Vitamin K₂

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. 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.[2] However, these menaquinones synthesized by bacteria in the gut appear to contribute minimally to overall vitamin K status.[3][4] MK-4 and MK-7 are both found in the United States in dietary supplements for bone health.

The U.S. Food and Drug Administration (FDA) has not approved any form of vitamin K for the prevention or treatment of osteoporosis. MK-4 (45 mg daily) has been approved by the Ministry of Health in Japan since 1995 for the prevention and treatment of osteoporosis.[5]

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".[6] 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.

Vitamin K structures. MK-4 and MK-7 are both subtypes of K₂.

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).

Carboxylation reaction – the Vitamin K cycle

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.

MK-4 or MK-7 has a protective effect on bone mineral density and reduced risk of hip, vertebral and non-vertebral fractures.[7] These effects appear to be accentuated when combined with vitamin D and in the setting of osteoporosis.[1] The possible health benefits suggested for further investigation relate to bone strength and arterial health (reducing calcification or even decalcifying, with a possible reduction in blood pressure).[8]

Vitamin K is absorbed along with dietary fat from the small intestine and transported by chylomicrons in the circulation. 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.[9]

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. Evidence suggests K₂ is converted from dietary K₁ in mice, so they might not require a dietary intake of K₂.[10]

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).[11][12] This hypothesis is supported by the fact that the majority of the Western population exhibits a substantial fraction of undercarboxylated extra-hepatic proteins. 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.[13][14]

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; therefore the toxic level is not a described problem. All data available as of 2017 demonstrate that vitamin K has no adverse effects in healthy subjects. 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.[15]

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). (Here and following it is noteworthy that most food assays measure only fully unsaturated menaquinones.)

MK-4 is synthesized by animal tissues and is found in meat, eggs, and dairy products.[16] 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.[9] In one report, no substantial differences in MK-4 levels were observed between wild game, free-range animals, and factory farm animals.[17]

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.[18] As of 2001, the richest known source of natural K₂ was nattō fermented using the nattō strain of Bacillus subtilis,[19] which is reportedly a good source of long-chain MK-7. 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.[20] As of this date, it is unknown whether B. subtilis will produce K₂ using other legumes (e.g., chickpeas, or lentils).

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; the intense smell and strong taste of nattō appear to make this soya food a less attractive source of K₂ for Western tastes.

With regard to utilisation, reports suggest that Vitamin K₂ is preferred by the extrahepatic tissues (bone, cartilage, vasculature), which may be produced as MK-4 by the animal from K₁, or it may be of bacterial origin (from MK-7, MK-9, and other MKs). Discussion is ongoing as to what extent K₂ produced by human intestinal bacteria contributes to daily vitamin K₂ needs.

Supplement companies sell nattō extract reportedly standardized with regards to K₂ content, in capsule form.

Prawns, herring, kale, spinach, broccoli, green peas, bananas, apples, oranges, margarine, corn oil, sunflower oil, olive oil, rye bread, wheat bread, sourdough bread, and tea contain vitamin K₁ but not vitamin K₂. Skimmed milk and coffee do not contain any vitamin K.[17]

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.[24]

Furthermore, OAC is often linked to undesired soft-tissue calcification in both children and adults.[25][26] This process has been shown to be dependent upon the action of K vitamins. Vitamin K deficiency results in undercarboxylation of MGP. Vascular calcification was shown to appear in warfarin-treated experimental animals within two weeks.[27] Also in humans on OAC treatment, two-fold more arterial calcification was found as compared to patients not receiving vitamin K antagonists.[28][29] 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.[30][31] Coumarins, by interfering with vitamin K metabolism, are thought by some to lead to excessive calcification of cartilage and the tracheobronchial arteries.

Anticoagulant therapy is usually instituted to avoid life-threatening diseases, and high vitamin K intake interferes with anticoagulant effects. Patients on warfarin (Coumadin) or being treated with other vitamin K antagonists are therefore advised not to consume diets rich in K vitamins. However, the latest research proposes to combine vitamins K with OAC to stabilize the International normalized ratio (INR, a laboratory test measure of blood coagulation).

Individuals taking anticoagulant medications, such as warfarin (coumarins), should consult their doctor before taking vitamin K₂.

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.[32]

• Vitamin K
• Vitamin K₁
• Vitamin K₃