Cobalamin (B12)

Question 1

How is cobalamin made/synthesised?

Answer 1

Exclusively synthesised by bacteria

Ruminants take B12 from soil and rumen bacteria can synthesise it

Plants cannot make cobalamin

Question 2

What is the synthetic cobalamin called?

Answer 2

Cyanocobalamin

Question 3

Explain how cobalamin is absorbed and the mechanisms involved?

Answer 3

Cobalamin is bound to proteins in food

In the stomach HCl and pepsin release cobalamin from protein

Free cobalamin binds to R-protein in the stomach

Then in the small intestine the pancreatic proteases cleave cobalamin from R-protein

Free cobalamin bind to intrinsic factor (IF) in duodenum

IF and cobalamin travel to the distal ileum where they are absorbed by endocytosis

Question 4

How is cobalamin transported through the body?

Answer 4

Trans-cobalamin proteins

Question 5

Where is cobalamin stored?

Answer 5

In the liver (enough for 2-3 years)

Question 6

What are the functions of cobalamin

Answer 6

5-deoxyadenosylcobalamin is required by L-methylmalonyl-coenzyme A mutase to catalyse the conversion of L-methylmalonyl-CoA to succinyl-coezyme A
- if this does not occur it gets converted to MMA which causes the synthesis of abnormal synthesis of fatty acids which damage myelin sheaths (severe CNA and PNS dysfunction)

Required as a coenzyme for methionine synthase for the conversion of homosysteine to methionine

Question 7

Explain what is meant by the methyl folate trap

Answer 7

It involves the donation of a methyl group from 5-methyl THFA to cobalamin to form THFA and methyl-cobalamin which are both the active forms of the vitamins

Me-cobalamin then donates the methyl group to homocysteine to form methionine

If either folate or cobalamin are deficient, the other is ‘trapped’ in the inactive form

Question 8

What is caused by a cobalamin deficiency?

Answer 8

• Pernicious Anemia (megaloblastic) due to a lack of mature RBCs
• Atrophic Gastritis
• Nerve degeneration

Question 9

What are potential causes of a cobalamin deficiency?

Answer 9

• Absence of R-protein
• poor binding of cobalamin to IF
• presence of B12 consuming bacteria in ilium
• anti-acid medications reduce pH in stomach and make it harder to split cobalamin from protein
• vegetarian or vegan diets

Question 10

What are sources of cobalamin?

Answer 10

• oysters, muscles, clams
• beef, lamb, chicken
• eggs