Page 75 - Essential Haematology

Chapter 5 Macrocytic anaemias / 61

Diet
Folate polyglutamates
Food–B 12

Stomach IF Parietal
cell

Enterohepatic Bile
circulation of
B 12 and folate
Methyl
Pancreatic THF
secretion Duodenum
and jejunum
IF-B 12
Portal
blood

Ileum

IF receptor B 12 + TC
(cubulin/
amnionless)

Figure 5.2 The absorption of dietary vitamin B 12 after combination with intrinsic factor (IF), through the ileum.
Folate absorption occurs through the duodenum and jejunum after conversion of all dietary forms to methyl-
tetrahydrofolate (methyl THF). TC, transcobalamin.

Methylation of DNA, myelin, amines, proteins, etc.
S-adenosyl homocysteine S-adenosyl methionine

Methyl B 12
Homocysteine Methionine
Methyl THF THF

Ado B 12
Propionyl CoA Methylmalonyl CoA Succinyl CoA

Figure 5.3 The biochemical reactions of vitamin B 12 in humans. Ado B 12 , deoxyadenosylcobalamin;

CoA, coenzyme A; THF, tetrahydrofolate.