Vitamin K₂ or menaquinone (MK) (/ˌmɛnəˈkwɪnoʊn/) is one of three types of vitamin K, the other two being vitamin K₁ (phylloquinone) and K₃ (menadione). K₂ is both a tissue and bacterial product (derived from vitamin K₁ in both cases) and is usually found in animal products or fermented foods.^[1]

The number n of isoprenyl units in their side chain differs and ranges from 4 to 13, hence Vitamin K₂ consists of various forms.^[2] It is indicated as a suffix (-n), e. g. MK-7 or MK-9. The most common in the human diet is the short-chain, water-soluble menatetrenone (MK-4), which is usually produced by tissue and/or bacterial conversion of vitamin K₁, and is commonly found in animal products. It is known that production of MK-4 from dietary plant vitamin K₁ can be accomplished by animal tissues alone, as it proceeds in germ-free rodents.

However, at least one published study concluded that "MK-4 present in food does not contribute to the vitamin K status as measured by serum vitamin K levels. MK-7, however significantly increases serum MK-7 levels and therefore may be of particular importance for extrahepatic tissues."^[3]

Long-chain menaquinones (longer than MK-4) include MK-7, MK-8 and MK-9 and are more predominant in fermented foods such as natto and cheonggukjang.^[4] Longer-chain menaquinones (MK-10 to MK-13) are produced by anaerobic bacteria in the colon, but they are not well absorbed at this level and have little physiological impact.^[1]

When there are no isoprenyl side chain units, the remaining molecule is vitamin K₃. This is usually made synthetically, and is used in animal feed. It was formerly given to premature infants, but due to inadvertent toxicity in the form of hemolytic anemia and jaundice,^{[failed verification]} it is no longer used for this purpose.^[1] K₃ is now known to be a circulating intermediate in the animal production of MK-4.^[5]

Description

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

MK-7 and other long-chain menaquinones are different from MK-4 in that they are not produced by human tissue. MK-7 may be converted from phylloquinone (K₁) in the colon by Escherichia coli bacteria.^[6] However, these menaquinones synthesized by bacteria in the gut appear to contribute minimally to overall vitamin K status.^[7][8] MK-4 and MK-7 are both found in the United States in dietary supplements for bone health.

All K vitamins are similar in structure: 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".^{[9][full citation needed]} The number of repeating units is indicated in the name of the particular menaquinone (e.g., MK-4 means that four isoprene units are repeated in the carbon tail). The chain length influences lipid solubility and thus transport to different target tissues.

Mechanism of action

The mechanism of action of vitamin K₂ is similar to vitamin K₁. K vitamins were first recognized as a factor required for coagulation, but the functions performed by this vitamin group were revealed to be much more complex. K vitamins play an essential role as cofactor for the enzyme γ-glutamyl carboxylase, which is involved in vitamin K-dependent carboxylation of the gla domain in "Gla proteins" (i.e., in conversion of peptide-bound glutamic acid (Glu) to γ-carboxy glutamic acid (Gla) in these proteins).^[10]

Carboxylation of these vitamin K-dependent Gla-proteins, besides being essential for the function of the protein, is also an important vitamin recovery mechanism since it serves as a recycling pathway to recover vitamin K from its epoxide metabolite (KO) for reuse in carboxylation.

Several human Gla-containing proteins synthesized in several different types of tissues have been discovered:

• Coagulation factors (II, VII, IX, X), as well as anticoagulation proteins (C, S, Z). These Gla-proteins are synthesized in the liver and play an important role in blood homeostasis.
• Osteocalcin. This non-collagenous protein is secreted by osteoblasts and plays an essential role in the formation of mineral in bone.
• Matrix gla protein (MGP). This calcification inhibitory protein is found in numerous body tissues, but its role is most pronounced in cartilage and in arterial vessel walls.
• Growth arrest-specific protein 6 (GAS6). GAS6 is secreted by leucocytes and endothelial cells in response to injury and helps in cell survival, proliferation, migration, and adhesion.
• Proline-rich Gla-proteins (PRGP), transmembrane Gla-proteins (TMG), Gla-rich protein (GRP) and periostin. Their precise functions are still unknown.

Health effects

MK-4 or MK-7 has a protective effect on bone mineral density and reduced risk of hip, vertebral and non-vertebral fractures.^[11] These effects appear to be accentuated when combined with vitamin D and in the setting of osteoporosis.^[1]

Utilisation

With regard to utilisation, reports suggest^{[clarification needed]} that Vitamin K₂ is preferred by the extrahepatic tissues (bone, cartilage, vasculature), which may be produced as MK-4 by the animal from K₁,^{[citation needed]} or it may be of bacterial origin (from MK-7, MK-9, and other MKs).^{[citation needed]}

Absorption profile

Vitamin K is absorbed along with dietary fat from the small intestine and transported by chylomicrons in the circulation.^[12] Most of vitamin K₁ is carried by triacylglycerol-rich lipoproteins (TRL) and rapidly cleared by the liver; only a small amount is released into the circulation and carried by LDL and HDL. MK-4 is carried by the same lipoproteins (TRL, LDL, and HDL) and cleared fast as well. The long-chain menaquinones are absorbed in the same way as vitamin K₁ and MK-4, but are efficiently redistributed by the liver in predominantly LDL (VLDL). Since LDL has a long half life in the circulation, these menaquinones can circulate for extended times resulting in higher bioavailability for extra-hepatic tissues as compared to vitamin K₁ and MK-4. Accumulation of vitamin K in extra-hepatic tissues has direct relevance to vitamin K functions not related to hemostasis.^[13]

