a basic introduction to folate and b12 metabolism

1. Folate and B12
metabolism
Dr JG Nel
Department of Haematology and
Cell Biology
5 June 2008

2. Folate and Vitamin B12
 History
 Chemical characteristics
 Sources
 Absorption
 Transport in the body

3. Folate and Vitamin B12
 Stores and requirements
 Cobalamin and Folate in cellular metabolism

5. Vitamin B12

6. The History of Vitamin B12
 George Whipple showed that beef liver improved the formation
of red corpuscles in anaemic dogs.
 1926: Minot and Murphy announced that a daily diet of a pound
of liver can control pernicious anaemia.
 1930s: antipernicious factor- B12 named before it was isolated.
 1948: Bacterial assay for the determination of B12 developed by
Karl Folkers
 Vitamin B12 Isolated by two groups at the same time

7. The History of Vitamin B12
 1956: Dorothy Hodgkin determines B12 structure using X-ray
crystallography.
 1971: Robert B. Woodward synthesizes B12

8. Chemical characteristics of
Vitamin B12
 Three primary
domains
1. Corrin nucleus
2. Nucleotide
 B group linked to Co
1. CN
2. OH
3. CH3*
4. Adenosyl*

9. Chemical characteristics of
Vitamin B12
 Product of bacterial biosynthesis
 Not destroyed by heat
 Mol. Mass : 1355.37g/mol
 Metabolism : hepatic
 Excretion : renal
 Half life : 6 days, 400 days in liver

10. Sources of Vitamin B12
 Foods of animal origin
 Milk
 Meat –esp liver and shellfish
 eggs
 Unwashed plants
 Fortified food products eg. cereals
 Cyanocobalamin supplements –oral, intranasal and IM

11. Vitamin B12 Absorption
Two mechanisms
1. Passive
2. Active

12. Vitamin B12 Absorption
Passive absorption
 mucosal surfaces
 rapid
 inefficient (less than 1% of oral dose)

13. Vitamin B12 Absorption
Intrinsic factor
 Produced by parietal cells in fundus and body of
stomach
 Nucleotide portion of Co fits into a pit on the surface of
the protein
 Does not bind pseudocoabalamin compounds

14. Vitamin B12 Absorption
Active
 Localised to ileum in humans
 Efficient for small oral doses of cobalamin
 Normal mechanism by which the body
acquires cobalamin

15. Vitamin B12 Absorption
 B12 freed from non-specific
binding to dietary protein by
peptic digestion
 Rapidly binds to pepsin-
resistant high affinity R-
protein
 Duodenum R-protein
degraded by pancreatic
trypsin
 IF binds exogenous
cobalamin and endogenous
cobalamin from bile by it’s
C-terminal

16. Vitamin B12 Absorption
 In terminal ileum IF-
cobalamin binds to Cubilin
and enters the cell via
receptor mediated
endocytosis
 Cobalamin dissociates from
IF and binds to
Transcoballamin
 IF is degraded
 Cobalamin transported into
the portal circulation after 6
hours.
 Limited receptor sites

17. Enterohepatic Circulation of
Vitamin B12
 0.5ug-5ug enters bile each day
 Binds to IF and a portion gets reabsorbed with
Cobalamin derived from sloughed intestinal cells

18. Transport of Vitamin B12
 Transported mainly by two proteins
 Haptocorrin (formerly Transcobalamin I) 80%
 Transcobalamin (formerly Transcobalamin II) 20%

19. Transport of Vitamin B12
Haptocorrin
 Glycoprotein
 Related to other HCs
 Derived from specific granules of neutrophils
 Bound tighly to Co
 Removed from plasma by hepatic
glycoprotein receptors

20. Transport of Vitamin B12
Transcobalamin
 B2 globulin
 Synthesized by: liver,macrophages,ileum,
endothelium
 Readily gives up cobalamin
 Enters cells via receptor mediated
endocytosis

21. Vitamin B12 Stores
 Normal Western Diet 5-30ug Cobalamin daily
 Losses (urinary and fecal) 1-3ug (0.1% of body stores)
 Requirement 1-3ug daily
 Body stores 2-3mg

23. Folate

24. The history of Folate
 1931 Lucy Wills: dietary yeast prevents anaemia in
pregnant women–“Wills factor”
 1941 Henry K. Mitchell: isolates folic acid from
spinach leaves.
 1946 Yellapragada Subbarao synthesized folic acid.
 1980s evidence that the addition of folic acid in the
diets of pregnant women can prevent birth defects.

25. Chemical properties of Folic
acid
 Folate = ionic form of folic
acid lacking one H atom
 Pteroylmonoglutamic acid
 Occurs as poly- and
monoglutamic variants
 Watersoluble
 Destroyed by heat and light
exposure

26. Sources of Folate
 Leafy vegetables
 Dried beans and peas
 Fortified cereal products
 Sunflower seeds
 Eggs, liver and milk
 Yeast

27. Absorption of Folate
 dietary folates ingested as
polyglutamates
 absorbed as
monoglutamate.
 Hydrolase in brush border
of jejunal and ileal epithelial
cells deconjugates the
polyglutamate to
monoglutamate form.
 enters the cell by simple
diffusion or by folate binding
proteins.

29. Transport of Folate
 Reduced to tetrahydrofolate in mucosal cell
 Methylated before release into the portal circulation
 Loosely bound to albumin
 Methylated THF binds to specific folate receptors
and enters the cell via receptor mediated
endocytosis.
 Released from the receptor, transferred to
cobalamin

30. Transport of Folate
 In the cell folate needs to polyglutamated in
order to aid retention of the molecule

31. Folate Stores
 Requirement 100-
500ug/day
 Hepatic stores 8-20mg
 Enterohepatic
circulation.
 With dietary deprivation
stores begin to decline
in 3-4 weeks.
 RDA 400ug

33. Vitamin B12 and Folate in
Cellular metabolism

34. Biological role of Vitamin B12
Only two reactions in the body are known to
require cobalamin:
1. Methylmalonyl CoA isomerization
2. The methylation of homocysteine to
methionine

35. Biological role of Vitamin B12
Methylmalonyl CoA isomerization
Adenosylcobalamin
Methylmalonyl CoA Succinyl CoA
Methylmalonyl CoA Mutase

36. Biological role of Vitamin B12
The methylation of homocysteine to methionine
• requires
 5-Methyltetrahydrofolate as methyldonor
 Methylcobalamin as coenzyme
• Irreversable
• 1st
step in the conversion
Methyltetrahydrofolate intracellular folate Coenzymes

37. Biological role of Folate
 Purine synthesis DNA,RNA
 Pyrimidine synthesis DNA
 Amino acid interconversion
 Serine-Glycine
 Homocysteiene to Methionine
 Forminoglutamic acid to glutamic acid