Vitamin K

Vitamin K
Drug class
Class identifiers
Use Vitamin K deficiency, Warfarin overdose
ATC code B02BA
Biological target Gamma-glutamyl carboxylase
Clinical data
Drugs.com Medical Encyclopedia
External links
MeSH D014812
In Wikidata
Vitamin K1 (phylloquinone) – both forms of the vitamin contain a functional naphthoquinone ring and an aliphatic side chain. Phylloquinone has a phytyl side chain.
Vitamin K2 (menaquinone). In menaquinone, the side chain is composed of a varying number of isoprenoid residues. The most common number of these residues is four, since animal enzymes normally produce menaquinone-4 from plant phylloquinone.
A sample of phytomenadione for injection, also called phylloquinone
Vitamin K structures. MK-4 and MK-7 are both subtypes of K2.

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.[1] 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.

Bacteria in the gut flora can also convert K1 into vitamin K2. In addition, bacteria typically lengthen the isoprenoid side chain of vitamin K2 to produce a range of vitamin K2 forms, most notably the MK-7 to MK-11 homologues of vitamin K2. All forms of K2 other than MK-4 can only be produced by bacteria, which use these forms in anaerobic respiration. The MK-7 and other bacterially derived forms of vitamin K2 exhibit vitamin K activity in animals, but MK-7's extra utility over MK-4, if any, is unclear and is a matter of investigation.

Three synthetic types of vitamin K are known: vitamins K3, K4, and K5. Although the natural K1 and all K2 homologues and synthetic K4 and K5 have proven nontoxic, the synthetic form K3 (menadione) has shown toxicity.[2]

Discovery of vitamin K1

Vitamin K1 was identified in 1929 by Danish scientist Henrik Dam when he investigated the role of cholesterol by feeding chickens a cholesterol-depleted diet.[3] After several weeks, the animals developed hemorrhages and started bleeding. These defects could not be restored by adding purified cholesterol to the diet. A second compound—together with the cholesterol—apparently had been 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.

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.[4] 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[5][6] and in parenterally-administered K1 in rats.[7][8] In fact, tissues that accumulate high amounts of MK-4 have a remarkable capacity to convert up to 90% of the available K1 into MK-4.[9][10] There is evidence that the conversion proceeds by removal of the phytyl tail of K1 to produce menadione as an intermediate, which is then condensed with an activated geranylgeranyl moiety (see also prenylation) to produce vitamin K2 in the MK-4 (menatetrione) form.[11]

Vitamin K2

Main article: Vitamin K2

Vitamin K2 (menaquinone) includes several subtypes. The two subtypes most studied are menaquinone-4 (menatetrenone, MK-4) and menaquinone-7 (MK-7).

Chemical structure

The three synthetic forms of vitamin K are vitamins K3 (Menadione), K4, and K5, which are used in many areas, including the pet food industry (vitamin K3) and to inhibit fungal growth (vitamin K5).[12]

Physiology

Vitamin K1, the precursor of most vitamin K in nature, is a stereoisomer of phylloquinone, an important chemical in green plants, where it functions as an electron acceptor 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), but it occurs in far smaller quantities in other plant tissues (roots, fruits, etc.). Iceberg lettuce contains relatively little. The function of phylloquinone in plants appears to have no resemblance to its later metabolic and biochemical function (as "vitamin K") in animals, where it performs a completely different biochemical reaction.

Vitamin K (in animals) 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.[13]

At this time, 17 human proteins with Gla domains have been discovered, and they play key roles in the regulation of three physiological processes:

Like other lipid-soluble vitamins (A, D, E), vitamin K is stored in the fat tissue of the human body.

Absorption and dietary need

Previous theory held that dietary deficiency is extremely rare unless the small bowel was heavily damaged, resulting in malabsorption of the molecule. Another at-risk group for deficiency were those subject to decreased production of K2 by normal intestinal microbiota, as seen in broad spectrum antibiotic use.[21] Taking broad-spectrum antibiotics can reduce vitamin K production in the gut by nearly 74% in people compared with those not taking these antibiotics.[22] Diets low in vitamin K also decrease the body's vitamin K concentration.[23] Those with chronic kidney disease are at risk for vitamin K deficiency, as well as vitamin D deficiency, and particularly those with the apoE4 genotype.[24] Additionally, in the elderly there is a reduction in vitamin K2 production.[25]

