Vitamin B12

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Source: Wikipedia, the free encyclopedia.
This article is about the family of vitamers. For individual forms, see hydroxocobalamin, cyanocobalamin, methylcobalamin, and adenosylcobalamin.
"B12" and "Cbl" redirect here. For other uses of B12, see B12 (disambiguation). For the musical group, see Carbon Based Lifeforms.

Vitamin B12
General skeletal formula of cobalamins
Stick model of cyanocobalamin (R = CN) based on the crystal structure[1]
Clinical data
Other namesVitamin B12, vitamin B-12, cobalamin
AHFS/Drugs.comMonograph
MedlinePlusa605007
License data
intramuscular (IM), intranasal
ATC code
Legal status
Legal status
  • UK: OTC
  • US: OTC
transcobalamins plasma proteins.
Binding of hydroxocobalamin is slightly higher than cyanocobalamin.
MetabolismLiver
Elimination half-lifeApproximately 6 days
(400 days in the liver).
ExcretionKidney
Identifiers
  • α-(5,6-Dimethylbenzimidazolyl)cobamidcyanide
JSmol)
  • NC(=O)C[C@@]8(C)[C@H](CCC(N)=O)C=2/N=C8/C(/C)=C1/[C@@H](CCC(N)=O)[C@](C)(CC(N)=O)[C@@](C)(N1[Co+]C#N)[C@@H]7/N=C(C(\C)=C3/N=C(/C=2)C(C)(C)[C@@H]3CCC(N)=O)[C@](C)(CCC(=O)NCC(C)OP([O-])(=O)O[C@@H]6[C@@H](CO)O[C@H](n5cnc4cc(C)c(C)cc45)[C@@H]6O)[C@H]7CC(N)=O
  • InChI=1S/C62H90N13O14P.CN.Co/c1-29-20-39-40(21-30(29)2)75(28-70-39)57-52(84)53(41(27-76)87-57)89-90(85,86)88-31(3)26-69-49(83)18-19-59(8)37(22-46(66)80)56-62(11)61(10,25-48(68)82)36(14-17-45(65)79)51(74-62)33(5)55-60(9,24-47(67)81)34(12-15-43(63)77)38(71-55)23-42-58(6,7)35(13-16-44(64)78)50(72–42)32(4)54(59)73–56;1–2;/h20-21,23,28,31,34-37,41,52-53,56-57,76,84H,12-19,22,24-27H2,1-11H3,(H15,63,64,65,66,67,68,69,71,72,73,74,77,78,79,80,81,82,83,85,86);;/q;;+2/p-2/t31?,34-,35-,36-,37+,41-,52-,53-,56-,57+,59-,60+,61+,62+;;/m1../s1 checkY
  • Key:RMRCNWBMXRMIRW-WYVZQNDMSA-L checkY

Vitamin B12, also known as cobalamin, is a water-soluble

amino acid metabolism.[3] It is important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the circulatory system in the maturation of red blood cells in the bone marrow.[2][4] Plants do not need cobalamin and carry out the reactions with enzymes that are not dependent on it.[5]

Vitamin B12 is the most chemically complex of all vitamins,[6] and for humans the only vitamin that must be sourced from animal-derived foods or supplements.[2][7] Only some archaea and bacteria can synthesize vitamin B12.[8] Vitamin B12 deficiency is a widespread condition that is particularly prevalent in populations with low consumption of animal foods. This can be due to a variety of reasons, such as low socioeconomic status, ethical considerations, or lifestyle choices such as veganism.[9]

Foods containing vitamin B12 include meat,

eggs, and dairy products.[2] Many breakfast cereals are fortified with the vitamin.[2] Supplements and medications are available to treat and prevent vitamin B12 deficiency.[2] They are usually taken by mouth, but for the treatment of deficiency may also be given as an intramuscular injection.[2][6]

Vitamin B12 deficiencies have a greater effect on the pregnant, young children, and elderly people, and are more common in middle and lower developed countries due to malnutrition.

H2 blockers or other antacids are at increased risk.[13]

The diets of vegetarians and vegans may not provide sufficient B12 unless a dietary supplement is taken.

neuropathy or a blood disorder called pernicious anemia, a type of anemia in which red blood cells become abnormally large.[2] This can result in fatigue, decreased ability to think, lightheadedness, shortness of breath, frequent infections, poor appetite, numbness in the hands and feet, depression, memory loss, confusion, difficulty walking, blurred vision, irreversible nerve damage, and many others.[14] If left untreated in infants, deficiency may lead to neurological damage and anemia.[2] Folate levels in the individual may affect the course of pathological changes and symptomatology of vitamin B12 deficiency. Vitamin B12 deficiency in pregnant women is strongly associated with an increased risk of spontaneous abortion, congenital malformations such as neural tube defects, problems with brain development growth in the unborn child.[10]

