Vitamin K

Vitamin K is a family of structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-synthesis modification of certain proteins that are required for blood coagulation ("K" from Danish koagulation, for "coagulation") and for controlling binding of calcium in bones and other tissues. The complete synthesis involves final modification of these "Gla proteins" by the enzyme gamma-glutamyl carboxylase that uses vitamin K as a cofactor.

Vitamin K is used in the liver as the intermediate VKH₂ to deprotonate a glutamate residue and then is reprocessed into vitamin K through a vitamin K oxide intermediate. The presence of uncarboxylated proteins indicates a vitamin K deficiency. Vitamin K₂, in turn, consists of several related chemical subtypes, with differing lengths of carbon side chains made of isoprenoid groups of atoms. The two most studied are menaquinone-4 (MK-4) and menaquinone-7 (MK-7). Vitamin K has several roles: an essential nutrient absorbed from food, a product synthesized and marketed as part of a multi-vitamin or as a single-vitamin dietary supplement, and a prescription medication for specific purposes.

In the European Union, adequate intake is defined the same way as in the US. For women and men over age 18 the adequate intake is set at 70 μg/day, for pregnancy 70 μg/day, and for lactation 70 μg/day. For children ages 1–17 years, adequate intake values increase with age from 12 to 65 μg/day. Japan set adequate intakes for adult women at 65 μg/day and for men at 75 μg/day. The European Union and Japan also reviewed safety and concluded – as had the United States – that there was insufficient evidence to set an upper limit for vitamin K.

For US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percentage of daily value. For vitamin K labeling purposes, 100% of the daily value was 80 μg, but on 27 May 2016 it was revised upwards to 120 μg, to bring it into agreement with the highest value for adequate intake. Compliance with the updated labeling regulations was required by 1 January 2020 for manufacturers with US$10 million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales. A table of the old and new adult daily values is provided at Reference Daily Intake.

Fortification

According to the Global Fortification Data Exchange, vitamin K deficiency is so rare that no countries require that foods be fortified. The World Health Organization does not have recommendations on vitamin K fortification.

Sources

Vitamin K₁ is primarily from plants, especially leafy green vegetables.

Vitamin K₁

{|class="wikitable"

|-

!Plant-sourced The MK-4 form is from conversion of plant-sourced vitamin K₁ in various tissues in the body.

!Amount K₂<br /> MK-4 to MK-7<br /> (μg / 100 g)

|-

|Goose || 31

|-

|Chicken || 8.9

|-

|Pork || 2.1

|-

|Beef || 1.1

|-

|Salmon || 0.5

|-

|Egg yolk || 32

|-

|Egg white || 0.9

|}

{|class="wikitable"

|-

!Animal source

Medical uses

thumb|Injectable solutions of vitamin K

Treating vitamin deficiency in newborns

Vitamin K₁ is given as an injection to newborns to prevent vitamin K deficiency bleeding.

Human milk contains 0.85–9.2&nbsp;μg/L (median 2.5&nbsp;μg/L) of vitamin K₁, while infant formula is formulated in range of 24–175&nbsp;μg/L. Vitamin K deficiency bleeding occurs more frequently in the Asian population compared to the Caucasian population.

Managing warfarin therapy

Warfarin is an anticoagulant drug. It functions by inhibiting an enzyme that is responsible for recycling vitamin K to a functional state. As a consequence, proteins that should be modified by vitamin K are not, including proteins essential to blood clotting, and are thus not functional. Some foods are so high in vitamin K₁ that medical advice is to avoid those (examples: collard greens, spinach, turnip greens) entirely, and for foods with a modestly high vitamin content, keep consumption as consistent as possible, so that the combination of vitamin intake and warfarin keep the anti-clotting activity in the therapeutic range. Oral vitamin K is used in situations when a person's International normalized ratio is greater than 10 but there is no active bleeding. The newer anticoagulants apixaban, dabigatran and rivaroxaban are not vitamin K antagonists.