Dietary Reference Intake

The Food and Nutrition Board of the U.S. Institute of Medicine updated an estimate of what constitutes an Adequate Intake (AI) for vitamin K in 2001. At that time there was not sufficient evidence to set the more rigorous Estimated Average Requirement (EAR) or Recommended Dietary Allowance (RDA) given for most of the essential vitamins and minerals. The current AIs for vitamin K for women and men ages 18 and up are 90 μg/day and 120 μg/day, respectively. AI for pregnancy and lactation is 90 μg/day. For infants up to 12 months the AI is 2.0-2.5 μg/day. and for children ages 1–18 years the AI increases with age from 30 to 75 μg/day. As for safety, the FNB also sets Tolerable Upper Intake Levels (known as ULs) for vitamins and minerals when evidence is sufficient. In the case of vitamin K no UL is set, as evidence for adverse effects is not sufficient. Collectively EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes.[26] The European Food Safety Authority reviewed the same safety question and did not set an UL.[27]

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For vitamin K labeling purposes 100% of the Daily Value was 80 μg, but as of May 2016 it has been revised to 120 μg. A table of the pre-change adult Daily Values is provided at Reference Daily Intake. Food and supplement companies have until July 28, 2018 to comply with the change.

Anticoagulant drug interactions

Phylloquinone (K1)[28][29] or menaquinone (K2) are capable of reversing the anticoagulant activity of the anticoagulant warfarin (tradename Coumadin). Warfarin works by blocking recycling of vitamin K, so that the body and tissues have lower levels of active vitamin K, and thus a deficiency of vitamin K.

Supplemental vitamin K (for which oral dosing is often more active than injectable dosing in human adults) reverses the vitamin K deficiency caused by warfarin, and therefore reduces the intended anticoagulant action of warfarin and related drugs.[30] Sometimes small amounts of vitamin K are given orally to patients taking warfarin so that the action of the drug is more predictable.[30] The proper anticoagulant action of the drug is a function of vitamin K intake and drug dose, and due to differing absorption must be individualized for each patient. The action of warfarin and vitamin K both require two to five days after dosing to have maximum effect, and neither warfarin or vitamin K shows much effect in the first 24 hours after they are given.[31]

The newer anticoagulants dabigatran and rivaroxaban have different mechanisms of action that do not interact with vitamin K, and may be taken with supplemental vitamin K.[32][33]

Food sources

Vitamin K1

Food Serving size Vitamin K1[34] micrograms (μg) Food Serving size Vitamin K1[34] micrograms (μg)
Kale, cooked 1/2 cup 531 Parsley, raw 1/4 cup 246
Spinach, cooked 1/2 cup 444 Spinach, raw 1 cup 145
Collards, cooked 1/2 cup 418 Collards, raw 1 cup 184
Swiss chard, cooked 1/2 cup 287 Swiss chard, raw 1 cup 299
Mustard greens, cooked 1/2 cup 210 Mustard greens, raw 1 cup 279
Turnip greens, cooked 1/2 cup 265 Turnip greens, raw 1 cup 138
Broccoli, cooked 1 cup 220 Broccoli, raw 1 cup 89
Brussels sprouts, cooked 1 cup 219 Endive, raw 1 cup 116
Cabbage, cooked 1/2 cup 82 Green leaf lettuce 1 cup 71
Asparagus 4 spears 48 Romaine lettuce, raw 1 cup 57
Table from "Important information to know when you are taking: Warfarin (Coumadin) and Vitamin K", Clinical Center, National Institutes of Health Drug Nutrient Interaction Task Force.[35]

Vitamin K1 is found chiefly in leafy green vegetables such as dandelion greens (which contain 778.4 μg per 100 g, or 741% of the recommended daily amount), spinach, swiss chard, lettuce and Brassica (e.g. cabbage, kale, cauliflower, broccoli, and brussels sprouts) and often the absorption is greater when accompanied by fats such as butter or oils; some fruits, such as avocado, kiwifruit and grapes, are also high in vitamin K. By way of reference, two tablespoons of parsley contain 153% of the recommended daily amount of vitamin K.[36] Some vegetable oils, notably soybean, contain vitamin K, but at levels that would require relatively large calorific consumption to meet the USDA-recommended levels.[37] Colonic bacteria synthesize a significant portion of humans' vitamin K needs; newborns often receive a vitamin K shot at birth to tide them over until their colons become colonized at five to seven days of age from the consumption of their mother's milk.