Vitamin B12 was discovered as a result of pernicious anemia, an

autoimmune disorder in which the blood has a lower than normal number of red blood cells, due to a deficiency of vitamin B12.[5][15] The ability to absorb the vitamin declines with age, especially in people over 60.[16]

Definition

Vitamin B12 is a

coenzyme, meaning that its presence is required for some enzyme-catalyzed reactions.[12][18]

Cyanocobalamin is a manufactured form of B12. Bacterial fermentation creates AdoB12 and MeB12, which are converted to cyanocobalamin by the addition of potassium cyanide in the presence of sodium nitrite and heat. Once consumed, cyanocobalamin is converted to the biologically active AdoB12 and MeB12. The two bioactive forms of vitamin B
12 are

mitochondria
.

Cyanocobalamin is the most common form used in dietary supplements and food fortification because cyanide stabilizes the molecule against degradation. Methylcobalamin is also offered as a dietary supplement.[12] There is no advantage to the use of adenosylcobalamin or methylcobalamin forms for the treatment of vitamin B12 deficiency.[19][20][4]

Hydroxocobalamin can be injected intramuscularly to treat vitamin B12 deficiency. It can also be injected intravenously for the purpose of treating cyanide poisoning, as the hydroxyl group is displaced by cyanide, creating a non-toxic cyanocobalamin that is excreted in urine.

"Pseudovitamin B12" refers to compounds that are corrinoids with a structure similar to the vitamin but without vitamin activity.[21] Pseudovitamin B12 is the majority corrinoid in spirulina, an algal health food sometimes erroneously claimed as having this vitamin activity.[22]

Deficiency

Main article: Vitamin B12 deficiency

Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system.

fatigue, feeling weak, lightheadedness, dizziness, breathlessness, headaches, mouth ulcers, upset stomach, decreased appetite, difficulty walking (staggering balance problems),[14][24] muscle weakness, depression, poor memory, poor reflexes, confusion, and pale skin, feeling abnormal sensations, among others, especially in people over age 60.[6][14][25] Vitamin B12 deficiency can also cause symptoms of mania and psychosis.[26][27] Among other problems, weakened immunity, reduced fertility and interruption of blood circulation in women may occur.[28]

The main type of

triad of symptoms
:

  1. purines and thymidine
    ).
  2. Gastrointestinal symptoms: alteration in bowel motility, such as mild
    autoimmune attack on the parietal cells of the stomach in pernicious anemia. There is an association with gastric antral vascular ectasia (which can be referred to as watermelon stomach), and pernicious anemia.[31]
  3. Neurological symptoms: sensory or motor deficiencies (absent reflexes, diminished vibration or soft touch sensation) and
    involuntary movements and hypotonia.[33]

Vitamin B12 deficiency is most commonly caused by malabsorption, but can also result from low intake, immune gastritis, low presence of binding proteins, or use of certain medications.

stomach acid as they age, a condition known as achlorhydria, thereby increasing their probability of B12 deficiency due to reduced absorption.[2]

Nitrous oxide overdose or overuse converts the active monovalent form of vitamin B12 to the inactive bivalent form.[37]

Pregnancy, lactation and early childhood

The U.S. Recommended Dietary Allowance (RDA) for pregnancy is 2.6 μg/d, for lactation 2.8 μg/d. Determination of these values was based on an RDA of 2.4 μg/d for non-pregnant women, plus what will be transferred to the fetus during pregnancy and what will be delivered in breast milk.[12][38]: 972  However, looking at the same scientific evidence, the European Food Safety Authority (EFSA) sets adequate intake (AI) at 4.5 μg/d for pregnancy and 5.0 μg/d for lactation.[39] Low maternal vitamin B12, defined as serum concentration less than 148 pmol/L, increases the risk of miscarriage, preterm birth and newborn low birth weight.[40][38] During pregnancy the placenta concentrates B12, so that newborn infants have a higher serum concentration than their mothers.[12] As it is recently absorbed vitamin content that more effectively reaches the placenta, the vitamin consumed by the mother-to-be is more important than that contained in her liver tissue.[12][41]

Women who consume little animal-sourced food, or who are vegetarian or vegan, are at higher risk of becoming vitamin depleted during pregnancy than those who consume more animal products. This depletion can lead to anemia, and also an increased risk that their breastfed infants become vitamin deficient.[41][38] Vitamin B12 is not one of the supplements recommended by the World Health Organization for healthy women who are pregnant,[10] however vitamin B12 is often suggested during pregnancy in a multivitamin along with folic acid[42][43] especially for pregnant mothers who follow a vegetarian or vegan diet.[44]

