Vascular calcification and osteoporosis share similar etiopathogenetic mechanisms. Vitamin K2 deficiency could be responsible of the so called “calcium paradox”, that is the lack of calcium in the bone and its storage in the vessel wall. These events may have clinically relevant consequences, such as cardiovascular accidents, and bone fractures.
Vitamin K is a family of structurally similar, fat-soluble, 2-methyl-1,4-naphthoquinones, including phylloquinone (K1), menaquinones (K2), and menadione (K3). The structural difference is in the substituent R group.
The best-known member of the vitamin K family is phylloquinone (K1), also known as phytonadione because of its relationship with photosynthesis. Phylloquinone is found in higher plants and algae, with the highest concentrations found in green leafy vegetables.
Something more to say about calcium homeostasis: the role of vitamin K2 in vascular calcification and osteoporosis. 2013
Menaquinones (K2) also occur naturally, but are produced by an array of bacteria, not by higher plants. Recent studies have determined menaquinones can be produced in limited quantities by animals, and probably by humans, from the conversion of other forms of vitamin K. The most common form of vitamin K2 in animals is menaquinone 4 (menatetrenone; MK-4), produced by the processing of exogenous and bacterial naphthoquinones. It also occur in cheese, meat, and fermented soya derivatives.
Compared to vitamin K1, dietary contribution of vitamin K2 is much less. Dietary sources of vitamin K2 include chicken, egg yolk, dairy products, cow liver, and natto. Vitamin K2 can also be of microbiological origin, found primarily in fermented foods, or can be produced by bacteria of the gastrointestinal tract. The petitioner proposes that menaquinone be marketed in the form of a vitamin K2-containing oil, which is produced from the fermentation of soybean protein isolate and corn starch in the presence of Gram-positive bacterium Bacillus subtilis natto.
In fact several studies were conducted in which circulating level of menaquinone were monitored following ingestion of natto or natto bacilli powder. In eastern Japanese women, identified as frequent natto consumers, serum menaquinone levels were significanty higher.
Vitamins K1 and K2 differ only in the substituent R grouP. Vitamin K1 possesses a phytyl R group (partially saturated polyisoprenoid group), while K2 possesses a repeating, unsaturated trans-polyisoprenyl group. Vitamin K2 as referred to in this article means MK-4.
Menadione (K3) is not considered a natural vitamin K, but a synthetic analogue that acts as a provitamin. It possesses a much simpler structure, with no aliphatic R group chain.
MVitamin K and Bone Health. 2013
Menaquinones (MK) are evolutionarily the most ancient type of isoprenoid quinones and the most widespread bacterial respiratory quinones. They are synthesized by a limited number of obligate and facultative anaerobic bacteria some of which occupy a niche in the microflora of the human gut. The family contains a wide spectrum of isoprenologues in which the side chain comprises a polymer of repeating prenyl units.
Many bacterial strains synthesize MK in which one or more of the prenyl units is saturated. The additional hydrogen atoms are indicated by a prefix (dihydro-, tetrahydro- etc) and are abbreviated MK-n(H2), MK-n(H4) etc. The contribution of partially saturated MK to vitamin K nutrition has been largely ignored but they are synthesised by members of the gut microflora and by bacteria used in food fermentation processes.
VitaminK occurs in the natural world in several forms, including a plant form, phylloquinone (PK), and a bacterial form,menaquinones (MKs). Inmanyspecies, including humans,PKis aminor constituent of hepatic vitaminK content, with most hepatic vitaminK content comprising long chainMKs. Menaquinone-4 (MK-4) is ubiquitously present in extrahepatic tissues, with particularly high concentrations in the brain, kidney and pancreas of humans and rats1–3. It has consistently been shown that PK is endogenously converted to MK-4. This occurs either directly within certain tissues or by interconversion to menadione (K3), followed by prenylation to MK-4. However, the molecular mechanisms for these conversion reactions are unclear.