Phylloquinone's tight binding to thylakoid membranes in chloroplasts makes it less bioavailable. For example, cooked spinach has a 5% bioavailability of phylloquinone, however, fat added to it increases bioavailability to 13% due to the increased solubility of vitamin K in fat.[38]

Deficiency

Main article: Vitamin K deficiency

Average diets are usually not lacking in vitamin K, and primary deficiency is rare in healthy adults. Newborn infants are at an increased risk of deficiency. Other populations with an increased prevalence of vitamin K deficiency include those who suffer from liver damage or disease (e.g. alcoholics), cystic fibrosis, or inflammatory bowel diseases, or have recently had abdominal surgeries. Secondary vitamin K deficiency can occur in people with bulimia, those on stringent diets, and those taking anticoagulants. Other drugs associated with vitamin K deficiency include salicylates, barbiturates, and cefamandole, although the mechanisms are still unknown. Vitamin K1 deficiency can result in coagulopathy, a bleeding disorder.[39] Symptoms of K1 deficiency include anemia, bruising, and bleeding of the gums or nose in both sexes, and heavy menstrual bleeding in women.

Osteoporosis[40][41] and coronary heart disease[42][43] are strongly associated with lower levels of K2 (menaquinone). Vitamin K2 (as menaquinones MK-4 through MK-10) intake level is inversely related to severe aortic calcification and all-cause mortality.[44]

Toxicity

Although allergic reaction from supplementation is possible, no known toxicity is associated with high doses of the phylloquinone (vitamin K1) or menaquinone (vitamin K2) forms of vitamin K, so no tolerable upper intake level (UL) has been set.[45]

Blood clotting (coagulation) studies in humans using 45 mg per day of vitamin K2 (as MK-4)[46] and even up to 135 mg/day (45 mg three times daily) of K2 (as MK-4),[47] showed no increase in blood clot risk. Even doses in rats as high as 250 mg/kg body weight did not alter the tendency for blood-clot formation to occur.[48]

Unlike the safe natural forms of vitamin K1 and vitamin K2 and their various isomers, a synthetic form of vitamin K, vitamin K3 (menadione), is demonstrably toxic at high levels. The U.S. FDA has banned this form from over-the-counter sale in the United States because large doses have been shown to cause allergic reactions, hemolytic anemia, and cytotoxicity in liver cells.[2]

Biochemistry

Function in animals

Mechanism of action for K1-vitamin.

The function of vitamin K2 in the animal cell is to add a carboxylic acid functional group to a glutamate 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 (carboxylate) groups on the same carbon in the gamma-carboxyglutamate residue allows it to chelate calcium ion. The binding of calcium ion 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.

Within the cell, vitamin K undergoes electron reduction to a reduced form called vitamin K hydroquinone by the enzyme vitamin K epoxide reductase (VKOR).[49] Another enzyme then oxidizes vitamin K hydroquinone to allow carboxylation of Glu to Gla; this enzyme is called the gamma-glutamyl carboxylase[50][51] 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 reconverted to vitamin K by VKOR. The reduction and subsequent reoxidation of vitamin K coupled with carboxylation of Glu is called the vitamin K cycle.[52] Humans are rarely deficient in vitamin K1 because, in part, vitamin K 1 is continuously recycled in cells.[53]

Warfarin and other 4-hydroxycoumarins block the action of the VKOR.[54] This results in decreased concentrations of vitamin K and vitamin K hydroquinone in the 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 overdose.

Gamma-carboxyglutamate proteins

Main article: gla domain

The following human Gla-containing proteins ("gla proteins") have been characterized to the level of primary structure: the blood coagulation factors II (prothrombin), VII, IX, and X, the anticoagulant proteins C and S, and the factor X-targeting protein Z. The bone Gla protein osteocalcin, the calcification-inhibiting matrix Gla protein (MGP), the cell growth regulating growth arrest specific gene 6 protein (Gas6), and the four transmembrane Gla proteins (TMGPs), the function of which is at present unknown. Gas6 can function as a growth factor to activate the Axl receptor tyrosine kinase and stimulate cell proliferation or prevent apoptosis in some cells. In all cases in which their function was known, the presence of the Gla residues in these proteins turned out to be essential for functional activity.