Low vitamin concentrations in human milk occur in families with low socioeconomic status or low consumption of animal products.[38]: 971, 973  Only a few countries, primarily in Africa, have mandatory food fortification programs for either wheat flour or maize flour; India has a voluntary fortification program.[45] What the nursing mother consumes is more important than her liver tissue content, as it is recently absorbed vitamin that more effectively reaches breast milk.[38]: 973  Breast milk B12 decreases over months of nursing in both well-nourished and vitamin-deficient mothers.[38]: 973–974  Exclusive or near-exclusive breastfeeding beyond six months is a strong indicator of low serum vitamin status in nursing infants. This is especially true when the vitamin status was poor during the pregnancy and if the early-introduced foods fed to the still breastfeeding infant are vegan.[38]: 974–975 

Risk of deficiency persists if the post-weaning diet is low in animal products.[38]: 974–975  Signs of low vitamin levels in infants and young children can include anemia, poor physical growth and neurodevelopmental delays.[38]: 975  Children diagnosed with low serum B12 can be treated with intramuscular injections, then transitioned to an oral dietary supplement.[38]: 976 

Gastric bypass surgery

Various methods of gastric bypass or gastric restriction surgery are used to treat morbid obesity. Roux-en-Y gastric bypass surgery (RYGB) but not sleeve gastric bypass surgery or gastric banding, increases the risk of vitamin B12 deficiency and requires preventive post-operative treatment with either injected or high-dose oral supplementation.[46][47][48] For post-operative oral supplementation, 1000 μg/d may be needed to prevent vitamin deficiency.[48]

Diagnosis

According to one review: "At present, no 'gold standard' test exists for the diagnosis of vitamin B12 deficiency and as a consequence the diagnosis requires consideration of both the clinical state of the patient and the results of investigations."

polymorphonuclear leukocytes may be seen. Diagnosis is supported based on vitamin B12 blood levels below 150–180 pmol/L (200–250 pg/mL) in adults.[50] However, serum values can be maintained while tissue B12 stores are becoming depleted. Therefore, serum B12 values above the cut-off point of deficiency do not necessarily confirm adequate B12 status.[2] For this reason, elevated serum homocysteine over 15 micromol/L and methylmalonic acid (MMA) over 0.271 micromol/L are considered better indicators of B12 deficiency, rather than relying only on the concentration of B12 in blood.[2] However, elevated MMA is not conclusive, as it is seen in people with B12 deficiency, but also in elderly people who have renal insufficiency,[27] and elevated homocysteine is not conclusive, as it is also seen in people with folate deficiency.[51] In addition, elevated methylmalonic acid levels may also be related to metabolic disorders such as methylmalonic acidemia.[52] If nervous system damage is present and blood testing is inconclusive, a lumbar puncture may be carried out to measure cerebrospinal fluid B12 levels.[53]

Serum haptocorrin binds 80-90% of circulating B12, rendering it unavailable for cellular delivery by transcobalamin II. This is conjectured to be a circulating storage function.[54] Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B12, while at the same time potentially manifesting as a symptomatic vitamin deficiency because of insufficient vitamin bound to transcobalamin II which transfers the vitamin to cells.[55]

Medical uses

A vitamin B12 solution (hydroxocobalamin) in a multi-dose bottle, with a single dose drawn up into a syringe for injection. Preparations are usually bright red.

Treatment of deficiency

Severe vitamin B12 deficiency is initially corrected with daily intramuscular injections of 1000 μg of the vitamin, followed by maintenance via monthly injections of the same amount or daily oral dosing of 1000 μg. The daily dose is far in excess of the vitamin requirement because the normal transporter protein mediated absorption is absent, leaving only very inefficient intestinal passive absorption.[56][57] Injection side effects include skin rash, itching, chills, fever, hot flushes, nausea and dizziness. Oral maintenance treatment avoids this problem and significantly reduces cost of treatment.[56][57]

Cyanide poisoning

For

intravenously and sometimes in combination with sodium thiosulfate.[58][59] The mechanism of action is straightforward: the hydroxycobalamin hydroxide ligand is displaced by the toxic cyanide ion, and the resulting non-toxic cyanocobalamin is excreted in urine.[60]

Dietary recommendations

Some research shows that most people in the United States and the United Kingdom consume sufficient vitamin B12.

developing countries
are at particular risk due to increased requirements during growth coupled with diets low in animal-sourced foods.