The contribution of vitamin K2 (menaquinones) produced by the intestinal microflora to human nutritional requirements for vitamin K. 1994
Two pathways for the conversion of PK to MK-4 are possible: first, side-chain removal occurs during intestinal absorption and then the released K3 is transferred to tissues through the bloodstream and is subsequently prenylated to formMK-4; and second, after the transfer of PK to tissues, side-chain cleavage and geranylgeranylation occur simultaneously within tissues.
In E. coli, MKs are involved in several anaerobic electron transport systems: it is the major transporter of electrons under anaerobic growth conditions. Six genes (menA, menB, menC, menD, menE and menF) are involved in the biosynthetic pathway of MKs, and a key reaction for menaquinone biosynthesis in E. coli is the conversion of 1,4-dihydroxy-2-naphthoic acid (a bicyclic naphthalenoid) to the membrane-bound demethylmenaquinone. The key enzyme catalysing
this reaction is encoded by menA.
Several human homologues of the E. coli menaquinone biosynthetic enzymes exist in the human genome database. However, the biological functions of these homologous genes remain unknown. UBIAD1 was the first identified mammalian homologue of E. coli menA. In E. coli, the menA gene encodes a prenyltransferase that is involved in the vitaminK biosynthetic pathway. UBIAD1 is a human biosynthetic enzyme for MK-4.
Identification of UBIAD1 as a novel human menaquinone-4 biosynthetic enzyme. 2010
Vitamin K is a cofactor in a number of biochemical pathways. Those most commonly associated with vitamin K are the vitamin K-dependent carboxylation reactions. In these reactions the reduced form of vitamin K (hydroquinone) de-protonates glutamate via the gamma-glutamylcarboxylase enzyme. The epoxide formed is recycled via vitamin K epoxide reductase and quinone reductase, and glutamic acid-containing proteins, such as coagulation factors II (prothrombin), VII, IX, and X, protein C, and
protein S, are carboxylated. Compared to the other vitamin K analogues, vitamin K2 has the most potent gamma-carboxylation activity.
Vitamin K functions in the posttranslational modification of a number of vitamin K dependent proteins such as osteocalcin, a bone protein containing gamma-carboxyglutamic acid, discovered in 1975. Gamma-carboxylation of the glutamic acid in osteocalcin is vitamin K dependent and involves the conversion of glutamic acid residues (Glu) to gamma-carboxyglutamic acid residues (Gla). A number of calcium-binding proteins, such as calbindin and osteocalcin, contain gamma-carboxyglutamate. These proteins are involved with calcium uptake and bone mineralization. Osteocalcin is synthesized only in osteoblasts. Because osteocalcin that is not carboxylated cannot bind to hydroxyapatite, serum levels of osteocalcin are a good biochemical marker of the metabolic turnover of bone.
An elevated level of serum glutamic acid-under-carboxylated osteocalcin is indicative of vitamin K deficiency and is associated with reduced hip bone mineral density (BMD) and increased fracture risk in healthy elderly women.
Recent trends in the metabolism and cell biology of vitamin K with special reference to vitamin K cycling andmenaquinone-4 biosynthesis. 2014
Vitamin K in the treatment and prevention of osteoporosis and arterial calcification. 2005
The role of vitamin K in soft-tissue calcification. 2012
The transformation of vascular smooth muscle cells (VSMC) into osteoblastic-like cell is widely documented in humans and experimental models of atherogenesis,
and is the final step of the transformation of VSMC from the contractile phenotype to the secretory phenotype. The result is the increase in arterial wall stiffness and a consequent negative influence on hemodynamics, which together with the thromboembolic risk of atherosclerotic plaques and the effects of vascular stenosis or obstruction contributes to increase cardiovascular disease risk. This is the background of the development of systolic hypertension, hypertrophy/dilation of left cardiac
ventricle, risk of myocardial infarction, risk of cerebral hemorrhage, hypertensive nephropathy.
A common pathogenic mechanism between calcific atherosclerosis (CA) and osteoporosis (OP) has recently been identified, but it has been only partially clarified; it is represented by subclinic deficiency of vitamin K2 (VK2), which could have an important role in the development of “calcium paradox”.