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.[55] 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.[56]

Methods of assessment

Vitamin K status can be assessed by:

but an article by Schurgers et al. reported no correlation between FFQ and plasma phylloquinone.[60]

Function in bacteria

Many bacteria, such as Escherichia coli found in the large intestine, can synthesize vitamin K2 (menaquinone-7 or MK-7, up to MK-11),[64] but not vitamin K1 (phylloquinone). In these bacteria, menaquinone transfers two electrons between two different small molecules, during oxygen-independent metabolic energy production processes (anaerobic respiration).[65] 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 a menaquinone. The menaquinone, with the help of another enzyme, then transfers these two electrons to a suitable oxidant, such fumarate or nitrate (also called an electron acceptor). Adding two electrons to fumarate or nitrate converts the molecule to succinate or nitrite + water, respectively.

Some of these reactions generate a cellular energy source, ATP, in a manner similar to eukaryotic cell aerobic respiration, except the final electron acceptor is not molecular oxygen, but fumarate or nitrate. In aerobic respiration, the final oxidant is molecular oxygen (O2), which accepts four electrons from an electron donor such as NADH to be converted to water. E. coli, as facultative anaerobes, can carry out both aerobic respiration and menaquinone-mediated anaerobic respiration.

Injection in newborns

The blood clotting factors of newborn babies are roughly 30 to 60% that of adult values; this may be due to the reduced synthesis of precursor proteins and the sterility of their guts. Human milk contains 1–4 μg/L of vitamin K1, while formula-derived milk can contain up to 100 μg/L in supplemented formulas. Vitamin K2 concentrations in human milk appear to be much lower than those of vitamin K1. Occurrence of vitamin K deficiency bleeding in the first week of the infant's life is estimated at 0.25 to 1.7%, with a prevalence of two to 10 cases per 100,000 births.[66] Premature babies have even lower levels of the vitamin, so they are at a higher risk from this deficiency.

Bleeding in infants due to vitamin K deficiency can be severe, leading to hospitalization, blood transfusions, brain damage, and death. Supplementation can prevent most cases of vitamin K deficiency bleeding in the newborn. Intramuscular administration is more effective in preventing late vitamin K deficiency bleeding than oral administration.[67][68]

USA

As a result of the occurrences of vitamin K deficiency bleeding, the Committee on Nutrition of the American Academy of Pediatrics has recommended 0.5 to 1.0 mg vitamin K1 be administered to all newborns shortly after birth.[68]

UK

In the UK vitamin K supplementation is recommended for all newborns within the first 24 hours.[69] This is usually given as a single intramuscular injection of 1 mg shortly after birth but as a second-line option can be given by three oral doses over the first month.[70]

Controversy

Controversy arose in the early 1990s regarding this practice, when two studies suggested a relationship between parenteral administration of vitamin K and childhood cancer,[71] however, poor methods and small sample sizes led to the discrediting of these studies, and a review of the evidence published in 2000 by Ross and Davies found no link between the two.[72] Doctors reported emerging concerns in 2013,[73] after treating children for serious bleeding problems. They cited lack-of newborn Vitamin K administration, as the reason that the problems occurred, and recommended that breast-fed babies could have an increased risk unless they receive a preventative dose.

Health effects

Osteoporosis

A review of 2014 concluded that there is positive evidence that monotherapy using MK4, one of the forms of Vitamin K2, reduces fracture incidence in postmenopausal women with osteoporosis, and suggested further research on the combined use of MK4 with bisphosphonates. In contrast, an earlier review article of 2013 concluded that there is no good evidence that vitamin K supplementation helps prevent osteoporosis or fractures in postmenopausal women.[74]

A Cochrane systematic review of 2006 suggested that supplementation with Vitamin K1 and with MK4 reduces bone loss; in particular, a strong effect of MK4 on incident fractures among Japanese patients was emphasized.[75]

A review article of 2016 suggested to consider, as one of several measures for bone health, increasing the intake of foods rich in vitamins K1 and K2.[76]

Cardiovascular health

Adequate intake of vitamin K is associated with the inhibition of arterial calcification and stiffening,[77] but there have been few interventional studies and no good evidence that vitamin K supplementation is of any benefit in the primary prevention of cardiovascular disease.[78]

One 10 year population study, the Rotterdam Study, did show a clear and significant inverse relationship between the highest intake levels of menaquinone (mainly MK-4 from eggs and meat, and MK-8 and MK-9 from cheese) and cardiovascular disease and all-cause mortality in older men and women.[44]

