This article is about the family of vitamers. For vitamin K1, the form most commonly used as a dietary supplement or in a multi-vitamin, seePhytomenadione.
Vitamin K is used in the liver as the intermediate VKH2 to deprotonate aglutamate residue and then is reprocessed into vitamin K through a vitamin K oxide intermediate.[3] The presence of uncarboxylated proteins indicates avitamin K deficiency. Carboxylation allows them to bind (chelate)calcium ions, which they cannot do otherwise.[4] Without vitamin K, blood coagulation is seriously impaired, and uncontrolled bleeding occurs. Research suggests that deficiency of vitamin K may also weaken bones, potentially contributing toosteoporosis, and may promotecalcification of arteries and other soft tissues.[2][4][5]
Chemically, the vitamin K family comprises 2-methyl-1,4-naphthoquinone (3-)derivatives. Vitamin K includes two natural vitamers:vitamin K1 (phylloquinone) andvitamin K2 (menaquinone).[4] Vitamin K2, in turn, consists of a number of related chemical subtypes, with differing lengths of carbon side chains made ofisoprenoid groups of atoms. The two most studied are menaquinone-4 (MK-4) and menaquinone-7 (MK-7).
Vitamin K1 is made by plants, and is found in highest amounts ingreen leafy vegetables, being directly involved inphotosynthesis. 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, variant MK-4.Bacteria in thegut flora can also convert K1 into K2. All forms of K2 other than MK-4 can only be produced by bacteria, which use these duringanaerobic respiration. Vitamin K3 (menadione), a synthetic form of vitamin K, was used to treat vitamin K deficiency, but because it interferes with the function ofglutathione, it is no longer used in this manner in human nutrition.[2]
Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. "Vitamin K" include several chemical compounds. These are similar in structure in that they share a quinone ring, but differ in the length and degree of saturation of the carbon tail and the number of repeating isoprene units in the side chain (see figures in Chemistry section). Plant-sourced forms are primarily vitamin K1. Animal-sourced foods are primarily vitamin K2.[1][6][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.[1]
The USNational Academy of Medicine does not distinguish between K1 and K2 – both are counted as vitamin K. When recommendations were last updated in 1998, sufficient information was not available to establish anestimated average requirement orrecommended dietary allowance, terms that exist for most vitamins. In instances such as these, the academy definesadequate intakes (AIs) as amounts that appear to be sufficient to maintain good health, with the understanding that at some later date, AIs will be replaced by more exact information. The current AIs for adult women and men ages 19 and older are 90 and 120 μg/day, respectively, for pregnancy is 90 μg/day, and for lactation is 90 μg/day. For infants up to 12 months, the AI is 2.0–2.5 μg/day; for children ages 1–18 years the AI increases with age from 30 to 75 μg/day. As for safety, the academy setstolerable upper intake levels (known as "upper limits") for vitamins and minerals when evidence is sufficient. Vitamin K has no upper limit, as human data for adverse effects from high doses are not sufficient.[5]
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.[8] Japan set adequate intakes for adult women at 65 μg/day and for men at 75 μg/day.[9] 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.[9][10]
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.[11][12] Compliance with the updated labeling regulations was required by 1 January 2020 for manufacturers withUS$10 million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales.[13][14] A table of the old and new adult daily values is provided atReference Daily Intake.