VK1 and VK2 are activators of specific hepatic and extra-hepatic proteins with important biological functions, called “Gla-proteins”, which are VK dependent (VKDP). The “Gla-proteins” owe their name and differences to the richness of glutammic acid residues, that are carboxylated in a VK-dependent process necessary for the acquisition of the biological function. Thus, un-carboxylated forms of VKDP are biologically inactive, and their presence in the blood is a sign of relative or absolute VK deficiency.
Role of vitamin K and vitamin K-dependent proteins in vascular calcification. 2001
The first VK2DP have a well-characterized biological function, essential for the maintenance of the normal structure of arterial wall, osteoarticular system, teeth, and for the regulation of cell growth. In particular, the Matrix-glaprotein (MGP) is the gatekeeper of vascular “ossification”. MGP inhibits the precipitation of calcium, in the form of hydroxiapatite crystals, at the site of elastic lamellae, an event that blocks the removal of the initial foci of calcification.
The second VK2DP with a relevant biological function is the Bone-gla-protein (osteocalcin). It acts in the bone, favoring mineralization of trabecula, and in the tooth, favoring synthesis of enamel.
DEFINITION AND FUNCTION OF GLA-PROTEINS
Matrix gla protein (MGP) is a 10-kDa vitamin K-dependent protein found in numerous body tissues that requires vitamin K for its optimum function. It is present in bone (together with the related vitamin K-dependent protein osteocalcin), as well as in heart, kidney and lung. In bone, its production is increased by vitamin D.
The MGP was linked to the short arm of chromosome 12
Abnormalities in the MGP gene have been linked with Keutel syndrome, a rare condition characterised by abnormal calcium deposition in cartilage, peripheral stenosis of the pulmonary artery, and midfacial hypoplasia.
Matrix gla protein (MGP) inhibits arterial and cartilaginous calcification. A threonine to alanine (Thr83Ala) polymorphism (codon 83) in MGP is associated with myocardial infarction and femoral artery calcification.
Matrix gla protein gene polymorphism is associated with increased coronary artery calcification progression. 2013
Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein (BGLAP), is a noncollagenous protein found in bone and dentin. In humans, the osteocalcin is encoded by the BGLAP gene. Its receptor is GPRC6A. This protein contains 47-50 amino acid residues (molecular weight 5,200-5,900) depending on the species.
Osteocalcin is secreted solely by osteoblasts and thought to play a role in the body's metabolic regulation and is pro-osteoblastic, or bone-building, by nature. It is also implicated in bone mineralization and calcium ion homeostasis.
MGP and osteocalcin are both calcium-binding proteins that may participate in the organisation of bone tissue. Both have glutamate residues that are post-translationally carboxylated by the enzyme gamma-glutamyl carboxylase in a reaction that requires Vitamin K hydroquinone. This process also occurs with a number of proteins involved in coagulation: prothrombin, factor VII, factor IX and factor X, protein C, protein S and protein Z.
The enzyme peptidyl-glutamate 4-carboxylase can use various vitamin-K derivatives, including menaquinone, but does not contain iron. In the reverse direction the mechanism appears to involve the generation of a strong base by oxygenation of vitamin K. It catalyses the post-translational modification of several proteins of the blood-clotting system. 9--12 glutamate residues are converted to 4-carboxyglutamate (Gla) in a specific domain of the target protein. The 4-pro-S hydrogen of the glutamate residue is removed and there is an inversion of stereochemistry at this position.
In conclusion vitamin K2 has been demonstrated to be involved in the inhibition
of vascular foci of calcification, especially through the carboxylation of proteins that regulate calcium deposition in atherosclerotic plaques.
In all patients diagnosed with calcific atherosclerosis, the assessment of the overall cardiovascular risk is mandatory, and an accurate evaluation of VK2 dietary intake is crucial for the investigation of carential status. Probably, particular attention should be paid to patients suffering from intestinal dysmicrobism.
Nimambumbu Preçia, Mampasi