Vitamin B12

 

Vitamin B 12

PKGhatak, MD

Vitamin B12 is a water soluble vitamin, a chemical compound containing a cobalt atom. Vitamin B12 is essential for humans and must be supplied in food. Liver, meat, milk, egg, herring and mackerel fish are the chief source of B12; vegetables are nearly devoid of B12. Stomach acid and pepsin release B12 from the food, B12 then combines with a glycoprotein. B12 is absorbed in the terminal ileum. The liver is the principal store of B12. Breast milk contains enough B12 to meet a developing child's requirements. The daily requirement for adults is 3 micrograms, for infants 0.3 micrograms and for children the amount varies according to age. Adults have about 3 year reserve of B12 in the liver.

Chemistry:

B12 was originally obtained from the liver by Dr. Castleman in 1948. The isolated compound was light sensitive but when combined with a cyanide group, B12 becomes a stable crystalline form - cyanocobalamin. The central part of the molecule is four pyrrole rings surrounding a single six valent cobalt atom. This ring is called the Corrin ring system. The corrin ring is synthesized from delta aminolaevulinic acid by a process where the methyl group is supplied by methionine (amino acid). The central cobalt atom is attached to the N molecule of each of the four pyrrole rings. The 5th valent of Cobalt is attached to the N molecule

 of 5,6-dimethyl benzimidazole ribose phosphate and the 6th valent of the cobalt is attached to a variable group, R - cyano or 5-deoxyadenosyl or a methyl group. These forms are interchangeable with the cells. The R binding compound gives the type of B12 that will be formed eg- cyanocobalamin and adenosylcobalamin. In food, B12 presents as Adenosyl B12, Hydoxyadenosyl B12, Methyl B12, Cyano B12 and Sulphito B12, of which the first two forms are commonly found.

In nature, B12 is also present as hydroxocobalamin, a red color compound and nitrocobalamin. Methylcobalamin and  Hydroxycobalamin are therapeutically active and last longer in the body. B12 in plasma is bound to plasma proteins in variable amounts. The intracellular B12 level in the mitochondria is dependent on the amount of free B12 present in the serum. 

Physiology:

Acid and pepsin in the stomach release B12 from the food. The intrinsic factor (IF) of Castle, a glycoprotein, secreted by the parietal cells of the stomach, combines with B12 in the duodenum and prevents B12 from digestion by GI enzymes and keeps it safe from gut bacteria using it. As B12 reaches the terminal ilium, the receptors present on the surface epithelial cell of the ilium bind with it, then release B12 from the IF. Only free B12 is available for absorption. About 1 to 2 % B12 is absorbed by simple diffusion, the rest is ferried across the cells by an active process and carried by the Transporter I protein. The Transporter II protein carries B12 to the liver, bone marrow and other tissues. The proportion of B12 absorbed from food is limited by the amount of available IF. As a result, only about 20% is absorbed; that amount generally satisfies the daily 3 micrograms of B12 requirement. In macrocytic anemia or pernicious anemia, 1500 to 3000 micrograms of B12 are prescribed and 1 to 2% absorption by diffusion meets the therapeutic needs. The only source of B12 for the herbivorous animal is that produced by gut bacteria. 

B12 inside the cells.

B12 with the attached transport protein enters the cells. The lysozymes free the B12. The B12 then is taken up by mitochondria. Here B12 first converted to Cobalamin II alamin. Then cobalamin II alamin is converted to adenosylcobalamin and methylcobalamin. Hydroxycobalamin is also present in the cytoplasm. 

The action of coenzymes:

Methylcobalamin is the cofactor of methionine synthase. Methionine synthase converts Homocysteine to Methionine. In another reaction methionine synthase transfer the methyl group from 5 methyltetrahydrofolate (THF) to homocysteine. THF is the active form of vitamin Folic Acid. THF is essential in DNA synthesis.

The adenosylcobalamin is the cofactor of methylmalonyl CoA mutase (CoA means coenzyme A). Methylmalonyl CoA mutase converts methylmalonyl CoA to Succinyl CoA in the Krebs tricarboxylic acid cycle.  Energy supplying amino acid metabolism requires to pass through succinyl CoA step. Similarly, the fatty acid metabolism is also dependent on this methyl B12 coenzyme.

The function of B12:

B12 coenzymes are required for the synthesis of 1. Purine Nucleotides, 2. metabolism of several amino acids and fatty acids 3. the normal growth and development of cells.

Myelin sheath of nerve fibers.

In the initial steps of myelin sheath synthesis, the B12 coenzyme produces methylmalonyl-CoA from branched amino acids threonine, isoleucine and valine. In B12 deficiency malonyl CoA is reduced and methylmalonyl-CoA is substituted. This results in defective myelin and early breakdown of the myelin sheath.

Causes of B12 deficiency.

Dietary.

 B12 deficiency is expected in strict vegetarian Hindus in India. In the west, vegans are similarly affected.

Diseases of the stomach.

Acid suppression by pantoprazole over a prolonged period, atrophic gastritis, partial gastrectomy,

Intrinsic factor (IF) deficiency.

Congenital absence of IF.

Acquired autoimmune disease producing antibodies to the parietal cells or antibodies to IF.

Diseases of the terminal ileum.

Crohn's disease. Surgical removal of terminal ilium, Celiac disease. Tropical sprue. Malabsorption syndrome. HIV infection, Frequent bacterial enteritis. Change of gut bacterial population.

Intestinal parasite. Diphyllobothrium latum, a fish water fluke, infestation of the gut.

Diseases associated with Megaloblastic anemia.

Type 1 diabetes mellitus, Graves disease, Hypothyroidism, Addition's disease. Parkinson syndrome.

Risk factors. North European ancestry. Alcoholism.

Diagnosis of B12 deficiency:

1. Blood test.

B12 deficiency is easily detected by obtaining blood B12 levels.

2. Associated abnormalities of blood picture.

CBC. Any or all of these features may be present. Anemia, Macrocytic RBCs, 

Increased RWD (Red blood cell width distribution), Ovalocytes, Leukopenia, and hypersegmented neutrophils. Thrombocytopenia, pancytopenia. Increased blood LDH, Bilirubin. and AST. Decreased Haptoglobin, and increased serum Methylmalonic acid and Homocysteine.

Some neurological symptoms are suspected to be from the B12 deficiency but the serum B12 levels may be normal. The B12 deficiency is confirmed by finding a blood methylmalonic acid over 1000 mcg/ml and high homocysteine levels.

Diagnosis of Pernicious Anemia.  

1. Presence of megaloblastic anemia and 2. deficiency of Intrinsic factor.

Intrinsic factor deficiency may be due to congenital or hereditary.  Acquired causes are due to the presence of antibodies. Antibodies are of two types. 1. Complement fixing antibodies to parietal cells and, 2. antibodies against the Intrinsic factor.

Symptoms of B12 deficiency.

Weakness, glossitis, anemia.

Neurological.

Some or none of the following symptoms are usual findings.

Peripheral neuritis, Depression, dementia, Psychosis, Cerebellar ataxia, Subacute combined degeneration of the spinal cord. Optic atrophy. Cranial nerve nephropathy.

In general, Megaloblastic anemia is common in B12 deficiency, neurological manifestations are variable and occasionally appear before hematological changes.

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