Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Vitamins
Fat-soluble vitamins
Vitamin K group

According to biological chemistry nomenclature, the Vitamin K group includes Two Types of Quinones with side chains represented by isoprenoid units: Vitamins K₁ and K₂*. The cyclic core of both vitamins is based on a 1,4-naphthoquinone ring. It is worth noting that animal Tissues are capable of synthesizing isoprenoid side chains, but cannot synthesize the naphthoquinone component. In most Bacteria, vitamin K serves as a component of the Respiratory Chain instead of ubiquinone.

Vitamin K₁ retains the name "phylloquinone", while the K₂ vitamins are designated as "menaquinone", accompanied by the number of isoprenoid units**. Specifically, vitamin K₂ with six isoprenoid units in its side chain is recommended to be named "menaquinone-6", where the number 6 indicates the length of the side chain.

Vitamin K₁ (phylloquinone) was first isolated from alfalfa. It is a derivative of 2-methyl-1,4-naphthoquinone containing a 20-carbon phytyl radical at the 3-position:

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Vitamin K₂ was discovered in plants and animals and contains 6 to 9 isoprenoid units in its side chain.

* E. Doisy and H. Dam were awarded the Nobel Prize in 1943 for the Discovery of the antihemorrhagic action of vitamin K.

** When designating the length of the isoprenoid side chain, the number of isoprenoid units is used as the basis rather than the number of carbon atoms.

Vitamin K₁ is a pale yellow liquid that is unstable when heated in an alkaline medium and upon exposure to light, whereas vitamin K₂ forms yellow crystals and is likewise unstable. Both compounds are insoluble in Water, but readily soluble in organic Solvents such as benzene, chloroform, acetone, hexane, etc.

In addition to vitamins K₁ and K₂, certain naphthoquinone derivatives exhibit vitamin activity and high antihemorrhagic potency. For instance, the synthetic analogue of vitamin K lacking a side chain at the 3-position is called vitamin K₃ (menadione, or 2-methyl-1,4-naphthoquinone); it essentially acts as a provitamin. Since vitamin K₃ is insoluble in water, dozens of water-soluble derivatives have been synthesized from it, one of which has found widespread clinical application—vicasol, the sodium bisulfite derivative of vitamin K₃ synthesized by A.V. Palladin:

Vitamin K is an antihemorrhagic factor intimately linked to Blood Coagulation, significantly prolonging clotting time. Consequently, vitamin K deficiency leads to spontaneous parenchymal and capillary hemorrhages (such as nosebleeds and internal bleeding). Furthermore, any vascular trauma (including surgical Procedures) in The Setting of vitamin K deficiency can result in profuse bleeding. Vitamin K deficiency in humans is rarer than other vitamin deficiencies. This is explained by two factors: first, a mixed diet is relatively rich in vitamin K (group K vitamins are synthesized in green plants and certain microorganisms); second, The amount of vitamin K synthesized by the intestinal microflora is quite sufficient to prevent deficiency. Deficiencies typically develop when intestinal fat absorption is impaired. Infants frequently experience severe subcutaneous bleeding and hemorrhages, which are also observed in hemorrhagic diathesis resulting from maternal coagulation factor deficiency.

Biological role. Vitamin K is involved in the hepatic synthesis of prothrombin, most likely via an enzymatic system. Evidence has been obtained that vitamin K is required as a stimulator for the hepatic Biosynthesis of at least four protein-Enzymes participating in the complex blood coagulation cascade: factors II, VII, IX, and X. Specifically, data indicate that the molecules of these factors invariably contain carboxyglutamic acid residues; active prothrombin contains 10 such residues. Prothrombin, acting as a proteolytic enzyme, cleaves specific peptide bonds in the soluble blood protein fibrinogen to form insoluble fibrin (see Chapter 17). It has been demonstrated that the γ-carboxylation of glutamic acid residues in protein molecules, particularly prothrombin, occurs post-translationally with the participation of γ-glutamyl carboxylase, which requires vitamin K; HCO₃^- serves as the source of CO₂. In this reaction, vitamin K apparently Functions as a cofactor.

The post-synthetic carboxylation of the γ-carboxyl group of glutamate also plays a crucial role in binding Ca²⁺ ions to the protein molecule, as this generates additional negatively charged carboxyl groups. It should be noted that biotin does not participate in this carboxylation reaction.

One of the potent vitamin K antagonists is the natural substance dicoumarol (dicumarol). Its administration causes a sharp decrease in blood levels of prothrombin and several other protein clotting factors, consequently leading to hemorrhages. The synthetic vitamin K analogue warfarin exhibits a similar anticoagulant property, acting as a competitive inhibitor of thrombus formation.

The ability of dicoumarol and warfarin to reduce blood coagulability was subsequently widely adopted for treating human disorders characterized by hypercoagulability. Specifically, in cases of coronary thrombosis and thrombophlebitis, both drugs help dissolve blood clots, exerting an effective therapeutic action. If bleeding occurs following the administration of dicoumarol or warfarin, patients are treated with vitamin K preparations.

Natural occurrence and daily requirement. Vitamin K is most abundant in plants, particularly the green leaves of chestnut, nettle, and alfalfa. Plant sources rich in vitamin K include cabbage, spinach, pumpkin, green tomatoes, peanut oil, rowan berries, etc. Among animal products, aside from pork Liver, it is virtually absent. The daily human requirement for vitamin K has not been precisely established because it is synthesized by intestinal microorganisms; an intake of about 1 mg is generally considered sufficient.