According to the Global Fortification Data Exchange, vitamin K deficiency is so rare that no countries require that foods be fortified.[15] TheWorld Health Organization does not have recommendations on vitamin K fortification.[16]
Vitamin K1 is primarily from plants, especially leafy green vegetables. Small amounts are provided by animal-sourced foods. Vitamin K2 is primarily from animal-sourced foods, with poultry and eggs much better sources than beef, pork or fish.[7] One exception to the latter isnattō, which is made from bacteria-fermented soybeans. It is a rich food source of vitamin K2 variant MK-7, made by the bacteria.[17]
Animal-sourced foods are a source of vitamin K2.[18][20] The MK-4 form is from conversion of plant-sourced vitamin K1 in various tissues in the body.[21]
Because vitamin K aids mechanisms for blood clotting, its deficiency may lead to reduced blood clotting, and in severe cases, can result in reduced clotting, increased bleeding, and increasedprothrombin time.[2][5]
Normal diets are usually not deficient in vitamin K, indicating that deficiency is uncommon in healthy children and adults.[4] An exception may be infants who are at an increased risk of deficiency regardless of the vitamin status of the mother during pregnancy and breast feeding due to poor transfer of the vitamin to the placenta and low amounts of the vitamin in breast milk.[19]
Secondary deficiencies can occur in people who consume adequate amounts, but have malabsorption conditions, such ascystic fibrosis or chronic pancreatitis, and in people who haveliver damage or disease.[2] Secondary vitamin K deficiency can also occur in people who have a prescription for a vitamin K antagonist drug, such as warfarin.[2][4] A drug associated with increased risk of vitamin K deficiency iscefamandole, although the mechanism is unknown.[22]
Vitamin K is given as an injection to newborns to preventvitamin K deficiency bleeding.[19] The blood clotting factors of newborn babies are roughly 30–60% that of adult values; this appears to be a consequence of poor transfer of the vitamin across the placenta, and thus low fetal plasma vitamin K.[19] Occurrence of vitamin K deficiency bleeding in the first week of the infant's life is estimated at between 1 in 60 and 1 in 250.[23]
Human milk contains 0.85–9.2 μg/L (median 2.5 μg/L) of vitamin K1, while infant formula is formulated in range of 24–175 μg/L.[19] Late onset bleeding, with onset 2 to 12 weeks after birth, can be a consequence of exclusive breastfeeding, especially if there was no preventive treatment.[19] Late onset prevalence reported at 35 cases per 100,000 live births in infants who had not received prophylaxis at or shortly after birth.[24] Vitamin K deficiency bleeding occurs more frequently in the Asian population compared to the Caucasian population.[19]
Bleeding in infants due to vitamin K deficiency can be severe, leading to hospitalization,brain damage, and death. Intramuscular injection, typically given shortly after birth, is more effective in preventing vitamin K deficiency bleeding than oral administration, which calls for weekly dosing up to three months of age.[19]
Warfarin is ananticoagulant 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.[25] The purpose of the drug is to reduce risk of inappropriate blood clotting, which can have serious, potentially fatal consequences.[2] The proper anticoagulant action of warfarin is a function of vitamin K intake and drug dose. Due to differing absorption of the drug and amounts of vitamin K in the diet, dosing must be monitored and customized for each patient.[26] 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.[27]
Coumarin is used in the pharmaceutical industry as a precursor reagent in the synthesis of a number of synthetic anticoagulant pharmaceuticals.[31] One subset,4-hydroxycoumarins, act asvitamin 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 generationrodenticides ("rat poison"). Death occurs after a period of several days to two weeks, usually from internal hemorrhaging.[31] For humans, and for animals that have consumed either the rodenticide or rats poisoned by the rodenticide, treatment is prolonged administration of large amounts of vitamin K.[32][33] This dosing must sometimes be continued for up to nine months in cases of poisoning by "superwarfarin" rodenticides such asbrodifacoum. Oral vitamin K1 is preferred over other vitamin K1 routes of administration because it has fewer side effects.[34]
An increase inprothrombin time, a coagulation assay, has been used as an indicator of vitamin K status, but it lacks sufficient sensitivity and specificity for this application.[35] Serum phylloquinone is the most commonly used marker of vitamin K status. Concentrations <0.15 μg/L are indicative of deficiency. Disadvantages include exclusion of the other vitamin K vitamers and interference from recent dietary intake.[35] Vitamin K is required for the gamma-carboxylation of specific glutamic acid residues within the Gla domain of the 17 vitamin K–dependent proteins. Thus, a rise in uncarboxylated versions of these proteins is an indirect but sensitive and specific marker for vitamin K deficiency. If uncarboxylated prothrombin is being measured, this "Protein induced by Vitamin K Absence/antagonism (PIVKA-II)" is elevated in vitamin K deficiency.
The test is used to assess risk of vitamin K–deficient bleeding in newborn infants.[35]Osteocalcin is involved in calcification of bone tissue. The ratio of uncarboxylated osteocalcin to carboxylated osteocalcin increases with vitamin K deficiency. Vitamin K2 has been shown to lower this ratio and improvelumbar vertebraebone mineral density.[36] Matrix Gla protein must undergo vitamin K dependent phosphorylation and carboxylation. Elevated plasma concentration of dephosphorylated, uncarboxylated MGP is indicative of vitamin K deficiency.[37]
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 notolerable upper intake levels needs to be set.[5][9][10] However, vitamin K1 has been associated with severe adverse reactions such asbronchospasm andcardiac arrest when given intravenously. The reaction is described as a nonimmune-mediatedanaphylactoid reaction, with incidence of 3 per 10,000 treatments. The majority of reactions occurred when polyoxyethylatedcastor oil was used as the solubilizing agent.[38]
4-amino-2-methyl-1-naphthol ("K5") is not natural and hence not a "vitamin". Research with "K5" suggests it may inhibitfungal growth in fruit juices.[41]
Vitamin K1 (phylloquinone) – both forms of the vitamin contain a functionalnaphthoquinone ring and analiphatic side chain. Phylloquinone has aphytyl side chain.Vitamin K2 (menaquinone). In menaquinone, the side chain is composed of a varying number ofisoprenoid residues. The most common number of these residues is four, since animalenzymes normally produce menaquinone-4 from plant phylloquinone.