The US

dietary reference intakes (DRIs).[12]

The European Food Safety Authority (EFSA) refers to the collective set of information as "dietary reference values", with population reference intake (PRI) instead of RDA, and average requirement instead of EAR. AI and UL are defined by EFSA the same as in the United States. For women and men over age 18 the adequate intake (AI) is set at 4.0 μg/day. AI for pregnancy is 4.5 μg/day, for lactation 5.0 μg/day. For children aged 1–14 years the AIs increase with age from 1.5 to 3.5 μg/day. These AIs are higher than the U.S. RDAs.[39] The EFSA also reviewed the safety question and reached the same conclusion as in the United States—that there was not sufficient evidence to set a UL for vitamin B12.[62]

The Japan National Institute of Health and Nutrition set the RDA for people ages 12 and older at 2.4 μg/day.[63] The World Health Organization also uses 2.4 μg/day as the adult recommended nutrient intake for this vitamin.[64]

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 B12 labeling purposes, 100% of the daily value was 6.0 μg, but on May 27, 2016, it was revised downward to 2.4 μg.

US$10 million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales.[67][68] A table of the old and new adult daily values is provided at Reference Daily Intake
.

Sources

Bacteria and archaea

Vitamin B12 is produced in nature by certain bacteria, and archaea.[69][70][71] It is synthesized by some bacteria in the gut microbiota in humans and other animals, but it has long been thought that humans cannot absorb this as it is made in the colon, downstream from the small intestine, where the absorption of most nutrients occurs.[72] Ruminants, such as cows and sheep, are foregut fermenters, meaning that plant food undergoes microbial fermentation in the rumen before entering the true stomach (abomasum), and thus they are absorbing vitamin B12 produced by bacteria.[72][73]

Other mammalian species (examples:

guinea pigs) consume high-fibre plants which pass through the gastrointestinal tract and undergo bacterial fermentation in the cecum and large intestine. In this hindgut fermentation, the material from the cecum is expelled as "cecotropes" and are re-ingested, a practice referred to as cecotrophy. Re-ingestion allows for absorption of nutrients made available by bacterial fermentation, and also of vitamins and other nutrients synthesized by the gut bacteria, including vitamin B12.[73]

Non-ruminant, non-hindgut herbivores may have an enlarged forestomach and/or small intestine to provide a place for bacterial fermentation and B-vitamin production, including B12.

cobalt.[74] Soil that is deficient in cobalt may result in B12 deficiency, and B12 injections or cobalt supplementation may be required for livestock.[75]

Animal-derived foods

Animals store vitamin B12 from their diets in their

organ meats from lamb, veal, beef, and turkey; also shellfish and crab meat.[6][61][79]

Plants and algae

There is some evidence that bacterial fermentation of plant foods and symbiotic relationships between algae and bacteria can provide vitamin B12. However, the Academy of Nutrition and Dietetics considers plant and algae sources "unreliable", stating that vegans should turn to fortified foods and supplements instead.[34]

Natural plant and algae sources of vitamin B12 include fermented plant foods such as tempeh[80][81] and seaweed-derived foods such as nori and laverbread.[82][83][84] Methylcobalamin has been identified in Chlorella vulgaris.[85] Since only bacteria and some archea possess the genes and enzymes necessary to synthesize vitamin B12, plant and algae sources all obtain the vitamin secondarily from symbiosis with various species of bacteria,[5] or in the case of fermented plant foods, from bacterial fermentation.[80]

Fortified foods

Foods for which vitamin B12-fortified versions are available include

breakfast cereals, plant-derived milk substitutes such as soy milk and oat milk, energy bars, and nutritional yeast.[79] The fortification ingredient is cyanocobalamin. Microbial fermentation yields adenosylcobalamin, which is then converted to cyanocobalamin by addition of potassium cyanide or thiocyanate in the presence of sodium nitrite and heat.[86]

As of 2019, nineteen countries require food fortification of wheat flour, maize flour or rice with vitamin B12. Most of these are in southeast Africa or Central America.[45]

Vegan advocacy organizations, among others, recommend that every vegan consume B12 from either fortified foods or supplements.[6][36][87][88]

Supplements

A blister pack of 500 µg methylcobalamin tablets

Vitamin B12 is included in multivitamin pills; in some countries grain-based foods such as bread and pasta are fortified with B12. In the US, non-prescription products can be purchased providing up to 5,000 µg each, and it is a common ingredient in energy drinks and energy shots, usually at many times the recommended dietary allowance of B12. The vitamin can also be supplied on prescription and delivered via injection or other means.[2]

Sublingual methylcobalamin, which contains no cyanide, is available in 5 mg tablets. The metabolic fate and biological distribution of methylcobalamin are expected to be similar to that of other sources of vitamin B12 in the diet.[89] The amount of cyanide in cyanocobalamin is generally not a concern, even in the 1,000 µg dose, since the amount of cyanide there (20 µg in a 1,000 µg cyanocobalamin tablet) is less than the daily consumption of cyanide from food, and therefore cyanocobalamin is not considered a health risk.[89]