Treating rodenticide poisoning

Coumarin is used in the pharmaceutical industry as a precursor reagent in the synthesis of a number of synthetic anticoagulant pharmaceuticals. One subset, 4-hydroxycoumarins, act as vitamin K antagonists. They block the regeneration and recycling of vitamin K. Some of the 4-hydroxycoumarin anticoagulant class of chemicals are designed to have high potency and long residence times in the body, and these are used specifically as second generation rodenticides ("rat poison"). Death occurs after a period of several days to two weeks, usually from internal hemorrhaging. This dosing must sometimes be continued for up to nine months in cases of poisoning by "superwarfarin" rodenticides such as brodifacoum. Oral vitamin K₁ is preferred over other vitamin K₁ routes of administration because it has fewer side effects.

Methods of assessment

An increase in prothrombin time, a coagulation assay, has been used as an indicator of vitamin K status, but it lacks sufficient sensitivity and specificity for this application.

Serum phylloquinone is the most commonly used marker of vitamin K status. Concentrations <0.15&nbsp;μg/L are indicative of deficiency. Disadvantages include exclusion of the other vitamin K vitamers and interference from recent dietary intake.

• Matrix Gla protein must undergo vitamin K dependent phosphorylation and carboxylation. Elevated plasma concentration of dephosphorylated, uncarboxylated MGP is indicative of vitamin K deficiency.

Synthetic forms

Some synthetic compounds also have vitamin K activity in humans. One synthetic drug with vitamin K activity still used in developed countries (the UK) is menadiol sodium diphosphate. Unlike natural vitamin K, it is water-soluble and can be absorbed without the help of bile salts.

Side effects

No known toxicity is associated with high oral doses of the vitamin K₁ or vitamin K₂ forms of vitamin K, so regulatory agencies from US, Japan and European Union concur that no tolerable upper intake levels needs to be set.

Non-human uses

Menadione, a natural compound sometimes referred to as vitamin K₃, is used in the pet food industry because once consumed it is converted to vitamin K₂. The US Food and Drug Administration has banned this form from sale as a human dietary supplement because overdoses have been shown to cause allergic reactions, hemolytic anemia, and cytotoxicity in liver cells.

Chemistry

thumb|class=skin-invert-image|Vitamin K₁ (phylloquinone)&nbsp;– both forms of the vitamin contain a functional [[naphthoquinone ring and an aliphatic side chain. Phylloquinone has a phytyl side chain.]]

thumb|class=skin-invert-image|Vitamin K₂ (menaquinone). In menaquinone, the side chain is composed of a varying number of [[isoprene#isoprenoids|isoprenoid residues. The most common number of these residues is four, since animal enzymes normally produce menaquinone-4 from plant phylloquinone.]]

The structure of phylloquinone, vitamin K₁, is marked by the presence of a phytyl sidechain. Vitamin K₁ appears as a yellow viscous liquid at room temperature due to its absorption of violet light in the UV–visible spectra obtained by ultraviolet–visible spectroscopy. The structures of menaquinones, vitamin K₂, are marked by the polyisoprenyl side chain present in the molecule that can contain four to 13 isoprenyl units. MK-4 is the most common form.

thumb|A sample of phytomenadione for injection, also called phylloquinone

Conversion of vitamin K₁ to vitamin K₂

In animals, the MK-4 form of vitamin K₂ is produced by conversion of vitamin K₁ in the testes, pancreas, and arterial walls. While major questions still surround the biochemical pathway for this transformation, the conversion is not dependent on gut bacteria, as it occurs in germ-free rats and in parenterally administered K₁ in rats. There is evidence that the conversion proceeds by removal of the phytyl tail of K₁ to produce menadione (also referred to as vitamin K₃) as an intermediate, which is then prenylated to produce MK-4.

Physiology

In animals, vitamin K is involved in the carboxylation of certain glutamate residues in proteins to form gamma-carboxyglutamate (Gla) residues. The modified residues are often (but not always) situated within specific protein domains called Gla domains. Gla residues are usually involved in binding calcium, and are essential for the biological activity of all known Gla proteins.