The structure of phylloquinone, Vitamin K1, is marked by the presence of aphytyl sidechain.[5] Vitamin K1 has an (E) trans double bond responsible for its biological activity, and two chiral centers on the phytyl sidechain.[42] Vitamin K1 appears as a yellow viscous liquid at room temperature due to its absorption of violet light in the UV-visible spectra obtained byultraviolet–visible spectroscopy.[43] 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.[5] The large size of Vitamin K1 gives many different peaks in mass spectroscopy, most of which involve derivatives of the naphthoquinone ring base and the alkyl side chain.[44]
A sample of phytomenadione for injection, also called phylloquinone
In animals, the MK-4 form of vitamin K2 is produced by conversion of vitamin K1 in thetestes,pancreas, andarterial walls.[21] While major questions still surround the biochemical pathway for this transformation, the conversion is not dependent ongut bacteria, as it occurs in germ-free rats[45] and in parenterally administered K1 in rats.[46][47] 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 thenprenylated to produce MK-4.[48]
In animals, vitamin K is involved in thecarboxylation of certainglutamate residues in proteins to formgamma-carboxyglutamate (Gla) residues. The modified residues are often (but not always) situated within specificprotein domains calledGla domains. Gla residues are usually involved in bindingcalcium, and are essential for the biological activity of all known Gla proteins.[49]
Vitamin K is absorbed through thejejunum andileum in thesmall intestine. The process requiresbile andpancreatic 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 – foods identified as having a high vitamin K content – are on the order of 4% to 17% regardless of whether raw or cooked.[4] Less information is available for absorption of vitamin K2 from foods.[4][5]
The intestinal membrane proteinNiemann–Pick C1-like 1 (NPC1L1) mediates cholesterol absorption. Animal studies show that it also factors into absorption of vitamins E and K1.[57] The same study predicts potential interaction between SR-BI and CD36 proteins as well.[57] The drugezetimibe inhibits NPC1L1 causing a reduction in cholesterol absorption in humans, and in animal studies, also reduces vitamin E and vitamin K1 absorption. An expected consequence would be that administration of ezetimibe to people who take warfarin (a vitamin K antagonist) would potentiate the warfarin effect. This has been confirmed in humans.[57]
This sectionis missing information about invertebrates. Please expand the section to include this information. Further details may exist on thetalk page.(January 2021)
In both cases R represents the isoprenoid side chain.
Vitamin K is distributed differently within animals depending on its specific homologue. Vitamin K1 is mainly present in the liver, heart and pancreas, while MK-4 is better represented in the kidneys, brain and pancreas. The liver also contains longer chain homologues MK-7 to MK-13.[58]
The function of vitamin K2 in the animal cell is to add acarboxylic acidfunctional group to aglutamate (Glu)amino acid residue in aprotein, to form agamma-carboxyglutamate (Gla) residue. This is a somewhat uncommonposttranslational 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 tochelatecalcium 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.[59]
Within the cell, vitamin K participates in a cyclic process. The vitamin undergoes electronreduction to a reduced form called vitamin K hydroquinone (quinol), catalyzed by the enzymevitamin K epoxide reductase (VKOR).[60] Another enzyme thenoxidizes vitamin K hydroquinone to allow carboxylation of Glu to Gla; this enzyme is calledgamma-glutamyl carboxylase[61] 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.[62] Humans are rarely deficient in vitamin K because, in part, vitamin K2 is continuously recycled in cells.[63]
Warfarin and other4-hydroxycoumarins block the action of VKOR.[25] 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 theamino termini of these factors, they no longer bind stably to the blood vesselendothelium and cannot activateclotting 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.[26]
The following human Gla-containing proteins ("Gla proteins") have been characterized to the level of primary structure: blood coagulation factors II (prothrombin), VII, IX, and X, anticoagulantprotein C andprotein S, and the factor X-targetingprotein Z. The bone Gla proteinosteocalcin, the calcification-inhibitingmatrix Gla protein (MGP), thecell growth regulating growth arrest specific gene 6 protein, and the four transmembrane Gla proteins, the function of which is at present unknown. The Gla domain is responsible for high-affinity binding ofcalcium ions (Ca2+) to Gla proteins, which is often necessary for their conformation, and always necessary for their function.[59]