Dietary intake in humans

The European Food Safety Authority (EU) and the US Institute of Medicine, on reviewing existing evidence, have decided there is insufficient evidence to publish a dietary reference value for vitamin K or for K₂. They have, however, published an adequate intake (AI) for vitamin K, but no value specifically for K₂.^{[citation needed]}

Parts of the scientific literature, dating back to 1998, suggest that the AI values are based only on the hepatic requirements (i.e. related to the liver).^[14][15] This hypothesis is supported by the fact that the majority of the Western population exhibits a substantial fraction of undercarboxylated extra-hepatic proteins.^{[citation needed]} Thus, complete activation of coagulation factors is satisfied, but there does not seem to be enough vitamin K₂ for the carboxylation of osteocalcin in bone and MGP in the vascular system.^[16][17]

There is no known toxicity associated with high doses of menaquinones (vitamin K₂). Unlike the other fat-soluble vitamins, vitamin K is not stored in any significant quantity in the liver. All data available as of 2017^[update] demonstrate that vitamin K has no adverse effects in healthy subjects.^{[citation needed]} The recommendations for the daily intake of vitamin K, as issued recently by the US Institute of Medicine, also acknowledge the wide safety margin of vitamin K: "a search of the literature revealed no evidence of toxicity associated with the intake of either K₁ or K₂". Animal models involving rats, if generalisable to humans, show that MK-7 is well tolerated.^[18]

Dietary sources

Apart from animal livers, the richest dietary source of menaquinones are fermented foods (from bacteria, not molds or yeasts); sources include cheeses consumed in Western diets (e.g., containing MK-8 and MK-9) and fermented soybean products (e.g., in traditional nattō consumed in Japan, containing MK-7).^{[citation needed]} (Here and following it is noteworthy that most food assays measure only fully unsaturated menaquinones.^{[citation needed]})

MK-4 is synthesized by animal tissues and is found in meat, eggs, and dairy products.^[19] Cheeses have been found to contain MK-8 at 10–20 μg per 100 g and MK-9 at 35–55 μg per 100 g.^[13] In one report, no substantial differences in MK-4 levels were observed between wild game, free-range animals, and factory farm animals.^[20]

In addition to its animal origins, menaquinones are synthesized by bacteria during fermentation and so, as stated, are found in most fermented cheese and soybean products.^{[21][non-primary source needed]} As of 2001, the richest known source of natural K₂ was nattō fermented using the nattō strain of Bacillus subtilis,^[22] which is reportedly a good source of long-chain MK-7.^{[citation needed]} In nattō, MK-4 is absent as a form of vitamin K, and in cheeses it is present among the vitamins K only in low proportions.^{[relevant?][23][better source needed]} Still it is unknown whether B. subtilis will produce K₂ using other legumes (e.g., chickpeas, or lentils) or even B. subtilis fermented oatmeal.

Food frequency questionnaire-derived estimates of relative intakes of Vitamins K in one northern European country suggest that for that population, about 90% of total vitamin K intakes are provided by K₁, about 7.5% by MK-5 through MK-9 and about 2.5% by MK-4;^{[citation needed]} the intense smell and strong taste of nattō appear to make this soya food a less attractive source of K₂ for Western tastes.

Supplement companies sell nattō extract reportedly standardized with regards to K₂ content, in capsule form.^{[citation needed]}

Analysis of foods

Notes:

• † – The reported amounts in comparable milk from the USA and the Netherlands differ by more than 40 times, so these numbers should be considered suspect.

Anticoagulants

Recent studies found a clear association between long-term oral (or intravenous) anticoagulant treatment (OAC) and reduced bone quality due to reduction of active osteocalcin. OAC might lead to an increased incidence of fractures, reduced bone mineral density or content, osteopenia, and increased serum levels of undercarboxylated osteocalcin.^[27]

Furthermore, OAC is often linked to undesired soft-tissue calcification in both children and adults.^[28][29] This process has been shown to be dependent upon the action of K vitamins. Vitamin K deficiency results in undercarboxylation of MGP. Also in humans on OAC treatment, two-fold more arterial calcification was found as compared to patients not receiving vitamin K antagonists.^[30][31] Among consequences of anticoagulant treatment: increased aortic wall stiffness, coronary insufficiency, ischemia, and even heart failure. Arterial calcification might also contribute to systolic hypertension and ventricular hypertrophy.^[32][33] Anticoagulant therapy is usually instituted to avoid life-threatening diseases, and high vitamin K intake interferes with anticoagulant effects.^{[citation needed]} Patients on warfarin (Coumadin) or being treated with other vitamin K antagonists are therefore advised not to consume diets rich in K vitamins.^{[citation needed]}

In other organisms

Many bacteria synthesize menaquinones from chorismic acid. They use it as a part of the electron transport chain, playing a similar role as other quinones such as ubiquinone. Oxygen, heme, and menaquinones are needed for many species of lactic acid bacteria to conduct respiration.^[34]

Variations in biosynthetic pathways mean that bacteria also produce analogues of Vitamin K₂. For example, MK9_(II-H), which replaces the second geranylgeranyl unit with a saturated phytyl, is produced by Mycobacterium phlei. There also exists a possibility of cis–trans isomerism due to the double bonds present. In M. phlei, the 3'-methyl-cis MK9_(II-H) form seems to be more biologically active than trans MK9_(II-H).^[35] However, with human enzymes, the naturally abundant trans form is more efficient.^[36]

One hydrogenated MK that sees relevant amounts of human consumption is MK-9(4H), found in cheese fermented by Propionibacterium freudenreichii. This variation has the second and third units replaced with phytyl.^[37]