Seventeen human proteins with Gla domains have been discovered; they play key roles in the regulation of three physiological processes:

• Blood coagulation: prothrombin (factor II), factors VII, IX, and X, and proteins C, S, and Z
• Bone metabolism: osteocalcin, matrix Gla protein (MGP), periostin, and Gla-rich protein.
• Vascular biology: Matrix Gla protein, growth arrest – specific protein 6 (Gas6)
• Unknown functions: proline-rich γ-carboxyglutamyl proteins 1 and 2, and transmembrane γ-carboxy glutamyl proteins 3 and 4.

Absorption

Vitamin K is absorbed through the jejunum and ileum in the small intestine. The process requires bile and pancreatic juices. Estimates for absorption are on the order of 80% for vitamin K₁ in its free form (as a dietary supplement) but much lower when present in foods. For example, the absorption of vitamin K from kale and spinach&nbsp;– foods identified as having a high vitamin K content&nbsp;– are on the order of 4% to 17% regardless of whether raw or cooked. The same study predicts potential interaction between SR-BI and CD36 proteins as well.

The function of vitamin K₂ in the animal cell is to add a carboxylic acid functional group to a glutamate (Glu) amino acid residue in a protein, to form a gamma-carboxyglutamate (Gla) residue. This is a somewhat uncommon posttranslational modification of the protein, which is then known as a "Gla protein". The presence of two −COOH (carboxylic acid) groups on the same carbon in the gamma-carboxyglutamate residue allows it to chelate calcium ions. The binding of calcium ions in this way very often triggers the function or binding of Gla-protein enzymes, such as the so-called vitamin K–dependent clotting factors discussed below. Another enzyme then oxidizes vitamin K hydroquinone to allow carboxylation of Glu to Gla; this enzyme is called gamma-glutamyl carboxylase or the vitamin K–dependent carboxylase. The carboxylation reaction only proceeds if the carboxylase enzyme is able to oxidize vitamin K hydroquinone to vitamin K epoxide at the same time. The carboxylation and epoxidation reactions are said to be coupled. Vitamin K epoxide is then restored to vitamin K by VKOR. The reduction and subsequent reoxidation of vitamin K coupled with carboxylation of Glu is called the vitamin K cycle. Humans are rarely deficient in vitamin K because, in part, vitamin K₂ is continuously recycled in cells.

Warfarin and other 4-hydroxycoumarins block the action of VKOR. This results in decreased concentrations of vitamin K and vitamin K hydroquinone in tissues, such that the carboxylation reaction catalyzed by the glutamyl carboxylase is inefficient. This results in the production of clotting factors with inadequate Gla. Without Gla on the amino termini of these factors, they no longer bind stably to the blood vessel endothelium and cannot activate clotting to allow formation of a clot during tissue injury. As it is impossible to predict what dose of warfarin will give the desired degree of clotting suppression, warfarin treatment must be carefully monitored to avoid underdose and overdose.

Gla proteins are known to occur in a wide variety of vertebrates: mammals, birds, reptiles, and fish. The venom of a number of Australian snakes acts by activating the human blood-clotting system. In some cases, activation is accomplished by snake Gla-containing enzymes that bind to the endothelium of human blood vessels and catalyze the conversion of procoagulant clotting factors into activated ones, leading to unwanted and potentially deadly clotting.

Another interesting class of invertebrate Gla-containing proteins is synthesized by the fish-hunting snail Conus geographus. These snails produce a venom containing hundreds of neuroactive peptides, or conotoxins, which is sufficiently toxic to kill an adult human. Several of the conotoxins contain two to five Gla residues. The Gla-modification signal is different from the vertebrate Gla domain.

Function in plants and cyanobacteria

Vitamin K₁ is an important chemical in green plants (including land plants and green algae) and some species of cyanobacteria, where it functions as an electron acceptor transferring one electron in photosystem I during photosynthesis. For this reason, vitamin K₁ is found in large quantities in the photosynthetic tissues of plants (green leaves, and dark green leafy vegetables such as romaine lettuce, kale, and spinach),

