Vitamin K Chemistry Metabolism and Functions

The antihemorrhagic properties of vitamin K were first described in the 1930s; the newly discovered factor was called "coagulation vitamin." Besides the naturally occurring vitamins Ki (phylloqui-none) and K2 (menaquinone), some additional quinones have vitamin K-like effects.

The basic structure of vitamin K (A) is 1,4-naphtoquinone. Its vitamin activity depends on the methyl group in position two, while its fat solubility and other properties are due to the long side chains. Vitamin K2 is a collective term for menaquinones with side chains of different lengths—always with an isoprene unit attached. From a pharmacological perspective, only the fat-soluble vitamins K1 and K2 are of practical significance. The synthetic, water-soluble K3 and its derivatives are no longer used.

Vitamin K supplied with food is actively absorbed in the proximal small intestine during lipid digestion and transported to the liver in chylomicrons. It also circulates in all other lipoprotein fractions (particularly VLDL). Its absorption rate is 20-70 %. Vitamin K2 is also synthesized by intestinal bacteria in the distal small intestine and colon. In that part of the digestive tract, however, the concentration of bile acids is so low that the absorption of fat-soluble K2 from intestinal bacteria is of no great significance.

The first function of vitamin K to be discovered was its role in blood coagulation, and it has been used for therapeutic purposes in this context for a long time. Inactive precursors of the coagu lation factors are synthesized in the liver and activated there by y-glutamyl carboxylase (B). Vitamin K is a cofactor required for this reaction, in which it is converted to 2,3-epoxide. Active vitamin K is subsequently regenerated from the latter in a two-step reaction. Coagulation factors II, VII, IX, and X, as well as the coagulation inhibitors, proteins C and S, are released into the blood, where they attach to phospholipid membranes.

All components of the blood coagulation system are present in the normal neonate's body; however, the vitamin K-dependent factors will not reach the activity levels found in adults for several weeks or months. Additionally, low contents in mother's and cow's milk tend to cause deficiencies in infants. For these reasons, prophylactic vitamin K supplementation has been common practice for a long time. Vitamin K is injected IM right after birth or administered orally in an oily solution. Antagonists that block vitamin K effects also have a long history of therapeutic use: coumarin derivatives are widely used in thrombosis prevention.

In the past decades, additional vitamin K effects have been discovered. It is involved in the formation (carboxyla-tion) of osteocalcin, a bone protein that inhibits Ca2+ mobilization from bone during postmenopause. Matrix Gla-proteins (MGP), which have been found in bone, kidneys, lungs, and heart among others, are also vitamin K dependent.

A. Chemistry

Vitamin K1 (phylloquinone) (2-methyl-3-phytyl-1,4-naphtoquinone)

e.g., in green plants, lettuce, cabbage

Vitamin K1 (phylloquinone) (2-methyl-3-phytyl-1,4-naphtoquinone)

e.g., in green plants, lettuce, cabbage

0 Vitamin K2 (menaquinone)

(2-methyl-3-(prenyl)n-1,4-naphtoquirine)

only in bacteria

Naphtoquinone

Vitamin K3 (menadion) (2-methyl-1,4-naphtoquinone)

B. Effects of Vitamin K

Vitamin Function
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