIONS

Vit A and E antagonize vit K and interfere with absorption.

EXCRETIONS

K1 undergoes stepwise oxidation of the phytyl side chain
Conjugated with glucuronic acid for excretion in bile.

RDA

120 mcg/day (male) 90 (female)

DEFICIENCY

Rare

TOXICITY

No effects

ASSESSMENT of STATUS

No consistent markers - coag testing