Function in other bacteria

Many bacteria, including Escherichia coli found in the large intestine, can synthesize vitamin K₂ (MK-7 up to MK-11), but not vitamin K₁. In the vitamin K₂ (menaquinone)–synthesizing bacteria, menaquinone transfers two electrons between two different small molecules during oxygen-independent metabolic energy production processes (anaerobic respiration). For example, a small molecule with an excess of electrons (also called an electron donor) such as lactate, formate, or NADH, with the help of an enzyme, passes two electrons to menaquinone. The menaquinone, with the help of another enzyme, then transfers these two electrons to a suitable oxidant, such as fumarate or nitrate (also called an electron acceptor). Adding two electrons to fumarate or nitrate converts the molecule to succinate or nitrite plus water, respectively. He initially replicated experiments reported by scientists at the Ontario Agricultural College. McFarlane, Graham and Richardson, working on the chick feed program at OAC, used chloroform to remove all fat from chick chow. They noticed that chicks fed only fat-depleted chow developed hemorrhages and started bleeding from tag sites. Dam found that these defects could not be restored by adding purified cholesterol to the diet. It appeared that&nbsp;– together with the cholesterol&nbsp;– a second compound was extracted from the food, and this compound was called the coagulation vitamin. The new vitamin received the letter K because the initial discoveries were reported in a German journal, in which it was designated as Koagulationsvitamin. Edward Adelbert Doisy of Saint Louis University did much of the research that led to the discovery of the structure and chemical nature of vitamin K. Dam and Doisy shared the 1943 Nobel Prize for medicine for their work on vitamin K₁ and K₂ published in 1939. Several laboratories synthesized the compound(s) in 1939.

For several decades, the vitamin K–deficient chick model was the only method of quantifying vitamin K in various foods: the chicks were made vitamin K–deficient and subsequently fed with known amounts of vitamin K–containing food. The extent to which blood coagulation was restored by the diet was taken as a measure for its vitamin K content. Three groups of physicians independently found this: Biochemical Institute, University of Copenhagen (Dam and Johannes Glavind), University of Iowa Department of Pathology (Emory Warner, Kenneth Brinkhous, and Harry Pratt Smith), and the Mayo Clinic (Hugh Butt, Albert Snell, and Arnold Osterberg).

The first published report of successful treatment with vitamin K of life-threatening hemorrhage in a jaundiced patient with prothrombin deficiency was made in 1938 by Smith, Warner, and Brinkhous.

The precise function of vitamin K was not discovered until 1974, when prothrombin, a blood coagulation protein, was confirmed to be vitamin K dependent. When the vitamin is present, prothrombin has amino acids near the amino terminus of the protein as γ-carboxyglutamate instead of glutamate, and is able to bind calcium, part of the clotting process.

Research

Osteoporosis

Vitamin K is required for the gamma-carboxylation of osteocalcin in bone. The risk of osteoporosis, assessed via bone mineral density and fractures, was not affected for people on warfarin therapy&nbsp;– a vitamin K antagonist. Studies investigating whether vitamin K supplementation reduces risk of bone fractures have shown mixed results.

Cardiovascular health

Matrix Gla protein is a vitamin K-dependent protein found in bone, but also in soft tissues such as arteries, where it appears to function as an anti-calcification protein. In animal studies, animals that lack the gene for MGP exhibit calcification of arteries and other soft tissues. These observations led to a theory that in humans, inadequately carboxylated MGP, due to low dietary intake of the vitamin, could result in increased risk of arterial calcification and coronary heart disease. and arterial stiffness. Lower dietary intakes of vitamin K₁ and vitamin K₂ were also associated with higher coronary heart disease. When blood concentration of circulating vitamin K₁ was assessed there was an increased risk in all cause mortality linked to low concentration. In contrast to these population studies, a review of randomized trials using supplementation with either vitamin K₁ or vitamin K₂ reported no role in mitigating vascular calcification or reducing arterial stiffness. The trials were too short to assess any impact on coronary heart disease or mortality.

Other

Population studies suggest that vitamin K status may have roles in inflammation, brain function, endocrine function and an anti-cancer effect. For all of these, there is not sufficient evidence from intervention trials to draw any conclusions. There are conflicting reviews as to whether agonists reduce the risk of prostate cancer.

References

External links

• Vitamin K:
• Vitamin K1 (phylloquinone, phytomenadione):
• Vitamin K2 (menaquinone 6):
• Vitamin K3 (menadione):