Intramuscular or intravenous injection

Injection of

hydroxycobalamin is often used if digestive absorption is impaired,[2] but this course of action may not be necessary with high-dose oral supplements (such as 0.5–1.0 mg or more),[90][91] because with large quantities of the vitamin taken orally, even the 1% to 5% of free crystalline B12 that is absorbed along the entire intestine by passive diffusion may be sufficient to provide a necessary amount.[92]

A person with cobalamin C disease (which results in combined

methylmalonic aciduria and homocystinuria) may require treatment with intravenous or intramuscular hydroxocobalamin or transdermal B12, because oral cyanocobalamin is inadequate in the treatment of cobalamin C disease.[93]

Nanotechnologies used in vitamin B12 supplementation

Conventional administration does not ensure specific distribution and controlled release of vitamin B12. Moreover, therapeutic protocols involving injection require health care people and commuting of patients to the hospital thus increasing the cost of the treatment and impairing the lifestyle of patients. Targeted delivery of vitamin B12 is a major focus of modern prescriptions. For example, conveying the vitamin to the bone marrow and nerve cells would help myelin recovery. Currently, several nanocarriers strategies are being developed for improving vitamin B12 delivery with the aim to simplify administration, reduce costs, improve pharmacokinetics, and ameliorate the quality of patients' lives.[94]

Pseudovitamin-B12

Pseudovitamin-B12 refers to B12-like analogues that are biologically inactive in humans.[21] Most cyanobacteria, including Spirulina, and some algae, such as Porphyra tenera (used to make a dried seaweed food called nori in Japan), have been found to contain mostly pseudovitamin-B12 instead of biologically active B12.[22][95] These pseudo-vitamin compounds can be found in some types of shellfish,[21] in edible insects,[96] and at times as metabolic breakdown products of cyanocobalamin added to dietary supplements and fortified foods.[97]

Pseudovitamin-B12 can show up as biologically active vitamin B12 when a microbiological assay with Lactobacillus delbrueckii subsp. lactis is used, as the bacteria can utilize the pseudovitamin despite it being unavailable to humans. To get a reliable reading of B12 content, more advanced techniques are available. One such technique involves pre-separation by silica gel and then assessment with B12-dependent E. coli bacteria.[21]

A related concept is

antivitamin B12, compounds (often synthetic B12 analogues) that not only have no vitamin action, but also actively interfere with the activity of true vitamin B12. The design of these compounds mainly involve replacement of the metal ion with rhodium, nickel, or zinc; or the attachment of an inactive ligand such as 4-ethylphenyl. These compounds have the potential to be used for analyzing B12 utilization pathways or even attacking B12-dependent pathogens.[98]

Drug interactions

H2-receptor antagonists and proton-pump inhibitors

Gastric acid is needed to release vitamin B12 from protein for absorption. Reduced secretion of

H2 blocker or proton-pump inhibitor (PPI) drugs, can reduce absorption of protein-bound (dietary) vitamin B12, although not of supplemental vitamin B12. H2-receptor antagonist examples include cimetidine, famotidine, nizatidine, and ranitidine. PPIs examples include omeprazole, lansoprazole, rabeprazole, pantoprazole, and esomeprazole. Clinically significant vitamin B12 deficiency and megaloblastic anemia are unlikely, unless these drug therapies are prolonged for two or more years, or if in addition the person's dietary intake is below recommended levels. Symptomatic vitamin deficiency is more likely if the person is rendered achlorhydric (a complete absence of gastric acid secretion), which occurs more frequently with proton pump inhibitors than H2 blockers.[99]

Metformin

Reduced serum levels of vitamin B12 occur in up to 30% of people taking long-term

anti-diabetic metformin.[100][101] Deficiency does not develop if dietary intake of vitamin B12 is adequate or prophylactic B12 supplementation is given. If the deficiency is detected, metformin can be continued while the deficiency is corrected with B12 supplements.[102]

Other drugs

Certain medications can decrease the absorption of orally consumed vitamin B12, including colchicine, extended-release potassium products, and antibiotics such as gentamicin, neomycin and tobramycin.[103] Anti-seizure medications phenobarbital, pregabalin, primidone and topiramate are associated with lower than normal serum vitamin concentration. However, serum levels were higher in people prescribed valproate.[104] In addition, certain drugs may interfere with laboratory tests for the vitamin, such as amoxicillin, erythromycin, methotrexate and pyrimethamine.[103]

Chemistry

Methylcobalamin (shown) is a form of vitamin B12. Physically it resembles the other forms of vitamin B12, occurring as dark red crystals that freely form cherry-colored transparent solutions in water.