Cancer

Vitamin K has been promoted in supplement form with claims it can slow tumor growth; there is however no good medical evidence that supports such claims.[79]

As antidote for poisoning by 4-hydroxycoumarin

Vitamin K is part of the suggested treatment regime for poisoning by rodenticide.[80]

History of discovery

In 1929, Danish scientist Henrik Dam investigated the role of cholesterol by feeding chickens a cholesterol-depleted diet.[3] After several weeks, the animals developed hemorrhages and started bleeding. These defects could not be restored by adding purified cholesterol to the diet. It appeared that—together with the cholesterol—a second compound had been 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.[81] Dam and Doisy shared the 1943 Nobel Prize for medicine for their work on vitamin K (K1 and K2) published in 1939. Several laboratories synthesized the compound(s) in 1939.[82]

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).[83]

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.[84]

The precise function of vitamin K was not discovered until 1974, when three laboratories (Stenflo et al.,[85] Nelsestuen et al.,[86] and Magnusson et al.[87]) isolated the vitamin K-dependent coagulation factor prothrombin (Factor II) from cows that received a high dose of a vitamin K antagonist, warfarin. It was shown that, while warfarin-treated cows had a form of prothrombin that contained 10 glutamate amino acid residues near the amino terminus of this protein, the normal (untreated) cows contained 10 unusual residues that were chemically identified as gamma-carboxyglutamate, or Gla. The extra carboxyl group in Gla made clear that vitamin K plays a role in a carboxylation reaction during which Glu is converted into Gla.

The biochemistry of how vitamin K is used to convert Glu to Gla has been elucidated over the past thirty years in academic laboratories throughout the world.

References

  1. ↑ "Vitamin K Overview". University of Maryland Medical Center.
  2. 1 2 Higdon (February 2008). "Vitamin K". Linus Pauling Institute, Oregon State University. Retrieved 12 April 2008.
  3. 1 2 Dam, H. (1935). "The Antihæmorrhagic Vitamin of the Chick.: Occurrence And Chemical Nature". Nature. 135 (3417): 652–653. doi:10.1038/135652b0.
  4. ↑ Newman P., Shearer MJ; Newman, Paul (2008). "Metabolism and cell biology of vitamin K". Thrombosis and Haemostasis. 100 (4): 530–547. doi:10.1160/TH08-03-0147. PMID 18841274.
  5. ↑ Davidson, RT; Foley AL; Engelke JA; Suttie JW (1998). "Conversion of Dietary Phylloquinone to Tissue Menaquinone-4 in Rats is Not Dependent on Gut Bacteria1". Journal of Nutrition. 128 (2): 220–223. PMID 9446847.
  6. ↑ Ronden, JE; Drittij-Reijnders M-J; Vermeer C; Thijssen HHW. (1998). "Intestinal flora is not an intermediate in the phylloquinone-menaquinone-4 conversion in the rat". Biochimica et Biophysica Acta (BBA) - General Subjects. 1379 (1): 69–75. doi:10.1016/S0304-4165(97)00089-5. PMID 9468334.
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  17. ↑ Viegas, C. S. B.; Simes, D. C.; Laizé, V.; Williamson, M. K.; Price, P. A.; Cancela, M. L. (2008). "Gla-rich Protein (GRP), A New Vitamin K-dependent Protein Identified from Sturgeon Cartilage and Highly Conserved in Vertebrates". Journal of Biological Chemistry. 283 (52): 36655–36664. doi:10.1074/jbc.M802761200. PMC 2605998Freely accessible. PMID 18836183.
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  19. ↑ Hafizi S, Dahlbäck B (2006). "Gas6 and protein S. Vitamin K-dependent ligands for the Axl receptor tyrosine kinase subfamily". FEBS J. 273 (23): 5231–44. doi:10.1111/j.1742-4658.2006.05529.x. PMID 17064312.
  20. ↑ Kulman JD, Harris JE, Xie L, Davie EW (2007). "Proline-rich Gla protein 2 is a cell-surface vitamin K-dependent protein that binds to the transcriptional coactivator Yes-associated protein". Proc. Natl. Acad. Sci. U.S.A. 104 (21): 8767–72. doi:10.1073/pnas.0703195104. PMC 1885577Freely accessible. PMID 17502622.
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  26. ↑ Vitamin K. IN: Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press. 2001, PP.162-196.
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