Vitamin B12

It is used for Macrocytic anemia.
It is also called as cobalamin or cyanocobalamin. It is present in liver, milk, egg and fish. Physiologically active forms of vitamin B12 coenzymes are methylcobalamin and 5-deoxyadenosinecobalamine.


Daily requirement:
Daily B12 requirement is about 2 µg in normal adults.

Mechanism of action:
It acts as a co-factor in the formation of Succinyl-CoA from methylmalonyl-CoA.

Methylmalonyl-CoA is an intermediate product during the degradation process of many metabolites in the body. In the case of pernicious anemia, its amount is increased. Whereas, succinyl-CoA is a precursor in the synthesis of heme.

Accumulation of methylmalonyl-CoA leads to abnormal production of fatty acid as well as its incorporation to cell membranes. This causes demyelination of the nerve cells.

It acts as a co-factor in the formation of H4 folate and methionine leading to the formation of DNA.

Accumulation of 5-CH3-H4 folate leads to the deficiency of folate cofactors resulting in reduced DNA synthesis and megaloblastic anemia.

Administration:
It can be administered orally, IM or deep subcutaneously.

Pharmacokinetics:
Absorption:
Vitamin B12 gets bind to intrinsic factor (IF) and form a complex. This IF-B12 complex is then absorbed in distal ileum. This absorption is done by a very specific receptor mediated transport system.

Distribution:
Vitamin B12 is distributed to different cells of the body upon binding to transcobalamin II. It is stored in the liver in amount of 300-500 µg.

Elimination:
Normally, very little amount is excreted through urine or feces but upon parenteral administration large amount is found in urine.

Vitamin B12 deficiency:
Its deficiency causes pernicious anemia due to spoiled synthesis of red blood cells. Its deficiency can be caused by:

1. Low level of intake of Vitamin B12
2. Decreased absorption of vitamin due to less amount of intrinsic factor (IF) produced by gastric parietal cells
3. Lessened activity of the receptor for uptake of the vitamin from intestine
4. Non-specific malabsorption syndrome.

Therapeutic uses:
It is used in megaloblastic anemia and pernicious anemia as it is important in the formation of blood and growth. It is also important in proper neural functioning.

Dosage:
It is given in the dose of 100-1000 µg daily for 1-2 weeks than as a maintenance therapy the same dose is required once a month for life.

Adverse effects:
It has no adverse effects as extra amount is excreted via urine.