Vitamin B12 is the most chemically complex of all the vitamins.[6] The structure of B12 is based on a corrin ring, which is similar to the porphyrin ring found in heme. The central metal ion is cobalt. As isolated as an air-stable solid and available commercially, cobalt in vitamin B12 (cyanocobalamin and other vitamers) is present in its +3 oxidation state. Biochemically, the cobalt center can take part in both two-electron and one-electron reductive processes to access the "reduced" (B12r, +2 oxidation state) and "super-reduced" (B12s, +1 oxidation state) forms. The ability to shuttle between the +1, +2, and +3 oxidation states is responsible for the versatile chemistry of vitamin B12, allowing it to serve as a donor of deoxyadenosyl radical (radical alkyl source) and as a methyl cation equivalent (electrophilic alkyl source).[105]

Four of the six coordination sites are provided by the corrin ring, and a fifth by a

methyl group (–CH3) or a 5′-deoxyadenosyl group. Historically, the covalent carbon–cobalt bond is one of the first examples of carbon–metal bonds to be discovered in biology. The hydrogenases and, by necessity, enzymes associated with cobalt utilization, involve metal–carbon bonds.[106]
Animals have the ability to convert cyanocobalamin and hydroxocobalamin to the bioactive forms adenosylcobalamin and methylcobalamin by means of enzymatically replacing the cyano or hydroxyl groups.

The structures of the four most common vitamers of cobalamin, together with some synonyms. The structure of the 5'-deoxyadenosyl group, which forms the R group of adenosylcobalamin is also shown.

Methods for the analysis of vitamin B12 in food

Several methods have been used to determine the vitamin B12 content in foods including microbiological assays, chemiluminescence assays, polarographic, spectrophotometric and high-performance liquid chromatography processes.[107] The microbiological assay has been the most commonly used assay technique for foods, utilizing certain vitamin B12-requiring microorganisms, such as Lactobacillus delbrueckii subsp. lactis ATCC7830.[77] However, it is no longer the reference method due to the high measurement uncertainty of vitamin B12.[108]

Furthermore, this assay requires overnight incubation and may give false results if any inactive vitamin B12 analogues are present in the foods.[109] Currently, radioisotope dilution assay (RIDA) with labelled vitamin B12 and hog IF (pigs) have been used to determine vitamin B12 content in food.[77] Previous reports have suggested that the RIDA method is able to detect higher concentrations of vitamin B12 in foods compared to the microbiological assay method.[77][107]

Biochemistry

Coenzyme function

Vitamin B12 functions as a

coenzyme, meaning that its presence is required in some enzyme-catalyzed reactions.[12][18]
Listed here are the three classes of enzymes that sometimes require B12 to function (in animals):

  1. Isomerases
    Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. These use the adoB12 (adenosylcobalamin) form of the vitamin.[110]
  2. Methyltransferases
    Methyl (–CH3) group transfers between two molecules. These use the MeB12 (methylcobalamin) form of the vitamin.[111]
  3. Dehalogenases
    Some species of anaerobic bacteria synthesize B12-dependent dehalogenases, which have potential commercial applications for degrading chlorinated pollutants. The microorganisms may either be capable of de novo corrinoid biosynthesis or are dependent on exogenous vitamin B12.[112][113]

In humans, two major coenzyme B12-dependent enzyme families corresponding to the first two reaction types, are known. These are typified by the following two enzymes:

Simplified schematic diagram of the folate methionine cycle. Methionine synthase transfers the methyl group to the vitamin and then transfers the methyl group to homocysteine, converting that to methionine.

L-methylmalonyl-CoA to succinyl-CoA, an important step in the catabolic breakdown of some amino acids into succinyl-CoA, which then enters energy production via the citric acid cycle.[110] This functionality is lost in vitamin B12 deficiency, and can be measured clinically as an increased serum methylmalonic acid (MMA) concentration. The MUT function is necessary for proper myelin synthesis.[4] Based on animal research, it is thought that the increased methylmalonyl-CoA hydrolyzes to form methylmalonate (methylmalonic acid), a neurotoxic dicarboxylic acid, causing neurological deterioration.[114]

tetrahydrofolate (THF) and methionine.[111] This functionality is lost in vitamin B12 deficiency, resulting in an increased homocysteine level and the trapping of folate as 5-methyl-tetrahydrofolate, from which THF (the active form of folate) cannot be recovered. THF plays an important role in DNA synthesis, so reduced availability of THF results in ineffective production of cells with rapid turnover, in particular red blood cells, and also intestinal wall cells which are responsible for absorption. THF may be regenerated via MTR or may be obtained from fresh folate in the diet. Thus all of the DNA synthetic effects of B12 deficiency, including the megaloblastic anemia of pernicious anemia, resolve if sufficient dietary folate is present. Thus the best-known "function" of B12 (that which is involved with DNA synthesis, cell-division, and anemia) is actually a facultative function which is mediated by B12-conservation of an active form of folate which is needed for efficient DNA production.[111] Other cobalamin-requiring methyltransferase enzymes are also known in bacteria, such as Me-H4-MPT, coenzyme M methyltransferase.[115]

