Metabolism of Vitamin B12

1. COBALAMINE (B12]
Gandham. Rajeev
Department of Biochemistry,
Akash Institute of Medical Sciences
& Research Centre,
Devanahalli, Bangalore,
Karnataka, India.
E-Mail: gandhamrajeev33@gmail.com

3. Vitamin B12

4.  Cobalamin
 Extrinsic factor (EF) of castle
 Antipernicious anemia factor
 Chemistry:
 Vitamin B12 is water soluble, heat stable and red
in colour
 It contains 4.35% cobalt by weight
 Four pyrrole rings co-ordinated with a cobalt
atom is called as a Corrin ring

5.  The corrin ring has four pyrrol units, like porphyrin
 Two of the pyrrole units (A&D) are directly bound
to each other where as the other two (B & C) are
held by methene bridges
 The groups namely methyl, acetamide and
propionamide are the substituents on the pyrrole
rings
 Cobalt present at the centre of the corrin ring is
bonded to the four pyrrole nitrogens

6.  Cobalt also holds dimethyl benzimidazole (DMB)
containing ribose 5-Phosphate and amino
isopropanol
 A nitrogen atom dimethyl benzimidazole is
linked to cobalt
 The amide group of aminoisopropanol binds with
D ring of corrin
 The cobalt atom also possesses a sixth
substituent group located above the plane of the
corrin ring

7. A
D
B
C
Aminoisopropanol
Methyl, Adenosine, acetamide, propionamide

8. Aminoisopropanol
dimethyl benzimidazole
D
BA
C

9.  All the forms of vitamin B12 are called cobalamins
because of the presence of cobalt atom
 Different forms of cobalamins include
Cyanocabalamin:- It refers to the isolated form of
vitamin B12 in which cyanide was added to
promote crystallization of cobalamin during the
isolation process
 Hydroxycobalamin:- It was usually present in the
tissues and in the naturally occurring forms of
Vitamin B12

10.  Methylcobalamin :- Methyl group replaces
adenosyl group
 Adenosyl cobalamin (Ado-B12):-
 When taken up by the cells, these groups are
removed and deoxy adenosyl cobalamin or Ado-
B12 is formed

11.  In the food, vitamin B12 is present as a complex
with proteins
 The free form of vitamin B12 is released by
cooking, HCL present in gastric juice and
proteolysis by pepsin in the stomach
 Mechanism:-
 The absorbance of vitamin B12 requires intrinsic
factor
 Intrinsic factor is a glycoprotein secreted by parietal
cells of stomach

12.  Vitamin B12 combines with intrinsic factor
 The vitamin B12 – intrinsic factor complex reaches
the ileum where it is absorbed
 In the ileum, the complex attaches to a specific
receptor and is taken up by the mucosal cell.
 In the mucosal cell, vitamin B12 is released from
its complex and reaches the portal circulation

13.  In the portal blood, it is transported in combination
with transcobalamin II
 Vitamin B12 is presented to cells where it is taken
up by the cells through receptor mediated
endocytosis
 Storage:
 It is mainly stored in liver, leukocytes and gastric
mucosa
 It is stored as complex with transcobalamin-I&II

14. Protein
B12
B12
B12
B12
Protein
IF
IF
IF
Mucosal cell
B12
Methyl B12 B12 B12
TC 1-B12(90%)
TC 1-B12(90%)
TCII -B12(10%)
TCII -B12(10%)
Tissues
B12
TCII
Methyl
B12
Deoxyad
enosyl
B12
(LIVER)
GIT
Plasma Tissues

15.  About 10 enzymes requiring vitamin B12 have
been identified
 Most of them are found in bacteria ( mutase,
ribonucleotide reductase, etc.)
 There are two reactions in mammals that
dependent on vitamin B12

16.  Methyl cobalamin is essential for the conversion of
homocysteine to methionine and formation of FH4
from methyl FH4
 The reaction is catalyzed by homocysteine methyl
transferase
Homocysteine
Homocysteine methyl transferase
Methionine
methylcobalamin
THFN5 methyl THF

17.  The circulating methyl FH4 is converted to FH4
 FH4 is either used for storage as folylpolyglutamate
form or it is utilized for other reactions such as
formation of methylene FH4
 Methyl folate trap:-
 In B12 deficiency, impaired conversion of methyl
FH4 to FH4 results in accumulation of methyl
FH4 & is called as methyl folate trap
 Methyl folate trap results in decreased availability of
FH4 & FH4 derivatives that are

