Vitamin K

Vitamin K
Drug class
Class identifiers
Use	Vitamin K deficiency, Warfarin overdose
ATC code	B02BA
Biological target	Gamma-glutamyl carboxylase
Clinical data
AHFS/Drugs.com	Medical Encyclopedia
External links
MeSH	D014812

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. 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 colon (large intestine) 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.

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. After several weeks, the animals developed haemorrhages 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. 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. 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. 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.

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, K₄, and K₅, which are used in many areas, including the pet food industry (vitamin K₃) and to inhibit fungal growth (vitamin K₅).

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.

At this time, 16 human proteins with Gla domains have been discovered, and 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, also called bone Gla protein (BGP), matrix Gla protein (MGP), periostin, and the recently discovered Gla-rich protein (GRP).
• Vascular biology: growth arrest-specific protein 6 (Gas6)
• Unknown function: proline-rich g-carboxy glutamyl proteins (PRGPs) 1 and 2, and transmembrane g-carboxy glutamyl proteins (TMGs) 3 and 4.

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. Taking broad-spectrum antibiotics can reduce vitamin K production in the gut by nearly 74% in people compared with those not taking these antibiotics. Diets low in vitamin K also decrease the body's vitamin K concentration. 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. Additionally, in the elderly there is a reduction in vitamin K₂ production.

Recent research results also demonstrate that the small intestine and large intestine seem to be inefficient at absorbing vitamin K supplements in rat populations low in Vitamin K. These results are reinforced by human cohort studies, where a majority of the subjects showed inadequate vitamin K amounts in the body. This was revealed by the presence of large amounts of incomplete gamma-carboxylated proteins in the blood, an indirect test for vitamin K deficiency. And in an animal model MK-4 was shown to prevent arterial calcifications, pointing to its potential role in prevention of such calcification. In this study vitamin K₁ was also tested, in an attempt to make connections between vitamin K₁ intake and calcification reduction; only vitamin K₂ (as MK-4) was found to influence warfarin-induced calcification in this study.

Recommended amounts

The U.S. Dietary Reference Intake (DRI) for an Adequate Intake (AI) of vitamin K for a 25-year-old male is 120 micrograms (μg) per day. The AI for adult women is 90 μg/day, for infants is 10–20 μg/day, and for children and adolescents 15–100 μg/day. To get maximum carboxylation of osteocalcin, one may have to take up to 1000 μg of vitamin K₁.

Anticoagulant drug interactions

Phylloquinone (K₁) 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. Sometimes small amounts of vitamin K (one milligram per day) are given orally to patients taking warfarin so that the action of the drug is more predictable. 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.

In two separate studies in the rat model, after long term administration of warfarin to induce calcification of arteries in the rodents, supplemental vitamin K was found to reverse or prevent some of the arterial calcification attendant on the long-term blockade of vitamin K. A second study found that only vitamin K₂ as MK-4, and not vitamin K₁ was effective at preventing warfarin-induced arterial calcification in rats, suggesting differing roles for the two forms of the vitamin in some calcium-dependent processes.

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.

Food sources

Vitamin K₁

Food	Serving size	Vitamin K1 micrograms (μg)	Food	Serving size	Vitamin K1 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.

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. Some vegetable oils, notably soybean, contain vitamin K, but at levels that would require relatively large calorific consumption to meet the USDA-recommended levels. 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.

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 bulimics, 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. Symptoms of K₁ deficiency include anemia, bruising, and bleeding of the gums or nose in both sexes, and heavy menstrual bleeding in women.

Osteoporosis and coronary heart disease are strongly associated with lower levels of K₂ (menaquinone). Vitamin K₂ (MK-7) deficiency is also related to severe aortic calcification and all-cause mortality. Menaquinone is not inhibited by salicylates as happens with K₁, so menaquinone supplementation can alleviate the chronic vitamin K deficiency caused by long-term aspirin use.

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.

Blood clotting (coagulation) studies in humans using 45 mg per day of vitamin K₂ (as MK-4) and even up to 135 mg/day (45 mg three times daily) of K₂ (as MK-4), 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.

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. 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.

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). Another enzyme then oxidizes vitamin K hydroquinone to allow carboxylation of Glu to Gla; this enzyme is called the 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 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. Humans are rarely deficient in vitamin K₁ because, in part, vitamin K ₁ is continuously recycled in cells.

Warfarin and other 4-hydroxycoumarins block the action of the VKOR. 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. 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.

Methods of assessment

Vitamin K status can be assessed by:

• The prothrombin time (PT) test measures the time required for blood to clot. A blood sample is mixed with citric acid and put in a fibrometer; delayed clot formation indicates a deficiency. This test is insensitive to mild deficiency, as the values do not change until the concentration of prothrombin in the blood has declined by at least 50%.
• Undercarboxylated prothrombin (PIVKA-II), in a study of 53 newborns, found "PT (prothrombin time) is a less sensitive marker than PIVKA II", and as indicated above, PT is unable to detect subclinical deficiencies that can be detected with PIVKA-II testing.
• Plasma phylloquinone was found to be positively correlated with phylloquinone intake in elderly British women, but not men,

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

• Urinary γ-carboxyglutamic acid responds to changes in dietary vitamin K intake. Several days are required before any change can be observed. In a study by Booth et al., increases of phylloquinone intakes from 100 μg to between 377 and 417 μg for five days did not induce a significant change. Response may be age-specific.
• Undercarboxylated osteocalcin (UcOc) levels have been inversely correlated with stores of vitamin K and bone strength in developing rat tibiae. Another study following 78 postmenopausal Korean women found a supplement regimen of vitamins K and D, and calcium, but not a regimen of vitamin D and calcium, was inversely correlated with reduced UcOc levels.

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), 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). 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. 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.

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.

UK

In the UK vitamin K supplementation is recommended for all newborns within the first 24 hours. 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.

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, 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. Doctors reported emerging concerns in 2013, 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

There is no good evidence that vitamin K supplementation helps prevent osteoporosis or fractures in postmenopausal women.

Cardiovascular health

Although vitamin K deficiency has been linked to a build up of calcium deposits in blood vessels, there is no good evidence that vitamin K supplementation is of any benefit in helping to prevent cardiovascular disease.

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.

As antidote for poisoning by 4-hydroxycoumarin

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

History of discovery

In 1929, Danish scientist Henrik Dam investigated the role of cholesterol by feeding chickens a cholesterol-depleted diet. 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. 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.

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 three laboratories (Stenflo et al., Nelsestuen et al., and Magnusson et al.) 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.

External links

• Vitamin K: Another Reason to Eat Your Greens