Physiology

Absorption

Vitamin B12 is absorbed by a B12-specific transport proteins or via passive diffusion.

transcobalamin II (TC2), and respective membrane receptor proteins. HC is present in saliva. As vitamin-containing food is digested by hydrochloric acid and pepsin secreted into the stomach, HC binds the vitamin and protected it from acidic degradation.[12][116] Upon leaving the stomach the hydrochloric acid of the chyme is neutralized in the duodenum by bicarbonate,[117] and pancreatic proteases release the vitamin from HC, making it available to be bound by IF, which is a protein secreted by gastric parietal cells in response to the presence of food in the stomach. IF delivers the vitamin to receptor proteins cubilin and amnionless, which together form the cubam receptor in the distal ileum. The receptor is specific to the IF-B12 complex, and so will not bind to any vitamin content that is not bound to IF.[12][116]

Investigations into the intestinal absorption of B12 confirm that the upper limit of absorption per single oral dose is about 1.5 µg, with 50% efficiency. In contrast, the passive diffusion process of B12 absorption — normally a very small portion of total absorption of the vitamin from food consumption — may exceed the haptocorrin- and IF-mediated absorption when oral doses of B12 are very large, with roughly 1% efficiency. Thus, dietary supplement B12 supplementation at 500 to 1000 µg per day allows pernicious anemia and certain other defects in B12 absorption to be treated with daily oral megadoses of B12 without any correction of the underlying absorption defects.[116]

After the IF/B12 complex binds to cubam the complex is disassociated and the free vitamin is transported into the

portal circulation. The vitamin is then transferred to TC2, which serves as the circulating plasma transporter, Hereditary defects in production of TC2 and its receptor may produce functional deficiencies in B12 and infantile megaloblastic anemia, and abnormal B12 related biochemistry, even in some cases with normal blood B12 levels. For the vitamin to serve inside cells, the TC2-B12 complex must bind to a cell receptor protein and be endocytosed. TC2 is degraded within a lysosome, and free B12 is released into the cytoplasm, where it is transformed into the bioactive coenzyme by cellular enzymes.[116][118]

Malabsorption

Antacid drugs that neutralize stomach acid and drugs that block acid production (such as proton-pump inhibitors) will inhibit absorption of B12 by preventing release from food in the stomach.[119] Other causes of B12 malabsorption include intrinsic factor deficiency, pernicious anemia, bariatric surgery pancreatic insufficiency, obstructive jaundice, tropical sprue and celiac disease, and radiation enteritis of the distal ileum.[116] Age can be a factor. Elderly people are often achlorhydric due to reduced stomach parietal cell function, and thus have an increased risk of B12 deficiency.[120]

Storage and excretion

How fast B12 levels change depends on the balance between how much B12 is obtained from the diet, how much is secreted and how much is absorbed. The total amount of vitamin B12 stored in the body is about 2–5 mg in adults. Around 50% of this is stored in the liver. Approximately 0.1% of this is lost per day by secretions into the gut, as not all these secretions are reabsorbed. Bile is the main form of B12 excretion; most of the B12 secreted in the bile is recycled via enterohepatic circulation. Excess B12 beyond the blood's binding capacity is typically excreted in urine. Owing to the extremely efficient enterohepatic circulation of B12, the liver can store 3 to 5 years' worth of vitamin B12; therefore, nutritional deficiency of this vitamin is rare in adults in the absence of malabsorption disorders.[12] In the absence of enterohepatic reabsorption, only months to a year of vitamin B12 are stored.[121]

Cellular reprogramming

Vitamin B12 through its involvement in one-carbon metabolism plays a key role in cellular reprogramming and tissue regeneration and epigenetic regulation. Cellular reprogramming is the process by which somatic cells can be converted to a pluripotent state. Vitamin B12 levels affect the histone modification H3K36me3, which suppresses illegitimate transcription outside of gene promoters. Mice undergoing in vivo reprogramming were found to become depleted in B12 and show signs of methionine starvation while supplementing reprogramming mice and cells with B12 increased reprogramming efficiency, indicating a cell-intrinsic effect.[122][123]

Synthesis

Biosynthesis

Main article: Cobalamin biosynthesis

Vitamin B12 is derived from a

S-adenosyl methionine. It was not until a genetically engineered strain of Pseudomonas denitrificans was used, in which eight of the genes involved in the biosynthesis of the vitamin had been overexpressed, that the complete sequence of methylation and other steps could be determined, thus fully establishing all the intermediates in the pathway.[126][127]