18.  Needed for purine nucleotide & thymidylate
synthesis
 Thus vitamin B12 deficiency, results in secondary
folate deficiency

19. Methyl FH4
FH4
N5,10 methylene FH4
dUMP
dTMP
DNA
Homocysteine
Methionine
Methylfolate trap Methyl FH4
Homocysteine methyl transferase
FH4
Serine
Glycine
Thymidylate
synthase
B12

20.  Isomerization of methyl CoA to succinyl CoA:-
 The degradation of odd chain fatty acids and
some amino acids (valine, leucine etc) and
pyrimidines (thymine & Uracil) produce propionyl
CoA, an imp. Compound methylmalonyl CoA
 The methyl malonyl CoA mutase converts methyl
malonyl CoA to succinyl CoA in the presence of
Vitamin B12,deoxyadenosyl cobalamin

21.  In B12 deficiency, methyl malonyl CoA
accumulates and is excreted in urine as methyl
malonic acid
 This condition is called as methylmalonic
aciduria, occurs in B12 deficiency.
 Demyelination :- Myelination of nerves is impaired in
B12 deficiency due to accumulation of methylmalonyl
CoA
 Demyelination is due excessive accumulation of
methylmalonyl CoA

22. Odd chain FA
Propionyl CoA
Amino acids
(Val, Ile, Thr, Met) Thymine, uracil
Methyl malonyl CoA
Succinyl CoA
Methyl malonyl CoA
mutase
Methyl malonic acid
Excreted in urine
5-Deoxyadenosylcabalamin (of B12)
+ Methylmalonic aciduria

23.  Rich sources of vitamin B12 are meats, egg, milk,
sea foods
 B12 is synthesized by microorganisms
 Vitamin B12 is not present in Plant sources

24.  Adults -1 µg/day
 Pregnancy & lactation -2 µg/day
 Causes:
 Inadequate intake-seen in pure vegetarians and
rarely in alcoholism
 Impaired absorption
 This is mainly caused by lack of intrinsic factor
 Lack of intrinsic factor is called as pernicious
Deficiency

25.  anemia & it is caused by destruction of gastric mucosa
 Impaired absorption is also seen in small intestinal
disorders
 Impaired storage and transport:
 Inadequate utilization of vitamin occurs because of liver
diseases and abnormalities of transport proteins
 Increased requirements are seen in hyperthyroidism,
infancy & thalassemia
 Increased excretion occurs in nephrotic syndrome

26.  Vitamin B12 deficiency is characterized by
 Megaloblastic anemia:
 Feature of megaloblastic anemia include
pallor,fatigue, glossitis ( beefy red tongue) & slight
yellow discoloration of the conjunctiva due to
increased unconjugated bilirubin
 Progression of anemia may result in angina &
congestive cardiac failure

30.  Gastrointestinal dysfunction:
 GIT epithelial cells are undergoing rapid turnover
 & dependent on vitamin B12
 B12 deficiency results in weight loss & diarrhea
 Demyelination of nervous tissue
 Subacute combined degeneration:
 Damage to nervous system is seen in B12
deficiency

31.  There is demyelination affecting cerebral cortex as
well as dorsal column & pyramidal tract
 Symmetrical paresthesia of extremities, alterations
of tendon & deep senses & reflexes, loss of
position sense, unsteadiness in gait, positive
Romberg’s sign & positive Babinski’s sign are
seen
 Achlorhydria:
 Absence of acid in gastric juice is associated with
B12 deficiency

33.  Serum B12: It is quantitated by radio-
immunoassay or by ELISA
 Methyl melonic acid is excreted in urine
 FIGLU excretion test
 Peripheral smear: Peripheral blood & bone
marrow morphology shows magaloblastic anemia
 Homocysteinuria: Excretion of homocysteine in
urine

34.  Schilling test:
 Radioactive labelled (Cobalt-60) vitamin B12 1μ
g is given orally
 In gastric atrophy cases, there is no absorption,
hence the entire radioactivity is excreted in faeces
& radioactivity is not observed in liver
 If the cause is nutritional deficiency, there will be
increased absorption
 Then radioactivity is noted in the liver region, with
very little excretion in feces

36.  Harper’s Biochemistry 25th Edition.
 Fundamentals of Clinical Chemistry by Tietz.
 Text Book of Medical Biochemistry-A R Aroor.
 Text Book of Biochemistry-DM Vasudevan
 Text Book of Biochemistry-MN Chatterjea
 Text Book of Biochemistry-Dr.U.Satyanarana