Gla proteins are known to occur in a wide variety of vertebrates: mammals, birds, reptiles, and fish. Thevenom of a number ofAustralian snakes acts by activating the human blood-clotting system. In some cases, activation is accomplished by snake Gla-containing enzymes that bind to theendothelium of human blood vessels and catalyze the conversion of procoagulant clotting factors into activated ones, leading to unwanted and potentially deadly clotting.[64]
Another interesting class of invertebrate Gla-containing proteins is synthesized by the fish-hunting snailConus geographus.[65] These snails produce a venom containing hundreds of neuroactivepeptides, orconotoxins, which is sufficiently toxic to kill an adult human. Several of the conotoxins contain two to five Gla residues.[66]
Vitamin K1 is an important chemical in green plants (including land plants and green algae) and some species ofcyanobacteria, where it functions as anelectron acceptor transferring one electron inphotosystem I duringphotosynthesis.[67] For this reason, vitamin K1 is found in large quantities in the photosynthetic tissues of plants (greenleaves, and dark green leafy vegetables such asromaine lettuce,kale, andspinach), but it occurs in far smaller quantities in other plant tissues.[7][67]
Detection of VKORC1 homologues active on the K1-epioxide suggest that K1 may have a non-redox function in these organisms. In plants but not cyanobacteria, knockout of this gene show growth restriction similar to mutants lacking the ability to produce K1.[68]
Many bacteria, includingEscherichia coli found in thelarge intestine, can synthesize vitamin K2 (MK-7 up to MK-11),[69] but not vitamin K1. In the vitamin K2 synthesizing bacteria, menaquinone transfers twoelectrons between two different small molecules, during oxygen-independent metabolic energy production processes (anaerobic respiration).[70] For example, a small molecule with an excess of electrons (also called an electron donor) such aslactate,formate, orNADH, 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 asfumarate ornitrate (also called an electron acceptor). Adding two electrons tofumarate ornitrate converts the molecule tosuccinate ornitrite pluswater, respectively.[70]
In 1929, Danish scientistHenrik Dam investigated the role ofcholesterol by feeding chickens a cholesterol-depleted diet.[71] He initially replicated experiments reported by scientists at theOntario Agricultural College.[72] McFarlane, Graham and Richardson, working on the chick feed program at OAC, usedchloroform to remove all fat from chick chow. They noticed that chicks fed only fat-depleted chow developed hemorrhages and started bleeding from tag sites.[73] Dam found that these defects could not be restored by adding purified cholesterol to the diet. It appeared that – together with the cholesterol – 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 asKoagulationsvitamin.Edward Adelbert Doisy ofSaint Louis University did much of the research that led to the discovery of the structure and chemical nature of vitamin K.[74] Dam and Doisy shared the 1943Nobel Prize for medicine for their work on vitamin K1 and K2 published in 1939. Several laboratories synthesized the compound(s) in 1939.[75]
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 theMayo Clinic (Hugh Butt,Albert Snell, and Arnold Osterberg).[76]
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.[77]
The precise function of vitamin K was not discovered until 1974, whenprothrombin, 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 asγ-carboxyglutamate instead ofglutamate, and is able to bind calcium, part of the clotting process.[78]
Vitamin K is required for the gamma-carboxylation ofosteocalcin in bone.[79] The risk ofosteoporosis, assessed viabone mineral density and fractures, was not affected for people on warfarin therapy – a vitamin K antagonist.[80] Studies investigating whether vitamin K supplementation reduces risk of bone fractures have shown mixed results.[4][79][81][82]
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.[4] In humans,Keutel syndrome is a rarerecessive genetic disorder associated with abnormalities in the gene coding for MGP and characterized by abnormaldiffusecartilagecalcification.[83] 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.[4]
Inmeta-analyses of population studies, low intake of vitamin K was associated with inactive MGP,arterialcalcification[84] and arterial stiffness.[85][86] Lower dietary intakes of vitamin K1 and vitamin K2 were also associated with highercoronary heart disease.[37][87] When blood concentration of circulating vitamin K1 was assessed there was an increased risk in all cause mortality linked to low concentration.[88][89] 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.[90]
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.[4] From a review of observational trials, long-term use of vitamin K antagonists as anticoagulation therapy is associated with lower cancer incidence in general.[91] There are conflicting reviews as to whether agonists reduce the risk of prostate cancer.[92][93]
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