Vitamin K

Vitamin K is a group of structurally similar, fat-soluble vitamins the human body requires for complete synthesis of certain proteins that are prerequisites for blood coagulation that the body needs for controlling binding of calcium in bones and other tissues. The vitamin K-related modification of the proteins allows them to bind calcium ions, which they cannot do otherwise. Without vitamin K, blood coagulation is seriously impaired, and uncontrolled bleeding occurs. Low levels of vitamin K also weaken bones and promote calcification of arteries and other soft tissues.

Chemically, the vitamin K family comprises 2-methyl-1,4-naphthoquinone (3-) derivatives. Vitamin K includes two natural vitamers: vitamin K₁ and vitamin K₂. Vitamin K₂, in turn, consists of a number of related chemical subtypes, with differing lengths of carbon side chains made of isoprenoid groups of atoms.

Vitamin K₁, also known as phylloquinone, phytomenadione, or phytonadione, is synthesized by plants, and is found in highest amounts in green leafy vegetables because it is directly involved in photosynthesis. It may be thought of as the "plant" form of vitamin K. It is active as a vitamin in animals and performs the classic functions of vitamin K, including its activity in the production of blood-clotting proteins. Animals may also convert it to vitamin K₂.

Vitamin K2

Vitamin K₂ (the menaquinones) is a group name for a family of related compounds, generally subdivided into short-chain menaquinones (with MK-4 as the most important member) and the long-chain menaquinones, of which MK-7, MK-8 and MK-9 are nutritionally the most recognized.

Description

Vitamin K₂, the main storage form in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K₂ homologues are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues. For example, menaquinone-4 (abbreviated MK-4) has four isoprene residues in its side chain. Menaquinone-4 (also known as menatetrenone from its four isoprene residues) is the most common type of vitamin K₂ in animal products since MK-4 is normally synthesized from vitamin K₁ in certain animal tissues (arterial walls, pancreas, and testes) by replacement of the phytyl tail with an unsaturated geranylgeranyl tail containing four isoprene units, thus yielding menaquinone-4. This homolog of vitamin K₂ may have enzyme functions distinct from those of vitamin K₁.

Menadione

Menadione is a synthetic chemical compound sometimes used as a nutritional supplement because of its vitamin K activity. It is an analog of 1,4-naphthoquinone with a methyl group in the 2-position.

Terminology

It is sometimes called vitamin K₃, although derivatives of naphthoquinone are not naturally occurring chemicals and therefore do not qualify as vitamins, and without the side chain in the 2-position they cannot exert all the functions of the K vitamins. Menadione is metabolized by the human body into K₂ which uses alkylation to yield menaquinones (MK-n, n=1-13; K₂ vitamers), hence is better classified as a provitamin.

It is also known as "menaphthone".

Uses

Despite the fact that it can serve as a precursor to various types of vitamin K, menadione is generally not used as a nutritional supplement in economically developed countries. Menadione for human use at pharmaceutical strength is available in some countries with large lower income populations. It is used in the treatment of hypoprothrombinemia outside of the United States.