Vitamin B12, also known as cobalamin, is essential for normal cell metabolism and blood cell formation. It contains a cobalt ion at its core and different attachments to the cobalt determine the type of B12. The main sources are animal products. B12 requires intrinsic factor for absorption in the ileum. A deficiency can cause megaloblastic anemia and neurological problems due to impaired DNA synthesis and methylation reactions. Pernicious anemia is a type of B12 deficiency caused by a lack of intrinsic factor.

1. Vitamin b12

2. Structure of Vit.b12 of Vit.b12
• It contains 4 pyrrol rings
• Co is present in the centre with a
cordination no. 6
• 4 N of pyrol ring are attached by
coordinate linkage
• 2 pyrol rings 1 & 4 are directly joined
rather than methane bridge
• In 1st posn. It is attached by Dimethyl
benziamide ribonucleotide

3. • Ribonucleotide molecule in turn is
connected to propionic side chain of
4th
ring of pyrol through
aminoisopropanol.
• R groups is attached at 6th
posn
• Different R groups give different
type----
a) R=CN- cyanocobalin
b) R=OH- hydroxycobalmin
c) R=CH3-methylcobalmin
d) R=Cl – chlorocobalmin
e) R=5’deoxyadenosyl-5 adenosyl

4. Structure of vit. b12

5. Sources
• Exogenous – liver ,egg, milk, kidney,
curd
• Endogenous – intestinal bacteria

6. Daily requirements
Adults - 3µg/day
Preg. N lactation
-4µg/day
Children-0.5-
1.5µg/day

7. Absorption , transport and
excretion of Vit . b12

8. • Vit.b12 is absorbed from ileum
• It is mediated by receptor sites in
ileum.
• It requires a.)HCl b)intrinsic
factor
Intrinsic factor is a glycoprotein secreted by the
parietal cells of gastric mucosa and carries vit.B12
for absorption in the ileum.

9. Mechanism of absorption
• In the intestine , Vit B12 binds to cobalphilin ,
a protein secreted in the saliva
• In the duodenum cobalphilin is hydrolyzed
releasing vit for binding to the intrinsic factor
• I.F. binds to B12 for transport through
intestinal lumen to absorptive site of ileum
• In the ileum I.F. is attached to specific binding
site as a result B12 is removed from I.F. in the
presence of calcium ions and releasing the
factor secreted by the duodenum.

10. • B12 now enters the ileal mucosal
cells for absorption into duodenum.
• Small amounts 1-3% may be
absorbed by diffusion
• About 0.5-10µg of B12 is transported
after absorption.
• After absorption Vit B12 is
transported to various tissues in
combination with plasma protein
transcobalmin 2
• It is stored in liver bound to
transcobalmin 1

11. • After transport in the blood free cobalmin is
released in the cytosol cells as hydroxycobalmin
B12 which is converted to
• A) METHYLCOBALMIN in cytosol
or
• B) DEADENOSINE COBALMIN B12 in mitochondria
• Methylcobalmin is taken up by liver and is
converted to deadenosyl B12 where it is stored.
• B12 is mainly excreted through bile
• About 20-25µg/day is excreted in urine

12. Synthesis of methionine from
homocystein
Homocystein
N5 methyl THF
THF
methionine
METHYLCOBALMIN

13. FOLATE TRAP
B12 is necessary for conversion of
N5 methyl THF to THF to form
methylcobalminB12 which is
required for conversion of
homocystein to methionine.
In B 12 defeciency the above
reaction does not take place and
folate is permanently trapped as
N5 methyl TH4 resulting in the non
availabilty of TH4 required for 1C
metabolism which is required for
the synthesis of purines,
pyramidines n nucleic acid.this
process is called folate trap.
This leads to decrease in folic acid
resulting in
Megaloblastic anemia

15. SYNTHESIS OF SUCCINYL COA
FROM METHHYL MELONIN COA
• Deadenosine B12 is the coenzyme
required in the conversion of methyl
melonine coA to succinyl coA
• In B12 def. methly melonic acid
accumulates and is excreted in urine
therefore it is a sensitive index of B
12 def.

16. Other functions of
vit.B12
• Formation of RBC beyond megaloblastic
stage
• Conversion of ribonucleotides to deoxy
ribonucleotides
• Maintaince sulphahydryl group in reduced
state.
• It’s def. produces reduction of glutathione in
blood.
• In bacteria , acts coenzyme in conversion of
glutamate or aspartate.

17. Defeciency of vit.B12

18. Megaloblastic anemia
• It is due to def. of
folic acid/B12
• Cell division is
sluggish but
cytoplasmic
development is
normal
• ratio of
RNA/DNA
• Nucleolated RBC’s
are seen.

19. demyleination
• In B12 def. due to non availabilty of
active methionine , methylation of
phospatidyl ethanolamine to
phosphatidyl choline is not adequate
so ethanolamine is not adequately
converted to choline in the absence
of B12
• It leads to def. of myelin sheath
around nerves.

20. c
insufficie
ncy
It results in excretion
of cobalophilin

21. Pernicious anemia
• Type of
megaloblastic
anemia due to def.
of INTRINSIC
FACTOR resulting in
the failure of abs.
of vit.B12
• It is thought that
pernicious anemia
is caused by
autoimmune
reaction against