Species from the following

Aerobacter, Agrobacterium, Alcaligenes, Azotobacter, Bacillus, Clostridium, Corynebacterium, Flavobacterium, Lactobacillus, Micromonospora, Mycobacterium, Nocardia, Proteus
, Rhizobium, Salmonella, Serratia, Streptococcus and Xanthomonas.[128][129]

Industrial

Industrial production of B12 is achieved through

GRAS status) by the Food and Drug Administration of the United States.[133]

The total world production of vitamin B12 in 2008 was 35,000 kg (77,175 lb).[134]

Laboratory

Main article: Vitamin B12 total synthesis

The complete laboratory synthesis of B12 was achieved by Robert Burns Woodward[135] and Albert Eschenmoser in 1972.[136][137] The work required the effort of 91 postdoctoral fellows (mostly at Harvard) and 12 PhD students (at ETH Zurich) from 19 nations. The synthesis constitutes a formal total synthesis, since the research groups only prepared the known intermediate cobyric acid, whose chemical conversion to vitamin B12 was previously reported. This synthesis of vitamin B12 is of no practical consequence due to its length, taking 72 chemical steps and giving an overall chemical yield well under 0.01%.[138] Although there have been sporadic synthetic efforts since 1972,[137] the Eschenmoser–Woodward synthesis remains the only completed (formal) total synthesis.

History

Further information: Vitamin § History

Descriptions of deficiency effects

Between 1849 and 1887,

megaloblasts in the bone marrow, and Ludwig Lichtheim described a case of myelopathy.[139]

Identification of liver as an anti-anemia food

During the 1920s,

Edwin Cohn prepared a liver extract that was 50 to 100 times more potent in treating pernicious anemia than the natural liver products. William Castle demonstrated that gastric juice contained an "intrinsic factor" which when combined with meat ingestion resulted in absorption of the vitamin in this condition.[139] In 1934, George Whipple shared the 1934 Nobel Prize in Physiology or Medicine with William P. Murphy and George Minot for discovery of an effective treatment for pernicious anemia using liver concentrate, later found to contain a large amount of vitamin B12.[139][141]

Identification of the active compound

While working at the Bureau of Dairy Industry, U.S. Department of Agriculture,

Karl Folkers from that company, developed the LLD assay. This identified "LLD factor" as essential for the bacteria's growth.[142] Shorb, Folker and Alexander R. Todd, at the University of Cambridge, used the LLD assay to extract the anti-pernicious anemia factor from liver extracts, purify it, and name it vitamin B12.[143] In 1955, Todd helped elucidate the structure of the vitamin. The complete chemical structure of the molecule was determined by Dorothy Hodgkin based on crystallographic data and published in 1955[144] and 1956,[145] for which, and for other crystallographic analyses, she was awarded the Nobel Prize in Chemistry in 1964.[146] Hodgkin went on to decipher the structure of insulin.[146]

George Whipple, George Minot and William Murphy were awarded the Nobel Prize in 1934 for their work on the vitamin. Three other Nobel laureates, Alexander R. Todd (1957), Dorothy Hodgkin (1964) and Robert Burns Woodward (1965) made important contributions to its study.[147]

Nobel laureates for discoveries relating to vitamin B12

Commercial production

Industrial production of vitamin B12 is achieved through

fermentation of selected microorganisms.[130] As noted above, the completely synthetic laboratory synthesis of B12 was achieved by Robert Burns Woodward and Albert Eschenmoser in 1972, though this process has no commercial potential, requiring more than 70 steps and having a yield well below 0.01%.[138]

Society and culture

In the 1970s, John A. Myers, a physician residing in Baltimore, developed a program of injecting vitamins and minerals intravenously for various medical conditions. The formula included 1000 μg of cyanocobalamin. This came to be known as the Myers' cocktail. After his death in 1984, other physicians and naturopaths took up prescribing "intravenous micro-nutrient therapy" with unsubstantiated health claims for treating fatigue, low energy, stress, anxiety, migraine, depression, immunocompromised, promoting weight loss and more.[148] However, other than a report on case studies[148] there are no benefits confirmed in the scientific literature.[149] Healthcare practitioners at clinics and spas prescribe versions of these intravenous combination products, but also intramuscular injections of just vitamin B12. A Mayo Clinic review concluded that there is no solid evidence that vitamin B12 injections provide an energy boost or aid weight loss.[150]

There is evidence that for elderly people, physicians often repeatedly prescribe and administer cyanocobalamin injections inappropriately, evidenced by the majority of subjects in one large study either having had normal serum concentrations or had not been tested prior to the injections.[151]

See also

Further reading

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External links
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